Geochemical screening of Eocene bivalves: disentangling environmental signals from diagenetic overprint

The Eocene ´greenhouse´ climate represents the warmest period within the Cenozoic and has therefore become especially interesting as an analogue for estimated future climate scenarios. For paleo-climate reconstructions, bivalves represent valuable proxy archives with a high temporal resolution, due to their distinct, periodic layering (growth increments). However, interpreting environmental signals from fossil bivalves can be challenging, because of the species-specific mineralogy of the shells (calcite or aragonite) and its associated geochemical behaviour, as well as its resilience against diagenetic alteration. For the current study several middle Eocene (Lutetian) aragonitic valves of the species Venericor planicosta from different localities of the Anglo-Paris-Basin were analysed. To disentangle the environmental signals from possible diagenetic or biological influences, EPMA mapping, Cathodoluminescence (CL), as well as SEM imaging were employed. The CL and the SEM analyses revealed no increased incorporation of Mn or an extensive recrystallization respectively, suggesting a pristine preservation of the original shell material. These findings are further underlined by results of EPMA mapping of Mg, Na, Sr and S, which display oscillating element distribution patterns with increased element ratios (Me/Ca) along growth increment boundaries. Moreover, an increasing trend of the Sr/Ca ratio along the growth axis of each shell, points to potential ontogenetic effects. The Me/Ca ratios pattern are similar in all specimens, independent from their geological age or sample location and are not expected for diagenetically influenced material. Accordingly, the aragonitic shells are considered as excellent preserved archives with a high potential to resolve Eocene seasonality, e.g. by using δ18O and ∆47.