Cadmium (Cd) and isotope systematics are emerging tools for studying the biogeochemical cycling of micronutrients in the oceans, and sedimentary archives, as Cd concentrations in seawater show a nutrient-like behaviour, with surface depletion and deep water enrichments. However, the underlying processes are yet to be fully understood. The Cd concentration and isotopic composition of seawater are set by the balance of Cd inputs (and their isotopic composition) and the fractionation on removal to sedimentary sinks. The most favoured explanation is the Cd utilisation by marine phototrophic biomass, causing the surface water’s dissolved Cd pool depletion creating a depth gradient of increasing Cd concentrations and lighter isotopic compositions. Under incomplete oxidative recycling, organic matter may act as an effective Cd sink and authigenic minerals may store the ambient seawater’s Cd isotope composition.

Consequently, stable Cd isotope compositions in marine carbonates show broad variations linked to paleo-productivity and redox state changes. Additional fractionation processes govern the Cd isotopic compositions of marine sediments. Besides biological utilisation, experimental Cd partitioning into authigenic calcites or sulphides under variable redox and salinity conditions has been shown.  Therefore, when applying Cd isotopes in carbonates, other geochemical proxies must be evaluated very carefully to understand the involved Cd fractionation processes. This presentation aims to present the status quo of research done on authigenic and biologic carbonates and carbonate leachates in carbonatic shales to show the strengths and pitfalls of this new emerging bio-geoscience isotope proxy and its use for paleoenvironmental reconstructions on Earth and beyond.

How to cite: Hohl, S. V.: Status-quo of carbonate cadmium isotope compositions as a paleo-productivity proxy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-825, https://doi.org/10.5194/egusphere-egu21-825, 2021.

Stromatolites are laminated, presumably microbial, structures, consisting largely of an authigenic precipitate, thus providing potential geochemical archives of early Earth aqueous environments and their habitability. In this study, we report trace element and Sm/Nd isotope data from Palaeoarchean stromatolites and adjacent cherts of the Strelley Pool Formation (NW Australia), obtained by ICP-MS and TIMS, to test their reliability as archives for palaeo-environmental reconstruction and to understand authigenic mineral formation. Stromatolitic carbonates plot together with the stratigraphically underlying Marble Bar cherts on a linear Sm-Nd regression line yielding an age of 3253 ±320 Ma. In contrast, associated crystal-fan carbonates yield 2718 ±220 Ma, suggesting that their Sm-Nd isotope system was altered after deposition. Geochronological information via Sm-Nd dating of black and white cherts is limited, probably due to a reset of the isotope system during an unknown Paleoproterozoic or younger alteration event. Carbonates, as well as white cherts, show shale-normalized rare earth element and yttrium patterns (REY_SN; except for redox-sensitive Ce and Eu) parallel to those of modern seawater, indicating a seawater-derived origin. Positive Eu_SN anomalies (2.1 - 2.4), combined with heterogeneous ɛNd_3.35Ga values (-3.2 to +5.8) within alternating stromatolite laminae, support that seawater chemistry was affected by both continental weathering and high-temperature hydrothermal fluids that episodically delivered chemical elements from young mafic and older felsic rock sources into the stromatolite environment. In contrast, black cherts show REY_SN patterns characteristic of a non-seawater source and significant amounts of elements leached from the surrounding rocks, overprinting the pristine geochemical composition of ancient seawater. In conclusion, Archaean stromatolites indeed preserve pristine authigenic phases at the mm-scale that contain signatures representative of the water chemistry prevailing in the depositional environment of the time.

How to cite: Viehmann, S., Hohl, S. V., Tepe, N., Van Kranendonk, M., Reitner, J., Hofmann, T., Koeberl, C., and Meister, P.: Trace metals and Nd isotopes in 3.35 Ga old stromatolites of the Strelley Pool Formation (Australia) unravel the genesis of carbonates and chert, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1984, https://doi.org/10.5194/egusphere-egu21-1984, 2021.

