Paleoecology of Late Cretaceous Coccolithophores: Insights From the Shallow-Marine Record

Coccolithophores, common primary producers in Mesozoic, Cenozoic, and present oceans, are significant components of the earth's biogeochemical cycles. Being the most productive calcifying organism on earth, their carbonate oozes play a major role in fixing CO2, thereby forming the most important long-term CO2 sink throughout the last 100 Ma. Our understanding of their fossil ancestors is mainly based on data from chalk sequences. Generally, the Cretaceous chalks were deposited at water depths of >= 200 m under stable, pelagic conditions. Here we report on remarkably diverse fossil coccolithophore assemblages from Cenomanian-Campanian (100-72 Ma) nearshore deposits, with estimated water depths of <= 40 m. For interpreting the findings in a broader paleoenvironmental context, we additionally analyzed contemporaneous chalk material from the Santonian-Campanian (86-72 Ma) interval. Highly diverse coccolithophore associations (50-81 species/sample) characterize the nearshore setting, while the contemporaneous chalk samples are less diverse (43-54 species/sample). The absolute abundances of coccoliths from the nearshore setting are by a factor 2-10 lower than those from the chalk, diversity indices are by 20%-30% higher (Shannon Index = 2.94-3.46) in the nearshore sediments. The presence of highly diverse coccolithophore communities in nearshore settings provides new insights into their paleoecology and evolution and the paleoceanography of the Late Cretaceous. Our data revise the traditional view of coccolithophores being typical open ocean dwellers. Extreme nearshore settings of the Late Cretaceous greenhouse world were characterized by environmental conditions equating those of the hemipelagic realm but supplying a larger variety of ecological niches.

Automated summary

This study reports remarkably diverse fossil coccolithophore assemblages from Cenomanian-Campanian nearshore deposits characterized by a high degree of diversity, with lower absolute abundances but higher diversity indices compared to contemporaneous chalk samples.