Effects and causes in a black shale event — the Toarcian Posidonia Shale of NW Germany

Abstract

Black shale formation in the Toarcian of NW Germany is associated with a major turnover in phytoplankton assemblages interpreted as the response to lowered salinities in surface waters of the epicontinental sea. The resulting halocline leeds to anoxia of varying degrees at the sediment surface and in the water column controlling the extent of benthic life. The bulk of organic matter concentrated in the Posidonia Shale occurs as thin bituminous laminae considered to be the remnants of microbial mats stabilizing the redox boundary at the sediment surface. Possible links with oceanic circulation may provide a basis for correlation with contemporaneous events in other regions.