Shell microstructures (disturbance lines) of Arctica islandica (Bivalvia) : a potential proxy for severe oxygen depletion

The spread of oxygen deficiency in nearshore coastal habitats endangers benthic communities. To better understand the mechanisms leading to oxygen depletion and eventually hypoxia, predict the future development of affected ecosystems, and define suitable mitigation strategies requires detailed knowledge of the dissolved oxygen (DO) history. Suitable high-resolution DO archives covering coherent time intervals of decades to centuries include bivalve shells. Here, we explored if the microstructure, specifically disturbance lines, in shells of Arctica islandica from the Baltic Sea can be used as an alternative or complementary proxy to Mn/Cashell to track the frequency and severity of past low-DO events. Disturbance lines differ from periodic annual growth lines by the presence of fine complex crossed lamellae instead of irregular simple prisms. Aside from a qualitative assessment of microstructural changes, the morphology of individual biomineral units (BMUs) was quantitatively determined by artificial intelligence-assisted image analysis to derive models for DO reconstruction. As demonstrated, Mn-rich disturbance lines can provide a proxy for past deoxygenation events (i.e., DO < 45 μmol/L), but it currently remains unresolved if low DO leads to microstructurally distinct features that differ from those caused by other environmental stressors. At least in studied specimens from the Baltic Sea and Iceland, low temperature, salinity near the lower physiological tolerance, or food scarcity did not result in disturbance lines. With decreasing DO supply, disturbance lines seem to become more prominent, contain more Mn, and consist of increasingly smaller and more elongated BMUs with a larger perimeter-to-area ratio. Although the relationship between DO and BMU size or elongation was statistically significant, the explained variability (<1.5%) was too small and the error too large to reconstruct DO values. BMU parameters may reveal a closer relationship with DO if studied in three dimensions and if the DO content was determined at high resolution, directly at the position where the bivalves lived, something that future work should address

Strong coupling between biomineral morphology and Sr/Ca of Arctica islandica (Bivalvia) : implications for Shell Sr/Ca-based temperature estimates

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

Sr/Ca in shells of laboratory-grown bivalves (Arctica islandica) serves as a proxy for water temperature : implications for (paleo)environmental research?

Seawater temperature is an essential quantity for paleoclimatological and paleoecological studies. A potential archive that can provide century-long, temporally well-constrained and high-resolution temperature proxy data is available in the form of bivalve shells. However, the number of well-accepted and robust temperature proxies contained in shells is limited to stable oxygen isotopes and carbonate clumped isotopes. Many studies have therefore investigated the possibility to reconstruct temperature from element/Ca properties, specifically Sr/Ca ratios in case of aragonitic shells. As demonstrated here, in agreement with thermodynamic expectations and the lattice strain model, shell Sr/Ca of laboratory-grown Arctica islandica specimens is strongly positively coupled to water temperature. If ultrastructure-related bias is mathematically eliminated, up to 75% of the variability in shell Sr/Ca data can be explained by water temperature. However, in field-grown specimens, this relationship is superimposed by other environmental variables that can hardly be quantified and mathematically eliminated. The explained variability of Sr/Ca is reduced to merely 26% and the prediction uncertainty too large for reliable temperature estimates. Most likely, the equable, less biased conditions in the laboratory resulted in the production of a more uniform shell ultrastructure (with larger and more elongated biomineral units) which in turn was associated with less variable Sr/Ca values and a stronger link to water temperature. Without a detailed understanding and quantification of the factors controlling ultrastructural variations in field-grown bivalves, it remains impossible to employ shell Sr/Ca of wild A. islandica specimens for precise temperature estimates, merely a qualitative temperature reconstruction seems feasible

Shell microstructures (disturbance lines) of Arctica islandica (Bivalvia): a potential proxy for severe oxygen depletion

The spread of oxygen deficiency in nearshore coastal habitats endangers benthic communities. To better understand the mechanisms leading to oxygen depletion and eventually hypoxia, predict the future development of affected ecosystems, and define suitable mitigation strategies requires detailed knowledge of the dissolved oxygen (DO) history. Suitable high-resolution DO archives covering coherent time intervals of decades to centuries include bivalve shells. Here, we explored if the microstructure, specifically disturbance lines, in shells of Arctica islandica from the Baltic Sea can be used as an alternative or complementary proxy to Mn/Cashell to track the frequency and severity of past low-DO events. Disturbance lines differ from periodic annual growth lines by the presence of fine complex crossed lamellae instead of irregular simple prisms. Aside from a qualitative assessment of microstructural changes, the morphology of individual biomineral units (BMUs) was quantitatively determined by artificial intelligence-assisted image analysis to derive models for DO reconstruction. As demonstrated, Mn-rich disturbance lines can provide a proxy for past deoxygenation events (i.e., DO &lt; 45 µmol/L), but it currently remains unresolved if low DO leads to microstructurally distinct features that differ from those caused by other environmental stressors. At least in studied specimens from the Baltic Sea and Iceland, low temperature, salinity near the lower physiological tolerance, or food scarcity did not result in disturbance lines. With decreasing DO supply, disturbance lines seem to become more prominent, contain more Mn, and consist of increasingly smaller and more elongated BMUs with a larger perimeter-to-area ratio. Although the relationship between DO and BMU size or elongation was statistically significant, the explained variability (&lt;1.5%) was too small and the error too large to reconstruct DO values. BMU parameters may reveal a closer relationship with DO if studied in three dimensions and if the DO content was determined at high resolution, directly at the position where the bivalves lived, something that future work should address

