Inter-species variability in palaeoseasonality records from fossil bivalve shells: Evidence from Late Albian rudist and pectinid shells from the Lusitanian Basin, Portugal

Abstract

Palaeoclimate reconstructions are essential for improving our understanding of Earth’s climate. Complementaryto more commonly reported long term palaeoclimate reconstructions, much information can be obtained frompalaeoclimate and –environment reconstruction on a shorter, seasonal time scale. Calcite shells of bivalves canprovide the high resolution climate archives required for doing these reconstructions in deep-time. These organismsgrow by incrementally adding carbonate to their shell, and are shown to precipitate stable oxygen isotoperatios in equilibrium with the ambient sea water, hereby recording past sea water composition and temperature(Klein et al., 1996). However, additional proxies such as stable carbon isotope and trace element ratios are requiredto disentangle the impact of various environmental parameters on the shell geochemistry (e.g. de Winter et al.,2017). The incorporation of these proxies into bivalve shells is not fully understood and often complicated by bothspecies- or specimen-specific vital effects, diagenesis and variations in sea water concentrations (e.g. Gillikin etal., 2005; Freitas et al., 2008).The current study aims at combining multi-proxy sclerochronological records (stable isotopes and trace elements)of different Late Albian bivalve taxa (rudists and pectinids) derived from the same proto-North Atlanticpalaeoenvironmental setting in Portugal (Sao Julião section, Lusitanian Basin, Horikx et al., 2014). Micro-X-RayFluorescence (XRF) scanning yields quantitative line scans and semi-quantitative 2D maps that highlight thedistribution of trace elements in the fossil shells. Trace element and stable isotope profiles - measured along thegrowth axis - allow for the identification of seasonal cycles recorded in the shell.Here, a multivariate statistical approach is presented to identify the influence of different types of diagenesis onthese multi-proxy records, and to isolate the seasonal cyclicity recorded in the shells. Results of this multi-proxyapproach allow discussion of inter-species differences in preservation, seasonal cyclicity and incorporation ofpalaeoenvironmental proxies into the shell calcite. Most importantly, this approach sheds light on the feasibility ofusing the here investigated bivalve shells for future deep-time palaeoseasonality reconstructions.