Sedimentary Geology 453 (2023) 106440 Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo Geochemical approach for decoding the paleoenvironmental and depositional evolution of a coastal lacustrine Konservat-Lagerstätte (Early Cretaceous, south-Central Pyrenees) Alejandro Gil-Delgado a,f, David Cruset b, Oriol Oms a, Edgar Botero c, Jordi Ibáñez-Insa b, Xavier Delclòs d, Albert Sellés e,f, Àngel Galobart e,f, Ramon Mercedes-Martín a,⁎ a Universitat Autònoma de Barcelona, Geology Department, Edifici Cs, 08193 Cerdanyola del Vallès, Spain b Geosciences Barcelona (GEO3BCN-CSIC), Lluís Solé i Sabarís, s/n, 08028 Barcelona, Spain c Technische Universitaat Hamburg-Harburg, Institute of River and Coastal Engineering, Am Schwarzenber g-Campus 1, 21073 Hamburg, Germany d Universitat de Barcelona, Faculty of Earth Sciences, Department of Earth and Ocean Dynamics, C/i Franquès s/n, 08028 Barcelona, Spain e Institut Català de Paleontologia Miquel Crusafont, C/ Escola Industrial, 23, 08201 Sabadell, Spain f Museu de la Conca Dellà, C/ del museu, 4, 25650 Isona i Conca Dellà, Spain a r t i c l e i n f o a b s t r a c t Article history: The lithographic limestone of La Pedrera de Meià (LPM) in south-Central Pyrenees (NE Spain) is considered one Received 16 March 2023 of the best preserved lacustrine-coastal successions of the Early Cretaceous in Europe, hosting a taxonomically Received in revised form 22 May 2023 diverse record of Barremian biota. While this Konservat-Lagerstätte has been extensively surveyed for paleonto- Accepted 23 May 2023 logical purposes, little is known about the paleoenvironmental and depositional conditions prevailing during the Available online 29 May 2023 formation of this fossil lagerstätte deposit. LPM limestone is made up of a homogeneous, dark gray to pale brown, Editor: Dr. Brian Jones faintly laminated 50 m-thick succession of micrite-rich intervals. To shed light on the environmental conditions of deposition, 303 rock samples were studied using a multiproxy approach based on different techniques includ- ing petrography, SEM-EDX, XRD, ICP-MS, XRF, Rare Earth Element distributions and C and O stable isotope geo- Keywords: chemistry. The combination of sedimentological and geochemical data reveals a vertical shift of facies from Fossil lagerstätte littoral to profundal environments and then returning to littoral conditions. Profundal facies are made up of Lacustrine carbonates filament-rich mudstones originated from in situ precipitation of micrite either through biotically mediated or Stable isotopes physico-chemical processes in undisturbed low energy settings, under anoxic conditions, characterized by the Multiproxy geochemistry presence of transported terrestrial and lacustrine organisms that are exquisitely well-preserved. Littoral areas are characterized by the presence of in situ charophytes, smooth ostracods, and miliolid foraminifera that accu- mulated in mud-rich facies indicating low/moderate-energy, shallow-water, and relatively oxygenated condi- tions. Stable isotopic data and elemental geochemistry suggest a hydrologically closed to semi-closed lacustrine system with a restricted water circulation encouraging dysoxic to anoxic conditions in profundal sed- iments. Paleoproductivity proxies suggest a good correlation between catchment-derived siliciclastic discharge to the lake and primary productivity peaks. Furthermore, a decline in terrigenous input coupled with a reduction of meteoric water entrance to the lake is interpreted based on elemental proxies derived from paleosalinity and paleoredox analysis. Collectively, the strontium/barium (Sr/Ba) ratios, the sparse presence of miliolid foraminif- era, and the isotopic signatures indicate an increase in salinity toward the younger intervals of the lake infilling, although their ultimate causes are unclear (i.e., increased evaporation, or a combination of reduced meteoric input and sporadic marine inputs might explain our data). The present work shows the importance of performing detailed multiproxy geochemical-sedimentological studies to better constrain the unique paleoenvironmental and paleohydrological conditions associated with the exceptional preservation of fossil biotas in Konservat- Lagerstätte deposits in analogous depositional settings. © 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/). 1. Introduction ⁎ Corresponding author. Konservat-Lagerstätte deposits are fossil accumulations character- E-mail address: ramon.mercedes@uab.cat (R. Mercedes-Martín). ized by the exceptional preservation of their biomineralized remains https://doi.org/10.1016/j.sedgeo.2023.106440 0037-0738/© 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 (e.g., shells, carcasses) and/or soft-tissues (e.g., organs, cellular struc- 2. Geological setting of the study area tures) (Clements and Gabbott, 2022; Iniesto et al., 2013). Thus, they rep- resent windows of ancient life that can help to unravel the paleoecology, LPM Konservat-Lagerstätte (Fig. 1) is located in the Camarasa and paleobiology and evolution of metazoans, traditionally aided by tapho- Vilanova de Meià municipalities 875 m-high in the Montsec ranges nomic and paleontological studies (Fürsich et al., 2007; Olcott et al., (Lleida province, NE Spain). These ranges are part of the thrust 2022; Varejão et al., 2019). In addition, more robust paleoenvironmental sheet taking the same name and belonging to the south-Central Pyr- and paleoclimatic context for the exceptional preservation phenomena enees fold and thrust belt (Séguret, 1972; Muñoz et al., 2018). The can help better understand the factors that disrupted the normal organic studied section is part of a 2000 m thick succession that includes matter recycling and allowed delicate tissues and remains to become strata from the Triassic to Lower Eocene. The LPM site (see Fig. 1B) fossils. is a 40–50 m thick succession of continental mudstones that accumu- La Pedrera de Meià (hereafter LPM), also known as La Pedrera de lated in paleolakes on top of the ‘Lower Cretaceous unconformity’ Rúbies, is a renowned Konservat-Lagerstätte that has provided a which resulted from early halokinetic activity (Burrel and Teixell, huge reference fossil record from the Barremian age, including 2021), and deformed Jurassic carbonates (Fig. 1C). The lateral exten- Montsechia vidali (one of the most primitive angiosperm plants, sion of LPM carbonates is of around 300 m and results from the infill Gómez et al., 2015), Macryphantes cowdeni and Palaeouloborus of the small depocenters at the unconformity surface, which was first lacasae (the most ancient orb-weaver spiders, Selden, 1990), infilled by Early Cretaceous brecciated carbonates, and subsequently Meiatermes bertrani (with two termite castes, one of the most an- by a Barremian lacustrine lithographic limestone succession (Fig. 1C, cient eusocial insect, Martínez-Delclòs and Martinell, 1995) and in red). two different primitive bird taxa, significantly Noguerornis gonzalezi, Since its discovery at the end of the 19th century, the LPM and an unnamed specimen close to Sulcavis (Sanz et al., 1997; Konservat-Lagerstätte has been attributed to different geological Chiappe and Lacasa, 2002; Cambra-Moo et al., 2006). ages. In the late 19th and early 20th centuries it was postulated to Importantly, the site is also the type locality of 112 holotypes and 19 be Late Jurassic in comparison with the Solnhofen lagoon deposits paratypes of flora and fauna, raising a considerable interest among the (Vidal, 1915). In the 70s, based on ostracod associations (Brenner scientific community (Galobart et al., 2022; Gil-Delgado et al., 2023). et al., 1974) the unit was dated as Early Cretaceous (Berriasian– LPM