Benthic ecosystem response to the deposition of lower Aptian black shales in an epicontinental sea

Abstract

Oceanic Anoxic Events (OAEs) of the Mesozoic are manifested globally by deposition of laminated, organic-rich sediments at depths ranging from the deep ocean to the shallow, epicontinental seas. In the deep ocean, the deposition of the OAE black shales is known to have been associated with the development of anoxic or dysoxic conditions. However, little is known about the oxygenations of shallower epicontinental seas, which may have served as refuge for benthic organisms. Here we present high resolution data on changes in diversity and abundance of benthic foraminifera from a continuous profile throughout the Fischschiefer, a lower Aptian black shale correlated with OAE 1a, located on the southern margin of the epicontinental Lower Saxony Basin. Benthic foraminiferal diversity and abundance decreased and reached minimum values prior to the deposition of the laminated black shale. The persistent occurrence of foraminifera throughout the Fischschiefer indicates that in the epicontinental setting of the studied basin, the sea floor remained habitable to at least some benthic life throughout the OAE. The impoverished foraminiferal content of the Fischschiefer was replaced by rich assemblages during two “interruption” events. These observations are consistent with the occurrence of other benthic and nektonic fossils in the studied profile, where ammonites indicate a well-oxygenated water column during parts of the OAE and benthic molluscs hint at short-term re-oxygenation events within the Fischschiefer horizon. Collectively, these data indicate that unlike the deep ocean, at least part of the water column of epicontinental seas remained habitable for planktic, nektic and benthic life throughout OAE 1a, facilitating subsequent rapid recolonisation of the European boreal seaways. Nevertheless, benthic foraminifera indicate a severe decline in diversity and abundance during OAE 1a.