Environmental changes in the Late Ordovician-early Silurian: Review and new insights from black shales and nitrogen isotopes

Abstract

The Late Ordovician (Katian-Hirnantian) through earliest Silurian (Rhuddanian) interval was a time of varying climate and sea level, marked by a peak glacial episode in the early-mid Hirnantian. Synthesis of recently published data permits global correlation of at least two cycles of glacial advance and retreat with a distinct interglacial period that is recognizable in sequence stratigraphic and chemostratigraphic records worldwide. A period of warming and sea level rise during the late Katian is marked by the widespread occurrences of oceanic anoxia in paleotropical and subtropical localities, mostly confined to regions of inferred upwelling and semi-restricted marine basins. Nitrogen isotope data show that the regions of oceanic anoxia were marked by intense water column denitrification in which cyanobacteria were the principal source of fixed N. In the overlying peak glacial interval of the Hirnantian, sedimentary successions indicate that anoxia was much restricted. The shift to more positive N isotope values also suggests less intense water column denitrification. In the late Hirnantian and early Rhuddanian the black shales showing evidence of anoxia spread over all paleolatitudes and water depths indicating a Rhuddanian ocean anoxic event comparable to examples from the Mesozoic. It is accompanied by a return to intensely denitrifying conditions within the water column. The two phases of Hirnantian mass extinction coincide with rapid, climate-driven changes in oceanic anoxia. The first extinction occurs at the onset of glaciation and with the loss of anoxic conditions at the end of the Katian. The second extinction occurs at the demise of glaciation and coincides with the return of anoxic conditions during the late Hirnantian/early Rhuddanian. The N isotope data suggest that the extinctions may be traced to changes occurring at the base of the marine food web, i.e., redox driven changes in nutrient cycling and primary producer communities.