shares similar faunal species with the Las Hoyas Konservat- Valanginian), and currently, based on charophyte biostratigraphy Lagerstätte (Cuenca, Spain), also of Barremian age (Poyato-Ariza (Martin-Closas and López-Morón, 1995) the site is considered youn- and Buscalioni, 2016), with the common occurrence of arthropods ger (Early Barremian). (Austropotamobius llopisi, Delclosia roselli), insects (Iderdaegomphus LPM is the most important site, but there are smaller ones torcae, Noguerablatta fontllongae, Meiatermes), fish (Urocles woodwardi, located to the East such as La Cabroa, and El Reguer (Fig. 1C). The la- Vidalmia catalaunica, Notagogus ferreri, Rubiesichthys gregalis, Ascalobos, custrine deposits of LPM are known to fill paleotectonic depressions Leptolepis, Holophagus), or reptiles (Meyasaurus). The plant assemblage developed on top of a Jurassic–Cretaceous unconformity, with an un- includes common species as ferns (Weichselia reticulata, Ruffordia derlying Lower Cretaceous breccia (see CBeb code, in Pi et al., 2003). goepperti, Sphenopteris), coniferales (Frenelopsis rubiesensis, Sphenolepis As a result of their unique conditions of preservation, they have kurriana), Cycadoposida (Zamites) and angiosperms (Montsechia vidali). hosted many discoveries of exquisitely well-preserved fossils from The characterization of the LPM site as a freshwater coastal lake is based different taxa, including soft tissues, which are not preserved under on ostracod, insect, and frog assemblages (Brenner et al., 1974; regular taphonomic conditions. Some of the specimens found Martínez-Delclòs, 1991; Gómez and Lires, 2019), the charophyte and include charophytes, fish, and many other vertebrates including ranunculacean aquatic flora (Martin-Closas and López-Morón, 1995; frogs, lizards, crocodiles, and birds (Vidal, 1915; Lacasa, 1991; Martín-Closas, 2003), together with the total absence of Mesozoic Martínez-Delclòs, 1991, 1995; Lacasa, 2013; Gil-Delgado et al., marine fauna. While the site has been the focus of intense paleonto- 2023). logical research (Zeiler, 1902; Vidal, 1902; Texeira, 1954; Martínez- The thickness of the LPM type section ranges between 40 and 50 m Delcòs, 1989; Barale, 1989; Mercadé, 1991; Martínez-Delclòs and (Mercadé, 1991, Gil-Delgado et al., 2023) and is made up of laminated Martinell, 1995; Fregenal-Martínez and Meléndez, 1995; Galobart carbonates containing charophytes, ostracods, gastropods, bivalves, et al., 2022), no detailed analytical work has been performed to as- and foraminifera. Faunal associations and stratigraphic features enabled sess the lacustrine sedimentary dynamics characterizing these the interpretation of environments as formed in quiet, shallow, and Barremian Lagerstätte deposits. Thus, this study aims at providing a well‑oxygenated lacustrine settings (Martínez-Delclòs, 1995; Gibert et refined multiproxy geochemical-sedimentological framework to al., 2000). Moreover, the presence of framboidal pyrite, the sparsity of shed light on lake paleoenvironmental evolution integrating sedi- wave or current sedimentary structures and the exceptional fossil pres- mentary petrography, mineralogy, isotope geochemistry, bulk ervation has been interpreted as indicating a relatively stable anoxic chemical data, and organic matter content, across a 40 m-thick's lake (Mercadé, 1991). well-exposed sedimentary section. For this reason, the goal of this work is to obtain more quantitative 3. Material and methods constraints about the paleohydrological, paleoenvironmental and depo- sitional conditions prevailing during the formation of the LPM litho- 3.1. Material graphic limestones. The remarkable coincidence of LPM site -in terms of microfacies var- A total of 303 rock samples were obtained in 3 field campaigns. A iability, depositional setting, and exceptional fossil preservation- with first collection of 45 samples spaced at 1 to 2 m intervals was ob- other renowned Early Cretaceous Lagerstäte deposits like Las Hoyas in tained throughout the 50 m-thick section and labeled as “PM”, Central Spain (Meléndez, 1995; Poyato-Ariza and Buscalioni, 2016; (“Pedrera de Meià”). Another collection of 78 samples very closely Talbot et al., 1995) constitute a precedent to start applying multidisci- spaced (2 to 10 cm interval) was acquired in an interval of 5 m plinary approaches to compare and understand these two Konservat- and was named “PR”, (“Pedrera de Rúbies”). Finally, a further Lagerstätte. Additionally, the present analysis may provide researchers sampling of 180 levels was undertaken in order to achieve sam- with a refined geochemical methodology for future investigations in pling intervals of 25 cm, and was labeled with the acronym “R” similar lagerstätte deposits with limited stratigraphic and/or geochem- (“Rúbies”). ical resolution (e.g., Solnhofen in Germany; Nusplingen, Causse Méjean, Field samples were obtained and powdered with an agate planetary Cerin in France, or Crato in Brazil). mill. 2 A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 Fig. 1. Location, field views and fossils from the La Pedrera de Meià site (LPM). A). Site's location, B) aerial view of LPM, C) geological map of the Montsec range within the lithographic limestone sites and outcrops in red, the lithographic limestones model section, and legend (modified from Gil-Delgado et al., 2023). D) Image of a field excavation, E) outcrop image of LPM, F) thin-bedded limestones, G) Ilerdaegomphus, adult dragonfly, H) a teleostean fish, possibly Ascalabos sp. with part of the soft tissues preserved, I) Montsechia vidalii. 3 A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 3.2. Petrography conditions during sedimentation, alkalinity and detrital input was ob- tained by comparing the Pr/Pr* and Ce/Ce* ratios of samples following A microfacies study was carried out on 56 thin sections from the equations of Bau and Dulski (1996) and Lawrence et al. (2006), throughout the 50 m-thick section (Nikon Eclipse E400 POL). Thin sec- respectively: tions were polished with diamond grinding paste to obtain reliable ½PrSN SEM-EDX data (see below). Pr=Pr∗ ¼ 0:5∗½CeSN þ 0:5∗½NdSN 3.3. Powder X-Ray diffraction (XRD) ½CeSN Ce=Ce∗ ¼ 2 Powder X-Ray diffraction was performed throughout the LPM lime- ½PrSN =½NdSN stones for qualitative and quantitative mineralogical analysis. All sam- ples were carefully powdered and homogenized. XRD measurements The logCe/Ce* also indicates if LPM carbonates precipitated under were recorded at GEO3BCN-CSIC analytical facilities (Barcelona) by oxic or anoxic conditions using the limit of −0.1 established by using a Bragg-Brentano diffractometer (Bruker D8-A25) equipped Elderfield and Pagett (1986), and Wright et al. (1987) as a threshold be- with a LynxEye position sensitive detector. All measurements were ac- tween both conditions, whereas Y/Ho ratios shed light about the poten- quired with Cu Kα radiation, applying a tube voltage and current of 40 tial influence of seawater (>36), detrital sediment inputs (between 20 kV and 40 mA, respectively, and using a Ni filter to attenuate Cu Kβ ra- and 30) or oxyhydroxides (<25) (Tostevin et al., 2016). diation. The scans were typically recorded from 4° to 60° in 2Theta, with a step size of 0.035°, and an equivalent counting time of 192 s per step. 3.5. Scanning electron microscopy-energy dispersive X-ray spectroscopy Phase identification was carried out with the DIFFRAC.EVA software from Bruker in combination with the Powder Diffraction File PDF-2. Eight thin sections were carbon coated for pyrite framboid analyses Rietveld refinements (Young, 1993) for the calcite phase were carried using a SEM-EDX at the UAB Microscopy Labs, operating with an accel- out