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Geochimica et cosmochimica acta 308, 326-352 (2021). doi:10.1016/j.gca.2021.06.012, Published by Elsevier, New York, NY [u.a.]

The shells of long-lived bivalves record environmental variability in their geochemical signatures and are thus used extensively in marine high-resolution paleoclimate studies. To possibly overcome the limitations of the commonly employed temperature proxy, the δ18Oshell value, which requires knowledge of the seawater δ18O signature and is prone to diagenetic overprint, the shell microstructures and the morphological properties of individual biomineral units (BMUs) recently attracted research interest as an alternative paleoclimate proxy. In shells of A. islandica, one of the most extensively used and best studied sclerochronological archives, the size of the BMUs increases in warmer temperatures under laboratory circumstances. This study assesses whether this relationship persists under natural growth conditions or whether additional environmental and physiological factors control the BMU size and bias temperature reconstructions. For this purpose, shells from the surface waters of NE Iceland and the Baltic Sea, as well as from deeper waters of the North Sea (100 and 243 m) were analyzed by means of SEM. The BMU sizes were measured by means of image processing software. Results demonstrate a strong effect of temperature on the BMU size at NE Iceland and in the North Sea at 100 m depth. At 243 m depth, however, temperature variability was likely too low (1.2°C) to evoke a microstructural change. At the Baltic Sea, the BMUs remained small, possibly due to physiological stress induced by low salinity and/or hypoxia. Thus, the size of BMUs of A. islandica shells only serves as a relative temperature indicator in fully marine habitats, as long as seasonal temperature amplitudes exceed ca. 1°C. Furthermore, BMU size varied through lifetime with the largest units occurring during age seven to nine. This pattern is possibly linked to the shell growth rate or to the amount of metabolic energy invested in shell growth.

Bivalve shells serve as powerful high-resolution paleoclimate archives. However, the number of reliable temperature proxies is limited. It has remained particularly difficult to extract temperature signals from shell Sr/Ca, although Sr is routinely employed in other biogenic aragonites. In bivalves, Sr/Ca is linked to the prevailing microstructure and is sometimes affected by kinetics. Here, the hypothesis is tested that temperature can be reconstructed from shell Sr/Ca once microstructure and/or growth-rate-related bias has been mathematically eliminated. Therefore, the relationship between Sr/Ca and increment width, as well as biomineral unit size, has been studied in three different shell portions of field-grown Arctica islandica specimens. Subsequently, microstructure and/or growth-rate-related variation was removed from Sr/Ca data and residuals compared to temperature. As demonstrated, the hypothesis could not be verified. Even after detrending, Sr/Ca remained positively correlated to water temperature, which contradicts thermodynamic expectations and findings from inorganic aragonite. Any temperature signal potentially recorded by shell Sr/Ca is overprinted by other environmental forcings. Unless these variables are identified, it will remain impossible to infer temperature from Sr/Ca. Given the coupling with the biomineral unit size, a detailed characterization of the microstructure should remain an integral part of subsequent attempts to reconstruct temperature from Sr/Ca.

The shells of oysters (Family Ostreidae) are predominantly composed of two different calcite microstructures: A dense foliated structure consisting of sheet-like folia (“foliated” microstructure) and a more porous microstructure consisting of less well organized leaf-shaped crystals (“chalky” microstructure). These unique characteristics of oyster shells have been subject to a number of studies, with some authors hypothesizing that the chalky structures are mineralized by bacteria living in the shell (Vermeij, 2014). The formation of these microstructures is of great interest, because the phenomenon is unique in the mollusk phylum and because the shells of oysters are popular archives for paleoclimate and paleoenvironment reconstructions (e.g. Bougeois et al., 2018; de Winter et al., 2018). Previous authors have challenged the bacterially mediated mineralization hypothesis through microstructural observations of different parts of the oyster shell (Checa et al., 2018).Here, we expand on this structural evidence by adding detailed observations of differences in chemical composition between the foliated and chalky microstructures. We combine information on trace element concentrations with stable carbon, oxygen, nitrogen and sulfur isotope ratios as well as carbonate clumped isotope analyses of samples from foliated and chalky structures in multiple modern specimens of Magallana gigas, the Pacific oyster. These analyses shed light on the chemical variability within the oyster shell and how it relates to the occurrence of various calcite microstructures. Given the unique isotopic signature of bacterially mediated calcite, our isotopic analysis results allow us to definitively conclude whether the chalky shell structure in modern oysters was precipitated via symbiotic microbes. Furthermore, the degree of intra-shell chemical variability has implications for paleoclimate and paleoenvirionmental reconstructions from fossil oyster shells, for which the applied trace element and isotope systems function as important proxies. The results of this study therefore yield important recommendations for sampling fossil oyster shells for reconstructions, and provide a baseline for the investigation of chemical variability between shell microstructures throughout the Ostreidae family and the mollusk phylum. ReferencesBougeois, L., Dupont-Nivet, G., De Rafélis, M., Tindall, J. C., Proust, J.-N., Reichart, G.-J., de Nooijer, L. J., Guo, Z. and Ormukov, C.: Asian monsoons and aridification response to Paleogene sea retreat and Neogene westerly shielding indicated by seasonality in Paratethys oysters, Earth and Planetary Science Letters, 485, 99–110, 2018.Checa, A. G., Harper, E. M. and González-Segura, A.: Structure and crystallography of foliated and chalk shell microstructures of the oyster Magallana: the same materials grown under different conditions, Scientific reports, 8(1), 7507, 2018.Vermeij, G. J.: The oyster enigma variations: a hypothesis of microbial calcification, Paleobiology, 40(1), 1–13, 2014.de Winter, N., Vellekoop, J., Vorsselmans, R., Golreihan, A., Soete, J., Petersen, S., Meyer, K., Casadio, S., Speijer, R. and Claeys, P.: An assessment of latest Cretaceous Pycnodonte vesicularis (Lamarck, 1806) shells as records for palaeoseasonality: a multi-proxy investigation, Climate of the Past, 14(6), 725–749, 2018.