The sedimentary environment and notably the climate conditions that pertained during deposition of the Mesoproterozoic (~1200 Ma) Torridonian Stoer Group have been subject to debate for some time. On one hand it has been proposed that, despite the low palaeolatitude, the Group is largely represented by fluvio-lacustrine sediments deposited under cold, possibly glacial conditions.  On the other hand, evidence and arguments have been put forward in favour of either a marine, or arid to semi-arid terrestrial environment. Contributing to this debate, in this study we focus on thin calcitic layers within the Clachtoll formation and younger Poll a’ Mhuilt member that may represent stromatolites, or stromatolite like deposits. Whilst recent work has cast doubt on the biogenic origin of these calcite layers, suggesting they may be either evaporitic or detrital in origin, we believe that much of the petrographic and isotope evidence is equivocal. Focusing on large scale morphology, sedimentary structures, micro-fabrics and mineralogy we  present new evidence for the biogenicity of these deposits. A key difficulty is resolving both diagentic (pressure solution, stylolite development and neomorphism) and later grain growth fabrics associated with low grade metamorphism from unaltered fabrics and grains. In combination with bulk (δ¹³C and δ¹⁸O) and clumped isotope (Δ₄₇) studies we find that whilst the Stoer Group has undergone low grade metamorphism with maximum temperatures of ca. 120^oC the isotope system has remained closed with respect to exchange with diagenetic and metamorphic fluids. The implication is that the very depleted δ¹⁸O values for the calcites of -18‰_VPDB are characteristic of the original environmental conditions. Meteoric water values would need to be as low as -15 to -20‰_VSMOW for precipitation of the calcite at ambient Earth surface temperatures. This is prima facie evidence that the deposits are terrestrial and not marine and at face value also implies cold conditions with isotopically depleted rainfall. We cannot rule out, however, that precipitation sourced from a global ocean that is significantly depleted in ¹⁸O as suggested by some models may contribute to explaining the very depleted isotope signatures and apparent low temperatures.

How to cite: Dennis, P., Duchesne, B., and Marca, A.: Palaeoenvironmental signals from stromatolites of the Mesoproterozoic Stoer Group, N.W. Scotland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4122, https://doi.org/10.5194/egusphere-egu21-4122, 2021.

How to cite: Pollier, C., Ariztegui, D., Nuñez Guerrero, A., and Rabassa, J.: Living and fossil microbialites in Laguna de Los Cisnes (Southernmost Chile): A duel between biotic and abiotic processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8911, https://doi.org/10.5194/egusphere-egu21-8911, 2021.

Stromatolites represent some of the earliest evidence for life and are valuable geochemical archives for understanding the rise of oxygen on early Earth. Metal redox proxies in carbonates, such as stable uranium isotopes (²³⁸U/²³⁵U), are useful for assessing the oxidation state of ancient waterbodies, but may also be sensitive to local water chemistry and early sedimentary diagenesis. This requires the validation of such proxies in modern environments before applying them to ancient carbonates. Here we measure the U isotopic composition of modern stromatolites forming in Hamelin Pool in Shark Bay, Western Australia – a large hypersaline marine embayment and the largest modern example of stromatolite development globally. Actively-growing stromatolite tops from Shark Bay exhibited a narrow range of δ²³⁸U from -0.30 to -0.33‰, corresponding to an offset of ca. +0.1‰ from seawater. Such an offset has not been found in other biotic marine carbonates, which exhibit seawater-like δ²³⁸U (ca. -0.4‰), but is consistent with findings from carbonate co-precipitation experiments. One hypothesis for our measured +0.1‰ offset is the elevated Ca concentration of the hypersaline Shark Bay seawater relative to open seawater. This results in a greater proportion of dissolved U present as Ca₂UO₂(CO₃)₃, which is expected to be isotopically lighter than other U species and not incorporated during carbonate mineral formation. Higher δ²³⁸U up to +0.11‰ were measured in the deeper stromatolite laminae, consistent with the expected U isotope signatures for U reduction. Stromatolite radiocarbon ages show that the diagenetic modification of U occurs within ~1 ka and may be considered syndepositional on geological timescales. These results from the deeper stromatolite laminae support the application of a ca. -0.4‰ correction factor to the δ²³⁸U of stromatolites formed in oxic waterbodies, similar to other biotic carbonates. It is unclear whether the additional +0.1‰ offset found in stromatolite tops is particular to seawater chemistry of Shark Bay or a general feature of microbial carbonate precipitation. This warrants investigation of the δ²³⁸U proxy in other modern environments where stromatolites proliferate.