by using TOPAS 4.2 software from Bruker. The Rietveld refinements eration voltage of 20 kV and working with an Oxford Instruments INCA allowed extracting the unit-cell parameters and degree of crystallinity x-Act EVO MA10 software. EDX was used to identify pyrite and other of calcite along the studied succession, thus providing additional proxies mineralogical estimations. Characterization of pyrite framboids is a po- to constrain the depositional conditions at LPM. tential tool to distinguish between anoxic, dysoxic and oxic conditions in a diversity of depositional environments (Suits and Wilkin, 1998; 3.4. Inductively coupled plasma mass spectrometry and X-ray fluorescence Wilkin and Barnes, 1997, 1996). Pyrite framboid diameter and charac- teristics were used to define the following redox categories within Bulk elemental characterization of LPM section was undertaken with LPM limestones (Bond and Wignall, 2010): euxinic conditions (mean XRF and ICP-MS (see Table S1). XRF analyses were performed on all the 3–5 μm), anoxic conditions (mean 4–6 μm), lower dysoxic conditions samples by using a portable device. The aim of the XRF measurements (mean 6–10 μm), and oxic conditions (no framboids). was to obtain a preliminary semiquantitative estimate of the elemental geochemistry of LPM. For this purpose, the intensity of the relevant XRF 3.6. Loss on ignition peaks was extracted from the spectra (e.g. Ibáñez-Insa et al., 2017). The resulting data was normalized to the intensity of Ca, which allowed an Forty-seven PM samples were prepared for Loss on ignition (LOI) estimate of the relative content along the succession for P, K, Ti, Mn, analyses at the GEO3BCN-CSIC labs. LOI is a common method to esti- Fe, Rb and Sr elements among others. The XRF measurements were per- mate the organic content in sediments (Heiri et al., 2001) and it is formed on the same powdered samples used for XRD by using a Bruker used as an alternative to Total Inorganic Carbon and Total Organic Tracer Geo-IV at GEO3BCN-CSIC Barcelona. Carbon calculation (TIC and TOC respectively). LOI can be a good ap- In turn, ICP-MS was used to obtain an accurate multi-elemental de- proximation and a better tool for many types of samples (Dean, 1974; termination in 46 of the collected samples as wt% (weight percent) of Dabrio et al., 2004). major elements in the form of oxides (e.g. TiO2, Al2O3, Fe2O3, etc.) and for minor and trace elements (e.g. Li, Be, Sr, rare-earth elements, etc.) 3.7. Carbon and oxygen isotope analysis as parts per million (ppm). For this purpose, the powdered samples were analyzed with a Thermo Fisher Scientific Element XR High- Carbon and oxygen isotope analyses were performed in continuous Resolution Inductively Coupled Plasma-Mass spectrometer (HR-ICP- flow mode following the procedure described in Breitenbach and MS). The samples were initially dried at 40 °C during 24 h and subse- Bernasconi (2011), using a GasBench II coupled to a ThermoScientifc quently acid-digested in closed polytetrafluoroethylene (PTFE) vessels MAT 253 mass spectrometer at the ICTA-UAB Stable isotopes Lab with a combination of HNO3 + HF + HClO4 (2.5 mL:5 mL:2.5 mL v/v). (LAIE, Barcelona, Spain). Due to the very low organic content of the orig- The samples were then evaporated and, to make a double inal samples, no pre-treatment to remove organics was performed. Be- evaporation, 1 mL of HNO3 was added to them, which were tween 150 and 400 μg sample powder (homogenized fine fraction), redissolved and diluted with MilliQ water (18.2 MΩ cm−1) and 1 mL depending on the carbonate content was weighed into borosilicate of HNO3 in a 100 mL volume flask. A tuning solution of 1 μg L−1 Li, B, glass vials and oven-dried at 104 °C overnight. Samples were run at Na, K, Sc, Fe, Co, Cu, Ga, Y, Rh, In, Ba, Tl, U was measured to improve 70 °C for 2.5 h together with international standards NBS19, NBS18, the sensitivity of the ICP-MS, and 20 mg L−1 of a monoelemental solu- IAEA603 and a working lab standard. All results are reported with re- tion of 115In were added to the samples as internal standard. spect to the Vienna Pee Dee Belemnite (VPDB) standard. The external The HR-ICP-MS results were employed to determine the Rare-Earth standard deviation for oxygen is <0.15 ‰, and for carbon <0.08 ‰. Elements and yttrium elemental concentration (REE + Y) of LPM lime- stones. The REE + Y contents were normalized to the Post-Archean 4. Results Australian Shale (PAAS) from McLennan (1989) to obtain elemental anomalies that may provide potential information about lacustrine 4.1. Sedimentary microfacies paleoredox conditions (Ce), marine influence (Y), lake alkalinity (La, Lu, and Gd) and hydrothermal alteration of samples (Eu) (e.g., Möller Based on the sedimentary features, texture, and composition of the and Bau, 1993; Bau and Dulski, 1996; Bau et al., 1998; Kim et al., studied thin sections and outcrops, three sedimentary microfacies 2012; Tostevin et al., 2016). Additional information about the redox have been recognized in the LPM lacustrine succession (Figs. 2 and 3): 4 A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 bioclastic wackestone, Microfacies 1 (F1); microbial filament-rich mud- stone, Microfacies 2 (F2); and laminated silt-rich mudstone, Microfacies 3 (F3). Although most of the fossil specimens collected during previous paleontological campaigns were not tied to a specific stratigraphical section, recent surveys show that biota can be found across all the facies in the lithographic limestones section. 4.1.1. Microfacies 1: Bioclastic wackestone This microfacies is dominant at the base of the section (0 to 2 m), and at the top (41 to 48 m), and occurs as pale gray, tabular and plane parallel beds ranging between 5 and 40 cm in thickness. They are characterized by poorly sorted, organic-rich wackestone to floatstone rich in fragments of charophyte thalli, scarce charophyte gyrogonite debris, freshwater ostracod and brackish miliolid shells, muddy intraclasts, fecal pellets, together with undetermined invertebrate remains (Figs. 2; 3A and B). This microfacies is characterized by relatively variable LOI contents, between 1 and 1.9 % in the first 2 m of the section, and between 0.55 and 1.44 % in the last 7 m. Work in progress indicates that the fossil assemblage from F1 is composed by teleostean fish as Rubiesichthys gregalis and Ascalabos sp., crustaceans as Delclosia roselli, Montsechia vidalii remains are very abundant, Ranunculus ferreri, Podozamites sp., Brachyphyllum sp. and some seeds are also recognized in this microfacies; undetermined coprolites are very abundant. 4.1.2. Microfacies 2: Filament-rich mudstone This microfacies is the most extensive and characterizes the middle part of the LPM sedimentary log, spanning from 2 to 38 m. These lime- stones display a pale to dark gray color and are organized in plane parallel beds, meter-scale slumped beds (around meter 28), centimetric-scale syn-sedimentary fractures, and centimeter-scale cf. stromatolitic struc- tures. Beds range between 1 and 40 cm in thickness. Petrographically, this microfacies is made up of dark gray to pale brown homogeneous, faintly laminated micrite-rich intervals that occasionally alternate with organic-matter and clastic-rich laminae containing small sand-sized quartz and opaque mineral grains and silt, fragments of fish bones, fine plant debris, and thin-shelled mollusks (Figs. 2 and 3-C and D). Water scape structures and some slumping levels are concentrated in this microfacies, which is distinguished by the conspicuous occurrence of irregularly shaped, contorted filamentous structures that are present as vertically oriented objects (average length of 144.3 μm, n = 76; average diameter of 1.06 μm, n = 13, Fig. 3D). They occur interspersed in micrite sediment, although some intervals are densely populated by fila- mentous structures that are accommodated forming dark sub-horizontal laminae. In some samples, fining-upwards millimeter-thick bands are recognized grading from clastic-rich to filament-rich mudstones. Filamentous structures may correspond to sparite crystals with hollow centers as observed in SEM images (Fig. 3D). This microfacies is characterized by variable LOI contents ranging between 2.78 % and 0.66 %, from base to top section. Ongoing research lists fossil assemblages in F2 of fish Rubiesichthys gregalis, Ascalabos sp., and Ichthyemidion sp. among others. The mayfly larvae Mesopalingea leridae is very common, and some stratiomyomorphan fly larvae are also found. Plant remains include Brachyphylum sp., Podozamites sp., Weichselia? and some unidentified ferns. Undetermined coprolites are also present. 