Out of the trenches: towards disentangling gradual and catastrophic changes across the K/T boundary

In a recent synthesis, it was convincingly shown that the impact of a large asteroid in the Gulf of Mexico coincided with the Cretaceous/Tertiary (K/T) boundary, 66 million years ago, and with the sudden biodiversity decline known as the K/T mass extinction when numerous organisms became extinct. This mass extinction set the stage for the development of modern ecosystems and biota on land and in the sea. Yet, extinction patterns of various fossil groups (especially multicellular animals) are considered incompatible with an instantaneous cataclysm decimating life and suggest environmental deterioration prior to the K/T boundary. Accordingly, various studies suggest that gradual but distinct climatic and/or sea-level changes lead to an accumulation of environmental stress perturbing latest Cretaceous ecosystems prior to the K/T boundary and triggering a longer term onset of extinctions. In this project, we study several K/T boundary successions and especially focus on assessing the extent and effects of gradual environmental and biotic changes in shallow marine ecosystems by evaluating quantitative microfossil records in three separated study areas, the US Gulf Coast and Atlantic Coast, Argentina and Turkey. These studies are complemented by geochemical analyses to identify climatic and paleoceanographic instabilities that are unrelated to the impact at the K/T boundary.