Bivalves offer outstanding potential as environmental archives. However, vital effects exert a strong control on the incorporation of many trace and minor elements into the shell so that their use as environmental proxies is currently limited. Furthermore, Sr and Mg show a strong relationship to the micrometer-sized shell architecture (shell microstructure), i.e., near growth lines, which are typically dominated by irregular simple/spherulitic prismatic microstructures, the concentrations of these elements are significantly higher than in portions between growth lines (= growth increments, which are microstructurally more complex). In contrast, Ba is uncoupled from the prevailing shell microstructure. To shed more light on these issues, we conducted a combined element chemical (in-situ analysis by means of LA-ICP-MS) and microstructural analyses (using SEM) of shells of Arctica islandica collected alive in NE Iceland.According to our findings, (1) contemporaneous shell portions in the hinge and ventral margin (both belonging to the outer shell layer) within individual specimens showed nearly identical Sr/Ca and Mg/Ca values, but Ba/Ca was 1.5 – 2.5 times higher in the ventral margin than in the hinge. (2) In agreement with previous studies, Sr and Mg were strongly elevated near annual growth lines. (3) Along an isochronous transect from the inner portion of the outer shell layer near the myostracum toward the outer shell surface (in the ventral margin), Si/Ca values increased, on average, by 75% ± 11%, whereas Na/Ca values decreased by 7% ± 1%. Along this transect, the shell microstructure gradually changed from crossed-acicular to homogeneous suggesting that Si and Na are linked to the prevailing nanometer-sized shell architecture or underlying physicochemical processes controlling their formation. (4) In the hinge, Ba/Ca, Sr/Ca, Mn/Ca and Mg/Ca attained highest values along the axis of maximum growth, but gradually decreased in slower growing (contemporaneous) shell portions away from that axis. (5) In contemporaneous shell portions (in either the hinge or the ventral margin), the concentration of some elements varied significantly among specimens, whereas others showed little variability. For example, in similar and contemporaneous shell portions of different specimens, Na/Ca values exhibited only little variation (17.4 – 23.7 mmol/mol), whereas Sr/Ca and B/Ca differed more severely (0.3 – 1.6 mmol/mol and 0.04 – 0.07 mmol/mol, respectively; both within growth increments). Despite these inter-specimen chemical differences, the shell microstructure remained largely invariant.Our findings firstly suggest that the extrapallial fluid, if it exists at all, is chemically inhomogeneous. This could result from differences in the efficiency of transmembrane ion transport or to differences in shell formation rate along the growing margin (e.g., faster growth in the outer portion of the outer shell layer than in portions closer to the myostracum). Secondly, chemical differences among specimens may be attributed to physiological differences. Thirdly, some elements such as Ba are uncoupled to microstructural properties, but co-vary strongly among specimens suggesting an environmental control on the uptake and incorporation of this element into the shell.

