Biogeosciences, 16, 309–327, 2019 https://doi.org/10.5194/bg-16-309-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. The export flux of particulate organic carbon derived from 210 Po/210Pb disequilibria along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise Yi Tang1,2 , Nolwenn Lemaitre3 , Maxi Castrillejo4,5 , Montserrat Roca-Martí5,6 , Pere Masqué5,7 , and Gillian Stewart2,1 1 Earth and Environmental Sciences, the Graduate Center, City University of New York, New York, USA 2 School of Earth and Environmental Sciences, Queens College, City University of New York, Flushing, USA 3 Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH-Zürich, Zürich, Switzerland 4 Laboratory of Ion Beam Physics, ETH-Zürich, Otto Stern Weg 5, Zürich, 8093, Switzerland 5 Institut de Ciència i Tecnologia Ambientals and Departament de Física, Universitat Autònoma de Barcelona, Barcelona, Spain 6 Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA 7 School of Science and Centre for Marine Ecosystems Research, Edith Cowan University, Joondalup, Western Australia, Australia Correspondence: Gillian Stewart (gillian.stewart@qc.cuny.edu) Received: 7 July 2018 – Discussion started: 20 August 2018 Revised: 3 January 2019 – Accepted: 7 January 2019 – Published: 24 January 2019 Abstract. The disequilibrium between 210 Po activity and phytoplankton community. The particularly low export effi- 210 Pb activity in seawater samples was determined along ciencies in the Iberian Basin, on the other hand, were ex- the GEOTRACES GA01 transect in the North Atlantic dur- plained by the dominance of smaller phytoplankton, such ing the GEOVIDE cruise (May–June 2014). A steady-state as cyanobacteria or coccolithophores. POC fluxes estimated model was used to quantify vertical export of particulate from the 210 Po/210 Pb and 234 Th/238 U disequilibria agreed 210 Po. Vertical advection was incorporated into one version within a factor of 3 along the transect, with higher POC es- of the model using time-averaged vertical velocity, which timates generally derived from 234 Th. The differences were had substantial variance. This resulted in large uncertain- attributed to integration timescales and the history of bloom ties for the 210 Po export flux in this model, suggesting that events. those calculations of 210 Po export fluxes should be used with great care. Despite the large uncertainties, there is no ques- tion that the deficits of 210 Po in the Iberian Basin and at the 1 Introduction Greenland Shelf have been strongly affected by vertical ad- vection. Using the export flux of 210 Po and the particulate The oceans play an essential role in the regulation of atmo- organic carbon (POC) to 210 Po ratio of total (> 1 µm) par- spheric CO2 and the buffering of the global climate system ticles, we determined the POC export fluxes along the tran- (e.g., Sabine, 2004) by removing carbon from the atmosphere sect. Both the magnitude and efficiency of the estimated POC via dissolution and photosynthesis in the surface ocean, and export flux from the surface ocean varied spatially within storing it in the dissolved or particulate forms. An important our study region. Export fluxes of POC ranged from negligi- component of this oceanic sequestration is the biological car- ble to 10 mmol C m−2 d−1 , with enhanced POC export in the bon pump, driven by sinking particles from the surface to Labrador Sea. The cruise track was characterized by overall the deep ocean (e.g., Falkowski et al., 1998; Ducklow et al., low POC export relative to net primary production (export 2001). efficiency < 1 %–15 %), but relatively high export efficien- The magnitude of particulate organic carbon (POC) ex- cies were seen in the basins where diatoms dominated the port flux from the upper ocean was traditionally obtained Published by Copernicus Publications on behalf of the European Geosciences Union. 310 Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect from time-series sediment traps (e.g., Honjo et al., 2008) and the natural radiotracer pair, 234 Th/238 U (e.g., Bhat et al., 1968; Buesseler et al., 1992). Here we focus on the application of another natural radionuclide pair: polonium- 210 (210 Po, T1/2 = 138.4 days) and its progenitor lead-210 (210 Pb, T1/2 = 22.3 years). The 210 Po/210 Pb pair has a dif- ferent particle-binding dynamic compared to the 234 Th/238 U pair since both isotopes are particle reactive, whereas 238 U is conservative and remains dissolved in seawater (Djogic et al., 1986). However, the nature of the particle association differs between the isotopes. Lead-210 and 234 Th are only adsorbed to particle surfaces, whereas 210 Po is both adsorbed to sur- faces and biologically reactive so it can be assimilated by or- ganisms and even bioaccumulated (Fisher et al., 1983; Cher- rier et al., 1995; Stewart and Fisher, 2003a, b). This behav- ior leads to a higher partitioning coefficient (relative associa- Figure 1. Map of stations occupied during the GA01 transect in the tion between the isotope and the particulate vs. the dissolved North Atlantic. The red squares indicate the stations where 210 Po phase) of 210 Po compared to that of 210 Pb (e.g., Masqué et and 210 Pb activities were measured, as discussed in this study. The al., 2002; Wei et al., 2014; Tang et al., 2017). transect is divided into the Iberian Basin (stations 1, 13), the West- Lead-210 in the water column comes both from atmo- ern European Basin (stations 21, 26), the Iceland Basin (stations 32, spheric deposition and in situ production via the decay of 38), the Irminger Basin (stations 44, 60), and the Labrador Basin 226 Ra. The residence time of 210 Pb in the atmosphere is only (stations 64, 69, 77). of days to weeks (Moore et al., 1974; Turekian et al., 1977). Polonium-210 (produced by decay of 210 Pb via 210 Bi) ac- tivity in aerosols, and the subsequent fluxes to the surface fluxes estimated from 210 Po/210 Pb disequilibria were com- ocean, are only about 10 %–20 % of those of 210 Pb (Masqué pared with those derived from 234 Th/238 U disequilibria. et al., 2002). The large difference in their particle reactivity and half-lives often leads to a disequilibrium between 210 Po 2 Methods and 210 Pb activities in the upper water column as particles sink. 2.1 Cruise track and hydrographic setting This deviation from secular equilibrium, often in the form of a deficit of 210 Po activity with respect to 210 Pb activity, The GEOVIDE cruise (GEOTRACES GA01 transect) was can be used to estimate POC export in a similar manner to the carried out in May–June 2014 from Lisbon to Newfound- application of the 234 Th/238 U disequilibrium (Friedrich and land (Fig. 1). Seawater and particulate samples for 210 Po and Rutgers van der Loeff, 2002; Verdeny et al., 2009; Wei et al., 210 Pb activity analysis were collected from the water column 2011). Particle export fluxes estimated from the 234 Th/238 U at 11 stations (Fig. 1). The GA01 transect can be separated and the 210 Po/210 Pb disequilibria integrate export that has into five sections according to its biogeochemical character- occurred on timescales of weeks to months prior to the sam- istics, described in detail by Lemaitre et al. (2018). From east pling time, respectively. The use of both isotope pairs could to west, these are the Iberian Basin (stations 1, 13), the West- provide complementary information on the causes, timing, ern European Basin (stations 21, 26), the Iceland Basin (sta- and efficiency of export fluxes of POC (e.g., Murray et al., tions 32, 38), the Irminger Basin (stations 44, 60), and the 2005; Stewart et al., 2007; Roca-Martí et al., 2016). Labrador Basin (stations 64, 69, 77). In this study along the GEOTRACES GA01 transect in the North Atlantic, we first used a traditional scavenging model 2.2 Radionuclides sampling and analysis with the assumptions of steady-state and negligible physical transport to derive 210 Po fluxes over different depths of the Radionuclide data were produced by two collaborating labo- water column at 11 stations. Then, vertical advection (pri- ratories to ensure higher counting statistics for 210 Po activity marily upwelling) was considered, and its impact on 210 Po in the samples: the Laboratori de Radioactivitat Ambiental flux was assessed. Using the POC concentration, and particu- at Universitat Autònoma de Barcelona (UAB) (samples from late 210 Po activity in the particles collected by in situ pumps, stations 1, 13, and 21) and the Stewart Laboratory at Queens sinking fluxes of POC were then calculated. The magnitude College (QC) (samples from stations 26, 32, 38, 44, 60, 69, and efficiency of carbon export derived from the 210 Po/210 Pb and 77). The sampling method for total and particulate 210 Po disequilibrium was considered in relation to the composition and 210 Pb samples and the determination of the radionuclide of the phytoplankton community. Finally, the POC export activity was described in Tang et al. (2018). In brief, wa- ter samples (5–10 L each) for total 210 Po and 210 Pb activity Biogeosciences, 16, 309–327, 2019 www.biogeosciences.net/16/309/2019/ Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect 311 were collected using Niskin bottles at 10 full water column that assumes the negligible atmospheric input of 210 Po ac- stations (16–22 depths/station) and at 1 station to 1000 m tivity and ignores advection and diffusion. In this case, the (9 depths), for a total of 200 samples. Particulate 210 Po and 210 Po flux (P ) can be simplified as follows: 210 Pb were collected at 3–10 depths per station between 15 and 800 m by using McLane in situ pumps equipped with a P = λPo (IPb − IPo ). (2) 53 µm PETEX screen to capture the large-size particles and The influences of advection and non-steady-state (NSS) pro- a 1 µm quartz fiber QMA filter to capture small particles. cesses on the overall 210 Po activity balance are discussed be- The average equivalent volume filtered for particulate 210 Po low in Sect. 4.1 and 4.2. and 210 Pb samples through the PETEX screen was 200 L and Many previous studies have used a single fixed integration through the QMA filter was 70 L. depth for export calculations at all sampling locations (e.g., For water samples, Po and Pb isotopes (including the 100 m in the Antarctic Circumpolar Current, Rutgers van der added chemical yield tracers of 209 Po and stable lead) were Loeff et al., 1997; 120 m in the central equatorial Pacific, co-precipitated with cobalt–ammonium pyrrolidine dithio- Murray et al., 2005). The GA01 transect, however, crossed carbamate (Co-APDC) (Fleer and Bacon, 1984) at sea, but