Inoceramid sclerochronology - Advancing Late Cretaceous climate reconstructions

点击翻译按钮获取中文摘要

The Cretaceous serves a classic example of a greenhouse world and belongs to one of the best studied time intervals of Earth history. However, it still remains uncertain which actual temperatures prevailed at different latitudes and water depths, and almost nothing is known about the seasonal and inter-annual environmental variability in extratropical oceans at that time. Such information is essential to constrain meridional temperature gradients, verify the presence or absence of polar ice, test hypotheses on atmospheric and oceanic circulation patterns and further refine numerical climate models. It may also be used to study the link between sedimentation and orbital forcing in more detail, because the latter modifies seasonal temperature patterns. Furthermore, high-resolution temperature (and other environmental) data can help to better understand species distribution patterns, the paleobiology and life history traits of organisms as well as potential causes for the rise and demise of taxa. A promising archive for deciphering seasonal to inter-annual climate dynamics in extratropical Cretaceous oceans - whose full potential has by far not been explored - is contained in shells of inoceramid bivalves. This peculiar species-group appeared in the Permian and, for yet unknown reasons, became extinct several million years prior to the Cretaceous/Paleogene boundary. Despite decades of research, not particularly much is known about the biology, ecology and life-history traits of the inoceramids, but they thrived in the greenhouse climate of the late Mesozoic. During the Late Cretaceous, they clearly dominated the extratropical megafauna. With enormous shell sizes of up to 3m (!), inoceramids belong to the largest bivalves that ever lived. Inoceramids inhabited nearly any water depth and even occurred abundantly in hostile, dysoxic environments. By applying state-of-the-art sclerochronological techniques combined with a broad spectrum of diagenesis screening tests, the proposed study will pave the way toward a routine use of inoceramid bivalves in high-resolution paleoclimatology. To demonstrate the potential of this archive, we will exemplarily study how seasonal temperatures changed (1) during the Cretaceous hothouse, i.e. before, during and after OAE2 (Cenomanian/Turonian stage boundary; Münsterland Basin) and (2) during the Campanian stage (Hokkaido). (3) In addition, it will be analyzed if a difference exists between seasonal environmental variables reconstructed from shells of different rock types (Münsterland Basin). If sedimentation was orbitally forced, seasonal temperatures, primary productivity etc. should have differed between times of limestone and marl deposition. (4) Furthermore, the life history traits of selected inoceramid taxa will be determined including ontogenetic ages as well as the timing and rate of seasonal shell growth.