How to cite: Martin, A. N., Markowska, M., Chivas, A. R., and Weyer, S.: The uranium isotopic composition of modern stromatolites forming in Shark Bay, Western Australia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14810, https://doi.org/10.5194/egusphere-egu21-14810, 2021.

Biogeochemical disruptions across the Cretaceous-Paleogene boundary : insights from sulfur isotopes

Arbia JOUINI^1*, Guillaume PARIS¹, Guillaume CARO¹, Annachiara BARTOLINI²

¹ Centre de Recherches Pétrographiques et Géochimiques, CRPG-CNRS, UMR7358, ,15 rue Notre Dame des Pauvres, BP20, 54501Vandoeuvre-lès-Nancy, France, email:Arbia.jouini@univ-lorraine.Fr

² Muséum National D’Histoire Naturelle, Département Origines & Evolution, CR2P MNHN, CNRS, Sorbonne Université, 8 rue Buffon CP38, 75005 Paris, France

The Cretaceous–Paleogene (KPg) mass extinction event 66 million years ago witnessed one of the ‘Big Five’ mass extinctions of the Phanerozoic. Two major catastrophic events, the Chicxulub asteroid impact and the Deccan trap eruptions, were involved in complex climatic and environmental changes that culminated in the mass extinction including oceanic biogenic carbonate crisis, sea water chemistry and ocean oxygen level changes. Deep understanding of the coeval sulfur biogeochemical cycle may help to better constrain and quantify these parameters.

Here we present the first stratigraphic high resolution isotopic compositions of carbonate associated sulfate (CAS) based on monospecific planktic and benthic foraminifers' samples during the Maastrichtian-Danian transition from IODP pacific site 1209C. Primary δ34SCAS data suggests that there was a major perturbation of sulfur cycle around the KPg transition with rapid fluctuations (100-200kyr) of about 2-4‰ (±0.54‰, 2SD) during the late Maastrichtian followed by a negative excursion in δ34SCAS of 2-3‰ during the early Paleocene.

An increase in oxygen levels associated with a decline in organic carbon burial, related to a collapse in primary productivity, may have led to the early Paleocene δ34SCAS negative shift via a significant drop in microbial sulfate reduction. Alternatively, Deccan volcanism could also have played a role and impacted the sulfur cycle via direct input of isotopically light sulfur to the ocean. A revised correlation between δ34SCAS data reported in this study and a precise dating of the Deccan volcanism phases would allow us to explore this hypothesis.

Keywords : KPg boundary, Sulphur cycle, cycle du calcium, Planktic and benthic foraminifera

How to cite: Jouini, A.: Biogeochemical disruptions across the Cretaceous-Paleogene boundary : insights from sulfur isotopes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14669, https://doi.org/10.5194/egusphere-egu21-14669, 2021.