diverse physical and biogeochemical conditions. Thus, in- digested using concentrated HCl and HNO3 back at the home vestigating export at a single fixed depth for every station laboratories. Particulate samples were spiked with 209 Po and may bias the spatial comparisons of particle export. In this stable lead before acid digestion (UAB: HNO3 /HCl/HF, study, four site-specific integration depths were used for each QC: HNO3 /HCl). Polonium isotopes (209 Po and 210 Po) were station: the mixed-layer depth (MLD), the depth of the eu- plated by deposition onto a sliver disc (Flynn, 1968) and their photic zone (Z1 % ), the primary production zone (PPZ), and activity was determined by alpha spectrometry. After remov- the 234 Th-238 U equilibrium depth (ThEq). The MLD was de- ing any remaining Po isotopes by running the plating solution fined as a change in potential density of 0.03 kg m−3 relative through an anion exchange column, the solution was respiked to the potential density at 10 m (Weller and Plueddemann, with 209 Po and stored for at least 6 months. Lead-210 activity 1996). Z1 % was defined as the depth at which photosyn- was determined by plating the ingrowth of 210 Po from 210 Pb. thetic available radiation was 1 % of its surface value (Jerlov, The activities of 210 Po and 210 Pb at the sampling date were 1968). PPZ was the depth at which the fluorescence reaches determined by correcting for nuclide decay, ingrowth, chem- 10 % of its maximum (Owens et al., 2015). ThEq was the ical recoveries, detector backgrounds, and blank contamina- depth at the bottom of the total 234 Th water column deficit, tion (Rigaud et al., 2013). where the activity of 234 Th equals that of 238 U (data from Lemaitre et al., 2018). These depths were used both to calcu- 2.3 The 210 Po flux method late 210 Po and POC export and in order to compare the POC The export flux of 210 Po was estimated from total 210 Po and export fluxes estimated from the 210 Po/210 Pb disequilibria to 210 Pb activities using a one-box model (Broecker et al., 1973; those derived from the 234 Th/238 U disequilibria. Among the Matsumoto, 1975; Savoye et al., 2006). The 210 Po activity 11 stations, the depths of the MLD (23 ± 7 m) were similar in the surface ocean is the result of a balance between atmo- to those at Z1 % (31 ± 9 m), whereas the depths of the PPZ spheric input, continuous production from the decay of 210 Pb (72 ± 29 m) and ThEq (95 ± 43 m) were deeper and compa- in seawater, radioactive decay of 210 Po, removal onto sink- rable to each other. For the depths of MLD, Z1 % , PPZ, and ing particles, and transport into or out of the box by advec- ThEq at which total radionuclides data are not available, the tion and diffusion. Therefore, the general form of the mass measured values of total 210 Po and 210 Pb activities were lin- balance equation for 210 Po between sources and sinks is as early interpolated (Table 1). follows: The 210 Po flux was then used to derive the flux of POC by multiplying the deficit of 210 Po by the ratio of POC concen- ∂Po/∂t = FPo + λPo IPo − λPo IPo − P + V , (1) tration to 210 Po activity (POC/210 Po) of the total and large particulate material. Particulate 210 Po and POC data were where ∂Po/∂t is the change in 210 Po activity with time, FPo not always available at the depths of the MLD, Z1 % , PPZ, (dpm m−2 d−1 ) is the atmospheric flux of 210 Po to the sea and ThEq at our study sites. To estimate POC/210 Po ratios at surface, λPo is the decay constant of 210 Po (0.005 d−1 ), IPb these depths, a regression was performed between the mea- and IPo (dpm m−2 ) are the inventories of 210 Pb and 210 Po sured POC/210 Po ratios and depth for each basin using a sin- activities, P (dpm m−2 d−1 ) is the removal flux of 210 Po via gle power law function. sinking particles, and V (dpm m−2 d−1 ) is the sum of the ad- vective and diffusive fluxes. 2.4 Quantification of the influence of the vertical The atmospheric flux of 210 Po is usually ignored as it rep- advection on 210 Po export resents only ∼ 2 % of the in situ production of 210 Po from 210 Pb in the upper water column of the open ocean (e.g., Cyclonic or anticyclonic eddies constantly impact the hori- Cochran, 1992; Masqué et al., 2002; Murray et al., 2005; zontal velocity fields at our study sites (Zunino et al., 2017), Verdeny et al., 2008). We first used a steady-state (SS) model changing the current directions and making it difficult to es- www.biogeosciences.net/16/309/2019/ Biogeosciences, 16, 309–327, 2019 312 Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect Table 1. The mixed-layer depth (MLD, defined as a change in potential density of 0.03 kg m−3 relative to the potential density at 10 m), the depth of the euphotic zone (Z1 % , defined as the depth at which photosynthetic available radiation was 1 % of its surface value), the primary production zone (PPZ, at which the fluorescence reaches 10 % of its maximum), and the 234 Th-238 U equilibrium depth (ThEq) at each station along the GA01 transect. Together with the 30-day (30 days prior to the sampling date) average vertical velocity within the 20 m under the corresponding depths (w20 , 10−6 m s−1 , downwards as positive direction). Primary production (PP) and net primary production (NPP) rates derived from 24 h bottle incubations (Fonseca-Batista et al., 2018; Lemaitre et al., 2018) and from the VGPM products, respectively, are also presented. Note that the NPP rates were averaged for the previous 138 days (210 Po half-life) prior to the sampling date. Dates are in mm/dd/yy format. Integration depth (m) w20 (10−6 m s−1 ) Production (mmol C m−2 d−1 ) St. Sampling Basin MLD Z1 % PPZ ThEq MLD Z1 % PPZ ThEq PP ± NPP ± date 1 5/19/14 Iberian Basin 15 40 136 90 −1 ± 5 −1 ± 4 −3 ± 2 −2 ± 6 33 2 69 43 13 5/24/14 Iberian Basin 35 40 90 110 0.1 ± 3.1 0.1 ± 3.1 −2 ± 5 0.4 ± 54.6 79 3 61 32 21 5/31/14 Western European 15 32 64 110 −1 ± 2 −1 ± 4 −1 ± 5 0.1 ± 5 135 2 109 112 Basin 26 6/4/14 Western European 30 30 98 100 −2 ± 3 −2 ± 2 −5 ± 5 −5 ± 4 174 19 58 57 Basin 32 6/7/14 Iceland Basin 30 31 70 120 −1 ± 9 −1 ± 9 −4 ± 20 −3 ± 20 105 11 48 36 38 6/10/14 Iceland Basin 30 30 69 80 1±3 1±3 3±4 3±5 68 7 44 37 44 6/13/14 Irminger Basin 26 22 44 40 1±2 1±2 2±3 2±3 137 2 46 44 60 6/18/18 Irminger Basin 17 20 36 100 −14 ± 20 −14 ± 20 −36 ± 40 −11 ± 70 166 32 50 51 64 6/19/14 Labrador Basin 20 47 80 80 2±6 7±7 3±7 3±7 54 18 47 49 69 6/22/14 Labrador Basin 20 28 44 40 −2 ± 1 −2 ± 3 −2 ± 3 −2 ± 3 27 5 46 56 77 6/26/14 Labrador Basin 15 20 59 80 4±5 4±7 7 ± 10 9 ± 20 80 21 50 56 timate the magnitude of horizontal velocities. This constant Total 210 Po fluxes at each depth, therefore, are the sum variability, together with the patchiness of sampling resolu- of the steady-state values based only on the 210 Po deficit tion, meant we could not assess the influence of horizontal (Eq. 2), λPo (IPb −IPo ), and vertical advective flux (Eq. 3), advective processes on 210 Po export estimates. w20 × A2Po − A1Po . However, because we had relatively high depth resolution The ECCO2 vertical velocities were obtained from the at each station, we did attempt to assess the influences of Asia-Pacific Data-Research Center (APDRC, http://apdrc. vertical advection on 210 Po inventories at all the investigated soest.hawaii.edu/las/v6/dataset?catitem=1, last access: 18 depths by measuring the vertical gradient of 210 Po activity January 2019). The ECCO2 model configuration uses a and multiplying it by a time-averaged vertical velocity. Be- cube–sphere grid projection with 18 km horizontal grid spac- cause the water column inventory of 210 Po represents an in- ing and 50 vertical levels among which there are 12 equal tegration of the changes over approximately the mean life vertical layers from the surface to 120 m (Menemenlis et al., of the isotope, we did not use the vertical velocity measured 2008). We selected the ECCO2 grid points closest to the sta- by the acoustic doppler current profiler (ADCP) at the sam- tion and extracted vertical velocities from the depths between pling time, but a time-averaged vertical velocity from the Es- z and (z + 20 m) during 30 days prior to the sampling date at timating the Circulation and Climate of the Ocean, Phase II each station. Because the deficit of 210 Po activity in the wa- (ECCO2). The activity gradient of 210 Po below the depth z ter column weighs the changes that occurred shortly prior to (i.e., the MLD, Z1 % , PPZ, and ThEq) at each station was the sampling time more heavily than those that occurred fur- calculated from the depth z (using the average activity in the ther back in time (Verdeny et al., 2009), we chose to average layer of 0-z m) as starting point (A1Po ) and linearly interpo- the vertical velocity over 1 month rather than over the mean lated through the measurements 20 m below z (A2Po ) at each life of 210 Po (200 days). The 30-day averaged vertical veloc- station. A positive gradient (A2Po − A1Po > 0) was defined as ity was then used to calculate vertical advective 210 Po export higher activity at the depth of (z + 20 m) than the starting flux via Eq. (3) at each station. point. We labeled the vertical velocity w20 , which was the 30-day (30 days prior to the sampling date) average vertical velocity between the depths of z and (z + 20 m). The flux of 210 Po due to vertical advection (F ) was calculated as the w following:   Fw = w20 × A2Po − A1Po . (3) Biogeosciences, 16, 309–327, 2019 www.biogeosciences.net/16/309/2019/ Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect 313 2.5 Satellite-based net primary production and over, this could also be due to shorter blooms in the Irminger phytoplankton composition and Labrador basins where the phytoplankton growth was light-limited during winter compared to the conditions in the The 8-day net primary production (NPP) data with a spatial Iberian and Western European basins. resolution of 0.083 by 0.083◦ were obtained from the Oregon The satellite-derived phytoplankton species composition State University Ocean Productivity standard products (http: demonstrated unique features within the basins (Fig. 2). The //www.science.oregonstate.edu/ocean.productivity/, last ac- Iberian Basin was dominated (> 60 %) by coccolithophores cess: 18 January 2019), wherein NPP was estimated by the between October 2013 and July 2014, but had a gradual in- Vertically Generalized Production Model (VGPM) (Behren- crease in the contribution of diatoms until April 2014 and a feld and Falkowski, 1997). Due to some missing data be- decreasing contribution after that. In the Western European tween November 2013 