4.1.3. Microfacies 3: Laminated silt-rich mudstone This microfacies constitutes the uppermost part of the studied sedi- mentary section, roughly between 38 and 41 m. These limestones dis- play a pale to dark gray color and are arranged in plane parallel beds ranging between 1 and 20 cm in thickness. They comprise a poorly con- tinuous wavy to irregular laminated texture formed by the alternation of millimeter to sub-millimeter-thick dark gray, organic, and peloidal- Fig. 2. Sedimentary section of LPM with sampling sites, microfacies types, sedimentary rich laminae with pale gray, bioclast and silt-rich laminae in graded in- structures, and fossils. See Gil-Delgado et al. (2023) for details. tervals (Figs. 2 and 3E-F). Some levels contain thin infilled burrowed 5 A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 Fig. 3. Microfacies of LPM limestones. A–B) F1: Bioclastic wackestone to floatstone with charophyte thalli, scarce gyrogonite charophyte debris, freshwater ostracod (yellow arrow) and miliolid shell remains (white arrow), intraclasts and fecal pellets. C–D) F2: Filament-rich mudstone constituted by abundant, irregularly shaped, and contorted filamentous structures. Some horizons are densely populated (yellow arrows). SEM photomicrograph of this microfacies showing micrite embedding sparite coatings around hollow centers (white arrows). E–F) F3: Laminated silt-rich mudstone constituted by the alternation of millimeter to sub-millimeter-thick dark gray, organic, and peloidal-rich laminae with pale gray, bioclast and silt-rich laminae in graded intervals. Some thin burrows are observed (yellow arrows). cavities and poorly sorted intraclasts or bioclastic charophyte remains 4.2. Bulk chemical data with intraparticle porosity. Post depositional events are restricted to few small-scale slumps and water scape structures. This microfacies is The general trends displayed by the most relevant elemental characterized by variable LOI contents ranging between 2.11 % and abundancies obtained by ICP-MS are shown in Fig. 4 (seen also Tables 0.38 %, from the base to the top. The paleontological record from this S1 and S2). In Fig. 4A (from left to right) titanium (Ti), zirconium (Zr), microfacies has not yet been stratigraphically framed. potassium (K), and aluminum (Al) are grouped. These display a rather 6 A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 Fig. 4. Summary of geochemical proxies used in this work. A) Combined plot for Ti, Zr, K, and Al as determined from ICP-MS; Fe (ICP-MS vs XRF); Ga, Ba, and Sr/Ba ratio from ICP-MS; Ni, and Cu ICP-MS profiles; Loss on Ignition % (LOI); Mo, U, V, and U/Th ratio from ICP-MS; Y/Ho ratio (ICP-MS), and pyrite framboid diameters categorized as Anoxia (An.), Lower dysoxia (L. dys.) and Oxic zone with no pyrite (Ox.). B) Oxygen (δ16O) and carbon (δ13C) stable isotopes across the succession; Sr and Mg/Ca ratio (ICP-MS); Sr (from XRF) and calcite cell parameter (Rietveld refinement from XRD scans); Pr), log (Ce/Ce*), and Eu anomalies from ICP-MS. similar decreasing trend (although K is steadier). The very same to be steady up to 47 m, while pyrite framboid diameters show a rather fashion is observed for iron (Fe), both in the case of the ICP-MS decreasing trend up section. data (ppm values) and that of the more closely spaced XRF results. Fig. 4B shows δ18O and δ13C carbonate composition across the suc- It is worth mentioning that in the latter case the data are represented cession. Both isotopic species seem to exhibit a very similar behavior with intensity of the Fe Kα peak normalized to that of Ca Kα. As can along the bottom-middle part of the succession (microfacies F1 and be seen in Fig. 4, there is good congruence between the two methods. F2), and more negative values in the middle part of microfacies F2, More importantly, the abundance of these elements shows a statisti- and in microfacies F3. Detailed data on carbon and oxygen isotopes is cally significant positive correlation (see Figs. S1 and S2, Tables S1 shown in Figs. 5 and 6. and S2). In turn, the Sr and Mg/Ca ratio in Fig. 4 show a similar vertical trend Data for gallium (Ga) and barium (Ba) show a decreasing vertical and tend to exhibit increased values up section. This is also reflected in trend as occurs in the case of Fe and for the Ti, Zr, K and Al group the Sr content as inferred from the XRF intensity of the Kα peak for all (Fig. 4A). In contrast, nickel (Ni) and cupper (Cu) profiles are fairly the investigated samples, together with the basal lattice parameter of steady. Loss on Ignition (LOI) displays a smooth decline up section. In calcite, a (Å), which is expected to reflect the incorporation of impurities addition, molybdenum (Mo), uranium (U) and vanadium (V) reveal sta- in the crystal lattice of the mineral. ble values from 0 up to 35 m which coincides with a change from Decreased lattice parameters may be linked to the substitutional in- microfacies F2 to F3 (Fig. 4A). Above 35 m, an increase for these ele- corporation of Mg in Ca sites of the crystal structure of calcite (Titschack ments and for the U/Th is identified. In the case of V a saw-shaped pro- et al., 2011). Both Sr content and a (Å) show an inverse correlation that file can be recognized below 35 m. Regarding the Y/Ho ratio, it is found is slightly lost from meter 45 onwards (Fig. 4B). It should be noted that 7 A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 Fig. 5. δ 13C and δ 16O stable isotopes across LPM succession in ‰ VPDB. Data clustering (right) stratigraphically constrained for samples labeled as “R”, as obtained with Past software to define 3 zones: No lithographic limestones (No-lith.), stage A and stage B. Mg cannot be detected with the XRF spectrometer employed in this 5. Discussion work. In any case, Mg and Sr as determined by ICP-MS have a positive correlation (see Figs. S1 and S2). The inverse correlation found between 5.1. Paleoenvironmental and geochemical reconstruction of LPM Sr (XRF) and cell parameter a (Å) suggests that they are unsuitable to Konservat-Lagerstätte succession evaluate calcite compositional variations throughout the section (see Figs. S1 and S2). 