Bivalve shell microstructures - exploring a novel marine temperature proxy

Bivalve shells are unparalleled high-resolution climate archives. Not only can the environ- mental conditions prevailing during shell formation be stored in the physical, chemical, and, as recently shown, microstructural properties of the shells, but their paleoclimate records can also excellently be temporally contextualized by recurring growth lines, which form due to periodic shell growth. Nevertheless, most paleotemperature proxies employed in bivalve shells come with limitations. Temperature reconstructions based on the oxygen stable isotope (δ18Oshell) value of the shells, for example, also depend on the δ18Owater, which is often unknown. In addition, the δ18Oshell values of the shells are prone to alteration during diagenesis. The trace elemental composition of the shells, while correlating to the water temperature, were shown to be strongly affected by physiological processes such as variations in shell growth rate. The width of growth increments of the shells simultaneously inform about food conditions and temperature and are likewise strongly physiologically controlled. The microstructural properties of the shells, in contrast, might be less affected by factors other than temperature, and be preserved in fossil shells when geochemical signal such as the 18Oshell are already lost. However, a link between the shell microstructure and the ambient water temperature was hitherto only demonstrated in afew short-lived bivalve species. Before microstructural properties can confidently be used to infer paleotemperature, these proxies need to be adequately calibrated and tested. This thesis examined whether an influence of temperature on microstructural properties such as the biomineral unit (BMU) size is a common phenomenon across long-lived bivalve taxa commonly used in sclerochronological paleoclimate reconstructions. In addition, it was evaluated whether food availability and quality, salinity, shell growth rate and ontogenetic age also affect the microstructural properties of the shells, possibly overprinting temperature signals. The results of this project comprise three manuscripts published in international, peer-reviewed scientific journals. In the 1st manuscript, it is investigated whether the microstructure of the shells of Gly- cymeris bimaculata, a long-lived bivalve species broadly distributed in temperate coastal to brackish regions, is affected by changes in water temperature. G. bimaculata forms crossed-lamellar shells, a microstructure which occurs in over 90% of all marine mollusks in some form. Novel image processing techniques applied to scanning electron microscopy images combined with stable oxygen isotope analysis revealed that the size, length and width of BMUs of G. bimaculata indeed correlate strongly with the water temperature. This link can be used to infer water temperatures with up to 2.3°C precision, introducing a promising new paleotemperature proxy that could potentially apply to a wide range of mollusk taxa which also form crossed-lamellar shells. In manuscript two, shells of Arctica islandica, a well-studied sclerochronological archive known for its extreme longevity and wide distribution across the northern North Atlantic, were analyzed. In order to study the effects of water temperature on the shells in isolation of other factors, specimens were analyzed that were raised in a lab under different temperature settings, while keeping salinity and food conditions constant. These experiments revealed a direct influence of water temperature on the size of the BMUs and the size of pores incorporated into the shells of A. islandica, suggesting that a microstructural response to temperature variations is a common phenonmenon across different bivalve taxa. In Manuscript three, it was tested whether different environmental regimes can effectively be discriminated by the analysis of microstructural properties of A. islandica shells collected across multiple habitats. Analysis were also performed in different shell portions and across different ontogentic stages in order to determine physiological influences on the shell microstruce. This way, it could be revealed that temperature changes as small as approx. 1-2 °C trigger alterations of the shell microstructure. In naturally grown shells however, temperature signals can be obstructed when unfavorable growth conditions grown such as small as low and variable or low dissolved oxygen content. In addition, the microstructural properties of the shells change strongly during ontogeny. In summary, the experiments of this thesis revealed an influence of the water tempera- ture on the microstructure of bivalve shells. However, the microstructural properties are also strongly coupled to the physiology of the bivalve and to its biomineralization processes, which complicates temperature reconstructions. The methods developed in this study strongly facilitate quantitative analysis of microstructural properties of carbonate shells and open up plenty of research applications not only in paleoclimatology but also in biomineralization research. I carried out all laboratory work (oxygen stable isotope analysis, laser ablation–inductively coupled plasma–mass spectrometry measurements, growth pattern analysis, scanning electron microscopy and image analysis) of this thesis. I also developed the image analysis methods used for BMU morphometry, analyzed and interpreted the data and produced the figures. I wrote all text for manuscripts one and three and contributed substantially to the text of manuscript two. Contributions of other authors are stated at the beginning of each manuscript.xxiii, 223 Seiten, Illustrationen, Diagramm

