In order to assess the potential of the honeycomb oyster Pycnodonte vesicularis for the reconstruction of palaeoseasonality, several specimens recovered from late Maastrichtian strata in the Neuquén Basin (Argentina) were subject to a multi-proxy investigation, involving scanning techniques and trace element and isotopic analysis. Combined CT scanning and light microscopy reveals two calcite microstructures 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 of the vesicular calcite. This renders the vesicular portions not suitable for palaeoenvironmental reconstruction. In contrast, stable isotope and trace element compositions show that the original chemical composition of the foliated calcite is wellpreserved and can be used for the reconstruction of palaeoenvironmental conditions. Stable oxygen and clumped isotope thermometry on carbonate from the dense hinge of the shell yield sea water temperatures of 11 • C, while previous TEX H 86

18In order to assess the potential of the honeycomb oyster Pycnodonte vesicularis for the 19 reconstruction of palaeoseasonality, several specimens recovered from the late Maastrichtian 20Neuquén Basin (Argentina) were subject to a multi-proxy investigation, involving scanning 21 techniques, trace element and isotopic analysis. Combined CT scanning and light microscopy reveals 22 two major calcite micromorphologies in P. vesicularis shells (vesicular and foliated calcite O of about 1‰, which is 35 ascribed to a combination of varying salinity due to fresh water input in the winter and spring season 36 and a moderate temperature seasonality. Attempts to independently verify the seasonality in sea 37 water temperature by Mg/Ca ratios of shell calcite are hampered by significant uncertainty due to 38 the lack of proper transfer functions for pycnodontein oysters. The multi-proxy approach employed 39 here enables us to differentiate between well-preserved and diagenetically altered portions of the 40 shells and provides an improved methodology for reconstructing palaeoenvironmental conditions in 41 deep time. While establishing a chronology for these shells was severely complicated by growth 42

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