and February 2014, NPP for each sta- Basin, station 26 was dominated by diatoms all year round, tion was averaged for the previous 138 days (210 Po half-life) while station 21 was dominated by diatoms, except in Oc- instead of 200 days (210 Po mean life). tober 2013 and July 2014 when the combination of chloro- Monthly average concentrations of diatoms, coccol- phytes and coccolithophores contributed 35 %–77 % to the ithophores, cyanobacteria, chlorophytes, and total chloro- total chlorophyll concentration. The stations in the Iceland, phyll with the spatial resolution of 0.67 × 1.25◦ were ob- Irminger, and Labrador basins were all dominated (> 98 %) tained from the Goddard Earth Science Data and Information by diatoms between October 2013 and July 2014. Services Center Interactive Online Visualization and Analy- sis Infrastructure (Giovanni) (https://giovanni.gsfc.nasa.gov/ 3.2 One-month averaged vertical velocity w20 giovanni/ last access: 18 January 2019, Acker and Lep- toukh, 2007). Time-series (October 2013–July 2014, cover- The 1-month averaged vertical velocities w20 ranged from ing > 200 days before sampling) data are averages over lon- −36 × 10−6 to 9 × 10−6 m s−1 along the transect (negative is gitude for each month. We extracted data for the five basins upwelling, positive is downwelling, Table 1). The standard individually and calculated the fraction of each phytoplank- deviations of w20 were generally of the same order as the ton group at each station as the ratio of their concentration to values of w20 . Particularly large standard deviations, which total chlorophyll concentration. exceed the typical values of the vertical velocity by a full order of magnitude, were found at stations 13 (35–55, 110– 130 m) and 21 (110–130 m). These high standard deviations 3 Results suggest that the data on w20 should be used with great care. Downwelling was seen at stations 38, 44, 64, and 77 with 3.1 Satellite-derived seasonal NPP and phytoplankton the velocities in the range of 1 × 10−6 to 9 × 10−6 m s−1 . composition Upwelling was seen at the remaining stations, with high- est intensity at station 60 near Greenland (absolute value: The VGPM modeled NPP data along the GA01 transect 11–36 × 10−6 m s−1 ). The upwelling velocities were roughly was averaged over ∼ 138 days prior to the sampling date equivalent at stations 1, 13, 21, 26, 32, and 69 (absolute (see Sect. 2.5, Table 1). Seasonal NPP at each station var- value: 1–5 × 10−6 m s−1 ). ied from low values of 44–79 to a maximum value of 109 mmol C m−2 d−1 at station 21. The Western European 3.3 Total 210 Po deficits Basin had the highest seasonal NPP, followed by the Iberian Basin, while the Iceland Basin, the Irminger Basin, and the The vertical profiles of total 210 Po and 210 Pb activity at each Labrador Basin all had similar NPP values in the range of 45– station have been described in a companion article (Tang et 49 mmol C m−2 d−1 . There was a shift in the biological com- al., 2018). Here we show the section view of the water col- munity towards larger phytoplankton (e.g., diatoms) from umn 210 Po deficit (dpm 100 L−1 ), which was calculated as east to west along the transect (Fig. 2). The basins for which total 210 Pb activity minus total 210 Po activity (Fig. 3). There diatoms were the dominant phytoplankton group did not nec- were small 210 Po deficits in the upper 100 m (including the essarily have higher seasonal production relative to the basins majority of the depths of MLD, Z1 % , PPZ, and ThEq at all where smaller phytoplankton (e.g., coccolithophores) were stations) at stations 1, 13, and 21, whereas a relatively large more abundant. Indeed, the Iberian Basin had the second excess of 210 Po was observed at 100–400 m depth. Station 60 highest seasonal NPP, despite the fact that the majority of had the highest deficits of 210 Po (∼ 8 dpm 100 L−1 , n = 5) at chlorophyll was produced by coccolithophores. Despite the 40–120 m depth. A large surface deficit of 210 Po was found evidence that earlier blooms may have been driven by di- at station 64 (8 dpm 100 L−1 ) and a surface excess was found atoms (see Sect. 4.2), these observations highlight the possi- at station 38 (−3.5 dpm 100 L−1 ). There were positive 210 Po ble contribution of small particles to production, and possibly deficits throughout most of the water column at stations in the to export (proportional to their role in production according Irminger and Labrador basins, whereas large 210 Po excesses to Richardson and Jackson, 2007), along the transect. More- (negative deficits) below 100 m were generally seen in the www.biogeosciences.net/16/309/2019/ Biogeosciences, 16, 309–327, 2019 314 Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect Figure 2. Satellite-derived monthly average fraction of major phytoplankton groups from October 2013 to July 2014 along the GA01 transect: fdia , fcoc , fcya , and fchl are the fractions of diatoms (purple), coccolithophores (blue), cyanobacteria (gray), and chlorophytes (orange), respectively. Data are from the Giovanni online data system https://giovanni.gsfc.nasa.gov/giovanni/ (last access: 18 January 2019). Iberian Basin and Western European basins. Such 210 Po ex- or very low at stations 1, 21, and 38. At the other stations cess was likely related to the Iberian upwelling, which may the 210 Po fluxes averaged 3.7 ± 1.4, 4.6 ± 2.6, 9.5 ± 4.9, and have provided a source of 210 Po activity. 14.4 ± 12 dpm m−2 d−1 at the MLD, Z1 % , PPZ, and ThEq, respectively. The 210 Po fluxes tended to increase with depth 3.4 The 210 Po flux calculated from the deficit of 210 Po at 7 out of 11 stations (26, 38, 44, 60, 64, 69, and 77). At alone the MLD, Z1 % and PPZ, the largest 210 Po fluxes were all found in the Labrador Basin. The other four basins had rela- Using the data of total 210 Po and 210 Pb activities, the amount tively similar 210 Po export fluxes (2.1–2.8 dpm m−2 d−1 ) at of 210 Po escaping from the surface ocean via particles (210 Po the MLD and Z1 % . The West European Basin had much fluxes, dpm m−2 d−1 ) was calculated using Eq. (2) assum- higher 210 Po flux (8.7 dpm m−2 d−1 ) relative to that in the ing steady state and ignoring advection and diffusion (Ta- Iberian Basin (−0.1 dpm m−2 d−1 ) at the PPZ. At the ThEq, ble 2, 210 Po/210 Pb term). The 210 Po fluxes were negligible Biogeosciences, 16, 309–327, 2019 www.biogeosciences.net/16/309/2019/ Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect 315 Table 2. The total 210 Po flux as the sum of the flux calculated from the deficit and vertical advection, together with POC/210 Po ratios in particles > 1 µm (derived from the power law function in Fig. 5) and POC fluxes derived from 210 Po at the corresponding depths. The uncertainties of 210 Po export flux are associated with the activity uncertainty of the radionuclides. The error for the calculated particulate POC/210 Po ratio in each basin is the standard error of regression. The uncertainties of the 210 Po-derived POC flux were estimated based on the propagation of error. Integration depth (m) 210 Po flux (dpm m−2 d−1 ): 210 Po/210 Pb term 210 Po flux (dpm m−2 d−1 ): vertical advection term St. MLD Z1 % PPZ ThEq MLD ± Z1 % ± PPZ ± ThEq ± MLD ± Z1 % ± PPZ ± ThEq ± 1 15 40 136∗ 90∗ 1.1 0.3 1.5 0.8 −4.5 2.2 −0.9 1.6 2.4 19.7 3.6 14.7 6.8 4.8 4.6 16.2 13 35∗ 40 90∗ 110∗ 3.4 0.9 4.1 0.9 4.3 1.8 3.7 2.0 −0.2 5.2 −0.2 5.6 3.7 10.0 1.0 10.6 21 15 32∗ 64∗ 110∗ −0.6 0.5 −0.7 0.8 2.2 1.2 3.5 1.8 −1.1 4.0 −0.4 1.7 2.7 9.9 0.01 0.40 26 30 30 98∗ 100 4.8 1.5 4.8 1.5 15.2 3.1 26.4 4.8 −0.9 3.2 −0.9 3.2 4.0 4.0 2.8 4.0 32 30 31∗ 70∗ 120∗ 4.7 0.9 4.8 0.9 9.1 1.4 8.5 2.2 −1.6 12.2 −1.6 12.0 7.9 33.4 3.0 23.3 38 30 30 69∗ 80 −0.5 1.3 −0.5 1.3 3.7 2.5 5.2 2.6 0.4 1.8 0.4 1.8 −1.0 3.5 −0.9 4.9 44 26∗ 22∗ 44∗ 40 1.5 1.0 1.0 1.0 4.2 1.4 3.6 1.4 0.9 2.1 1.1 2.5 0.9 2.2 1.5 2.8 60 17∗ 20 36∗ 100 3.1 1.1 3.8 1.1 9.8 1.6 37 5.4 −24.9 49.6 −40.4 74.9 −36.2 69.0 14.1 87.1 64 20∗ 47∗ 80 80 5.8 0.8 9.8 2.1 17.8 3.2 18 3.2 −0.7 2.9 −4.3 8.8 −0.5 3.7 −0.5 3.7 69 20∗ 28∗ 44∗ 40 4.0 0.7 6.1 0.8 8.5 1.6 8.3 1.5 1.9 3.3 3.4 5.8 5.8 7.9 6.7 8.9 77 15∗ 20 59∗ 80 2.2 0.6 2.9 0.7 7.0 2.4 9.8 2.9 −0.6 5.2 0.3 6.4 3.0 9.9 −15 29 210 Po flux (dpm m−2 d−1 ): total flux POC/210 Po (mol dpm−1 ) St. MLD ± Z1 % ± PPZ ± ThEq ± MLD ± Z1 % ± PPZ ± ThEq ± 1 3.5 19.7 5.1 14.7 2.3 5.3 3.6 16.2 540 67 305 67 150 67 190 67 13 3.2 5.3 3.9 5.7 7.9 10.1 4.7 10.8 330 67 305 67 190 67 169 67 21 −1.7 4.1 −1.1 1.8 4.9 10.0 3.5 1.9 542 89 389 89 287 89 227 89 26 3.9 3.5 3.9 3.5 17.7 5.1 29.2 6.2 400 89 400 89 238 89 236 89 32 3.0 12.2 3.2 12.1 17.0 33.4 11.6 23.4 367 111 363 111 265 111 216 111 38 −0.2 2.3 −0.2 2.3 2.7 4.3 4.2 5.6 367 111 367 111 267 111 252 111 44 2.5 2.3 2.1 2.7 5.1 2.6 5.1 3.1 310 107 330 107 254 107 263 107 60 −21.8 49.6 −36.6 74.5 −26.4 69.0 51.2 87.2 364 107 342 107 274 107 187 107 64 5.1 3.0 5.5 9.0 17.4 4.9 17.4 4.9 675 152 375 152 261 152 261 152 69 5.9 3.4 9.4 5.8 14.4 8.0 15.0 9.0 675 152 536 152 393 152 419 152 77 1.5 5.2 3.1 6.4 10.1 10 −4.8 29.0 822 152 675 152 321 152 261 152 210 Po–POC flux (mmol C m−2 d−1 ): 210 Po/210 Pb term 210 Po–POC flux (mmol C m−2 d−1 ): total flux St. MLD ± Z1 % ± PPZ ± ThEq ± MLD ± Z1 % ± PPZ ± ThEq ± 1 0.6 0.2 0.4 0.3 −0.7 0.4 −0.2 0.3 1.9 10.7 1.5 4.5 0.3 0.8 0.7 3.1 13 1.1 0.4 1.3 0.4 0.8 0.4 0.6 0.4 1.0 1.8 1.2 1.7 1.5 2.0 0.8 1.9 21 −0.3 0.3 −0.3 0.3 0.6 0.4 0.8 0.5 −0.9 2.2 −0.4 0.7 1.4 2.9 0.8 0.5 26 1.9 0.7 1.9 0.7 3.6 1.5 6.2 2.6 1.5 1.4 1.5 1.4 4.6 2.0 6.9 3.0 32 1.7 0.6 1.7 0.6 2.4 1.1 1.8 1.1 1.1 4.5 1.1 4.4 4.5 9.1 2.5 5.2 38 −0.2 0.5 −0.2 0.5 1.0 0.8 1.3 0.9 −0.1 0.8 −0.1 0.8 0.7 1.2 1.1 1.5 44 0.5 0.4 0.3 0.4 1.1 0.6 1.0 0.5 0.8 0.8 0.7 0.9 1.3 0.9 1.4 1.0 60 1.1 0.5 1.3 0.5 2.7 1.1 6.9 4.1 −7.9 20 −12.5 25.9 −7.2 19.1 9.6 17.2 64 3.9 1.0 3.7 1.7 4.7 2.8 4.7 2.8 3.5 2.1 2.1 3.5 4.5 2.9 4.5 2.9 69 2.7 0.8 3.3 1.0 3.4 1.4 3.5 1.4 4.0 2.5 5.1 3.4 5.7 3.8 6.3 4.4 77 1.8 0.6 1.9 0.6 2.3 1.3 2.5 1.7 1.3 4.3 2.1 4.3 3.2 3.6 −1.3 7.6 ∗ For the depths at which total radionuclides data are not available, the measured values of total 210 Po and 210 Pb activities were linearly interpolated at the missing depths. www.biogeosciences.net/16/309/2019/ Biogeosciences, 16, 309–327, 2019 316 Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect Figure 3. Section plots of water column 210 Po deficits (dpm 100 L−1 , total 210 Pb activity minus total 210 Po activity) across the GA01 transect. Panel (a) is the upper 500 m. Panel (b) is 500–5500 m. Station numbers and basins are shown at the top of the upper panel. on the other hand, the Irminger Sea had the highest 210 Po 320 µmol