An assessment of latest Cretaceous Pycnodonte vesicularis (Lamarck, 1806) shells as records for palaeoseasonality: A multi-proxy investigation

In order to assess the potential of the honeycomb oyster Pycnodonte vesicularis for the reconstruction of palaeoseasonality, several specimens recovered from the late Maastrichtian Neuquén Basin (Argentina) were subject to a multi-proxy investigation, involving scanning techniques, trace element and isotopic analysis. Combined CT scanning and light microscopy reveals two major calcite micromorphologies in P. vesicularis shells (vesicular and foliated calcite). Micro-XRF analysis and cathodoluminescence microscopy show that reducing pore fluids were able to migrate through the vesicular portions of the shells (aided by bore holes) and cause recrystallization and precipitation of secondary carbonate in the porous micromorphology, thus rendering the vesicular portions not suitable for palaeoenvironmental reconstruction. In contrast, stable isotope and trace element compositions show that the original chemical composition of the shell is well-preserved in the denser, foliated portions, which can therefore be reliably used for the reconstruction of palaeoenvironmental conditions. Stable oxygen and clumped isotope thermometry on carbonate from the dense hinge region yield sea water temperatures of 11 °C, while previous TEX86H palaeothermometry yielded much higher temperatures. The difference is ascribed to seasonal bias in the growth of P. vesicularis, causing warm seasons to be underrepresented from the record, and TEX86H palaeothermometry being potentially biased towards warmer surface water temperatures. Superimposed on this annual mean is a seasonality in δ18O of about 1 ‰, which is ascribed to a combination of varying salinity due to fresh water input in the winter and spring season and a moderate temperature seasonality. Attempts to independently verify the seasonality in sea water temperature by Mg / Ca ratios of shell calcite are hampered by significant uncertainty due to the lack of proper transfer functions for pycnodontein oysters. The multi-proxy approach employed here enables us to differentiate between well-preserved and diagenetically altered portions of the shells and provides an improved methodology for reconstructing palaeoenvironmental conditions in deep time. While establishing a chronology for these shells was severely complicated by growth cessations and diagenesis, cyclicity in trace elements and stable isotopes allowed a tentative interpretation of the potential annual seasonal cycle in the late Maastrichtian palaeoenvironment of the Neuquén basin. Future studies of fossil ostreid bivalves should target dense foliated calcite rather than sampling bulk or vesicular calcite. Successful application of clumped isotope thermometry on fossil bivalve calcite in this study indicates that temperature seasonality in fossil ostreid bivalves may be constrained by the sequential analysis of well-preserved foliated calcite samples using this method.