Strong Coupling between Biomineral Morphology and Sr/Ca of Arctica islandica (Bivalvia)&mdash;Implications for Shell Sr/Ca-Based Temperature Estimates

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

Temperature-induced microstructural changes in shells of laboratory-grown Arctica islandica (Bivalvia).

Bivalve shells are increasingly used as archives for high-resolution paleoclimate analyses. However, there is still an urgent need for quantitative temperature proxies that work without knowledge of the water chemistry-as is required for δ18O-based paleothermometry-and can better withstand diagenetic overprint. Recently, microstructural properties have been identified as a potential candidate fulfilling these requirements. So far, only few different microstructure categories (nacreous, prismatic and crossed-lamellar) of some short-lived species have been studied in detail, and in all such studies, the size and/or shape of individual biomineral units was found to increase with water temperature. Here, we explore whether the same applies to properties of the crossed-acicular microstructure in the hinge plate of Arctica islandica, the microstructurally most uniform shell portion in this species. In order to focus solely on the effect of temperature on microstructural properties, this study uses bivalves that grew their shells under controlled temperature conditions (1, 3, 6, 9, 12 and 15°C) in the laboratory. With increasing temperature, the size of the largest individual biomineral units and the relative proportion of shell occupied by the crystalline phase increased. The size of the largest pores, a specific microstructural feature of A. islandica, whose potential role in biomineralization is discussed here, increased exponentially with culturing temperature. This study employs scanning electron microscopy in combination with automated image processing software, including an innovative machine learning-based image segmentation method. The new method greatly facilitates the recognition of microstructural entities and enables a faster and more reliable microstructural analysis than previously used techniques. Results of this study establish the new microstructural temperature proxy in the crossed-acicular microstructures of A. islandica and point to an overarching control mechanism of temperature on the micrometer-scale architecture of bivalve shells across species boundaries

Ecosystem changes through the Permian–Triassic and Triassic–Jurassic critical intervals: Evidence from sedimentology, palaeontology and geochemistry

ABSTRACTThe Permian–Triassic and Triassic–Jurassic critical intervals are among the most significant ecological upheavals in the Phanerozoic. Both evolutionary junctures are characterized by environmental deterioration associated with a marked biodiversity decline. In this study, Permian–Triassic and Triassic–Jurassic boundary sections from South China and the Northern Calcareous Alps were investigated. In order to reconstruct the interplay between biotic and abiotic processes, a multifaceted approach that included optical microscopy, X‐ray diffraction, Raman spectroscopy, stable carbon isotopes and lipid biomarkers was employed. The lower parts of these two sections are similar as both consist of limestone with abundant fossils of eukaryotic organisms. However, the Permian–Triassic record is dominated by dasyclad green algae and fusulinid foraminifera, while the Triassic–Jurassic record is typified by corals and coralline sponges. Moving upward, both sections consist mainly of micrite and marl. Concerning the Permian–Triassic section, it transits to volcanic ash intercalated by a distinct limestone bed with abundant calcispheres (tentatively attributed to ancestors of dinoflagellates). The Triassic–Jurassic section does not provide direct evidence for volcanic activity, but also becomes rich in calcisphere‐type cysts towards the top. Additionally, the section preserves abundant 4‐methyl sterenes (diagnostic for dinoflagellates) and C37–39 n‐alkanes (indicative for haptophytes). Hence, both critical intervals were associated with marked blooms of (ancestral) dinoflagellates and haptophytes (for example, coccolithophorids). These blooms were followed by ecological lag‐phases, as indicated by low carbonate contents and scarce fossils which only increased further up the sections. For both critical intervals, it is commonly assumed that the formation of voluminous volcanic provinces (Siberian Traps and Central Atlantic Magmatic Province, respectively), as well as associated processes (for example, burning of organic‐rich sediments such as coal), resulted in ecological devastation. However, results suggest that volcanism also had a positive effect on certain planktonic primary producers such as dinoflagellates and haptophytes, perhaps by delivering essential nutrients.China Council ScholarshipTeach@Tübingen Fellowshi