dpm−1 (n = 51, upper 800 m). The variability of fluxes followed by the West European Basin. The lowest POC/210 Po_TPF ratios in this study is in line with pre- 210 Po fluxes at all investigated depths were generally found vious observations in the Antarctic Circumpolar Current in the Iberian Basin. (300–1200 µmol dpm−1 for particles > 1 µm) (Friedrich and Rutgers van der Loeff, 2002) and the central Arctic (90– 3.5 POC/210 Po ratios in particles 1900 µmol dpm−1 for particles > 53 µm) (Roca-Martí et al., 2016). The average ratio of 290 µmol dpm−1 is compa- Most of the ratios of POC concentration to 210 Po ac- rable to those observed in the central equatorial Pacific tivity (µmol dpm−1 ) in the large-size fraction of particles (202±90 µmol dpm−1 for particles > 0.45 µm) (Murray et al., (POC/210 Po_LSF, > 53 µm) were comparable to or higher 2005), the North Atlantic (290 ± 70 µmol dpm−1 for parti- than those in the small-size fraction (POC/210 Po_SSF, 1– cles > 1 µm) (Rigaud et al., 2015), and the South Atlantic 53 µm), although a few samples at stations 13, 26, 44, 64, (113 ± 80 µmol dpm−1 for particles > 0.7 µm) (Sarin et al., and 77 had lower values of POC/210 Po_LSF than those of 1999). POC/210 Po_SSF (Table 3, Fig. 4a). The POC/210 Po ratio in The measured POC/210 Po ratios in total particles at each the total particles (> 1 µm, the combination of small and large station and depth were grouped into the five basins and fit- particles, POC/210 Po_TPF) was similar to that in the small ted against depth using a single power law function in each particles (SSF), within about 97 % (Table 3, Fig. 4b). This is basin (Fig. 5). The fit equations were used to calculate to- because over 80 % of the particulate 210 Po activity was asso- tal particulate POC/210 Po ratios at the investigated depths at ciated with the small-size fraction (Tang et al., 2018) likely each station (Table 3). due to the large surface area of abundant small particles. Be- cause of the possible link between small particles and ex- port along the transect discussed in Sect. 3.1, and the results 4 Discussion that scavenging of 210 Po was governed by the small particles (Tang et al., 2018), we propose to use this total particulate 4.1 Physical advection effects on 210 Po export fluxes fraction in addition to the more commonly used large-size fraction to calculate POC export along this cruise track. In the study region, there were consistent patterns of circula- The POC/210 Po in total particles (POC/210 Po_TPF) var- tion traveling through and near our sampling sites during the ied from 19 to 1300 µmol dpm−1 with a mean of 290 ± GEOVIDE cruise. From east to west the cruise track crossed Biogeosciences, 16, 309–327, 2019 www.biogeosciences.net/16/309/2019/ Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect 317 Table 3. The ratio of POC concentration to 210 Po activity (POC/210 Po) in the particles collected by in situ pumps. SSF is small-size fraction (1–53 µm); LSF is large-size fraction (> 53 µm); TPF is total particulate fraction (> 1 µm). POC/210 Po (µmol dpm−1 ) Station Depth (m) SSF ± LSF ± TPF ± 1 30 276 32 414 58 296 30 1 80 166 28 1040 159 355 44 1 550 41 4 31 4 39 4 1 800 18 17 19 4 19 10 1 120 108 14 222 42 117 13 1 250 65 7 63 9 65 6 13 60 289 29 216 26 281 25 13 100 206 20 132 14 198 17 13 200 79 7 50 8 76 7 13 450 73 7 35 7 69 6 21 80 622 51 13 405 2599 1280 96 21 120 133 18 2398 407 380 44 21 250 85 9 482 133 109 10 21 450 54 6 117 14 60 6 26 30 377 70 310 34 350 42 26 83 271 41 289 37 280 28 26 153 275 94 118 14 209 43 26 403 67 21 43 19 62 17 32 30 492 60 733 382 500 59 32 60 379 43 337 87 376 40 32 100 311 39 376 56 326 33 32 200 145 17 133 30 144 15 32 450 41 5 55 9 42 4 32 800 25 4 55 7 29 4 38 20 254 38 345 108 258 37 38 60 339 51 284 66 333 46 38 109 157 15 196 23 163 13 44 20 1025 115 3085 798 1176 124 44 40 463 58 1379 1787 475 59 44 80 140 14 90 23 137 13 44 150 102 18 97 56 102 17 44 300 47 7 25 7 45 6 60 8 306 30 1003 150 422 36 60 60 232 33 851 193 272 36 60 100 197 33 303 72 209 31 60 250 61 7 294 84 72 8 64 30 525 77 656 83 580 58 64 60 455 75 286 77 434 64 64 100 439 49 319 44 420 41 64 150 107 36 158 28 129 24 64 400 40 5 48 8 41 4 69 20 347 44 879 164 397 46 69 60 78 6 657 216 84 7 69 100 257 26 359 44 268 24 69 150 125 14 127 25 125 13 69 410 30 3 71 8 34 3 77 10 1281 309 917 150 1181 213 77 50 1372 357 1020 412 1339 320 77 80 512 63 544 103 516 57 77 200 84 13 217 79 92 13 77 460 22 3 59 6 27 3 www.biogeosciences.net/16/309/2019/ Biogeosciences, 16, 309–327, 2019 318 Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect alone (−5 to 37 dpm m−2 d−1 , Table 2). The magnitude of the uncertainty of the 210 Po export flux due to vertical advec- tion was influenced by the large variance in vertical veloc- ity field mentioned in Sect. 3.2. When excluding the three depths at stations 13 and 21, where the monthly vertical velocity average had substantial standard deviations (an or- der of magnitude greater than w20 ), the uncertainty of the 210 Po export flux was on average 2-fold larger than the cal- culated 210 Po export flux. The largest positive vertical ad- vective 210 Po fluxes were at station 1, where the Iberian up- welling increased the calculated flux by 150 %–500 %. The largest negative vertical advective 210 Po fluxes were seen at station 60 where upwelling decreased the 210 Po flux by 370 %–1100 % at the depths of the MLD, Z1 % , and PPZ. This is because the upwelling velocity was high at those depths (14–36×10−6 m s−1 , Table 1) and the upwelled water was depleted in 210 Po activity. The vertical advective trans- port was smaller at the MLD and Z1 % at station 13, at the ThEq at station 21, and at the PPZ and ThEq at station 64, with contributions lower than 6 % of the total 210 Po fluxes. Figure 4. Plots of the ratios of POC concentration to 210 Po activ- Including vertical advection in our flux estimates at all other ity in (a) the large (> 53 µm) particles (POC/210 Po_LSF) against depths, however, increased or decreased the 210 Po fluxes by the small (1–53 µm) particles (POC/210 Po_SSF), and in (b) the to- 10 %–180 %. Like with vertical advection, neglecting hori- tal (> 1 µm) particles (POC/210 Po_TPF) against the small particles. zontal advection can result in either an underestimate or over- The black lines indicate the 1 : 1 line. estimate of 210 Po export flux depending on whether the ad- vected water is enriched or depleted in 210 Po. However, be- cause our study region was characterized by distinct water the North Atlantic Current, the East Reykjanes Ridge Cur- masses separated over tens to hundreds of meters in the ver- rent, the Irminger Current, the Irminger Gyre, the eastern and tical plane, whereas those same water masses covered huge western boundary currents and the Labrador Current (Fig. 1 distances (hundreds to thousands of kilometers) in the hor- in García-Ibáñez et al., 2018). Additionally, short-lived ed- izontal plane (Fig. 4 in García-Ibáñez et al., 2018), vertical dies and fronts were also observed during the cruise, par- advection would most likely result in more change in physi- ticularly in the OVIDE section from Portugal to Greenland cal and chemical parameters over the scale of sampling than (García-Ibáñez et al., 2018; Zunino et al., 2017). In this dy- horizontal advection would. Because the advective 210 Po ex- namic region advective influences may be important to in- port flux was calculated as the product of the velocity of the clude in calculations of 210 Po export. Despite this knowl- water mass and the gradient of 210 Po activity in the corre- edge, we could not include horizontal advection in our model sponding direction, horizontal advection would most likely because the horizontal resolution of our sample sites was contribute a much smaller range of advective 210 Po flux esti- not sufficient to constrain reliable horizontal gradients of mates. 210 Po activity in the study region. This assumption of neg- Overall, the influence of physical advection on 210 Po ac- ligible horizontal physical transport has been made in most tivity may range from relatively unimportant to dominant de- 210 Po studies because of a similar lack of spatial resolution pending on the study area. In this study, we observed physical (e.g., Kim and Church, 2001; Stewart et al., 2010; Rigaud processes influencing 210 Po fluxes, in particular at stations et al., 2015), and may be justified in some open-ocean set- 1 and 60. For future studies of 210 Po and 210 Pb activity in tings where horizontal gradients in 210 Po activity are small regions of established upwelling or ocean margins, we sug- (e.g., Wei et al., 2011). For more dynamic regimes, such as gest designing the sampling plan so that the magnitude and along the GA01 transect, however, this assumption needs to variability of these processes may be incorporated into 210 Po be carefully evaluated, and the relative importance of advec- export models. At ocean margins, in particular, more water tive 210 Po flux should be assessed if possible. samples should be taken to improve the resolution of hori- We did, however, have enough sampling depths at each zontal features. station to assess the vertical variability in 210 Po activity and to estimate the impact of vertical advection on the 210 Po flux. 4.2 Non-steady-state effects on 210 Po export fluxes The range of 210 Po activity flux due to vertical advection (−40 to 14 dpm m−2 d−1 , Table 2) was of the same mag- To our knowledge, three time-series studies of 210 Po and nitude as the steady-state fluxes calculated from the deficit 210 Pb activities have been conducted to date and have as- Biogeosciences, 16, 309–327, 2019 www.biogeosciences.net/16/309/2019/ Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect 319 Figure 5. The ratios of POC concentration to 210 Po activity in the total particles vs. depth in each basin along the GA01 transect. Power law regression (red line) was fitted for POC/210 Po against depth in each plot: the Iberian Basin (stations 1, 13), West European Basin (stations 21, 26), Iceland Basin (stations 32, 38), Irminger Basin (stations 44, 60), and Labrador Basin (stations 64, 69, 77). The data points denoted as filled by black circles were outliers (points at a distance greater than 1.5 standard deviations from the power law model) and excluded from the power law regression. sessed the NSS effects on 210 Po fluxes. First, in the upper are needed because such a timescale could ensure the sensi- 500 m of the Sargasso Sea, Kim and Church (2001) found tivity for NSS estimates (Friedrich and Rutgers van der