Palaeogeography, Palaeoclimatology, Palaeoecology 615 (2023) 111447 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Agulhas leakage extension and its influences on South Atlantic surface water hydrography during the Pleistocene B. Nirmal a, K. Mohan b, *, Aradhna Tripati c, Beth A. Christensen d, P. Graham Mortyn e, David De Vleeschouwer f, M. Prakasam g, K. Saravanan b a School of Advanced Sciences, Vellore Institute of Technology, Chennai 600127, India b School of Civil Engineering, Vellore Institute of Technology, Chennai 600127, India c Departments of Earth, Planetary, and Space Sciences, Department of Atmospheric and Oceanic Sciences, Institute of the Environment and Sustainability, University of California, Los Angeles 90095, USA d Department of Environmental Science, School of Earth and Environment, Rowan University, Glassboro, NJ, USA e Institute of Environmental Science and Technology (ICTA) and Department of Geography, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain f Institute of Geology and Paleontology, University of Münster, Corrensstr 24, 48149 Münster, Germany g Wadia Institute of Himalayan Geology, 33, G. M. S. Road, Dehradun 248001, India A R T I C L E I N F O A B S T R A C T Editor: Prof. M Elliot The inter-ocean transfer of warm and salty water through the Agulhas leakage is regarded as a significant factor influencing the South Atlantic’s surface and intermediate water hydrography. To evaluate the extension of Keywords: Agulhas leakage for the past 1.2 Ma, planktonic foraminiferal assemblage from Ocean Drilling Program Hole Pleistocene 1088B, within the vicinity of AL, and Hole 1090B, located south of the significant Agulhas retroflection region, Agulhas leakage fauna were generated. The Hole 1088B shows higher abundance of tropical and sub-tropical assemblage. The com­ Subtropical Convergence bination of Globigerinoides ruber, Trilobatus sacculifer, Globigerinoides conglobatus and Globorotalia menardii has Deglacial warming Oceanic teleconnections previously been defined as the Agulhas leakage fauna [ALF]. The pulse-like amplification of the ALF record at major glacial-interglacial transitions indicates these were periods of strengthened Agulhas leakage with subse­ quent shedding of Agulhas rings and filaments as far as ~41◦ S. From ~450 ka, the frontal zone exhibits strong ALF variations with evident increase at glacial-interglacial transitions of Marine Isotopic Stages (MIS) 12, 10, 8 and 6, suggesting vigorous eddy mixing and a shift in the leakage zone towards the subantarctic waters. The migration of the Subtropical Convergence and Neogloboquadrina pachyderma (s) coiling ratios during these glacial-interglacial transitions supports the southward expansion of Agulhas leakage. The ALF record is sup­ ported by the δ18O record of Globigerina bulloides from hole 1088B, which identify a major anomaly at MIS 10, with depleted δ18O values towards the glacial termination. Simultaneously, the results from Hole 1090B show restricted presence of tropical-subtropical assemblage with their appearance reported only at glacial-interglacial transitions of the MIS 16, 12, 10 and 8. The curtailed ALF index and dominance of the subpolar species N. pachyderma (s) indicate the termination of Agulhas leakage before reaching the Subantarctic zone. The ODP sites chosen for the study reveal the intensification of Agulhas leakage and southward shift of the major leakage zone into the subantarctic zone during the Pleistocene glacial-interglacial transitions. 1. Introduction same time, the separated rings and filaments transport the Agulhas leakage (AL) water into the South Atlantic (Lutjeharms, 1996; de Ruijter The Agulhas current is a prominent warm water mass in the western et al., 1999). This incoming subtropical AL water mass is much warmer boundary of the Indian Ocean and it flows southward parallel to the than the surrounding surface waters of the South Atlantic. Thus, the AL South East coast of Africa. On reaching the Cape Basin, it experiences interlinks the upper layer waters of both oceans and thereby functions as retroflection with zero wind stress curl (Beal et al., 2011) by which the an interbasin circulation (Speich et al., 2002), which plays a funda­ larger part of the water mass moves back to the Indian Ocean. At the mental role in