Shell chemistry of the Boreal Campanian bivalve <i>Rastellum diluvianum</i> (Linnaeus, 1767) reveals temperature seasonality, growth rates and life cycle of an extinct Cretaceous oyster

Abstract

Abstract. The Campanian age (Late Cretaceous) is characterized by a warm greenhouse climate with limited land ice volume. This makes the Campanian an ideal target for the study of climate dynamics during greenhouse periods, which are essential for predictions of future climate change due to anthropogenic greenhouse gas emissions. Well-preserved fossil shells from the Campanian age (± 78 Ma) high paleolatitude (50° N) coastal faunas of the Kristianstad Basin (southern Sweden) offer unique snapshot of short-term climate and environmental variability during the Campanian, which complement traditional long-term climate reconstructions. In this study, we apply a combination of high-resolution spatially resolved trace element analyses (µXRF and LA-ICP-MS), stable isotope analyses (IRMS) and growth modelling to study short-term (seasonal) variations recorded in the oyster species Rastellum diluvianum from Ivö Klack. A combination of trace element and stable isotope records of 12 specimens sheds light on the influence of specimen-specific and age-specific effects on the expression of seasonal variations in shell chemistry and allows disentangling vital effects from environmental influences in an effort to refine palaeoseasonality reconstructions of Late Cretaceous greenhouse climates. Growth modelling based on stable isotope records from R. diluvianum further allows to discuss the mode of life, circadian rhythm and reproductive cycle of extinct oysters and sheds light on their ecology. This multi-proxy study reveals that mean annual temperatures in the Campanian high-latitudes were 17 to 19 °C with a maximum extent of seasonality of 14 °C. These results show that the latitudinal gradient in mean annual temperatures during the Late Cretaceous was steeper than expected based on climate models and that the difference in seasonal temperature variability between latitudes was much smaller in the Campanian compared to today. Our results also demonstrate that species-specific differences and uncertainties in the composition of Late Cretaceous seawater prevent trace element proxies (Mg / Ca, Sr / Ca, Mg / Li and Sr / Li) to be used as reliable temperature proxies for fossil oyster shells.