Lo- that the SS model may have overestimated and underesti- eff, 2002; Stewart et al., 2007). NPP values for the 2-month mated the 210 Po export fluxes in May and July 1997, respec- period were in the ranges of 51–184, 39–403, 22–131, 18– tively. Second, at the DYFAMED site of the northwestern 204, 16–210 mmol C m−2 d−1 in the Iberian Basin, the West Mediterranean Sea, the ∂Po/∂t term accounted for ∼ 50 % European Basin, the Iceland Basin, the Irminger Basin, and of 210 Po flux estimated by using the SS model (Stewart et al., the Labrador Basin, respectively, indicating the occurrence of 2007). Last, in the South China Sea, the 210 Po export fluxes blooms during this time period along the transect that might at 1000 m calculated from the SS and NSS models had simi- have influenced the 210 Po fluxes derived from Eq. (1). lar values within the uncertainties (Wei et al., 2014). In fact, SS may have underestimated the 210 Po export along the the SS model generally results in an underestimation of the GA01 transect depending on the stage of the bloom before 210 Po flux under conditions of decreasing 210 Po activity in sampling. For example, at station 21 the largest NPP peak the water column (i.e., when blooms switch from the pro- (403 mmol C m−2 d−1 ) occurred 2 weeks before our sam- ductive phase to the export phase), whereas the SS model pling date and diminished rapidly (∼ 100 mmol C m−2 d−1 overestimates the flux for conditions of increasing 210 Po ac- at sampling time). The combination of high phytoplankton tivity (i.e., high atmospheric deposition). export and a sudden decrease in NPP may have significantly Atmospheric aerosol deposition along the GA01 transect lowered the 210 Po activity in the upper waters, resulting in was reportedly low without the significant influence of the a negative ∂Po/∂t, and thus the SS model may have under- Saharan plume (Shelley et al., 2017). The influence of at- estimated the true 210 Po flux. Temporal variations were also mospheric deposition on the SS estimates obtained in this seen in the time-series phytoplankton community composi- study, therefore, can be ignored. However, it is important to tion, in particular at stations 1 and 13 (Fig. 2). Both stations assess the ∂Po/∂t term that was associated with the site- were dominated (> 60 %) by coccolithophores between Octo- specific bloom events during the cruise. Satellite estimates ber 2013 and July 2014, but appeared to have a diatom bloom of net primary production (VGPM model) for the eight 8- in April 2014 before sampling. Polonium-210 and 210 Pb tend day periods prior to the sampling date (∼ 2 months) were to bind to specific biopolymeric functional groups, leading to calculated at each station (Fig. 6). Two months of NPP data fractionation during their sorption onto particles (Quigley et www.biogeosciences.net/16/309/2019/ Biogeosciences, 16, 309–327, 2019 320 Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect Figure 6. Time-series (1 January–12 July 2014) satellite estimates of net primary production (NPP) between 1 January and 12 July in 2014 at each station along the GA01 transect (VPGM algorithm, http://www.science.oregonstate.edu/ocean.productivity/, last access: 18 January 2019). The shaded rectangle in each plot denotes NPP for about 2 months prior to the sampling date. Two months of NPP data are needed because this timescale could ensure the sensitivity for NSS estimates (Friedrich and Rutgers van der Loeff, 2002; Stewart et al., 2007). The vertical red line in each plot indicates the sampling date at each station. al., 2002; Chuang et al., 2013; Yang et al., 2013). The tem- 4.3 POC flux calculated from 210 Po flux poral variation of phytoplankton composition could therefore also lead to non-steady-state effects on the overall 210 Po ac- The POC export fluxes were calculated by multiplying both tivity balance, which are difficult to assess but deserve more the 210 Po export fluxes calculated from the deficit alone (SS attention. without advection) and the total 210 Po fluxes (sum of the The NSS effect on the 234 Th fluxes at the ThEq were eval- fluxes calculated from the 210 Po deficit and vertical advec- uated during the same cruise along the GA01 transect in tion) by the total particulate (> 1 µm) POC/210 Po ratios at the Lemaitre et al. (2018) by using the NSS model developed corresponding depths (Table 2, Fig. 7). The POC fluxes cal- in Savoye et al. (2006). Because the cruise plan did not al- culated from only the deficit term and the total term ranged low an opportunity to reoccupy the study areas over time, the from negligible to 7 mmol C m−2 d−1 and from negative to authors made the assumption that 234 Th activity was in equi- 10 mmol C m−2 d−1 , respectively. This is in good agreement librium with 238 U activity at the starting date of the bloom. with the SS fluxes derived via the 210 Po/210 Pb method ignor- Their results suggested that the NSS 234 Th fluxes were about ing advection in other regions of the world ocean (negligible 1.1 to 1.3 times higher than the SS estimates in the Iberian to 8.5 mmol C m−2 d−1 ) (e.g., Shimmield et al., 1995; Sarin and West European basins, and 1.4 to 2.1 times higher in et al., 1999; Kim and Church, 2001; Stewart et al., 2007; Ver- the Iceland, Irminger, and the Labrador basins. We did not deny et al., 2008; Roca-Martí et al., 2016; Subha Anand et attempt to apply the same technique to estimate NSS 210 Po al., 2017). fluxes in this study because the assumption of equilibrium The highest-estimated POC fluxes (Table 2) along the tran- between 210 Po activity and 210 Pb activity at the starting date sect were observed at most of the investigated depths in the of the bloom may be inappropriate and the 210 Po deficit in- Labrador Sea and at the Greenland Shelf, whereas the lowest tegrates over a longer time period (months) than a typical export was in the Iberian and West European basins. An ex- bloom event (days to weeks). ception to this pattern was found at station 26, where POC Biogeosciences, 16, 309–327, 2019 www.biogeosciences.net/16/309/2019/ Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect 321 Figure 8. Plot of POC export flux derived from the 210 Po method (210 Po–POC) versus satellite estimates of net primary production (NPP). The NPP values were averaged for the previous 138 days (210 Po half-life) prior to the sampling date. The sum of the 210 Po fluxes calculated from the 210 Po deficit and vertical advective flux, Figure 7. POC fluxes derived from 210 Po for the mixed-layer depth and the POC/210 Po ratios in the > 1 µm particles were used to de- (MLD), the base of the euphotic zone (Z1 % ), the base of the pri- rive POC fluxes. The 210 Po–POC fluxes were integrated within the mary production zone (PPZ), and the 234 Th-238 U equilibrium depth primary production zone (PPZ). Lines of export efficiency (EF) of (ThEq). (a) POC fluxes derived from the 210 Po fluxes that were cal- 10 %, 5 %, and 1 % are drawn in the plot. The numbers in the plot culated from the deficit alone; (b) POC fluxes derived from the sum are station numbers. The colored dots of the stations correspond to of the 210 Po fluxes that were calculated from the 210 Po deficit and the basins. vertical advective flux. Note that the > 1 µm particles were used to calculate the POC/210 Po ratios. The stations were plotted from west to east. sect where the phytoplankton community was not dominated by diatoms but by smaller phytoplankton, in particular coc- flux was actually similar in magnitude to the flux at sta- colithophores. Smaller phytoplankton cells could scavenge tions 64 and 69. Station 26 was located in the middle of the more 210 Po (higher particulate 210 Po activity relative to the Subarctic Front (SAF), a cold and fresh anomaly originating large particles) due to larger surface area per unit of volume, from subpolar water (Zunino et al., 2017). The hydrographic lowering their ratio of POC concentration to 210 Po activity. properties associated with the SAF appear to promote high primary production (174 ± 19 mmol C m−2 d−1 , Table 1) and 4.4 POC export efficiency subsequently high carbon export (Kemp et al., 2006; Rivière and Pondaven, 2006; Guidi et al., 2007; Waite et al., 2016). The POC export flux calculated from the total 210 Po flux at While stations on the Greenland Shelf (stations 60 and 64) the depth of the PPZ was compared to the satellite-derived had the greatest estimated carbon export at the depth of ThEq NPP over ∼ 138 days (see Sect. 2.5) at each station, and the (5–10 mmol C m−2 d−1 ), station 60 at the depth of Z1 % had ratio was reported as the POC export efficiency. the lowest POC flux (−12.5 to −8.4 mmol C m−2 d−1 ). The export efficiencies in this study were below 10 % at The negligible deficit of 210 Po at the MLD and Z1 % 10 out of 11 stations, averaging 6 ± 4 % (n = 10, excluding seen at stations 21, 38 and 44 leads to negligible 210 Po- the negative value at station 60, Fig. 8). Export efficiencies derived POC fluxes at those depths and stations (Ta- < 10 % observed here were similar to those found in the equa- ble 2, Fig. 7). The relatively low POC export (negligible – torial Pacific, the Arabian Sea, and at the BATS site (Bues- 1.7 mmol C m−2 d−1 ) at stations 1 and 13, on the other hand, seler, 1998; Subha Anand et al., 2017), but lower than those resulted from low particulate POC/210 Po ratios (Table 2). In reported at high-latitude sites (> 25 %), such as the Arctic fact, the Iberian Basin had the lowest measurements of par- (Gustafsson and Andersson, 2012; Moran et al., 1997; Roca- ticulate POC/210 Po ratios in both the small- and large-size Martí et al., 2016), the Bellingshausen Sea (Shimmield et al., fractions relative to the other four basins along the transect 1995), and the Antarctic Polar Front (Rutgers van der Loeff (Fig. 4). This basin was also the only region along the tran- et al., 1997). www.biogeosciences.net/16/309/2019/ Biogeosciences, 16, 309–327, 2019 322 Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect Export efficiencies ranged from 0.5 % to 2.5 % in the Iberian Basin, while the values in the Irminger Basin (3±3 %, excluding station 60) were similar to the export efficiencies in the Western European Basin (5 ± 5 %) and in the Iceland Basin (6 ± 6 %). The export efficiencies, in contrast, were larger in the Labrador Basin (10±3 %). The lowest export ef- ficiencies observed in the Iberian Basin were consistent with the dominance of smaller phytoplankton species there (coc- colithophores and cyanobacteria; Fig. 2). Indeed, small cells are usually slow-sinking