changing the surface hydrography of the South Atlantic * Corresponding author. E-mail address: mohan.kuppusamy@vit.ac.in (K. Mohan). https://doi.org/10.1016/j.palaeo.2023.111447 Received 5 September 2022; Received in revised form 5 January 2023; Accepted 8 February 2023 Available online 13 February 2023 0031-0182/© 2023 Elsevier B.V. All rights reserved. B. Nirmal et al. Palaeogeography, Palaeoclimatology, Palaeoecology 615 (2023) 111447 while facilitating the Atlantic Meridional Overturning Circulation 2. Study area (AMOC). The major contribution to AL is from the Indonesian Throughflow (ITF, 45%) and Tasman Leakage (TL, 30%; Durgadoo et al., The Pleistocene sediment successions from the two South Atlantic 2017). The former flows westward from the Pacific to the Indian Ocean Holes (ODP Leg 177) close to the Agulhas retroflection region were to join a Southern Hemisphere supergyre circulation that connects all examined for the study (Fig. 1). ODP Hole 1088B was drilled on the the ocean basins (Schott et al., 2009). Model simulations suggest about Agulhas ridge, (41◦ 8.16′ S, 13◦ 33′ E, 2082 m water depth) in the vicinity 90% of ITF waters reach the Agulhas current and exit to the South of the modern-day STF (Becquey and Gersonde, 2002). Although prox­ Atlantic in the upper 1500 m of water column (Durgadoo et al., 2017; imal to but not within the main AL, Site 1088 provides a record of Zhang et al., 2019). This warm water route brings fresh surface water changes in the STC. Surface waters here are transitional based on the and salty intermediate water to the South Atlantic through the AL (Le planktonic foraminiferal abundance and SST, and the site remains in the Bars et al., 2013; Durgadoo et al., 2017). The TL originates from south of reaches of Agulhas retroflection rings and filaments (Lutjeharms, 1996). Australia and flows westward along the Subtropical Front (STF) and The intermediate water depth places the Site 1088 in the boundary joins the Agulhas current via the south-east Madagascar current (Speich between North Atlantic Deep Water (NADW) and upper Circumpolar et al., 2002). More than half of water retroflects but TL contributes to AL Deep Water (CPDW) (Reid, 1989). The sediments recovered from Site through intermediate depths (Durgadoo et al., 2017). Of all the various 1088 are carbonate-rich with low sedimentation rate (0.1 cm/ka) inter-ocean transfers, the AL through the South Atlantic modulates the throughout the Pleistocene (Gersonde et al., 1999). The more southerly global ocean circulation balance by replenishing heat and salt to the Hole 1090B, drilled on the South Western flank of the Agulhas ridge, lies North Atlantic, thereby also modulating the strength of the AMOC (Beal in the central SAZ (42◦ 54′ S, 8◦ 53′ E, 3702 m water-depth), south of the et al., 2011). New observations suggest that AL is coupled with the Agulhas retroflection zone. Site 1090 is useful to record changes in the strength of the ITF, intensification of Asian monsoons and the strength of SAZ. This deeper site rests in the mixing zone of lower CPDW and NADW the South Indian sub-tropical gyre (Gordon et al., 2008; Beck et al., (Reid, 1989) and above the calcium carbonate compensation depth, 2018; Petrick et al., 2019). The atmosphere and ocean teleconnections which ensures preservation of carbonate rich sediments. The site records are significant at oceanic gateways such as the Agulhas corridor. Pleistocene sedimentation rates of 0.3 cm/ka, which is higher than the Most paleo AL reconstructions has been studied previously over the intermediate site due to the high opal deposition (Gersonde et al., 1999) centennial and millennial scales from the main leakage zone in the South and increased terrestrial deposition (Martínez-Garcia et al., 2011) Atlantic (Peeters et al., 2004; Martínez-Méndez et al., 2010; Caley et al., occurring at the subantarctic zone. Both Sites 1088 and 1090 are ideally 2012; Marino et al., 2013). The paleo reconstructions show enhanced located south of the prominent Agulhas corridor, which makes this re­ leakage and water mass transport during the glacial-interglacial transi­ gion much more sensitive for large scale changes in leakage intensity or tions of the Pleistocene. Sea surface temperature, salinity and Agulhas a southern shift in leakage zone. leakage