Valanginian Weissert oceanic anoxic event

Abstract

Biotic changes in nannofossils and radiolarians associated with the Valanginian δ¹³C anomaly are documented at Ocean Drilling Program Hole 1149B in the Pacific Ocean: they are coeval and similar to those previously documented in the Tethys, suggesting a global perturbation of marine ecosystems. A marked increase in abundance of Diazomatolithus, absence of nannoconids, and a Pantanellium peak characterize the Valanginian δ¹³C excursion. Such changes are interpreted as being due to global enhanced fertility and a biocalcification crisis under conditions of excess CO₂. The occurrence of organic C–rich black shales in the Southern Alps and in the Pacific in the interval corresponding to the δ¹³C excursion suggests a Valanginian oceanic anoxic event (OAE). Volcanism of the Paranà-Etendeka large igneous province (ca. 132 Ma) was presumably responsible for an increase of CO₂, triggering a climate change and accelerated hydrological cycling, possibly causing an indirect fertilization of the oceans. Widespread nutrification via introduction of biolimiting metals at spreading ridges could have significantly increased during the Gondwana breakup and simultaneous tectonic events in three separate oceans. There is no paleontological or δ¹⁸O evidence of warming during the Valanginian OAE. On the contrary, both nannofossils and oxygen isotopes record a cooling event at the climax of the δ¹³C excursion. Weathering of basalts and burial of organic C–rich black shales were presumably responsible for CO₂ drawdown and establishment of reversed greenhouse conditions.