particles that are likely more prone to degradation (Villa-Alfageme et al., 2016), leading to lower export efficiencies. Conversely, the higher export efficiencies at other stations, all generally dominated by diatoms (Fig. 2), support the idea that diatoms may be more efficient in export- Figure 9. Sinking fluxes of 210 Po (blue circles) and 234 Th (red ing POC than smaller phytoplankton (Buesseler, 1998). Dif- squares) integrated to the depth at which 234 Th activity returned to ferences in export efficiencies between the basins dominated equilibrium with 238 U activity (ThEq), assuming steady state and negligible physical transport along the GA01 transect. Note that the by diatoms suggest that other factors may also play some role stations are plotted from west to east, and the transect was sepa- (e.g., temporal decoupling between production and export). rated into the western (stations 44–77) and eastern (stations 1–38) The POC export efficiency could also vary widely within sections. the same basin. Taking the two stations in the Western Euro- pean Basin for instance, export efficiency at station 26 was ∼ 5-fold greater than that estimated at station 21, likely con- 4.5.1 210 Po flux vs. 234 Th flux sistent with a lower contribution of diatoms and a higher con- tribution of smaller phytoplankton at station 21 relative to The integrated 210 Po and 234 Th fluxes at the depth of ThEq those at station 26 (Fig. 2). But overall the time-series com- were compared (Fig. 9). There was a spatial trend of 234 Th position of the phytoplankton community at the two stations flux, but not 210 Po flux along the transect; 234 Th fluxes at was similar (Fig. 2). The site-specific environment may have stations 1 to 38 (eastern section, 1580 ± 430 dpm m−2 d−1 ) impacted the export of the same cell type to different degrees were significantly greater (Wilcoxon rank sum test, p value (Durkin et al., 2016). Station 26 was in the middle of the < 0.002) than the fluxes at stations 44 to 77 (western section, SAF, and the mesoscale physical processes (i.e., turbulence 710 ± 230 dpm m−2 d−1 ). The means of the 210 Po fluxes in and mixing) at the front can introduce nutrients into the lo- the western and eastern sections were not statistically differ- cal euphotic zone (Lévy et al., 2012). Large phytoplankton ent from each other (Wilcoxon rank sum test, p value = 0.3). species generally dominate in these nutrients-rich waters and However, the flux of 210 Po and 234 Th correlated with each can promote massive episodic particle export (e.g., Kemp et other better in the western (n = 5, R 2 = 0.6) than in the east- al., 2006; Guidi et al., 2007; Waite et al., 2016). ern (n = 6, R 2 = 0.01) sections. These relationships may be related to both the stage of the bloom and different half-lives of the two isotopes. Indeed, 234 Th fluxes integrate the conditions that occurred days to 4.5 Comparison of 210 Po and 234 Th derived POC fluxes weeks prior to the sampling date, while the 210 Po method in- tegrates the flux over the past few months. Within the Iberian The measurements of 234 Th/238 U disequilibrium to estimate Basin, stations 1 and 13 were sampled weeks to months af- POC export flux were simultaneously carried out during ter the bloom development (Fig. 6). The moderate to rel- the GEOVIDE cruise (Lemaitre et al., 2018). The authors atively high 234 Th fluxes are thus surprising. Lemaitre et discussed the influence of vertical advection on 234 Th ex- al. (2018) argue that the greater fluxes there might be related port flux and concluded it can be neglected. In the present to the proximity of the Iberian margin, where particle dy- study, estimates of 234 Th-derived POC (234 Th-POC) flux namics were intense and lithogenic particles were numerous were compared to 210 Po-derived POC (210 Po–POC) flux. To (Gourain et al., 2018). A temporal decoupling between pro- avoid discrepancies, both the 234 Th-POC and 210 Po–POC duction and export could be an alternative possibility. The flux estimates were calculated at the depth of ThEq using Western European and Icelandic basins were sampled dur- the POC / radionuclide ratio in total particles > 1 µm (TPF) ing bloom development, and the NPP peaks occurring just and large particles > 53 µm (LSF), and both methods ignored before sampling may have promoted the high fluxes. In fact, physical transport and assumed steady state, where any de- these basins have been characterized by the presence of fast- viation from secular equilibrium was created by sinking par- sinking particles during the bloom (Villa-Alfageme et al., ticles with an adsorbed and/or absorbed excess of the short- 2016), likely also explaining the high export. In contrast, the lived daughter isotope. lower export observed in the western section may be due to Biogeosciences, 16, 309–327, 2019 www.biogeosciences.net/16/309/2019/ Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect 323 small (1–53 µm, SSF), large (> 53 µm, LSF), or total (> 1 µm, TPF) particles, may all represent a combination of sinking and non-sinking particles. In the present study, the particulate POC / radionuclide ratio on the TPF and LSF were examined and used to calculate POC export flux. The POC/210 Po and POC/234 Th ratios in the particles at the depths of ThEq were derived from the power law functions in each basin (TPF POC/210 Po in Table 2, LSF POC/234 Th in Lemaitre et al., 2018; LSF POC/210 Po and TPF POC/234 Th not shown). The TPF POC/210 Po and TPF POC/234 Th ratios had very simi- lar spatial trends (n = 11, R 2 = 0.8, p value < 0.0001) along the transect, with the lowest POC / radionuclide ratios in the Iberian and Western European basins and the highest ratios in the Labrador Sea. In contrast, the LSF POC/210 Po ra- tios were not correlated with LSF POC/234 Th ratios (n = 11, R 2 = 0.3, p value = 0.07). The correlation of values within Figure 10. Plot of the POC flux derived from 210 Po (210 Po–POC) versus the POC flux derived from 234 Th (234 Th-POC) at 11 sta- the TPF but not the LSF suggests that the composition of tions along the GA01 transect. Both the fluxes of 210 Po and 234 Th large particles was different from that of the total particles, were calculated from the deficit term alone, assuming steady state and that the difference in particle association between POC and negligible physical transport. The POC / radionuclide ratios on and 210 Po and 234 Th was greater in large particles than in (a) total particulate fraction (TPF, > 1 µm) and (b) large-size frac- total particles. tion (LSF, > 53 µm) were used to calculate the POC flux. The fluxes were integrated down to the depth at which the 234 Th activity re- 4.5.3 210 Po-derived POC vs. 234 Th-derived POC turned to equilibrium with 238 U activity (ThEq). The numbers in the plot are station numbers. The colored dots of the stations corre- When the radionuclide fluxes were multiplied by the spond to the basins. POC / radionuclide values, the range of the calculated POC fluxes were negligible to 7 mmol C m−2 d−1 and negligible to 12 mmol C m−2 d−1 via the 210 Po method using the TPF the fact that the sampling occurred during the decline of the and LSF POC/210 Po ratios, respectively, and from 2.5 to bloom, probably with a decoupling between production and 13 mmol C m−2 d−1 and from 1 to 12 mmol C m−2 d−1 via export in the Labrador Basin, or during a particle retention the 234 Th method using the TPF and LSF POC/234 Th ra- event in the Irminger Basin. tios (Fig. 10). The 234 Th-POC and 210 Po–POC fluxes agreed Unlike the observations of higher 234 Th export flux in the within a factor of 3 along the transect, with higher POC esti- eastern than western sections, there were no significant dif- mates derived from the 234 Th method in 9 out of 11 stations. ferences in 210 Po export flux between the two sections. This This was consistent with previous studies that have typically observation supports the argument that the 210 Po deficit tends found higher-estimated POC flux via the 234 Th method (e.g., to smooth out episodic events due to integration over longer Shimmield et al., 1995; Stewart et al., 2007; Verdeny et al., time periods. The 210 Po deficit records seasonal changes in 2009). export fluxes, whereas the 234 Th deficit represents more re- When using the total particle POC / radionuclide ratios, cent changes in the water column (Verdeny et al., 2009; only stations 26 and 60 were characterized by slightly higher Hayes et al., 2018). Indeed, the 210 Po deficit integrates the 210 Po-derived POC flux estimates than 234 Th-derived esti- flux over months that include a period of lower flux prior to mates (0.2 and 1.5-fold, respectively). In contrast, at station the bloom along the GA01 transect, whereas the 234 Th deficit 1 the difference between the methods was greatest, with the integrates the flux only over weeks that include the bloom it- 210 Po-derived POC flux negligible and the 234 Th-POC flux self at most of the stations (Fig. 6). Therefore, the specific of about 7 mmol C m−2 d−1 . At stations 13, 21, and 44, the stage of the bloom shortly prior to the sampling date appears 234 Th-POC fluxes were greater than 210 Po–POC estimates by to have less influence on the 210 Po-derived than the 234 Th- almost 1-fold, whereas in the Iceland and Labrador basins, derived export flux along this transect. the 234 Th-POC fluxes were larger than 210 Po–POC estimates by 3- and 2-fold, respectively. When using the large parti- 4.5.2 POC/210 Po vs. POC/234 Th ratio cle POC / radionuclide ratios, only in the Irminger Basin was there higher 210 Po-derived POC flux than 234 Th-derived POC In order to calculate POC export flux, one needs both the ex- flux (> 0.2 to 3-fold). In the Iberian Basin, the greatest dif- port of the daughter nuclide at a defined depth as well as the ference between the methods was found at station 1, where particulate POC / radionuclide ratio on the sinking particles. 