Trace Fossils and Mid-Cretaceous Anoxic Events in the Atlantic Ocean

Patterns and intensity of bioturbation in marine deposits are useful indicators of the response of benthic organisms to fluctuating oxygen levels in bottom waters. Trace fossil assemblages in mid-Cretaceous (Barremian-Albian) Deep Sea Drilling Project (DSDP) core sections from the North and South Atlantic Ocean were examined in order to document the activities of burrowing infauna relative to anoxic episodes in deep Atlantic basins during that time.

Mid-Cretaceous anoxic events typically are represented in DSDP cores by dark, homogeneous or laminated organic muds, which alternate with moderately to heavily burrowed facies containing less organic carbon. Bioturbation intensity and trace fossil diversity appear to correlate inversely with the amount of unoxidized carbon and total sulfur in the sediment, suggesting that the more organic-rich facies were deposited under conditions where oxygen was a limiting factor for benthic infaunal macro-organisms.

The ichnogenus Chondrites commonly occurs, sometimes to the exclusion of all other kinds of burrows, in zones immediately above and (or) below unburrowed, laminated (oxygen-depleted) mud. It also occurs in heavily burrowed (oxygenated) limestones containing rich trace fossil faunas, including Zoophycos. Therefore, it appears that mid-Cretaceous Chondrites were created by animals possessing a broad range of oxygen tolerance. The presence of Chondrites alone in an organic-rich deposit probably indicates dysaerobic conditions.