fauna (ALF) are used as promising proxies in identifying the leakage variability and intensity (Peeters et al., 2004; Martínez-Méndez 3. Material and methods et al., 2010; Caley et al., 2012; Marino et al., 2013; Simon et al., 2013). The ALF, which is a group of tropical / sub-tropical planktonic forami­ 3.1. Age model and stable isotope analysis nifera species found at detached Agulhas rings and filaments (Peeters et al., 2004) is regarded as a reliable proxy in tracing the AL intensity The age models for ODP Holes 1088B and 1090B are based on into the south Atlantic. The leakage intensity can be altered by many biostratigraphic datums and oxygen isotope stratigraphy. We used the factors such as the velocity and volume of the Agulhas current, Southern age model calibrated in Nirmal et al., 2021 for Hole 1088B, where Hemisphere westerlies and fluctuations of hydrographic fronts. Of all shipboard calcareous nannofossils were corroborated with benthic δ18O these factors, the fluctuations of the STF influence the interocean data from (Billups, 2002) and age calibration was graphically con­ transfer with a northward position of the front acting as a choke point, structed against the LR04 benthic δ18O stack (Lisiecki and Raymo, by blocking the heat and salt transfer through AL (Bard and Rickaby, 2005). We also used the age model proposed by Martínez-Garcia et al. 2009). The STF is a region of vigorous interaction between atmosphere (2010) for Hole 1090B. and ocean circulation with intense AL retroflection eddy mixing south of For this study, we used the G. bulloides δ18O values previously Africa. A change in the Earth’s orbital parameters, the position of recorded by Nirmal et al. (2021). Overall, 120 samples (57 from 1088B westerlies, and solar insolation all exert control on the position of the and 63 from 1090B) were measured with a Finnigan MAT 253 plus mass STF and thereby leakage variability. The enhanced leakage during the spectrometer linked with an automated Kiel IV carbonate device at interglacials increases the strength of the AMOC and plays a pivotal role Wadia Institute of Himalayan Geology, Dehradun, India. The isotope in global heat transfer (Peeters et al., 2004; Bard and Rickaby, 2009; values were calibrated to the international standard Vienna Pee Dee Beal et al., 2011; Caley et al., 2012). Belemnite (VPDB) with the IAEA isotopic reference material NBS-18. Here we present a planktonic foraminiferal record from the vicinity Merck carbonate was also used as an in-house secondary standard. of the STF and within the Subantarctic Zone (SAZ). Ocean Drilling The external reproducibility was 0.08‰ for the δ18O analysis. Progam (ODP) Sites 1088 and 1090 along with previously studied Site 1089 (Cortese et al., 2004) are optimally located to observe the inter­ 3.2. Sample processing and foraminiferal census counts action between AL and the Southern Ocean, and also to monitor major shifts in leakage intensity. Here the mixing of warm, saline spillage with One hundred and forty samples (57 from Hole 1088B and 83 from cold, eutrophic sub-Antarctic waters creates an oceanographic divide Hole 1090B) covering 1.2 Ma were processed by following the standard with variable surface water hydrography. Today these sites are domi­ procedures for sample processing and wet sieving (Mohan et al., 2011). nated by subpolar and transitional planktonic foraminifera but based on Planktonic foraminiferal counts were performed over the >150 μm size previous studies (e.g., Peeters et al., 2004) it is anticipated that they fraction, for at least 300–500 specimens per sample. The relative should vary under different conditions. This study attempts to establish abundance record is then used to obtain the ALF index. The ALF at the the extent of AL into the SAZ and to identify the leakage termination southern STF comprised of Globigerinoides ruber, Trilobatus sacculifer, point. A 1.2 Ma record of planktonic foraminifera proxies was utilized to Globigerinoides conglobatus and Globorotalia menardii, as observed from unveil the late Pleistocene zonal shift in AL, covering the two climatic the study area (Table 1). transitions, the Mid Pleistocene Transition (MPT), ~1.25–0.7 Ma (Eld­ erfield et al., 2012; Diester-Haass et al., 2018) and the Mid Bruhnes Event (MBE), ~0.43 Ma (Barth et al., 2018). 