Abstract
Abstract. 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 well-preserved 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 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, while TEX86H palaeothermometry seems to be biased towards warmer surface water temperatures. 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 complicated by growth cessations and diagenesis, cyclicity in trace elements and stable isotopes allowed for a tentative interpretation of the seasonal cycle in late Maastrichtian palaeoenvironment of the Neuquén Basin. 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. 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.
Highlights
The Late Cretaceous is generally considered a greenhouse world (e.g. Hay, 2008)
Microscopic images of the foliated calcite microstructure (e.g. Fig. 3f–g) and comparison with modern oyster studies further show that the elongated crystal microstructure characteristic of pristine foliated shell calcite has not been compromised by diagenesis (Ullmann et al, 2010)
Elevated concentrations of Fe and Mn in the shells can be used as an indicator for recrystallization, since these elements are incorporated in secondary calcite from reducing pore waters in the sediment surrounding the shell during burial (Al-Aasm and Veizer, 1986a)
Summary
The Late Cretaceous is generally considered a greenhouse world (e.g. Hay, 2008). reconstructed global mean temperatures and atmospheric pCO2 concentrations for this period generally exceed those of the present-day climatePublished by Copernicus Publications on behalf of the European Geosciences Union.N. The Late Cretaceous is generally considered a greenhouse world The Late Cretaceous may be considered an analogue for climate of the near future if anthropogenic greenhouse gas emissions continue unabated (Hay, 2013; IPCC, 2014; Dlugokencky, 2017). Many studies have yielded reconstructions of Late Cretaceous climates using either climate models or a variety of proxies in temporally long archives, such as deepsea cores and continental sections (Pearson et al, 2001; Huber et al, 2002; Otto-Bliesner et al, 2002; Miller et al, 2003; Friedrich et al, 2012; de Winter et al, 2014; Vellekoop et al, 2016). It is important that these climate variations are understood on a shorter timescale
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