210 Po-derived POC flux was negligible, while 234 Th-derived In situ pump filtered particles, either operationally defined as POC flux was the highest along the transect but with a large www.biogeosciences.net/16/309/2019/ Biogeosciences, 16, 309–327, 2019 324 Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect uncertainty (12 ± 22 mmol C m−2 d−1 ), and at station 13 the Data availability. The total and particulate 210 Po and 210 Pb activ- 234 Th-POC flux was greater than the 210 Po–POC estimate ity data from the GEOVIDE cruise can be found in the companion by 4-fold. In the Western European, Iceland, and Labrador article (Table S2 in Tang et al., 2018). basins, the 234 Th-POC fluxes were larger than 210 Po–POC estimates by 5, 4, and 2-fold. Wilcoxon rank sum tests revealed that the 234 Th-POC es- Competing interests. The authors declare that they have no conflict timates were significantly greater than 210 Po-derived POC of interest. export at the stations from the Iberian Basin to the Iceland Basin (n = 6, TPF: p value < 0.01, LSF: p value < 0.02), but Special issue statement. This article is part of the special is- not at the stations from the Irminger Basin to the Labrador sue “GEOVIDE, an international GEOTRACES study along the Basin (n = 5, TPF: p value > 0.1, LSF: p value = 1). Since OVIDE section in the North Atlantic and in the Labrador Sea the ratios of POC to radionuclides for total particles had very (GA01)”. It is not associated with a conference. similar spatial trends along the transect, the discrepancy be- tween TPF 234 Th-POC and TPF 210 Po–POC flux estimates must be driven primarily by the discrepancy between the SS Acknowledgements. We thank the captain (Gilles Ferrand) and estimates of 234 Th and 210 Po fluxes, discussed in Sect. 4.5.1. crew of the R/V Pourquoi Pas? and the chief scientists (Gerál- In contrast, the discrepancy between the POC to isotope ra- dine Sarthou and Pascale Lherminier) of the GEOVIDE cruise. tios in the large particle may have led to some degree of dis- We also thank Pierre Branellec, Floriane Desprez de Gésin- crepancy between the LSF 234 Th-POC and LSF 210 Po–POC court, Michel Hamon, Catherine Kermabon, Philippe Le Bot, flux estimates. Stéphane Leizour, Olivier Ménage, Fabien Pérault, and Em- manuel de Saint-Léger for their technical support during the GEOVIDE expedition; Catherine Schmechtig for the GEOVIDE 5 Conclusions database management, and Phoebe Lam for providing two mod- ified McLane in situ pumps; Frédéric Planchon, Virginie Sanial, This study used the water column 210 Po and 210 Pb activity and Catherine Jeandel for their assistance with pump deploy- data to constrain the 210 Po particulate flux from the mixed- ments and particulate sample collection. The authors also thank layer depth, the base of the euphotic zone and primary pro- Arnout Roukaerts, Debany Fonseca-Batista, Florian Deman, and duction zone, and the 234 Th-238 U equilibrium depth. The Frank Dehairs for providing primary production data. Funding for the GEOVIDE cruise was provided by the French National Re- ratios of POC concentration to 210 Po activity on the to- search Agency (ANR-13-BS06-0014, ANR-12-PDOC-0025-01), tal particulate (> 1 µm) fraction and large particulate frac- the French National Center for Scientific Research (CNRS- tion (> 53 µm) were used to estimate POC export fluxes. LEFE-CYBER), the LabexMER (anr-10-LABX-19), and Ifremer. We have been able to include vertical advection into a Gillian Stewart and Yi Tang were supported by NSF award #OCE steady-state model to calculate the 210 Po flux along the 1237108. The Generalitat de Catalunya also helped through its transect. The scale of 210 Po fluxes due to vertical advec- grant 2017 SGR-1588. This work is contributing to the ICTA “Unit tion was of the same magnitude as the steady-state fluxes of Excellence” (MinECo, MDM2015-0552). Maxi Castrillejo and calculated from the 210 Po deficit alone. The 210 Po-derived Montserrat Roca-Marti were funded by an FPU PhD studentship POC export fluxes varied spatially, ranging from negligi- (AP-2012-2901 and AP2010-2510, respectively) from the Minis- ble to 10 mmol C m−2 d−1 along the transect, with the high- terio de Educación, Cultura y Deporte of Spain. Maxi Castrillejo est export fluxes in the Labrador Sea. POC export efficien- was also supported by the ETH Zurich Postdoctoral Fellowship Program (17-2 FEL-30), co-funded by the Marie Curie Actions cies (flux relative to production) also showed regional dif- for People COFUND Program. We also thank Gary Hemming ferences, ranging from negligible to 13 % along the transect. (Queens College) and Troy Rasbury (Stony Brook University) for Higher export efficiencies were seen in the basins where di- laboratory assistance with the ICP-MS analyses. Finally, we thank atoms dominated the phytoplankton community. The low- the associate editor and the anonymous reviewers for their helpful export efficiencies recorded in the Iberian Basin, on the other comments on how to improve the manuscript. hand, may be associated with the dominance of smaller phy- toplankton, such as coccolithophores. POC export fluxes es- Edited by: Gilles Reverdin timated from the water column 210 Po/210 Pb and 234 Th/238 U Reviewed by: two anonymous referees disequilibria agreed within a factor of 3 across our study re- gion, with higher POC estimates generally derived from the 234 Th method. The differences were attributed to integration timescales and the history of bloom events. References Acker, J. G. and Leptoukh, G.: Online Analysis Enhances Use of NASA Earth Science Data, Eos Trans. AGU, 88, 14–17, 2007. Behrenfeld, M. J. and Falkowski, P. G.: Photosynthetic rates de- rived from satellite-based chlorophyll concentration, Limnol. Biogeosciences, 16, 309–327, 2019 www.biogeosciences.net/16/309/2019/ Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect 325 Oceanogr., 42, 1–20, https://doi.org/10.4319/lo.1997.42.1.0001, Deep-Sea Res. Pt. I, 49, 101–120, https://doi.org/10.1016/S0967- 1997. 0637(01)00045-0, 2002. Bhat, S. G., Krishnaswamy, S., Lal, D., Rama, and Moore, W. S.: García-Ibáñez, M. I., Pérez, F. F., Lherminier, P., Zunino, P., 234Th/238U ratios in the ocean, Earth Planet. Sc. Lett., 5, 483– Mercier, H., and Tréguer, P.: Water mass distributions and trans- 491, 1968. ports for the 2014 GEOVIDE cruise in the North Atlantic, Broecker, W. S., Kaufman, A., and Trier, R. M.: The residence time Biogeosciences, 15, 2075–2090, https://doi.org/10.5194/bg-15- of thorium in surface sea water and its implications regarding 2075-2018, 2018. the rate of reactive pollutants, Earth Planet. Sc. Lett., 20, 35–44, Gourain, A., Planquette, H., Cheize, M., Lemaitre, N., Menzel Bar- https://doi.org/10.1016/0012-821X(73)90137-4, 1973. raqueta, J.-L., Shelley, R., Lherminier, P., and Sarthou, G.: Inputs Buesseler, K. O.: The decoupling of production and particulate ex- and processes affecting the distribution of particulate iron in the port in the surface ocean, Global Biogeochem. Cy., 12, 297–310, North Atlantic along the GEOVIDE (GEOTRACES GA01) sec- https://doi.org/10.1029/97GB03366, 1998. tion, Biogeosciences Discuss., https://doi.org/10.5194/bg-2018- Buesseler, K. O., Bacon, M. P., Kirk Cochran, J., and Livingston, 234, in review, 2018. H. D.: Carbon and nitrogen export during the JGOFS North At- Guidi, L., Stemmann, L., Legendre, L., Picheral, M., Prieur, L., and lantic Bloom experiment estimated from 234Th:238U disequi- Gorsky, G.: Vertical distribution of aggregates (> 110 µm) and libria, Deep-Sea Res. Pt. I, 39, 1115–1137, 1992. mesoscale activity in the northeastern Atlantic: Effects on the Cherrier, J., Burnett, W. C., and LaRock, P. A.: Uptake of polo- deep vertical export of surface carbon, Limnol. Oceanogr., 52, nium and sulfur by bacteria, Geomicrobiol. J., 13, 103–115, 7–18, https://doi.org/10.4319/lo.2007.52.1.0007, 2007. https://doi.org/10.1080/01490459509378009, 1995. Gustafsson, O. and Andersson, P.: 234Th-derived surface export Chuang, C., Santschi, P. H., Ho, Y., Conte, M. H., Guo, L., Schu- fuxes of POC from the Northern Barents Sea and the Eurasian mann, D., Ayranov, M., and Li, Y.: Role of biopolymers as major sector of the Central Arctic Ocean, Deep-Sea Res. Pt. I, 68, 1– carrier phases of Th, Pa, Pb, Po, and Be radionuclides in settling 11, https://doi.org/10.1016/j.dsr.2012.05.014, 2012. particles from the Atlantic Ocean, Mar. Chem., 157, 131–143, Hayes, C. T., Black, E. E., Andersen, R. A., Baskaran, M., Bues- 2013. seler, K. O., Charette, M. A., Cheng, H., Cochran, J. K., Ed- Cochran, J. K.: Uranium-Series Disequilibrium – Application to wards, R. L., Fitzgerald, P., Lam, P. J., Lu, Y., Morris, S. O., Earth, Marine, and Environmental Sciences, The oceanic chem- Ohnemus, D. C., Pavia, F. J., Stewart, G., and Tang, Y.: Flux of istry of the uranium and thorium-series nuclides, Oxford Univer- particulate elements in the North Atlantic Ocean constrained by sity Press, 334–395, 1992. multiple radionuclides, Global Biogeochem. Cy., 32, 1738–1758, Djogic, R., Sipos, L., and Branica, M.: Characterization of ura- https://doi.org/10.1029/2018GB005994, 2018 nium(VI) in seawater, Limnol. Oceanogr., 31, 1122–1131, Honjo, S., Manganini, S. J., Krishfield, R. A., and Francois, R.: Par- https://doi.org/10.4319/lo.1986.31.5.1122, 1986. ticulate organic carbon fluxes to the ocean interior and factors Ducklow, H., Steinberg, D., and Buesseler, K.: Upper Ocean Car- controlling the biological pump: A synthesis of global sediment bon Export and the Biological Pump, Oceanography, 14, 50–58, trap programs since 1983, Prog. Oceanogr., 76, 217–285, 2008. https://doi.org/10.5670/oceanog.2001.06, 2001. Jerlov, N. G.: Optical Oceanography, Elsevier Publishing Company, Durkin, C. A., Van Mooy, B. A. S., Dyhrman, S. T., and Bues- 1968. seler, K. O.: Sinking phytoplankton associated with carbon Kemp, A. E. S., Pearce, R. B., Grigorov, I., Rance, J., Lange, C. flux in the Atlantic Ocean, Limnol. Oceanogr., 61, 1172–1187, B., Quilty, P., and Salter, I.: Production of giant marine diatoms https://doi.org/10.1002/lno.10253, 2016. and their export at oceanic frontal zones: Implications for Si Falkowski, P. G., Barber, R. T., and Smetacek, V.: Biogeochemical and C flux from stratified oceans, Global Biogeochem. Cy., 20, Controls and Feedbacks on Ocean Primary Production, Science, GB4S04, https://doi.org/10.1029/2006GB002698, 2006. 281, 200–206, 1998. Kim, G. and Church, T. M.: Seasonal biogeochemical fluxes of Fisher, N. S., Burns, K. A., Cherry, R. D., and Heyraud, M.: Accu- 234Th and 210Po in the Upper Sargasso Sea: Influence from at- mulation and cellular distribution of 241Am, 210Po and 210Pb mospheric iron deposition, Global Biogeochem. Cy., 15, 651– in two marine algae, Mar. Ecol.-Prog. Ser., 11, 233–237, 1983. 