2 B. Nirmal et al. Palaeogeography, Palaeoclimatology, Palaeoecology 615 (2023) 111447 Fig. 1. Location of the study area and global Meridional Overturning Circulation (MOC). The South Atlantic ODP Holes 1088B (within AL) and 1090B (outside of the main AL region) are represented by Black dots. Other Sites (white dots) along the path of MOC are included for comparison. The prominent, modern Agulhas zone is shown by the thick red line with red circles indicating the warm Agulhas eddies and rings. The oceanic connection between ITF (Indonesian Throughflow), AL (Agulhas leakage) and AMOC (Atlantic Meridional Overturning Circulation) is shown (Modified from Broecker, 1991 and Rahmstorf, 2002). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 3.3. Principal components and foraminiferal index 3.3.1. Relative subtropical convergence (STC) index The latitude of STC migration is crucial for the AL to reach the South The relative abundance of planktonic species is taken as the primary Atlantic. It also determines the strength and displacement of Southern input for the PCA factor analysis. The analysis was performed for Holes Hemisphere westerlies (Bard and Rickaby, 2009). Peeters et al. (2004) 1088B and 1090B using the software STATISTICA, version 10.0 (Ta­ developed an index for this study area using the relative abundance of bles 1, 2). The factor loadings present the correlation between different transitional planktonic foraminifera. Subtropical to transitional waters species and samples (Kucera, 2003). The factors were chosen with ei­ exists at the STC and the location of Hole 1088B (~ 41◦ S) is optimal to genvalues >1, and the scores were rotated by the Varimax method to derive the STC index: G. truncatulinoides are found in abundance north of interpret environmental parameters (Fig. 2). the STC while transitional species G. inflata and N. pachyderma (dex.) dominate the assemblages to the south of the STCSTC index = 3 B. Nirmal et al. Palaeogeography, Palaeoclimatology, Palaeoecology 615 (2023) 111447 Table 1 reconstruct paleoceanographic and environmental changes (Darling Varimax factor loadings, 1088B, South Atlantic. et al., 2006). Neogloboquadrina pachyderma in subantarctic waters shows Species name Factor -1 Factor -2 Factor -3 Factor -4 two distinctive coiling patterns, which are linked with surface ocean temperature. The left-coiling morphotype of N. pachyderma is abundant Globigerina bulloides 0.312034 0.275422 0.750619 0.144094 Globigerinita glutinata 0.239875 0.317389 0.492279 0.221169 in polar waters while the right-coiling morphotype prefers more tran­ Globigerinoides 0.829511 0.029922 0.024323 0.209100 sitional and temperate waters (Darling et al., 2006; Itaki, 2016). At the conglobatus South Atlantic STC zone, the left-coiling variant dominates (> 95%) Globigerinoides ruber 0.765890 0.245871 0.050567 0.225735 with aberrant right coiling morphotypes (< 5%). The frontal charac­ Trilobatus sacculifer 0.921452 − 0.071687 − 0.062499 0.140845 Globorotalia 0.067700 0.175054 − 0.090081 0.813915 teristics of the study area and drastic temperature change at the STC may crassaformis have regulated the mixing of these morphotypes in the study area. The Globoconella inflata 0.331750 0.012871 0.632269 0.439716 ratio of dextral to sinistral variants of N. pachyderma from the Hole Globorotalia menardii 0.844009 0.058888 0.147162 − 0.141287 1088B is used as a paleoceanographic proxy in the South Atlantic. Globorotalia 0.540129 0.024391 0.300637 0.250125 truncatulinoides Neogloboquadrina 0.246889 0.589598 − 0.031369 − 0.126890 4. Results dutertrei Neogloboquadrina − 0.045222 0.880681 0.117020 0.127778 4.1. Planktonic foraminifera records from the Subtropical Convergence pachyderma (sin.) (STC), ODP Hole 1088B Turborotalia − 0.154709 0.910756 0.099204 0.071230 quinqueloba Orbulina universa 0.479552 0.130197 − 0.630181 0.067584 The relative abundance of major planktonic foraminifera from Hole Neogloboquadrina 0.564082 − 0.502563 0.268082 − 0.223459 1088B are used to examine the surface ocean changes in the STC zone. pachyderma (dex.) The species relative abundance is utilized to perform factor analysis (PCA) which condenses the data to various environmental preferences. Cumulative variance 33 54 64 72 (%) Four factors were retained with eigenvalues >1, which together