Stable oxygen isotopes (δ¹⁸O) of planktonic foraminifera are one of the most used tools to reconstruct environmental conditions of the water column. Since different species live and calcify at different depths in the water column, the δ¹⁸O of sedimentary foraminifera reflects to a large degree the vertical habitat and interspecies δ¹⁸O differences and can thus potentially provide information on the vertical structure of the water column. To fully unlock the potential of foraminifera as recorders of past surface water properties, it is necessary to understand how and under what conditions the environmental signal is incorporated into the calcite shells of individual species. Deep-dwelling species play a particularly important role in this context, since their calcification depth reaches below the surface mixed layer. Here we report δ¹⁸O measurements made on four deep-dwelling Globorotalia species collected with stratified plankton tows in the Eastern North Atlantic. Size and crust effects on the δ¹⁸O signal were evaluated showing that a larger size increases the δ¹⁸O of Globorotalia inflata and Globorotalia hirsuta, and a crust effect is reflected in a higher δ¹⁸O in Globorotalia truncatulinoides. The great majority of the δ¹⁸O values can be explained without invoking disequilibrium calcification. When interpreted in this way the data imply depth-integrated calcification with progressive addition of calcite with depth to about 300 m for G. inflata and to about 500 m for G. hirsuta. In Globorotalia scitula, despite a strong subsurface maximum in abundance, the vertical δ¹⁸O profile is flat and appears dominated by a surface layer signal. In G. truncatulinoides, the δ¹⁸O profile follows equilibrium for each depth, implying a constant habitat during growth at each depth layer. The δ¹⁸O values are more consistent with the predictions of the Shackleton (1974) paleotemperature equation, except in G. scitula, which shows values more consistent with the Kim and O’Neil (1997) prediction.  In all cases, we observe a difference between the level where most of the specimens were present and the depth where most of their shell appears to calcify.

How to cite: Rebotim, A., Voelker, A. H. L., Jonkers, L., Waniek, J. J., Schulz, M., and Kucera, M.: Calcification depth of deep-dwelling planktonic foraminifera from eastern North Atlantic: evidence from stable oxygen isotope ratios of shells from plankton tows , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10173, https://doi.org/10.5194/egusphere-egu21-10173, 2021.

Mar. Micropaleontol. 44, 141–162. https://doi.org/10.1016/S0377-8398(01)00039-1

How to cite: Saupe, A., Schmidt, J., Petersen, J., Bahr, A., and Grunert, P.: Biogeography of benthic foraminifera in contourite drift systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2618, https://doi.org/10.5194/egusphere-egu21-2618, 2021.

There is an increasing interest in understanding the role of coccolithophores, a group of major calcifying phytoplankton, in the marine carbon cycle: they have a dual contribution to the operation of the carbonate and biological pumps during their lifecycle. How the recent changes in seawater carbonate chemistry are affecting their production and calcification is a matter of debate in the scientific community. Culture experiments suggest that modern coccolithophore species (Emiliania huxleyi) is sensitive to such variations. Conversely, could past evolutionary or adaptative changes in the most important coccolithophore species have an impact on ocean chemistry?  

We focus on the interval comprising the MIS 14 to 7 (Mid-Brunhes, Pleistocene) when a remarkable increase in the amplitude of glacial/interglacial atmospheric CO₂ was recorded. We analyzed (i) the composition of the dominant coccolithophore Gephyrocapsa assemblages and (ii) the morphometric parameters (length, mass, and thickness) of its coccoliths (carbonated scales) in samples from a set of sediment cores (Sites IODP U1314, U1385and ODP 925 and 977) located in a north-south transect in the North Atlantic and the western Mediterranean Sea. We estimated the primary productivity conditions at the different regions and explore methodological approaches to measure the calcification of Gephyrocapsa coccoliths.

Preliminary results show a correlation between the abundance of coccoliths, assemblage composition, and coccolith morphology at different regions. A comparison with geochemical and sedimentological records suggests a significant role of Gephyrocapsa coccolithophore in marine organic and carbonate production throughout the interval. These observations open the discussion about the existence of a global environmental relationship between coccolithophore assemblages and coccolith morphometrical variations, but also, a possible impact of the changes in the Gephyrocapsa production and calcification on the ocean chemistry.

How to cite: González-Lanchas, A., Flores, J.-A., and Sierro, F. J.: Production and calcification variations of the key coccolithophore species Gephyrocapsa during the Late Pleistocene (MIS 14 to MIS 7), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12655, https://doi.org/10.5194/egusphere-egu21-12655, 2021.

