A comparison of bivalve, gastropod and fish otolith stable isotope profiles from the Aalter Sands, early Eocene southern North Sea Basin

Stable isotope sclerochemistry of mollusks and otoliths is key in reconstructing temperature and seasonality in past greenhouse climates. It is therefore crucial to understand the paleoecology and –environment of these organisms, and how these factors influence intra-and inter-taxon isotope variability and variation. To gain more insights in these factors, 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 successive 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. Studying multiple taxa and specimens in a death assemblage provides a more complete spectrum of isotope variation and variability.