explained 72% of variance in the dataset (Table 1). The factor loadings Factor loadings of PCA (Varimax-rotated). The scores represent species loading. >0.60 were grouped as species residing in similar environments (Fig. 2). Species marked in bold combine to form Agulhas leakage fauna. We observed glacial-interglacial variations in the foraminiferal assem­ Factor − 1, Agulhas leakage fauna (depth habitat - mixed layer, 0–250 m). Factor − 2, Species representing subantarctic to polar zone (depth habitat, blage after the onset of the MPT (~1250 ka), which indicates significant 0–200 m); *G. inflata depth habitat, 100–500 m. surface water changes at the STC during the late Pleistocene. The glacial Factor − 3, Species indicative for the transitional-subpolar zones (depth habitat, stages after the MPT show higher abundance in Polar assemblage 0–200 m). (N. pachyderma and T. quinqueloba) (Tolderlund, 1971; Schiebel and Factor − 4, Species indicative of the subtropical to transitional zones (depth Hemleben, 2017) reflecting cold and high productive waters, but the habitat, 100–500 m). maximum abundances (>40%) at MIS 12 and 6 may indicate the Depth habitat from plankton tows in Atlantic sector of Southern Ocean (Mortyn northward displacement of ACC fronts and intrusion of cold Antarctic and Charles, 2003). surface waters into the STC zone (Fig. 3a). The transitional group (G. bulloides, G. inflata and G. glutinata) that represents subpolar to Table 2 transitional assemblage in the South Atlantic (Tolderlund, 1971; Varimax factor loadings, 1090B, South Atlantic. Schiebel and Hemleben, 2017) indicates glacial-interglacial fluctuations Species name Factor -1 Factor -2 (Fig. 3b). The subtropical assemblage (G. crassaformis, G. truncatulinoides Globigerina bulloides 0.889997 0.002744 and N. dutertrei) increases in abundance at the glacial transitions after Globigerinita glutinata 0.837150 0.079330 the inception of the MBE (~430 ka; Fig. 3c). The fourth group (Factor 1) Globigerinita parkerae − 0.074839 − 0.159207 represents ALF (G. ruber, T. sacculifer, G. conglobatus and G. menardii), Globigerinita uvula 0.286006 0.081068 and occurs in pulse-like episodes at the major glacial - interglacial ter­ Globigerinoides ruber 0.014096 0.964826 minations between 1.2 and 0.5 Ma (Fig. 3d). After the intensification of Trilobatus sacculifer 0.062923 0.951153 Globorotalia crassaformis 0.112024 − 0.069772 the MBE, the ALF exhibits strong glacial-interglacial variations with Globoconella inflata 0.828621 0.158710 evident increases at glacial terminations of MIS 12, 10, 8 and 6. Globoconella puncticulata − 0.555377 − 0.314827 The planktonic δ18O records were used to corroborate the ALF index, Globorotalia truncatulinoides 0.544498 0.625942 but lacks high resolution to correlate at every glacial termination. Neogloboquadrina pachyderma (sin.) 0.019665 Pleistocene core sediments at the site were depleted and any high res­ ¡0.738409 Turborotalia quinqueloba 0.322214 0.158999 Orbulina universa 0.160695 0.162364 olution study is unattainable. The δ18O values range from 3.75 to 1.90‰ Neogloboquadrina pachyderma (dex.) 0.008631 − 0.015196 for the last 1.2 Ma and are consistent with what has previously been reported for this location (Hodell et al., 2003). The isotope record of Cumulative variance (%) 84 95 G. bulloides identifies an anomaly at MIS 10, suggesting near interglacial Factor loadings of PCA (Varimax-rotated). The scores represent species loading. δ18O values towards the glacial termination. Though the record is of low Species marked in bold combine to form ALF. resolution, MIS 10 at Hole 1088B represents the most depleted δ18O Factor − 1, positive loading represents species of transitional to subpolar zone signal during the late Pleistocene (Fig. 5g). (depth habitat, 0–200 m); *G. inflata depth habitat, 100–500 m. Negative loading indicate species of subantarctic to polar zone (depth habitat, 0–200 m). 