Instrumental climate data are only available for the last few hundred years. To extend this record back in time, climate proxies are used. However, on the geological timescale, the temporal resolution of most paleoclimate records does not provide information about seasonality, let alone events on the weather-timescale. These weather-timescale events are becoming more frequently integrated in models to predict future climate change, but reconstructions of variability with such short timescales in the geological record are extremely rare.

A recent study by de Winter et al. (2020) has revealed that the Eocene giant marine gastropod Campanile giganteum (Lamarck, 1804) had growth rates exceeding 600 mm/year along the helix, far exceeding those of most other modern and fossil molluscs. With such high growth rates, these giant gastropods have the unique potential to record weather-timescale variability in the Eocene greenhouse world. Therefore, we generated a high-resolution (mm-scale) δ¹⁸O record on a well-preserved specimen of C. giganteum from the Paris Basin in Fleury-la-Rivière, France, in order to generate a unique ultra-high resolution record of intra-annual, weather-timescale variability in the Eocene. Our preliminary results show a clear seasonal pattern with δ¹⁸O values ranging between 0.1‰ and -2.5‰, superimposed by weekly variations of up to 0.5‰. This could provide insights in weather patterns in the Eocene greenhouse climate and potentially allow the identification of extreme weather events.

Reference

de Winter N.J., Vellekoop J., Clark A.J., Stassen P., Speijer R.P., Claeys P., (2020) The Giant Marine Gastropod Campanile Giganteum (Lamarck, 1804) as a High‐Resolution Archive of Seasonality in the Eocene Greenhouse World., Geochemistry, Geophysics, Geosystems, 21(4), https://doi.org/10.1029/2019GC008794

How to cite: Van Horebeek, N., Vellekoop, J., Clark, A. J., de Winter, N. J., Stroobandt, Y., and Speijer, R. P.: A stable oxygen isotope record of weather-timescale variability in the Eocene greenhouse world, using the giant marine gastropod Campanile giganteum, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3268, https://doi.org/10.5194/egusphere-egu21-3268, 2021.

Increasing water temperatures are predicted around the globe with high amplitudes of warming in Subarctic and Arctic regions where Atlantic cod (Gadus morhua) populations currently flourish. We reconstructed population abundance, oxygen isotope and temperature chronologies from otoliths of the two largest cod populations in the world - the Icelandic and the Northeast Arctic (NEA) cod - to determine if their temperature selectivity over the last 100 years was driven by rising water temperatures and/ or changes in abundance. For δ¹⁸O_otolith analysis, individual annual growth increments from immature and mature life history stages of cod collected in southern Iceland and the Lofoten area (Norway) were micromilled from adult otoliths. Ambient temperatures of Icelandic and Norwegian cod were reconstructed using otolith δ¹⁸O. Linear mixed effect models were applied to identify and quantify the density-dependent temperature selectivity of both cod populations. The results indicated that Icelandic cod migrated into warmer waters with increasing abundance (p < 0.05), whereas NEA cod moved into colder waters (p < 0.001). The temperature selectivity of NEA cod was also significantly correlated with water temperatures at 0-200 m depth (p < 0.001), indicating that NEA cod were at least partially exposed to increasing ocean temperatures due to global warming. Stable oxygen isotope and ambient temperature chronologies can be an important tool for sustainable management plans in terms of future global warming as it can be used to predict re-distribution as oceans warm.

How to cite: von Leesen, G., Bogstad, B., Hjörleifsson, E., Ninnemann, U. S., and Campana, S. E.: A century of otolith-derived temperature exposure of Icelandic and Norwegian cod (Gadus morhua), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3114, https://doi.org/10.5194/egusphere-egu21-3114, 2021.

