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 and with the sudden biodiversity decline known as the K/T mass extinction (Schulte et al. 2010). This mass extinction set the stage for the development of modern marine and terrestrial ecosystems and biota (e.g., Krug et al., 2009; Meredith et al., 2011). Yet, extinction patterns of various fossil groups (especially metazoans) are considered incompatible with an instantaneous cataclysm decimating life and suggest environmental deterioration prior to the K/T boundary (e.g., Macleod et al., 1997). 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 (e.g. Keller, 2008). 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 continental margin marine ecosystems by evaluating quantitative microfossil records in three separated study areas, the US Gulf Coast, Tunisia and Turkey. These studies are complemented by geochemical analyses to identify climatic and paleoceanographic instabilities prior to the K/T boundary.