4.2. Planktonic foraminiferal records from the Subantarctic Zone (SAZ), Factor − 2, Agulhas leakage fauna (depth habitat - mixed layer, 0–250 m). ODP Hole 1090B Depth habitat from plankton tows in Atlantic sector of Southern Ocean (Mortyn and Charles, 2003). The relative abundance of major planktonic foraminifera from ODP Hole 1090B were used to examine the surface ocean changes in the SAZ. ratio of (G. truncatulinoides)/((N. pachyderma(dex.) + G. inflata + G. The PCA factor analysis retains four factors with eigenvalues >1, but the truncatulinoides)). first two factors explain 95% of the variance in the planktonic abun­ dance (Table 2). Factor 1 remains the most important with sub-polar to 3.3.2. N. pachyderma coiling ratio transitional assemblage (G. bulloides, G. inflata and G. glutinata) with The coiling ratio in N. pachyderma is used by micropaleontologists to positive correlation and polar species N. pachyderma showing a negative 4 B. Nirmal et al. Palaeogeography, Palaeoclimatology, Palaeoecology 615 (2023) 111447 Fig. 2. Principal component analysis. Scatter plot of PC 1 / PC 2 of planktonic foraminifera from ODP Hole 1088B. Species specific environmental conditions and their relationship is represented. The planktonic foraminifera are classified into polar (blue), transitional (green), sub-tropical to transitional (orange) and tropical to sub-tropical (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) correlation. Neogloboquadrina pachyderma (s.) almost individually sensitive to minor changes but to major variations in the AL. The ALF dominates both the glacial and interglacial assemblage from 1.2 to 0.5 record is subdued during the MPT (1.2–0.7 Ma; Fig. 5d), a period of shift Ma with 60–80% abundance recorded during major glacial intervals in intensity of glacial cycles (Ford and Chalk, 2020). This interval is (Fig. 3a). This suggests the northward migration of the STC and the characterized by enriched glacial δ18O values and relatively cold SST in subsequent expansion of cold water masses at the site. These migrations the South Atlantic (Fig. 5g, e). The ALF are depleted at the SAZ, at least and expansions were even more marked during pre-MBE times, as gla­ judging from the rare occurrence of subtropical to tropical species cials (and interglacials as well) had even higher amounts of polar recorded at Hole 1090B (Fig. 5c). The ALF index hints at the termination assemblage during that time interval. After the MBE the interglacial of AL waters beyond the STC zone, and a northward migration of fronts intervals indicate higher abundance of transitional assemblage (Fig. 3b) for most of the MPT indicated by the STC index (Fig. 5b). Previous but is often dominated by N. pachyderma (s.). The subtropical group studies report that southern sourced waters extensively dominated (G. crassaformis and G. truncatulinoides; Fig. 3c) and ALF (G. ruber and during the MPT (Farmer et al., 2019) resulting in subsequent northward T. sacculifer) are restricted during the MPT but suggest a period of sur­ excursion of SAZ. Southern Hemisphere summer insolation minima at face water change with slight yet vital occurrence at glacial transition of this time period point to expanded Antarctic Sea ice (Raymo et al., 2006) MIS 10 (Fig. 3d). The planktonic δ18O record from this central SAZ site which increased the water column stratification at SAZ (Hasenfratz varies from 4.00 to 2.5‰ (Fig. 5f) and agrees with previous studies et al., 2019). (Martínez-Garcia et al., 2010; Venz and Hodell, 2002). The enriched By MIS 18 (~0.7 Ma) the shift in glacial periodicity to 100 kyr cycles δ18O measurements compared to STC bound Site 1088 point to pre­ was complete. Deglacial warming was prominent at the STC during the dominant cold Antarctic surface water conditions south of the STC. transitions of MIS 18 and 16 (Fig. 5g). At these transitions, the ALF index at Hole 1088B was coupled with the depleted δ18O values of G. bulloides. 5. Discussion The ALF index from Hole 1090B suggests the SAZ remained out of the reaches from AL even after the MPT (0.7–0.45 Ma). The data are sup­ 5.1. Agulhas leakage export to the South Atlantic ported by the huge difference in sea surface temperature at both sites (Fig. 5e). Both transitions of MIS 18 and 16 suggest the southward shift The four factor scores retained from the STC bound site groups of the STC zone, suggesting the export of AL as far as modern day STC plannktonic foraminifera with similar ecology together. Factor 1 was (41◦ S). statistically significant in interpreting ALF and show a negative corre­ lation (r = − 0.61) between ALF and δ18O signature during post MBE 5.1.1. Post MBE super-interglacials glacial transitions (Fig. 4). In