Bivalve shells have a long-standing reputation as archives for high-resolution (seasonal scale) (paleo)climate variability due to their incremental growth, yielding accurate shell chronologies, and their abundance, diversity, and high preservation potential in the fossil record (Schöne and Surge, 2012). Capitalizing on innovations in geochemical techniques, high-resolution sclerochronology can now resolve changes in bivalve shell chemistry beyond the daily resolution (e.g. Sano et al., 2012; Warter et al., 2018). When applied on fossil shells, these ultra-high-resolution records have the potential to bridge the gap between climate and weather reconstructions and yield unprecedented information about bivalve paleobiology, extreme weather events in past climates and even astronomical cycles (Warter and Müller, 2017; de Winter et al., 2020; Yan et al., 2020).

However, studies of sub-daily scale shell chemistry are almost exclusively limited to giant clams (Tridacna spp.), due to their high growth rates. It is hitherto unknown if and how such diurnal cycles in chemistry differ in other genera across the bivalve clade and/or whether they are exclusive to photosymbiotic clams. In addition, it is not clear whether the daily cycles are formed in response to environmental conditions (e.g. light or temperature sensitivity) or reflect circadian rhythms.

To answer these questions, we combine ultra-high-resolution (hourly scale) Laser Ablation ICP-MS trace element profiles through shells of various tridacnid species from the tropical Gulf of Aqaba with profiles through the giant scallop (Pecten maximus) from the temperate Atlantic coast of northwestern France. We observe trace element cycles on in the daily frequency domain in both tridacnids and pectinids. This shows that these diurnal cycles are formed regardless of shell mineralogy (aragonite vs. calcite), living environment (tropical inter-tidal vs. temperate sub-tidal) and occur in highly unrelated bivalve taxa. Our data helps the interpretation of similar records from fossil shells in terms of past (extreme) weather events, climate, and shell growth.

References

de Winter, N. J. et al. Paleoceanography and Paleoclimatology 35, e2019PA003723 (2020).

Sano, Y. et al. Nature Communications 3, 761 (2012).

Schöne, B. R. & Surge, D. M. Treatise Online 24, Volume 1, Chapter 14 (2012).

Warter, V., Erez, J. & Müller, W. Palaeogeography, Palaeoclimatology, Palaeoecology 496, 32–47 (2018).

Warter, V. & Müller, W. Palaeogeography, Palaeoclimatology, Palaeoecology 465, 362–375 (2017).

Yan, H. et al. PNAS 117, 7038–7043 (2020).

How to cite: de Winter, N., Fröhlich, L., Killam, D., Boer, W., de Nooijer, L., Reichart, G.-J., and Schöne, B.: Daily cyclicity in bivalve shell chemistry: Paleo-weather record or circadian rhythm?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6168, https://doi.org/10.5194/egusphere-egu21-6168, 2021.

Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental and climatic changes. The oxygen isotopic composition (δ¹⁸O) of such carbonates depends on water temperature during carbonate precipitation, and on the δ¹⁸O of the lake water. Lake water δ¹⁸O, in turn, is controlled by the δ¹⁸O of precipitation in the catchment, water residence time and mixing, and by evaporation. A paleoclimate interpretation of carbonate δ¹⁸O records requires a site-specific calibration based on an understanding of the local conditions.

For this study, carbonates and water were sampled in the littoral zone of lake Locknesjön, central Sweden (62.99°N, 14.85°E, 328 m a.s.l.) along a water depth gradient from 1 to 8 m. We took samples from living organisms and sub-recent samples in surface sediments of the calcifying algae Chara hispida, the mollusk Pisidium, and adult and juvenile instars of two ostracod species, Candona candida and Candona neglecta.