5.1.1. Evaluation of the diagenetic overprint Finally, profiles for praseodymium (Pr) and log (Ce/Ce*) show a rel- Lacustrine carbonates are sensitive recorders shedding light on atively consistent steady trend, while europium (Eu) displays a rather depositional conditions, but their textures can be affected by saw-shaped vertical distribution. early diagenetic transformations compromising their usefulness for The REE + Y contents for the complete carbonate succession paleoenvironmental reconstructions. Therefore, original signatures results are normalized to the Post-Archean Australian Shale must be identified and differentiated from a possible diagenetic over- (PAAS) from McLennan (1989) and are shown in Fig. 7. Overall, print. LPM limestones display a set of features supporting a negligible the patterns for carbonate samples are rather flat (Fig. 7A). Most impact of early to burial diagenesis in their textures and geochemical samples show apparent positive to no Ce anomalies, although signatures. First, the original laminated fabric is preserved without evi- very few of them show negative anomalies. The patterns are also dence of recrystallization, neomorphism or cavity filling cements asso- characterized by the absence of Eu anomalies, negative to zero Y ciated with late-stage diagenesis. While some stylolites, and soft- anomalies, as well as apparent La, Lu and Gd positive anomalies. sediment deformation structures (water escape) are observed, they Additionally, one sample of section top shows a strong Ho negative are uncommon and concentrated to specific sedimentary intervals. anomaly. The occurrence of delicate carbonaceous and filament-rich structures The Pr/Pr* versus Ce/Ce* cross-plot in (Fig. 7B) shows that most sam- characterizing F2, the abundant and exquisitely well-preserved soft tis- ples display positive Ce anomalies. According to the plot of Ge et al. sues of fossil biota (Martínez-Delclòs, 1991; Gil-Delgado et al., 2023), (2010), which correlates the logCe/Ce* with redox conditions of sedi- and the paucity of cementation features or dolomitization textures mentation, most of the samples fall within the range of anoxic waters suggest that late diagenesis was not pervasive. In addition, the (Fig. 7C). anticorrelation between Sr concentrations and calcite lattice parameters 8 A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 Fig. 6. Cross plot of δ 13C and δ 16O for LPM limestones. Jurassic and Early Cretaceous dolostones from Cruset et al. (2021) are also shown for comparison. Gray rectangle represents the isotopic composition of marine carbonates from Veizer et al. (1999). Yellow circles show samples from stage A of Fig. 5 clustering. Green circles show samples that matches stage B of Fig. 5 clustering. Red triangles represent LPM Mesozoic bedrock signatures. (see below), and the good correlation between the detrital input proxies diagenesis (Gomes et al., 2020; Wintsch and Kvale, 1994) Therefore, the and the measured LOI throughout the sedimentary log reflects that LPM decreasing concentration of Al, Ti, and K throughout the LPM succession limestones (see below) are likely recording quite reliable primary depo- and their strong correlation with Fe points to a reduced input of terrig- sitional signatures (see Figs. S1 and S2). enous fractions up section probably associated with a lessening of fresh- water ingress into the lake (Cohen, 2003; Davison, 1993) (Fig. 4A). The 5.1.2. Detrital input vertical profile of Zr and Rb (not shown) also points to decreased en- In freshwater lakes, sedimentary profiles of Al, K, Fe and Ti trace el- richment in fine-grained clay material from the middle of the section ements can be commonly associated with terrestrial sediment dis- onwards. However, it should be noted that the presence of silt and charge into the lake reflecting the mineralogical by-products of soil sand-sized quartz grains is moderately sparse but homogenous across weathering (Cohen, 2003; Kylander et al., 2011; Moreno et al., 2007; the studied microfacies from petrographic observations. XRD and XRF Rohais et al., 2019; Tribovillard et al., 2006; Wintsch and Kvale, 1994). analyses confirm a very minor but steady contribution of siliceous ma- Thus, these elements can be used for reconstructions of detrital input ei- terials and/or clay to the calcareous mudstones which is also supported ther as fluvial or eolian contributions aided by their low mobility during by absence of negative Ce anomalies (Rieger et al., 2022). 9 A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 Fig. 7. A–B) PAAS-normalized REE + Y distribution patterns of carbonates of the LPM. C) PAAS-normalized Ce/Ce* versus Pr/Pr* cross-plot to analyze the Ce and Pr anomalies of carbonate samples. The method of Bau and Dulski (1996) modified by Webb and Kamber (2000) was used. D) Log(Ce/Ce*) of carbonate samples. The distinction between oxic and anoxic conditions at −0.10 is based on Elderfield and Pagett (1986) and Wright et al. (1987). 5.1.3. Paleosalinity and evaporation trends and Ba, and those observed for Al, Fe and Zr, suggesting that Ga and LPM limestones have been commonly described as having formed in Ba might have entered the lake through metal remobilization from extensive coastal lakes that evolved in close connection to the continent weathering of catchment lithologies (see Figs. S1 and S2). If this as- (Gibert et al., 2000). While fossil associations collectively suggest depo- sumption is correct, the increased inputs of Ba and Ga to the lake must sition in a non-marine lacustrine environment such as charophytes, os- have been more prominent throughout the first half of the section and tracods, mayfly and dragonfly nymphs, frogs, etc., (Gibert et al., 2000; have declined from the second half to the top, which is consistent Martínez-Delclòs, 1995), punctuated marine ingressions could have oc- with the overall reduction of the detrital input up section. Strontium curred. The scarce but recurrent presence of miliolid foraminifera in F1 could also derive from the chemical weathering of Sr-rich catchment (either in the lowermost and uppermost part of the section) may indi- minerals (i.e., plagioclase, gypsum, calcite, dolomite, celestine or stron- cate periods of saltier conditions developed in shallow freshwater tianite); however, a relatively constant abundance of Sr (between 150 lakes that evolve to coastal brackish lakes (Anadon, 1989; Pérez-Cano and 250 ppm) is observed in LPM mudstones. In addition, a very poor et al., 2022). Brackish waters are generated mainly through a spatially statistical significance with detrital-associated elements such as Al, Fe, heterogeneous mixing of freshwater and seawater which can be Zr, K and Ti has been recognized pointing toward an alternative source affected by seasonal evaporation increases (Anadón et al., 2002; of Sr other than limestone catchment weathering (see Figs. S1 and S2). Mackenzie et al., 1995; Rosen et al., 1996), so disentangling the origin Since Sr average concentrations in seawater are ~105 times greater of salinity departures in ancient lacustrine successions is challenging. compared to those in freshwater (Wei and Algeo, 2020), a marine- To estimate potential salinity fluctuations throughout LPM lake system, derived contribution of Sr could be plausible in a coastal lacustrine set- Ga, Ba, Sr, and Sr/Ba elemental concentrations in mudstones were ana- ting. Collectively, the Sr/Ba ratio has been employed as a successful lyzed (Fig. 4). Gallium is primarily derived from weathering processes proxy to identify salinity fluctuations in mudrocks (Wei and Algeo, on quartzose and feldspathic silicate rocks, and, due to strong particu- 2020) and organic-rich lacustrine deposits (Fathy et al., 2021; Li et al., late scavenging, Ga concentrations in seawater are usually lower than 2020). In this respect, LPM limestones show unusually high Sr/Ba values in freshwater systems (Wei and Algeo, 2020). In the same way, Ba prov- (from 10 to 25) at the topmost part of the section coincident with a enance in mudstones can be associated with the weathering reactions of change in depositional character from F2 to F3 going up section, naturally occurring Ba-rich catchment minerals: i.e., barite (BaSO4) or and also with the occurrence of some miliolids (Fig. 