the modern South Atlantic, ALF comprise After the inception of the MBE (~0.43 Ma), the South Atlantic saw an typical tropical to subtropical species present in the detached Agulhas intensification of glacial-interglacial variability. The influence of AL rings (Peeters et al., 2004). The presence of these tropical - subtropical strengthened at Hole 1088B based on the proxy records (ALF index, species is reported at various sites north of the modern day STC during surface δ18O and SST records) and the relationship between Factor 1 and the late Pleistocene, such as MD96–2081 (Peeters et al., 2004); MD δ18O, invigorated after the MBE (Fig. 4). The ALF index from the STC 96–2080 (Martínez-Méndez et al., 2010); ODP 1087 (Caley et al., 2012) bound site increases during the glacial – interglacial transitions of MIS (Supplementary fig. 1). The AL contribution to the STC is traced by the 12, 10 and 8 (Fig. 5d). The δ18O and SST record from the studied sites as presence of ALF along with warm and saline surface waters reaching well as adjacent Sites ODP 1087 (Caley et al., 2012; Petrick et al., 2018) ODP Site 1088. The ALF index from Sites 1088 and 1090 may not be and 1089 (Cortese et al., 2004) show the deglacial warming trend at 5 B. Nirmal et al. Palaeogeography, Palaeoclimatology, Palaeoecology 615 (2023) 111447 Fig. 3. The relative abundance of planktonic foraminifera from ODP Holes 1088B and 1090B over the past 1.2 My. (a) Sub Polar zone (N. pachyderma (s) and (d), T. quinqiloba); (b) Transitional zone (1088B - G. bulloides, G. inflata and G. glutinata; 1090B - G. bulloides, G. inflata, G. glutinata and G. puncticulata); (c) Sub-Tropical to transitional (1088B - G. truncatulinoides, G. crassaformis and N. dutertrei; 1090B - G. truncatulinoides and G. crassaformis); (d) ALF % (1088B - G. ruber, T. sacculifer, G. conglobatus and G. menardii; 1090B - G. ruber and T. sacculifer). The grey bars represent the Marine Isotopic stages (Lisiecki and Raymo, 2005). Polar assemblage started dominating the glacial intervals while glacial- interglacial transitions were marked by the AL export. The transition of MIS 10 signifies an anomaly in the δ18O record of G. bulloides at Hole 1088B, the lowest values reported at the site (Fig. 5g). A similar SST increase at MIS 10 is reported from an adjacent Site, ODP 1089 (Cortese et al., 2004) while southern Site 1090 shows the maximum ALF index during the late Pleistocene (Fig. 5c). This could explain a southward zone shift in AL, infiltrating the STC surface waters during the deglacial warming of MIS 12, 10 and 8. The southern shift of STC and the shift in westerlies (Martínez-Garcia et al., 2011) during these intervals also support the southern shift in the leakage zone. The glacial - interglacial transitions after MBE record an increase in dextral N. pachyderma abundance signifying warmer surface water conditions. Our ALF index clearly represents a choke point of AL at the SAZ (ODP 1090), with an imperceptible occurrence of Tropical-Sub-tropical planktonic assem­ blage. The pattern of ALF index and planktonic assemblage suggests the extension of AL as far as ~41◦ S (modern day STF). The subantarctic Fig. 4. Relationship between Factor 1 scores and the δ18O signature of surface waters remain independent of this interocean transfer other than G. bulloides (VPDB) from ODP Hole 1088B during the late Pleistocene. a period of major southern shift in the leakage zone. these transitions. This intensification in AL may be the cause of super- interglacials after MBE (Turney and Jones, 2010) and thereby 5.2. Response of the STC zone to late Pleistocene oceanic teleconnections strengthening of the AMOC (Peeters et al., 2004; Caley et al., 2012). The shift to 100 ky cycles by ~0.7 Ma saw severe glacial conditions 6 B. Nirmal et al. Palaeogeography, Palaeoclimatology, Palaeoecology 615 (2023) 111447 Fig. 5. Comparison of the 1.2 Ma relative abundance record of ALF index from South Atlantic sector with (a) Coiling ratio between dextral and sinistral N. pachyderma, ODP 1088B; (b) STC position derived from planktonic foraminiferal abundance (1088B); (c) 1090B ALF index; (d) 1088B ALF index; (e) SST values from ODP Site 1088 (Herbert et al., 2016) and ODP Site 1090 (Martínez-Garcia et al., 2010); (f) The δ18O values of G. bulloides (VPDB) from ODP Hole 