We show that neither the δ¹⁸O of carbonates nor the δ¹⁸O of water vary significantly with water depth, indicating a well-mixed epilimnion. The largest differences in the mean carbonate δ¹⁸O between species are caused by vital offsets, i.e. the species-specific deviation from the δ¹⁸O of inorganic carbonate which would have been precipitated in isotopic equilibrium with the water. After subtraction of these constant vital offsets, remaining differences in the mean carbonate δ¹⁸O between species can mainly be attributed to seasonal water temperature changes. The lowest δ¹⁸O values are observed in Chara encrustations, which form during the summer months when photosynthesis is most intense. Adult ostracods, which calcify their valves during the cold season, display the highest δ¹⁸O values. This is because an increase in temperature leads to a decrease in fractionation between carbonate and water, and therefore to a decrease in carbonate δ¹⁸O. An increase in temperature also leads to an increase in the δ¹⁸O of lake water through its effect on precipitation δ¹⁸O and on evaporation, and consequently to an increase in carbonate δ¹⁸O, opposite to the temperature effect on fractionation. However, the seasonal and inter-annual variability in lake water δ¹⁸O is small (0.5‰) due to the long water residence time. Seasonal changes in the temperature-dependent fractionation are therefore the dominant cause of carbonate δ¹⁸O differences between species.

Temperature reconstructions based on “paleo-temperature” equations for equilibrium carbonate precipitation using the mean δ¹⁸O of each species and the mean δ¹⁸O of lake water are well in agreement with the observed seasonal water temperature range. The high carbonate δ¹⁸O variability of samples within a species, on the other hand, leads to a large scatter in the reconstructed temperatures based on individual samples. This implies that care must be taken to obtain a representative sample size for paleo-temperature reconstructions.

How to cite: Labuhn, I., Tell, F., von Grafenstein, U., Hammarlund, D., Kuhnert, H., and Minster, B.: A modern snapshot of the isotopic composition of lacustrine biogenic carbonates: Records of seasonal water temperature variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13178, https://doi.org/10.5194/egusphere-egu21-13178, 2021.

Various elements of the biota of the early Pliocene Coralline Crag Formation (southern North Sea Basin, eastern England) have been taken to indicate a warm temperate marine climate, with summer surface temperatures above 20 °C and winter temperatures above 10 °C [1]. However, summer and winter temperature estimates from oxygen-isotope (δ¹⁸O) sclerochronology of benthic invertebrates are typically in the respective cool temperate range when calculated using a plausible modelled value for water δ¹⁸O of +0.1‰. For instance, examples of the bivalve mollusc Aequipecten opercularis from the Ramsholt Member indicate summer maximum temperatures of 11.0–15.7 °C and winter minimum temperatures of 4.4–7.1 °C [2]. Amongst other evidence, the pattern of microgrowth-increment variation in Ramsholt-Member A. opercularis points to a depth below the summer thermocline, hence the temperatures recorded for that season provide an underestimate of surface temperature; this may well have been in the warm temperate summer range [2], as suggested by the pelagic dinoflagellate biota [3]. However, the cool temperate benthic winter temperatures indicated by isotopic data are likely also to have obtained at the surface, pointing to a greater seasonal range in surface temperature (perhaps > 15 °C) than in the modern North Sea (< 13 °C) [2]. This conclusion is not changed by adoption of a different (invariant) value for water δ¹⁸O and also follows from data for a specific late Pliocene interval (Mid-Piacenzian Warm Period) elsewhere in the southern North Sea Basin (Belgium, Netherlands [4]). Here we present isotopic evidence of a seasonal range in surface temperature higher than now at other times in the late Pliocene. Examples of A. opercularis from several horizons in the Lillo Formation (Belgium) and the Oosterhout Formation (Netherlands) indicate seasonal ranges in benthic temperature of 10–14 °C. Seasonal variation in water δ¹⁸O can only plausibly account for about 1 °C of these ranges. Taking into consideration microgrowth-increment evidence of a setting below the summer thermocline, the seafloor ranges imply that the surface seasonal range was sometimes 17 °C or more. Other bivalves (Atrina fragilis, Arctica islandica, Pygocardia rustica, Glycymeris radiolyrata) do not indicate such a high seasonal range in benthic (and hence surface) temperature but this can be attributed to inadequate sampling—time-averaging or a failure to recover evidence of seasonal extremes because of growth breaks. The high surface temperature range could reflect a reduction in vigour of the North Atlantic Current and hence diminished oceanic supply of heat in winter.