4A). However, the witherite (BaCO3), since crustal concentrations are very low (Wei and REE + Y signatures of LPM limestones show negative to zero Y anoma- Algeo, 2020; Wolgemuth and Broecker, 1970). For this reason, Ba lies and apparent La, Lu and Gd anomalies, which could discard a persis- contents in freshwaters tend to be much higher than in marine waters tent marine influence during lacustrine deposition (Bau et al., 1998). (Wolgemuth and Broecker, 1970). Therefore, Ga and Ba might be Increased salinity could also be attributed to enhanced evaporation in potentially used as tracers of freshwater contributions to lake systems shallower coastal-lake environments which could leave a geochemical when compared with the abundances of Sr (Wei and Algeo, 2020). In signature in δ18O. While more positive δ18O values are correlated with LPM mudstones, strong correlations exist between the patterns of Ga the highest Sr/Ba values observed, it is difficult to ascertain whether 10 A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 this is related to increased evaporation or to a decline in continental of uranyl and Mo complexes into LPM mudstone sediments (Bennett freshwater circulation (Ingram et al., 1996). Additional work would be and Canfield, 2020; Tribovillard et al., 2006). In line with this, the partic- required to better assess the nature of salinity fluctuations in LPM ularly lower and flatter values of U and Mo for most of the lower and coastal-lacustrine succession (e.g., Sr isotopes). middle parts of the succession could be explained as a result of a hydrographically restricted lake water circulation which could have 5.1.4. Paleoproductivity limited the accumulation of these trace metals in deep-water sediments Primary productivity in lake systems can be very high but the con- favoring dysoxic to anoxic bottom conditions (Bennett and Canfield, centrations of organic matter in lacustrine sediments only record a frac- 2020; Tribovillard et al., 2006). A restricted water circulation during tion of the total biological production in surface waters. Thus, increases the early stages of LPM lacustrine sedimentation is also proposed here in primary production in ancient lacustrine successions can be associ- based on the observed covariance in the carbon and oxygen isotopic ated with fluctuations of the organic debris that escapes dissolution, trends (see discussion below). While V has been also commonly em- heterotrophic organism degradation or bacterial decay (Cohen, 2003; ployed for paleo-redox inferences (Jones and Manning, 1994; Rohais Lerman, 1978; Tribovillard et al., 2006). Despite these complications, et al., 2019), the broad patterns in V geochemistry across the LPM suc- the delivery of organic matter of the sediment-water interface and the cession show a different behavior to that of U and Mo (Fig. 4). The het- final burial flux can be considered proportional to surface-water pro- erogeneous saw-shaped profile in V concentrations from the first half of ductivity (Gierwsloski-Kordesch, 2010; Tribovillard et al., 2006). LPM the succession could be attributed to fluctuations in the Mn\\Fe cycling samples contain a conspicuous abundance of organic remains including typical during reducing conditions which can modify the adsorption of fragments of fish bones, clotted dark patches intermingled in V onto Fe\\Mn oxyhydroxides (Tribovillard et al., 2006), and carbonate charophyte floatstones, and an abundant assemblage of filament-rich sediments (Cohen, 2003; Hild and Brumsack, 1998). In such reducing mudstones suggesting a considerable lake productivity. In addition, conditions, a high solubility of Mn2+ in sediments may have resulted LOI values recorded in the different microfacies point to a decreasing in Mn being heterogeneously depleted in LPM mudstones. Additional concentration of organic matter up section. Such a decline could be as- evidence supporting suboxic conditions during LPM deposition is pro- sociated with either an apparent reduction in organic productivity in vided by the systematic study of pyrite framboid size distribution in younger sedimentary environments, or to a decreased preservation po- the different facies (Suits and Wilkin, 1998; Wilkin et al., 1996; Wilkin tential due to fluctuating redox conditions in the sediments. Moreover, and Barnes, 1997). The lower part of LPM succession shows mean pyrite Ni and Cu are increasingly used as paleoproductivity proxies in dif- framboids between 8.10 and 8.28 μm, the middle portion exhibits ferent lake and marine systems as these elements can be easily scav- framboid diameters between 5.12 and 6.76 μm, while the upper section enged by organic matter and dominantly delivered to the sediments contains mean framboids of 4.18 μm. Collectively, these data suggest (Li et al., 2020; Rohais et al., 2019; Tribovillard et al., 2006). In line the predominance of dysoxic conditions in the first half of the with this, the good correlation between Ni or Cu and LOI reinforces succession, and a wider redox behavior (from anoxic to oxygenic condi- the idea that relatively high contents of Ni and Cu in LPM lacustrine tions) in the second half and toward the upper part of LPM section. sediments could indicate a high flux of organic matter delivery to Moreover, the REE + Y contents show that the absence of negative Ce the basin (Figs. 4, S1 and S2). Interestingly, good correlations be- anomalies (Fig. 7A and B) could also indicate lacustrine sedimentation tween Ni and Cu with Al, Fe, Ti, and K, might suggest that detrital under anoxic conditions (Rieger et al., 2022) confirming the overall fluxes to the lake correlate well with primary productivity peaks as trend. has been previously proposed in other lacustrine basins (Wang et al., 2017) (see Figs. S1 and S2). 5.1.6. Paleohydrology The δ18 O composition of all the mudstone samples ranges be- 5.1.5. Paleoxygenation-redox conditions tween about −3 ‰, and −6 ‰, and therefore differs significantly Redox-sensitive trace proxies (V, U, Mo, U/Th ratios) have been ex- from Late Barremian–Early Aptian marine signatures (Veizer et al., tensively used as paleoredox indicators in ancient sedimentary systems 1999; Wissler et al., 2003) (Figs. 4 to 6). In addition, a non-marine (Bennett and Canfield, 2020; Tribovillard et al., 2012, 2006; Wang et al., or coastal terrestrial setting is supported by the lighter δ13C values 2017). Their usefulness is based on the fluctuations in the solubility and (between −1 ‰ and −4 ‰) typically recorded in these environ- redox state tied to specific chemical conditions at the time of sediment ments (Heimhofer et al., 2010; Paz and Rossetti, 2006) compared deposition (water-sediment interface) or during early diagenesis to marine equivalents which range from +2 ‰ to +4 ‰ (Veizer within sediment pore waters. Thus, the patterns of trace metal enrich- et al., 1999; Wissler et al., 2003). Similar negative oxygen isotope ment into authigenic minerals and sediments have proven valuable values have been interpreted to belong to continental freshwater tools to discriminate between oxic, dysoxic, or anoxic conditions settings in the geological record (Camoin et al., 1997; Gallois et al., (Jones and Manning, 1994; Rohais et al., 2019; Tribovillard et al., 2018; Paz and Rossetti, 2006; Talbot, 1990) although marine- 2006). In LPM samples a particular enrichment in the concentrations influenced continental environments might exhibit lighter values of U (1.12 ppm, V (8.32 ppm), and Mo (0.66 ppm) is recognized in the (Ingram et al., 1996). uppermost part of the succession (between 40 and 47 m) suggesting The balance between influx and evaporation is responsible of enhanced oxygen depletion above the