Chemical and structural variations of bivalve shells at the micrometer scale - taking sclerochronology to the next level

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Bivalve mollusks offer exceptional potential for paleoclimate research. They inhabit nearly all aquatic environments worldwide, occur abundantly in the fossil record and can live for a few months to several hundreds of years. Most importantly, their shells grow periodically and record environmental changes in precise chronological order. Yet, extracting specific environmental signals from the shells remains a very challenging task owing to limited number of reliable and well-accepted proxies that are currently available. In many other organisms, the trace and minor element (TME) content provides serviceable environmental proxy data. However, in bivalves their interpretation is notoriously difficult, because the uptake, transport and incorporation of TME into the shells is under strong biological control (vital effects) which superimposes the environmental signals. Through the strong control of TME, the bivalve maintains the outstanding mechanical properties of its shell - one of the strongest natural materials - and ensures that the individual biomineral units - nanocomposites of calcium carbonate and organic macromolecules - have a specific size (nm, µm), shape and orientation and are arranged in complex orders (shell microstructures).Fortunately, vital effects are reflected in the microstructure and in covariations of TME. The latter suggests mutual biological fractionation processes were at work. The Japanese collaboration partner will compute TME regression curves, and analyze/interpret the covariations. The goal is to identify and mathematically eliminate TME correlations in order to isolate environmental signals without the need to determine the fractionation mechanisms. TME concentrations are also strongly coupled to the shell microstructure which in turn, is controlled by biomineralization processes and integrates different TME fractionation processes. Some microstructure types contain higher levels of certain TME than others. In addition, the architecture of the shell varies strongly at the µm-scale which results in a small-scale chemical heterogeneity. Thus, contemporaneously formed shell portions deviate chemically from each other if their microstructure differs. To isolate environmentally controlled TME variations, we (at U Mainz) will conduct a combined and systematic mapping of shell TME and microstructure. Then, we will parameterize the typical TME levels in different microstructures in order to eliminate non-environmental variations from TME. Ultimately, we will jointly develop a multidimensional numerical model that combines the TME distribution, microstructure and environmental parameters.If it were possible to unlock the environmental information trapped in TME, bivalve sclerochronology would make a major leap forward and boost paleoclimate research. Proposed novel approaches and the complementary expertise of the applicants along with the analytical infrastructure at both institutions can turn this vision into reality.