1090B (Nirmal et al., 2021); (g) The δ18O values of G. bulloides (VPDB) from ODP Hole 1088B (Nirmal et al., 2021). The grey bars represent the Marine Isotopic stages (Lisiecki and Raymo, 2005), and the blue shading represents MIS stages indicating degla­ cial warming. Regional and Global events – Severe glacial conditions from MIS 22 - MIS 16 (Ford and Chalk, 2020); ITF strength (Holbourn et al., 2011: Petrick et al., 2019); Summer Monsoon intensifica­ tion from 0.5 Ma (Sun et al., 2010; Beck et al., 2018); W. Indian Ocean high salinity (Nuber et al., 2021); Equitorial and North Atlantic deglacial warming / AMOC strength (Diester-Haass et al., 2018); North Atlantic IRD and HE (Hodell et al., 2008). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) followed by weaker interglacials (Ford and Chalk, 2020). Before ~0.7 tropical high (Beck et al., 2018). A late Pleistocene Asian monsoon re­ Ma, the weak signals in ALF index at the STC site indicated restricted cord from Chinese Loess (Sun et al., 2010; Beck et al., 2018) shows an leakage into the South Atlantic. During this period the oceanic tele­ increase in monsoon intensity around 0.5 Ma. Also, the interglacial in­ connection was weak with large glacial ice build-up in the Northern tervals after MBE record intensified Asian monsoons and is regarded as Hemisphere, a decrease in ITF intensity, reduced Asian summer positive feedback to ITF strength. From the Transition of MIS 12, monsoon, curtailed AL, the extension of Antarctic Sea ice and weakened intensified deglacial warming and high salinity are reported from meridional overturning circulation (Petrick et al., 2019). By the end of western Indian Ocean Site U1476 (Nuber et al., 2021). This indicates the MPT (~0.7 Ma) the ALF index shows signs of deglacial warming at Hole addition of ocean salinity to the Agulhas corridor from Indian Ocean 1088B. This MIS 16 deglacial warming signal is also visible in the SST surface waters. The oceanic connection is well documented when the record from the west coast of Australia (IODP Site U1460, water depth, SST record and ALF index from Agulhas corridor and Hole 1088B (STC 215 m), which is a proxy record for enhanced ITF strength (DiNezio zone) indicate warming at the transitions of MIS 16, 12, 10 and 8. The et al., 2018; Petrick et al., 2019). As the ITF strengthens it moves MIS 12, 10 and 8 transitions show the maximum abundance of ALF westward and intensifies the South Equatorial Current (SEC) and the index at 1088B and 1090B are coupled with the increase in ITF, and the southern Indian sub-tropical gyre (Le Bars et al., 2013). Modern obser­ related feedback system. The MIS 10 anomaly visible at Sites 1088 and vations link Asian monsoons with the strength of the Indian Ocean sub- 1089 (this study; Cortese et al., 2004) suggests this as the period of 7 B. Nirmal et al. Palaeogeography, Palaeoclimatology, Palaeoecology 615 (2023) 111447 strongest feedback which occurred at the STC during the Pleistocene. At Appendix A. Supplementary data MIS 10, the STC expanded southwards, invigorating strong AL towards the SAZ. The increased AL and eddy circulation at transitions of MIS 12, Supplementary data to this article can be found online at https://doi. 10 and 8 had in turn supported the northward flow of AMOC. SST re­ org/10.1016/j.palaeo.2023.111447. cords from equatorial Atlantic Site ODP 925 (Bickert et al., 1997; Diester-Haass et al., 2018) and North Atlantic Site 980 (McManus et al., References 1999) reveal this northward advection of heat which strengthens the AMOC. By MIS 16 a considerable increase in ice rafted detritus was Bard, E., Rickaby, R.E., 2009. Migration of the subtropical front as a modulator of glacial climate. Nature 460 (7253), 380. https://doi.org/10.1038/nature08189. recorded from North Atlantic IODP Site U1308 (water depth, 3427 m). Barth, A.M., Clark, P.U., Bill, N.S., He, F., Pisias, N.G., 2018. Climate evolution across the The observed detrital layers (Hudson strait events) from the North mid-brunhes transition. Clim. Past 14 (12), 2071–2087. https://doi.org/10.5194/cp- Atlantic site also signify deglacial warming which destabilizes the Lau­ 14-2071-2018. Beal, L.M., De Ruijter, W.P., Biastoch, A., Zahn, R., Cronin, M., Hermes, J., rentide ice sheet. 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