References:

[1] Vignols et al. (2019), Chem. Geol. 526, 62–83. https://doi.org/10.1016/j.chemgeo.2018.05.034.

[2] Johnson et al. (2020), Palaeogeogr. Palaeoclimatol. Palaeoecol. 561. https://doi.org/10.1016/j.palaeo.2020.110046.

[3] Head (1997), J. Paleontol. 71, 165–193. https://doi.org/10.1017/S0022336000039123.

[4] Valentine et al. (2011), Palaeogeogr. Palaeoclimatol. Palaeoecol. 309, 9–16. https://doi.org/10.1016/j.palaeo.2011.05.015.

How to cite: Johnson, A., Valentine, A., Leng, M., Schöne, B., Sloane, H., and Goolaerts, S.: Sclerochronological evidence of pronounced seasonality from the Pliocene of the southern North Sea Basin, and its implication, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7247, https://doi.org/10.5194/egusphere-egu21-7247, 2021.

Sclerochronology provides valuable proxy data for investigating high-resolution paleoclimate dynamics at seasonal/(sub-)annual scale. Nevertheless, the interpretation of these proxy data is often hampered by the interplay between three main factors: (i) paleoenvironmental patterns; (ii) vital (physiological and kinetic) effects related to biomineralization pathways; and (iii) potential alteration during diagenetic modification of skeletal materials.

Because the interaction between environmental and metabolic factors is, at present, one of the most difficult to quantify, an ideal opportunity is brought forward to better understand the complexity of environment-metabolism interaction in marine biogenic carbonate archives. A project was tailored to investigate the impact of growth kinetics on geochemical proxy data (carbon and oxygen stable isotopes and main and trace elemental data) from oyster shells responding to changes in metabolism due to environmental fluctuations. Motivated by the exceptionally favourable circumstance that oyster farms are located near our Institution in Aveiro (Portugal), these will be used as a natural laboratory. Most interestingly, in Aveiro, oyster growth rates are significantly higher compared to those cultivated in France (Arcachon Bay). This is the case despite the fact, that the oysters grown in Aveiro are imported from France, and this will form the main study site of this project. In order to have a wider range of observational sites, a third oyster station in Southern Portugal (Olhão), influenced by warmer coastal waters will also be sampled. Finally, modern oyster specimen will be compared and contrasted with well-preserved ancient Crassostrea and Ostrea material in an attempt to bridge the gap between the Present and the Mesozoic.

State-of-the-art petrographic and geochemical research involving both modern and ancient oysters of the same genus will be performed. With reference to recent specimens, this must be performed in combination with a strict biological assessment of oyster metabolic performance, but with focus on carbonate archive research. An international Research Team (Portugal, Germany, France, Spain) was assembled, bringing together experts from a wide range of research fields, including Carbonate Geochemistry, Biomineralization, Sedimentology, Mineralogy, (Micro) Palaeontology, Sclerochronology, Biology, Ecology, Artificial Intelligence, and Data Modelling.

The goals of this project include: (i) establishing the link between modern environmental seasonal fluctuations, oyster growth rates and impact on the geochemical record of the shell, while additionally understanding non-linear responses (e.g., ontogenenetic evolution, effects of storms or other extreme events); (ii) compile information from a variety of proxies (bio-geochemical, petrographic, mineralogical, ecological), locations and times, aiming to test the best approaches for integration with a coherent framework; (iii) explore the link to ancient shell-archives, distinguishing between the various forcers of their geochemical signals, more specifically the interplay between paleoenvironmental conditions and vital effects.

How to cite: Coimbra, R., Rocha, F., Freitas, R., Immenhauser, A., Azerêdo, A. C., and Cabral, M. C.: Investigating the impact of growth kinetics on geochemical records of oyster shells, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7818, https://doi.org/10.5194/egusphere-egu21-7818, 2021.