Towards quantifiable temperatures from mollusk shells
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Abstract
Mollusk shells meet nearly all requirements of an ideal climate archive. For example, mollusks have an exceptionally wide geographical distribution, occurring from boreal to tropical regions as well as from the deep sea to the near shore. Shell parameters, such as the geochemical composition, vary relative to the physico-chemical state of the ambient water and thus record the prevailing environmental conditions during growth. Each shell portion and the corresponding proxy information can be precisely temporally aligned via annual, daily or circatidal growth patterns. Since growth of contemporaneous specimens of the same region is highly synchronized, annual growth increment chronologies can be combined into so-called master chronologies that allow high-resolution paleoclimate reconstruction over centuries and millennia. Hence, mollusk shells can provide means to study climate variability with a geographical and temporal resolution that is superior to other paleoclimatic archives.
However, quantified estimates of water temperature from mollusk shells are still limited. In fact, the only accepted paleothermometer, δ18Oshell, is a dual proxy that simultaneously records temperature and oxygen isotopic composition of the ambient water and thus only provides accurate environmental reconstructions if either of them is known. Other, less frequently applied temperature proxies such as growth rates, trace element ratios or clumped isotopes are likewise influenced by multiple parameters and need further refinements before they can be routinely applied. Consequently, it is currently impossible to benefit from the full potential offered by mollusk shells. This study tackles this deficit by developing new and optimizing existing proxies. Special emphasis is put on shell microstructures, element-to-element as well as Sr/Cashell ratios. Results are presented in three manuscripts, published (submitted in the case of manuscript III) in international, peer-reviewed scientific journals.
The first manuscript explores if shell microstructures of cultured specimens of the freshwater gastropod Viviparus viviparus are temperature-sensitive and may thus serve as an alternative temperature proxy. When water temperature was cold and variable, crossed-lamellar microstructures, i.e., the predominating microstructure in shells of V. viviparus, were highly unordered and heterogeneous. During warm and stable conditions, however, crossed-lamellar structures displayed a much more ordered and homogenous appearance. If new shell material was formed onto pre-existing, well-ordered crossed-lamellar structures, shell microstructures were always homogenous and ordered, irrespective of the prevailing temperature. It is hypothesized that the growth front forms an organic template that guides the appearance of crossed-lamellar structures. If this template is missing, for example when new material is formed de novo along the ventral margin, it is primarily the environment that controls the appearance of the microstructure. Hence, microstructures of specific shell portions can likely be used to estimate water temperature during growth.
Manuscript II tests if Sr/Lishell ratios in shells of the common cockle Cerastoderma edule can potentially serve as proxy for water temperature. Using LA-ICP-MS, strontium and lithium concentrations in shells of live-collected specimens from the intertidal zone of the North Sea (Schillig, Germany), were determined for the growing season of 2013 and compared to temperature records and other instrumental data. As expected, Sr/Cashell and Li/Cashell were vitally affected. However, Sr/Lishell ratios were strongly negatively and linearly correlated with water temperature, with up to 81% of explained variability. It was possible to reconstruct temperature based on Sr/Li to the nearest ±1.5 °C. It is hypothesized that normalizing Sr/Cashell to Li/Cashell mathematically reduces vital effects that hamper the application of traditional element/Cashell ratios as reliable proxies for water temperature in mollusk shells.
Manuscript III aims to advance Sr/Cashell ratios as proxy for water temperature in bivalve shells. In order to understand why strontium concentrations of bivalves are challenging to interpret, ultra-high-resolution geochemical and microstructural analyses were performed in a shell of Cerastoderma edule collected alive from the intertidal zone of the North Sea (Schillig, Germany). Results of this study demonstrate that strontium and sulfur (S = proxy for organics) are heterogeneously distributed and co-vary with microstructural changes. For example, circatidal growth lines of the outer portion of the outer shell layer (= irregular simple prisms; 2.9 ± 0.4 mmol/mol) contained much higher Sr levels than portions between consecutive growth lines (= growth increments; nondenticular prismatic structure; 2.5 ± 0.2 mmol/mol). In contrast, S/Cashell ratios displayed an inverted pattern with higher values at circatidal increments (2.4 ± 0.3 mmol/mol) and lower values at circatidal growth lines (2.1 ± 0.5 mmol/mol). It is hypothesized that microstructures or processes controlling their formation control the incorporation of Sr2+ ions into the shell. A lower sampling resolution that does not resolve these fine-scaled variations is therefore likely to result in Sr/Cashell ratios inadequate for temperature reconstructions, because strontium values of shell portions with different microstructures are averaged. It is suggested that Sr/Cashell-based temperature estimates can likely be improved by limiting chemical analyses to shell portions with the same microstructure.
This study proposes two alternatives to estimate water temperature from mollusk shells that aim to make better use of this highly versatile climate archive. The new proxies need to be rigorously tested in subsequent studies. Moreover, results of this study emphasize that a detailed knowledge on biomineralization of mollusk shells is required to improve the accuracy of temperature estimates.