Trochammina as opportunist foraminifera in the Lower Jurassic from north Siberia

Abstract

The ecostratigraphic analysis of foraminiferal assemblages from Upper Pliensbachian to Lower Toarcian (Lower Jurassic) mudstones, siltstones and black shales from northern Siberia allows for a better understanding of the response to the benthic biotic crisis related to the Toarcian Oceanic Anoxic Event in a high latitude context. The assemblages were dominated by agglutinated taxa with extremely low diversity values and dominance of Trochammina. These features suggest that the foraminiferal assemblages were adapted to restricted conditions, where the main limiting factors were salinity and oxygen degree. The opportunist behaviour of Trochammina enabled this genus to survive and adapt to unfavourable conditions. Trochammina proliferated in relation to the sea-level fall and probable changes in salinity in the Arctic palaeobasin during the Margaritatus Chron and at the beginning of the Viligaensis Chron (Late Pliensbachian). Another Trochammina proliferation is associated with the initial development of the restricted oxygen conditions related to the Toarcian Oceanic Anoxic Event.

Keywords

• r-strategists
• colonization
• anoxic event
• ecostratigraphy
• black shale