Making sense of variation in sclerochronological stable isotope profiles of mollusks and fish otoliths from the early Eocene southern North Sea Basin

Abstract. Stable isotope sclerochemistry of mollusks and otoliths is frequently used for the reconstruction of paleotemperature and seasonality. Constraints on the paleoecology and –environment of these organisms, and how these factors influence intra-and inter-taxon isotope variability and variation, are thus highly valuable. We measured seasonal changes in δ18O and δ13C compositions in multiple specimens of two carditid bivalve species, a turritelline gastropod species, and two species of otoliths from demersal fish, from two early Eocene (latest Ypresian, 49.2 Ma) coquinas in the inner neritic Aalter Sand Formation, located in the Belgian part of the southern North Sea Basin (paleolatitude ~41° N). Results demonstrate variability among taxa in average, amplitude and shape of intra-annual δ18O and δ13C values. This intertaxon variability is at least partly caused by growth cessation during winters in turritellines and otoliths, leading to an incomplete representation of the seasonal cycle in their growth increments, compared to carditid bivalves. Other contributing factors to isotopic variability include sedimentary transport, mobility, and the lifespan of the specimens. Specifically, ophidiid fish otolith isotope records appear to reflect environmental conditions over a wider range of habitats and environments, due to sedimentary transport and postmortem transport by free-swimming predatory fish. Our study therefore highlights the variability between different taxa and environments in the shallow marine realm, which has implications for seasonality reconstructions. We show that by studying multiple taxa and specimens in a death assemblage, a more complete spectrum of isotope variation and variability becomes apparent.