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

DE WINTER, NIELS J.; VELLEKOOP, JOHAN; VORSSELMANS, ROBIN; GOLREIHAN, ASEFEH; SOETE, JEROEN; PETERSEN, SIERRA V.; MEYER, KYLE W.; CASADIO, SILVIO; SPEIJER, ROBERT P.; CLAEYS, PHILIPPE

Climate of the Past

EGU

Año: 2018 vol. 14 p. 725 - 749

Abstract. In order to assess the potential of the honeycomboyster Pycnodonte vesicularis for the reconstructionof palaeoseasonality, several specimens recovered from lateMaastrichtian strata in the Neuquén Basin (Argentina) weresubject to a multi-proxy investigation, involving scanningtechniques and trace element and isotopic analysis. CombinedCT scanning and light microscopy reveals two calcitemicrostructures in P. vesicularis shells (vesicular andfoliated calcite). Micro-XRF analysis and cathodoluminescencemicroscopy show that reducing pore fluids were ableto migrate through the vesicular portions of the shells (aidedby bore holes) and cause recrystallization of the vesicularcalcite. This renders the vesicular portions not suitable forpalaeoenvironmental reconstruction. In contrast, stable isotopeand trace element compositions show that the originalchemical composition of the foliated calcite is wellpreservedand can be used for the reconstruction of palaeoenvironmentalconditions. Stable oxygen and clumped isotopethermometry on carbonate from the dense hinge of the shellyield sea water temperatures of 11C, while previous TEXH86palaeothermometry yielded much higher temperatures. Thedifference is ascribed to seasonal bias in the growth of P.vesicularis, causing warm seasons to be underrepresentedfrom the record, while TEXH86 palaeothermometry seems tobe biased towards warmer surface water temperatures. Themulti-proxy approach employed here enables us to differentiatebetween well-preserved and diagenetically altered portionsof the shells and provides an improved methodology forreconstructing palaeoenvironmental conditions in deep time.While establishing a chronology for these shells was complicatedby growth cessations and diagenesis, cyclicity in traceelements and stable isotopes allowed for a tentative interpretationof the seasonal cycle in late Maastrichtian palaeoenvironmentof the Neuquén Basin. Attempts to independentlyverify the seasonality in sea water temperature by Mg= Caratios of shell calcite are hampered by significant uncertaintydue to the lack of proper transfer functions for pycnodonteinoysters. Future studies of fossil ostreid bivalves should targetdense, foliated calcite rather than sampling bulk or vesicularcalcite. Successful application of clumped isotope thermometryon fossil bivalve calcite in this study indicates thattemperature seasonality in fossil ostreid bivalves may be constrainedby the sequential analysis of well-preserved foliatedcalcite samples using this method.