transition between F3 to F1 drastic changes in lake isotopic composition (Horton et al., 2016; Li (Fig. 4). A possible explanation for the higher U and Mo enrichments and Ku, 1997; Talbot, 1990) which in turn has been associated compared with lower and flatter values in F2 (mean of 0,4 μg/g and to fluctuations in lake levels and climate (Camoin et al., 1997; 0,2 μg/g, respectively) could be attributed to either: i) higher water re- Heimhofer et al., 2010; Paz and Rossetti, 2006). Trends in δ18O com- newal rates from adjacent water bodies (freshwater or marine) supply- position suggest that LPM mudstones can be subdivided in two ing dissolved U and Mo to the lake, or ii) higher organic carbon fluxes stages: i) a lowermost stage A (from 2 m to 18 m) showing a paucity associated to biologically-derived particulates in the water column in isotopic variability (between −5 ‰ and −3,5 ‰); and ii) an up- which are enriched in non-lithogenic U or Mo (Bennett and Canfield, permost stage B (from 18 m to 45 m) with more frequent and 2020; Tribovillard et al., 2006; Wagner et al., 2017). Furthermore, the wider variabilities (between −6 ‰ and −3 ‰). The data from the peak in redox-sensitive trace metals coincides with a peak in organic first interval could represent intermittent perturbations in lake productivity (LOI) and a remarkable change from F2 to F3. This levels with evaporative ( 18 O-rich values) and freshwater input microfacies shift could be attributed to a variation in the identity of (16O-rich values) events of similar magnitude (Fig. 5). Conversely, the primary producers and carbonate production potential, which in the uppermost interval seems to show rapid and wider fluctuations turn could have modified the depositional rates increasing the uptake in the hydrological cycle as also demonstrated by the reduced input 11 A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 of terrigenous fractions, a possible marine contribution to the lake, faintly laminated and homogeneous texture of this microfacies suggest and/or combined with enhanced evaporation. deposition under low-energy, calm and deeper conditions (Fig. 4). Al- Moreover, inorganic carbon isotope signatures of LPM limestones though changes in filament density and lateral arrangement are observed might have fluctuated according to different processes. The dissolution in some micrite intervals, it is challenging to decipher the reasons for such of the Jurassic-Cretaceous catchment limestones (with δ13C between a distribution. However, it is plausible to consider seasonal fluctuations in −0.5 ‰ and −2.5 ‰) may have had an influence on the carbon isotopic water chemistry, intensity of detrital events, redox conditions, or nutrient composition of the groundwaters inflowing to the lake explaining, at load as factors influencing the occurrence and preservation of the filamen- least in part, the −1 ‰ to −4 ‰ carbon isotopic values of LPM lime- tous structures in these profundal sediments. Since most of the sedimen- stones (Leng and Marshall, 2004). A progressive change in biological tary succession in LPM Konservat-Lagerstäte is constituted by F2, future productivity and carbonate factories (between F2 and F3) could have work is warranted to better characterize the potential geomicrobiological a significant impact of the δ13C of the dissolved inorganic carbon (DIC) roles that might be associated with the exceptional preservation of LPM pool of LPM lake. Preferred uptake of 12C by aquatic photoautotrophic fossil assemblages (Clements and Gabbott, 2022; Varejão et al., 2019). organisms (including charophytes and other littoral plants) would Finally, the depositional texture and sedimentary characteristics of F3 sug- lead to a progressive depletion of the carbon pool and enrichment of gest sedimentation in a moderate to low energy lacustrine setting episod- 13 C, as is observed in LPM carbonates across the uppermost stage B ically sourced by carbonate mud and silt-sized particles (Fig. 8). The (Giorgio, 2005; Kelts and Talbot, 1990). occurrence of tiny burrow cavities, scour surfaces and bioclastic compo- Finally, the modest positive covariance between the carbon and ox- nents collectively suggest deposition in shallower, more oxygenated, and ygen isotope curves of LPM limestones (with correlation coefficients of agitated environments than those forming F3 (Fig. 4). In summary, the 0.16 in stage A, and 0.22 in stage B) may be interpreted as possible de- vertical distribution of microfacies and sedimentological observations position in hydrologically closed to semi-closed basins (Cohen, 2003; suggest an early lake flooding event (F2 on top of F1) followed by a pro- Kelts and Talbot, 1990; Li and Ku, 1997; Talbot, 1990) (Figs. 5 and 6). gressive shallowing (F1 on top of F3) which is also supported by the distri- The temporal oscillations in oxygen and DIC isotopic compositions sug- bution of terrigenous fractions up-section (see Section 5.1.2), and the gest LPM waterbodies having relatively long residence times and more change in carbonate factories (see Figs. 4 and 8). frequent water level fluctuations. Moreover, the calcite mineralogy identified in LPM limestones may have resulted from an originally low Mg/Ca ratio of the Barremian lake 5.2. Carbonate production modes and LPM depositional evolution waters or porewaters (Chagas et al., 2016; Kelts and Hsü, 1978; Müller et al., 1972). The fact that Sr and Mg/Ca ratios of calcites are running in The analysis of microfacies and depositional textures indicates the parallel in the first half of the LPM succession probably attest that fluctu- predominance of two main carbonate sources in LPM sedimentary suc- ations in solution stoichiometry affected the incorporation of Mg and Sr cession: i) biogenic calcareous shells, and ii) chemically and biologically into calcite (Nehrke et al., 2007; Nielsen et al., 2013), as reflected in the mediated precipitation of authigenic carbonate minerals. In one side, F1 lattice parameters of calcite. However, such co-dependency is less re- is dominated by a carbonate factory including charophyte remains (thalli markable in the uppermost part of the section probably indicating: i) a and gyrogonites), ostracods, thin-shelled bivalves, and some miliolids change in the calcifying mechanisms from non-skeletal muds (F2–F3) to embedded in fine-grained, organic-rich mudstones (Fig. 8). This biotic as- charophyte calcification (F1); or ii) a diagenetic modification of the orig- sociation is typically encountered in littoral areas of the lake margins inal mineralogies at the transition between the two carbonate factories. characterized by low-energy, shallow-water, and oxygenated deposi- Furthermore, it is interesting to note the strong anticorrelation between tional environments colonized by aquatic photosynthetic organisms Sr and calcite lattice spacings, suggesting that mineralization patterns that may disintegrate forming carbonate muds (Kelts and Hsü, 1978; are affected by the pathways of Sr and/or Mg incorporation into calcite Platt and Wright, 1991; Valero-Garcés et al., 2014). On the other hand, (Wasylenki et al., 2005) (see Figs. S1 and S2). the formation of the filament-rich mudstones (F2) constituted by micrite embedding sparite coatings around hollow centers could have involved in 6. Conclusions situ precipitation of micrite mediated by picocyanobacteria inhabiting the lake (Dittrich and Obst, 2004), combined with chemical precipitation The limestones of La Pedrera de Meià (LPM) in south-Central Pyre- due to increased evaporation or lake water temperature changes nees provide one of the best-preserved successions of lacustrine- (Gierwsloski-Kordesch, 2010; Kelts and Talbot, 1990; Valero-Garcés coastal Barremian lagerstätte in Europe. This multiproxy study repre- et al., 2014; Verrecchia, 2002). The complete lack of bioturbation and sents an attempt to reconstruct the depositional environments and Fig. 8. Diagram showing the lacustrine sedimentary evolution and microfacies distributions of LPM from left to right. Shallow-water lacustrine carbonates (microfacies F1) characterized the first stage of the depositional evolution. Progressive deepening of the lake gave rise to deposition of profundal microfacies F2 under anoxic to dysoxic conditions, followed by deposition of microfacies F3 in shallower, more oxygenated, and agitated environments. The late stage is constituted by the deposition of shallow water carbonates (microfacies F1) in a coastal-la- custrine environment. 