ShellTrace v1.0 – A new approach for modelling growth and trace element uptake in marine bivalve shells: Model verification on pacific oyster shells (Crassostrea gigas)

Abstract. Bivalve shells record changes in their environment in the chemical composition of their shells and are frequently used as paleoclimate archives. However, many studies have shown that physiological changes related to growth of the bivalve may overprint these chemical tracers. In the present study, a new approach is presented that models growth and development of bivalve shells without a priori knowledge of the physiology of the species. The model uses digitized growth increments in a cross section of the shell together with basic assumptions of the shape of the shell in order to model changes in shell length, thickness, volume, mass and growth rate at a daily resolution through the lifetime of the bivalve. This approach reconstructs the growth history of bivalves based on their shell without the need for a culture experiment, paving the way for growth rate estimations based on fossil bivalve shells. Combination of the growth model with 2D X-Ray Fluorescence trace element mapping allows the incorporation of trace elements into the shell to be modelled in 3D through the bivalve's lifetime. This approach yields records of integrated total-shell trace element concentrations and accumulation rates, which shed light on the rates and mechanisms by which these trace elements are incorporated into the shells of bivalves. Application of growth and trace element modelling on a set of modern pacific oyster shells of well-known origin and comparison of model results with conventional trace element transects highlights the importance of considering heterogeneity in mineralogy, mineralization rates and chemical composition within the shells of bivalves. These insights lead to a better understanding of the complexity of trace element concentrations in bivalve shells, which can then be applied as proxies for the reconstruction of sub-annual changes in palaeoenvironmental conditions over geological timescales.