Quantitative analysis of calcareous microfossils across the Albian–Cenomanian boundary oceanic anoxic event at DSDP Site 547 (North Atlantic)

Nannofossil and planktonic and benthic foraminiferal assemblages were studied quantitatively in an expanded Albian to Cenomanian section at DSDP Site 547 in the eastern North Atlantic. The section spans a 0.5‰ positive δ13C excursion across the Albian–Cenomanian boundary, representing one of the mid-Cretaceous Oceanic Anoxic Events (OAE1d). The base of the carbon excursion at Site 547 coincides with a major faunal turnover in planktonic foraminifera, including extinctions and first appearances of several species as well as geographic entries. In contrast, nannofossil assemblage composition, which reflects conditions in the photic zone, shows little variation related to the carbon excursion, arguing against a noticeable change in surface water productivity during the event. Benthic foraminifera, which represent a paleowaterdepth of around 1 km, show that there was no change in environmental conditions at that water depth, and offer no support for increased deep water anoxia. Combined with published data, microfossils at Site 547 suggest that improved preservation of organic matter due to a decrease in oxygen content in a shallow oxygen minimum zone is a more likely cause for the Albian–Cenomanian boundary anoxic event than an increase in productivity.