Articles | Volume 14, issue 6
Clim. Past, 14, 725–749, 2018
Clim. Past, 14, 725–749, 2018

Research article 08 Jun 2018

Research article | 08 Jun 2018

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

Niels J. de Winter1,*, Johan Vellekoop1,2,*, Robin Vorsselmans2, Asefeh Golreihan2, Jeroen Soete2, Sierra V. Petersen3, Kyle W. Meyer3, Silvio Casadio4, Robert P. Speijer2, and Philippe Claeys1 Niels J. de Winter et al.
  • 1Analytical, Environmental and Geo-Chemistry (AMGC), Vrije Universiteit Brussel (VUB), Brussels, Belgium
  • 2Department of Earth and Environmental Science, KU Leuven, Heverlee, Belgium
  • 3Earth and Environmental Sciences Department, University of Michigan, Ann Arbor, Michigan, USA
  • 4Escuela de Geología, Paleontología y Enseñanza de las Ciencias, Universidad Nacional de Río Negro, CONICET, General Roca, Argentina
  • *These authors contributed equally to this work.

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.

Short summary
In this work, we apply a range of methods to measure the geochemical composition of the calcite from fossil shells of Pycnodonte vesicularis (so-called honeycomb oysters). The goal is to investigate how the composition of these shells reflect the environment in which the animals grew. Ultimately, we propose a methodology to check whether the shells of pycnodonte oysters are well-preserved and to reconstruct meaningful information about the seasonal changes in the past climate and environment.