Bivalve shells serve as excellent high-resolution archives of marine paleoclimate. Recently, ultrastructural features of the shells were investigated as potential temperature proxies that can overcome the limitations of the stable oxygen isotope method (i.e., missing data on past seawater oxygen isotope signature and diagenetic overprint). According to previous studies, the size of individual biomineral units of prismatic, nacreous and crossed-lamellar ultrastructures in cross-sections along the axis of maximum growth was solely related to water temperature. Despite being present in 90% of all mollusks, the crossed-lamellar ultrastructure was only studied for environmental relationships in one species (Glycymeris bimaculata) until now. To determine whether this new proxy can be applied to other bivalves with crossed-lamellar ultrastructure, further studies are needed.We analyzed the shells of other Glycymerididae collected at near-shore and shelf environments (G. nummaria and G. pilosa: Adriatic Sea, Croatia; G. glycymeris: Iroise Sea, France; Glycymeris sp: Southern Pacific, New Zealand) by means of SEM, using a previously developed automatic image analysis procedure. Morphological changes of the biomineral units of the shells were assessed for relationships with temperature, salinity and food availability. Additionally, the crossed-lamellar architectures of phylogenetically more distantly related taxa (Venus verrucosa and Callista chione: Adriatic Sea, Croatia) were assessed.Our results show that all studied Glycymerididae species, irrespective of environmental setting and locality, formed larger biomineral units in warmer waters. However, biomineral properties of ontogenetically old shell portions are more difficult to interpret, because declining growth rates condense the shell record and aggravate ultrastructural analyses. The crossed-lamellar shell layers of V. verrucosa and C. chione exhibited hierarchical organizations very similar to those of the Glycymerididae. The ultrastructural temperature proxy can therefore be applied to crossed-lamellar shells of bivalves from a wide range of coastal settings, preferably in ontogenetically young shell portions.

Bivalve shells are among the most promising archives for high-resolution seawater temperature reconstructions. However, despite major research advances in bivalve sclerochronology over the past decades, estimating water temperature from shells remains a challenging task. This is largely because the most frequently used and widely accepted temperature proxy in bivalves, i.e., the shell oxygen isotope (δ18Oshell) value, also requires knowledge of changes in δ18O of the water (δ18Owater) in which the bivalve lived, which is rarely available for ancient environments. According to a few recent studies, the size and shape of individual biomineral units (BMUs) of the shell ultrastructure may serve as an independent temperature proxy that is also less vulnerable to diagenetic changes than isotope chemical proxies. However, the crossed-lamellar ultrastructure, which occurs in ca. 90% of all mollusk species, has hitherto not been investigated. Here, we evaluate the potential use of morphological properties of the BMUs in Glycymeris bimaculata as a proxy for water temperature. For this purpose we introduce a suitable preparation technique (immersion for 2 m in 0.001 vol% formic acid followed by 20–30 m in 3.5 vol% H2O2) and an automated image processing technique that tremendously speed up ultrastructure analysis (5 s versus one to 2 h image processing time with automated and manual methods, respectively) and increase the robustness of BMU measurements. Glycymeris is a particularly useful target taxon, because it is a cosmopolitan genus with evolutionary roots in the Upper Cretaceous, and some species of Glycymeris can attain a lifespan of more than two hundred years, which allows for long-term, high-resolution paleoclimate reconstructions. As in other previously studied ultrastructures, larger and more elongated BMUs formed in warmer water permitting temperature estimates with an error of 2.3 °C (1 σ). Since the new temperature proxy is unaffected by other environmental variables, including salinity, it can potentially be applied to (modern and fossil) specimens from brackish environments.

The δ18O record of well-preserved shells of the inoceramid Inoceramus hercules from the Upohlavy working quarry (Czech Republic) provides an insight into the climate variability in the late Turonian benthic environment of the Bohemian Cretaceous Basin. Similar to modern bivalves, this inoceramid species built its shell near equilibrium with the oxygen isotope value of the ambient water. Due to the nearly year-round shell growth, sequentially sampled δ18O values allowed to estimate the narrowest sub-annual temperatures fluctuations that prevailed during lifetime of the organisms. In accordance with previous studies, reconstructed temperatures suggest colder water conditions (19.0 ± 0.9 °C) during the earliest late Turonian (coinciding with the Hyphantoceras Event; also known as Hitch Wood Event) than during the later part of the substage (20.9 ± 1.1 °C). Climate warming, however, did not affect the sub-annual temperature fluctuations evenly, but predominantly affected the warmest part of the year (at least +4.0 °C). On the contrary, the coldest recorded temperatures remained more or less invariant, varying not (much) more than +1.0 °C across the studied time interval. The observed changes in the benthic temperature estimates likely reflect shifts in seasonal sea surface temperature amplitudes in response to climate forcing. Given the widespread distribution of I. hercules in the upper Turonian–lower Coniacian hemipelagic strata of Europe, the present study sets the basis for future continent-wide sub-annually-resolved climate reconstructions.

Clostridium chromiireducens is an obligate, anaerobic, Gram-positive, rod-shaped, and spore-forming bacterium that is able to reduce Cr(VI). The draft genome consists of one chromosome (5,448 Mb) and contains 4,773 predicted protein-encoding genes.

Clostridium chromiireducens is an obligate, anaerobic, Gram-positive, rod-shaped, and spore-forming bacterium that is able to reduce Cr(VI). The draft genome consists of one chromosome (5,448 Mb) and contains 4,773 predicted protein-encoding genes.