Bivalve shell microstructures - exploring a novel marine temperature proxy

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Quantitative reconstruction of ocean temperature and salinity at annual and better temporal resolution during the last centuries to millennia is a pressing issue in paleoclimatology. Such data are essential to better understand the role of the ocean in global climate change and to constrain predictive numerical climate models. One of the most promising archives that can provide such data from nearly any part of the ocean has recently attracted considerable attention: shells of bivalve mollusks, specifically long-lived (> 30 yrs-old) species.Despite major research advances in bivalve sclerochronology during the last decade, the quantitative reconstruction of water temperature from shells remains a challenging task. For example, the most frequently used and widely accepted proxy in bivalves, i.e., the shell oxygen isotope value, simultaneously informs about changes of temperature and the oxygen isotope signature of the water (which is correlated to salinity). To reconstruct precise temperatures from shell oxygen isotope data, salinity (or the oxygen isotope signature of the water) must be known, an information which is rarely available for ancient environments as no suitable proxy exists.Recently, we have demonstrated that the microstructure of shells of the short-lived (ca. 6 yrs-old) Cerastoderma edule serves as a proxy for water temperature (Milano et al. 2017). As temperature rises, individual biomineral units become larger and more elongated. Our study was corroborated by Gilbert et al. (2017) for different species of Atrina and Pinna. Their results demonstrated that the nacre tablet thickness in these likewise fast-growing, relatively short-lived bivalves is also positively correlated to temperature. There are a number of advantages of this new tool over existing paleothermometers: (i) The increase in biomineral size follows thermodynamic predictions. (ii) The new temperature proxy can be applied to well-preserved fossils even if the original chemical signature is diagenetically lost. (iii) Once the image processing software is trained to recognize the biomineral units, a large number of measurements can be done in short time. With an independent temperature proxy, ocean salinity can be computed from shell oxygen isotope values.The main objective of the planned project is to develop transfer functions that can be used to quantitatively reconstruct ocean temperature from morphological properties of shell microstructures of long-lived bivalves. So far, the method has only been evaluated in a few short-lived bivalve species with crossed-lamellar and nacreous fabrics. For the broader application in paleoclimatology, it is necessary to test and calibrate this simple and easy-to-use but powerful novel method for long-lived bivalves (with different microstructures) from extratropical settings. Results of the planned study will significantly advance the field of bivalve sclerochronology.