661, https://doi.org/10.1029/2000GB001313, 2001. Fleer, A. P. and Bacon, M. P.: Determination of 210Pb and 210Po in Lemaitre, N., Planchon, F., Planquette, H., Dehairs, F., Fonseca- seawater and marine particulate matter, Nucl. Instrum. Methods, Batista, D., Roukaerts, A., Deman, F., Tang, Y., Mariez, C., and 223, 243–249, 1984. Sarthou, G.: High variability of particulate organic carbon ex- Flynn, W. W.: The determination of low levels of polonium-210 in port along the North Atlantic GEOTRACES section GA01 as environmental materials, Anal. Chim. Acta, 43, 221–227, 1968. deduced from 234Th fluxes, Biogeosciences, 15, 6417–6437, Fonseca-Batista, D., Li, X., Riou, V., Michotey, V., Deman, F., Frip- https://doi.org/10.5194/bg-15-6417-2018, 2018. iat, F., Guasco, S., Brion, N., Lemaitre, N., Tonnard, M., Galli- Lévy, M., Iovino, D., Resplandy, L., Klein, P., Madec, G., nari, M., Planquette, H., Planchon, F., Sarthou, G., Elskens, M., Tréguier, A. M., Masson, S., and Takahashi, K.: Large- Chou, L., and Dehairs, F.: Evidence of high N2 fixation rates in scale impacts of submesoscale dynamics on phytoplankton: productive waters of the temperate Northeast Atlantic, Biogeo- Local and remote effects, Ocean Modell., 43–44, 77–93, sciences Discuss., https://doi.org/10.5194/bg-2018-220, in re- https://doi.org/10.1016/j.ocemod.2011.12.003, 2012. view, 2018. Masqué, P., Sanchez-Cabeza, J. A., Bruach, J. M., Palacios, E., Friedrich, J. and Rutgers van der Loeff, M. M.: A two-tracer and Canals, M.: Balance and residence times of 210Pb and (210Po–234Th) approach to distinguish organic carbon and bio- 210Po in surface waters of the northwestern Mediterranean Sea, genic silica export flux in the Antarctic Circumpolar Current, www.biogeosciences.net/16/309/2019/ Biogeosciences, 16, 309–327, 2019 326 Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect Cont. Shelf Res., 22, 2127–2146, https://doi.org/10.1016/S0278- Sabine, C. L.: The Oceanic Sink for Anthropogenic CO2 , Science, 4343(02)00074-2, 2002. 305, 367–371, https://doi.org/10.1126/science.1097403, 2004. Matsumoto, E.: 234Th/238U radioactive disequilibrium in the sur- Sarin, M. M., Kim, G., and Church, T. M.: 210Po and 210Pb in the face layer of the ocean, Geochim. Cosmochim. Ac., 39, 205–212, South-equatorial Atlantic: distribution and disequilibrium in the https://doi.org/10.1016/0016-7037(75)90172-6, 1975. upper 500 m, Deep-Sea Res. Pt. II, 46, 907–917, 1999. Menemenlis, D., Campin, J., Heimbach, P., Hill, C., Lee, T., Savoye, N., Benitez-Nelson, C., Burd, A. B., Cochran, J. K., Nguyen, A., Schodlock, M., and Zhang, H.: ECCO2: High Reso- Charette, M., Buesseler, K. O., Jackson, G. A., Roy-Barman, M., lution Global Ocean and Sea Ice Data Synthesis, Mercator Ocean Schmidt, S., and Elskens, M.: 234Th sorption and export models Quarterly Newsletter, 31, 13–21, 2008. in the water column: A review, Mar. Chem., 100, 234–249, 2006. Moore, H. E., Poet, S. E., Martell, E. A., and Wilken- Shelley, R. U., Roca-Martí, M., Castrillejo, M., Sanial, V., Masqué, ing, M. H.: Origin of 222Rn and its long-lived daugh- P., Landing, W. M., van Beek, P., Planquette, H., and Sarthou, ters in air over Hawaii, J. Geophys. Res., 79, 5019–5024, G.: Quantification of trace element atmospheric deposition https://doi.org/10.1029/JC079i033p05019, 1974. fluxes to the Atlantic Ocean (> 40◦ N; GEOVIDE, GEOTRACES Moran, S. B., Ellis, K. M., and Smith, J. N.: 234Th/238U dise- GA01) during spring 2014, Deep-Sea Res. Pt. I, 119, 34–49, quilibrium in the central Arctic Ocean: implications for particu- https://doi.org/10.1016/j.dsr.2016.11.010, 2017. late organic carbon export, Deep-Sea Res. Pt. II, 44, 1593–1606, Shimmield, G. B., Ritchie, G. D., and Fileman, T. W.: The impact of https://doi.org/10.1016/S0967-0645(97)00049-0, 1997. marginal ice zone processes on the distribution of 210Pb, 210Po Murray, J. W., Paul, B., Dunne, J. P., and Chapin, T.: 234Th, 210Pb, and 234Th and implications for new production in the Belling- 210Po and stable Pb in the central equatorial Pacific: Tracers for shausen Sea, Antarctica, Deep-Sea Res. Pt. II, 42, 1313–1335, particle cycling, Deep-Sea Res. Pt. I, 52, 2109–2139, 2005. https://doi.org/10.1016/0967-0645(95)00071-W, 1995. Owens, S. A., Pike, S., and Buesseler, K. O.: Thorium-234 Stewart, G., Cochran, J. K., Miquel, J. C., Masqué, P., Szlosek, as a tracer of particle dynamics and upper ocean export J., Rodriguez y Baena, A. M., Fowler, S. W., Gasser, B., and in the Atlantic Ocean, Deep-Sea Res. Pt. II, 116, 42–59, Hirschberg, D. J.: Comparing POC export from 234Th/238U and https://doi.org/10.1016/j.dsr2.2014.11.010, 2015. 210Po/210Pb disequilibria with estimates from sediment traps Quigley, M. S., Santschi, P. H., Hung, C., Guo, L., and Honeyman, in the northwest Mediterranean, Deep-Sea Res. Pt. I, 54, 1549– B. D.: Importance of acid polysaccharides for 234Th complexa- 1570, https://doi.org/10.1016/j.dsr.2007.06.005, 2007. tion to marine organic matter, Limnol. Oceanogr., 47, 367–377, Stewart, G. M. and Fisher, N. S.: Bioaccumulation of polonium- https://doi.org/10.4319/lo.2002.47.2.0367, 2002. 210 in marine copepods, Limnol. Oceanogr., 48, 2011–2019, Richardson, T. L. and Jackson, G. A.: Small Phytoplankton and https://doi.org/10.4319/lo.2003.48.5.2011, 2003a. Carbon Export from the Surface Ocean, Science, 315, 838–840, Stewart, G. M. and Fisher, N. S.: Experimental stud- https://doi.org/10.1126/science.1133471, 2007. ies on the accumulation of polonium-210 by marine Rigaud, S., Puigcorbé, V., Camara-Mor, P., Casacuberta, N., phytoplankton, Limnol. Oceanogr., 48, 1193–1201, Roca-Martí, M., Garcia-Orellana, J., Benitez-Nelson, C. R., https://doi.org/10.4319/lo.2003.48.3.1193, 2003b. Masqué, P., and Church, T.: A methods assessment and rec- Stewart, G. M., Bradley Moran, S., and Lomas, M. ommendations for improving calculations and reducing un- W.: Seasonal POC fluxes at BATS estimated from certainties in the determination of 210Po and 210Pb ac- 210Po deficits, Deep-Sea Res. Pt. I, 57, 113–124, tivities in seawater, Limnol. Oceanogr.-Meth., 11, 561–571, https://doi.org/10.1016/j.dsr.2009.09.007, 2010. https://doi.org/10.4319/lom.2013.11.561, 2013. Subha Anand, S., Rengarajan, R., Shenoy, D., Gauns, M., Rigaud, S., Stewart, G., Baskaran, M., Marsan, D., and Church, and Naqvi, S. W. A.: POC export fluxes in the Arabian T.: 210Po and 210Pb distribution, dissolved-particulate exchange Sea and the Bay of Bengal: A simultaneous 234Th/238U rates, and particulate export along the North Atlantic US GEO- and 210Po/210Pb study, Mar. Chem., 198, 70–87, TRACES GA03 section, Deep-Sea Res. Pt. II, 116, 60–78, https://doi.org/10.1016/j.marchem.2017.11.005, 2017. https://doi.org/10.1016/j.dsr2.2014.11.003, 2015. Tang, Y., Stewart, G., Lam, P. J., Rigaud, S., and Church, T.: The Rivière, P. and Pondaven, P.: Phytoplankton size influence of particle concentration and composition on the frac- classes competitions at sub-mesoscale in a frontal tionation of 210Po and 210Pb along the North Atlantic GEO- oceanic region, J. Marine Syst., 60, 345–364, TRACES transect GA03, Deep-Sea Res. Pt. I, 128, 42–54, https://doi.org/10.1016/j.jmarsys.2006.02.005, 2006. https://doi.org/10.1016/j.dsr.2017.09.001, 2017. Roca-Martí, M., Puigcorbé, V., Rutgers van der Loeff, M. M., Tang, Y., Castrillejo, M., Roca-Martí, M., Masqué, P., Lemaitre, Katlein, C., Fernández-Méndez, M., Peeken, I., and Masqué, P.: N., and Stewart, G.: Distributions of total and size-fractionated Carbon export fluxes and export efficiency in the central Arctic particulate 210 Po and 210 Pb activities along the North At- during the record sea-ice minimum in 2012: a joint 234Th/238U lantic GEOTRACES GA01 transect: GEOVIDE cruise, Biogeo- and 210Po/210Pb study, J. Geophys. Res.-Oceans, 121, 5030– sciences, 15, 5437–5453, https://doi.org/10.5194/bg-15-5437- 5049, https://doi.org/10.1002/2016JC011816, 2016. 2018, 2018. Rutgers van der Loeff, M. M., Friedrich, J., and Bathmann, U. Turekian, K. K., Nozaki, Y., and Benninger, L. K.: V.: Carbon export during the Spring Bloom at the Antarctic Geochemistry of Atmospheric Radon and Radon Polar Front, determined with the natural tracer 234Th, Deep- Products, Annu. Rev. Earth Pl. Sc., 5, 227–255, Sea Res. Pt. II, 44, 457–478, https://doi.org/10.1016/S0967- https://doi.org/10.1146/annurev.ea.05.050177.001303, 1977. 0645(96)00067-7, 1997. Biogeosciences, 16, 309–327, 2019 www.biogeosciences.net/16/309/2019/ Y. Tang et al.: Carbon export along the GEOTRACES GA01 transect 327 Verdeny, E., Masqué, P., Maiti, K., Garcia-Orellana, J., Bruach, Wei, C.-L., Yi, M.-C., Lin, S.-Y., Wen, L.-S., and Lee, W.-H.: J. M., Mahaffey, C., and Benitez-Nelson, C. R.: Particle ex- Seasonal distributions and fluxes of 210 Pb and 210 Po in the port within cyclonic Hawaiian lee eddies derived from 210Pb– northern South China Sea, Biogeosciences, 11, 6813–6826, 210Po disequilibrium, Deep-Sea Res. Pt. II, 55, 1461–1472, https://doi.org/10.5194/bg-11-6813-2014, 2014. https://doi.org/10.1016/j.dsr2.2008.02.009, 2008. Weller, R. A. and Plueddemann, A. J.: Observations of the verti- Verdeny, E., Masqué, P., Garcia-Orellana, J., Hanfland, C., Kirk cal structure of the oceanic boundary layer, J. Geophys. Res.- Cochran, J., and Stewart, G. M.: POC export from ocean surface Oceans, 101, 8789–8806, https://doi.org/10.1029/96JC00206, waters by means of 234Th/238U and 210Po/210Pb disequilibria: 1996. A review of the use of two radiotracer pairs, Deep-Sea Res. Pt. Yang, W., Guo, L., Chuang, C., Schumann, D., Ayranov, M., and II, 56, 1502–1518, https://doi.org/10.1016/j.dsr2.2008.12.018, Santschi, P. H.: Adsorption characteristics of 210Pb, 210Po and 2009. 7Be onto micro-particle surfaces and the effects of macromolec- Villa-Alfageme, M., Soto, F. C., Ceballos, E., Giering, S. L. C., ular organic compounds, Geochim. Cosmochim. Ac., 107 47–64, Le Moigne, F. A. C., Henson, S., Mas, J. L., and Sanders, R. 2013. J.: Geographical, seasonal, and depth variation in sinking parti- Zunino, P., Lherminier, P., Mercier, H., Daniault, N., García-Ibáñez, cle speeds in the North Atlantic, Geophys. Res. Lett., 43, 8609– M. I., and Pérez, F. F.: The GEOVIDE cruise in May–June 8616, https://doi.org/10.1002/2016GL069233, 2016. 2014 reveals an intense Meridional Overturning Circulation over Waite, A. M., Stemmann, L., Guidi, L., Calil, P. H. R., Hogg, A. a cold and fresh subpolar North Atlantic, Biogeosciences, 14, M. C., Feng, M., Thompson, P. A., Picheral, M., and Gorsky, 5323–5342, https://doi.org/10.5194/bg-14-5323-2017, 2017. G.: The wineglass effect shapes particle export to the deep ocean in mesoscale eddies, Geophys. Res. Lett., 43, 9791–9800, https://doi.org/10.1002/2015GL066463, 2016. Wei, C.-L., Lin, S.-Y., Sheu, D. D.-D., Chou, W.-C., Yi, M.-C., Santschi, P. H., and Wen, L.-S.: Particle-reactive radionuclides (234 Th, 210 Pb, 210 ) as tracers for the estimation of export pro- duction in the South China Sea, Biogeosciences, 8, 3793–3808, https://doi.org/10.5194/bg-8-3793-2011, 2011. www.biogeosciences.net/16/309/2019/ Biogeosciences, 16, 309–327, 2019