12 A. Gil-Delgado, D. Cruset, O. Oms et al. Sedimentary Geology 453 (2023) 106440 paleoenvironmental fluctuations associated to this unique Konservat- analysis, Investigation; JI: Methodology, Software, Validation, Formal anal- Lagerstätte. Combining sedimentological and geochemical data several ysis, Investigation; XD: Funding acquisition, investigation, review; AS: In- conclusions can be drawn: vestigation, review; AG: investigation, Project Administration, review; RMM: Conceptualization, Validation, Methodology, Formal analysis, Inves- 1. LPM succession documents an overall evolution of a coastal lake tigation, Writing - original draft, review & editing, Visualization, Funding showing a transition from profundal to littoral lacustrine environ- acquisition. ments both recording the exceptional preservation of fossil biotic as- semblages. Littoral settings are characterized by low/moderate- Data availability energy, shallow-water, and oxygenated conditions where charophytes, ostracods and miliolids accumulated in mud-rich facies Data will be made available on request. (microfacies F1). Profundal facies are made up of filament-rich mud- stones (microfacies F2) originated from in situ precipitation of Declaration of competing interest micritic sediments either biotically mediated or physico-chemically induced and deposited in virtually undisturbed, low-energy environ- The authors declare the following financial interests/personal relation- ments under anoxic conditions. While the biota that lived in the ox- ships which may be considered as potential competing interests: ygenated areas of the lake is preserved, the terrestrial biota has been Alejandro Gil-Delgado reports financial support was provided by Autono- shed by flotation or by slow currents. mous University of Barcelona. Alejandro Gil-Delgado reports administra- 2. The analysis of stable δ13C and δ18O and redox-sensitive geochemical tive support was provided by Museu de la Conca Dellà. Alejandro elements (V, Mo, U, U/Th, REE + Y) points to deposition in a hydro- Gil-Delgado reports financial support was provided by International Asso- logically closed to semi-closed lacustrine basin with a restricted ciation of Sedimentologists. David Cruset reports financial support was water circulation (water stratification) favoring dysoxic to anoxic provided by Geosciences Barcelona. Ramon Mercedes-Martin reports fi- conditions in profundal sediments. These conditions were prevalent nancial support was provided by Autonomous University of Barcelona. during the entire deposition of LPM carbonates playing a key role in Xavier Delclos reports financial support was provided by University of the exceptional preservation of the biotas. Barcelona. Jordi Ibanez-Insa reports financial support was provided by 3. Paleoproductivity proxies (Ni, Cu, LOI) suggest that terrestrial sedi- Geosciences Barcelona. Albert Selles reports administrative support was ment discharge to the lake correlate well with primary productivity provided by Museu de la Conca Dellà. Angel Galobart reports administra- peaks. In addition, a decline in terrigenous input to the lake (fine- tive support was provided by Museu de la Conca Dellà. Oriol Oms reports grained clastic fraction) is here interpreted because of the progres- financial support was provided by Autonomous University of Barcelona. sive reduction of meteoric water entrance at the upper part of the succession. These trends are also supported by the Ba and Ga signa- Acknowledgements tures in LPM carbonates which points to reduced catchment weathering up section. This work has been funded by Spanish Government MCIN project 4. Whereas different lines of evidence advocate a salinity increase toward PGC2018-101575-B-I00. “Dynamics of transitional settings from Creta- the younger lake deposits (Sr/Ba ratios, presence of miliolids, isotopic ceous to Eocene in the south central Pyrenees”; and the Generalitat de signatures), it is difficult to determine whether this is related to in- Catalunya (CERCA Program). AGD was supported by the AGAUR pro- creased evaporation, or to a combination of reduced continental fresh- gram for the doctoral formation grant 2020 FI SDUR 00360. ASG re- water circulation to the lake, or to sporadic seawater inputs to the lake. search is supported by the project VIGEOCULT (PLEC2021-00793) funded by MCIN/AEI/10.13039/501100011033 and by the European The present study provides a framework to decode the Union NextGenerationEU/PRTR. ASG and AG acknowledges the project paleohydrological and paleoenvironmental fluctuations in similar litho- ARQ001SOL-173-2022 (“El Cretaci continental del Pirineu meridional graphic limestones worldwide. A straightforward application of this català: ecosistemes I evolució faunística. 2022-2025”) funded by the methodology could be the case of the renowned lagerstätte deposits Departament de Cultura of the Generalitat de Catalunya and the CERCA from Las Hoyas in central Spain (Meléndez, 1995; Poyato-Ariza and Program. We are indebted to Vilanova the Meià and Camarasa local au- Buscalioni, 2016; Talbot et al., 1995). thorities, as well as Orígens UNESCO Global Geopark to facilitate field- Additional targets might include other outstanding lithographic work in the Montsec area. AGD benefited from an International limestone sites such as Solnhofen and Nusplingen in Germany, Causse Association of Sedimentologists IAS Postgraduate Grant to cover field Méjean and Cerin in France, or Crato in Brazil. According to our results work and geochemical expenses. This work is part of the PhD Thesis the multiproxy study of these sites would benefit from a closely spacing of AGD (made in the framework of a PhD Doctoral Programme in Geol- geochemical sampling on the basis of ICP-MS, REE analysis, calcite cell ogy at UAB), and EB Undergraduate Thesis project. We would like to parameter, organic matter content, and stable isotope data. Future stud- thank Soledad Alvarez (GEO3BCN-CSIC) for technical support with the ies in LPM will improve the proxies' database by including Sr stable iso- XRD and XRF measurements. The constructive and detailed comments topes, biomarker signatures, or evaluating the possible involvement of by Prof. Giorgio Basilici and an anonymous reviewer contributed to microbial processes in biota preservation. Interestingly, the multiproxy sharpen the manuscript and are kindly acknowledged. framework presented may enable to precisely locate fossil specimens in the section by analyzing their host-rock geochemical signature. Finally, Appendix A. Supplementary data an integrated taphonomic study may be developed by considering the depositional model and the abiotic and biotic processes that appear to Supplementary data to this article can be found online at https://doi. influence the exceptional preservation phenomena. org/10.1016/j.sedgeo.2023.106440. 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