Middle Miocene Oxygen Minimum Zone Expansion Offshore West Africa: Evidence For Global Cooling Precursor Events

This reversal of climatic warming has proven difficult to understand because of the limited range of available records. The first records of this time interval from the Congo Fan, West Africa, may shed light on the events preceding the major cooling at 14 million years ago. The results of Kender et al. reveal two acidification events in the deep sea off West Africa about 16 and 15.5 million years ago, coincident with massive flood basalt release in the Columbia River basin, USA. The Columbia River Flood Basalts represent the largest group of eruptions to occur on Earth since the Paleogene, over 50 million years ago, and Kender et al. now suggest that they had a significant effect on the climate at this time.

The acidification events, which dissolved carbonate organisms on the seafloor, probably resulted from the accumulation of dead organic matter on the seafloor. Stronger offshore winds and increased primary productivity in the surface ocean off the west coast of Africa is likely to account for the accumulation and burial of this organic matter, which ultimately removed CO₂ from the atmosphere, leading to long-term cooling.

The implications of this study are that Columbia River volcanism likely triggered climatic cooling by sulphuric acid-induced volcanic winters, which would have increased the tradewinds off West Africa and, in turn, fertilized the ocean by transporting more nutrients such as sulphates to the ocean.

Large extinctions in deep-sea organisms (benthic foraminifera) associated with this time interval imply that increasing ocean productivity may have dramatic consequences for ocean ecology.

This reseach by S. Kender et al. of the British Geological Survey is published in the August issue of Geology on pages 699-702.