Black shale deposition, atmospheric CO2 drawdown, and cooling during the Cenomanian‐Turonian Oceanic Anoxic Event

Abstract

Oceanic Anoxic Event 2 (OAE2), spanning the Cenomanian‐Turonian boundary (CTB), represents one of the largest perturbations in the global carbon cycle in the last 100 Myr. The δ13 C carb, δ13 C org, and δ18 O chemostratigraphy of a black shale–bearing CTB succession in the Vocontian Basin of France is described and correlated at high resolution to the European CTB reference section at Eastbourne, England, and to successions in Germany, the equatorial and midlatitude proto‐North Atlantic, and the U.S. Western Interior Seaway (WIS). Δ 13 C (offset between δ13 C carb and δ13 C org) is shown to be a good p CO 2 proxy that is consistent with p CO 2 records obtained using biomarker δ13 C data from Atlantic black shales and leaf stomata data from WIS sections. Boreal chalk δ18 O records show sea surface temperature (SST) changes that closely follow the Δ 13 C p CO 2 proxy and confirm TEX 86 results from deep ocean sites. Rising p CO 2 and SST during the Late Cenomanian is attributed to volcanic degassing; p CO 2 and SST maxima occurred at the onset of black shale deposition, followed by falling p CO 2 and cooling due to carbon sequestration by marine organic productivity and preservation, and increased silicate weathering. A marked p CO 2 minimum (∼25% fall) occurred with a SST minimum (Plenus Cold Event) showing >4°C of cooling in ∼40 kyr. Renewed increases in p CO 2, SST, and δ13 C during latest Cenomanian black shale deposition suggest that a continuing volcanogenic CO 2 flux overrode further drawdown effects. Maximum p CO 2 and SST followed the end of OAE2, associated with a falling nutrient supply during the Early Turonian eustatic highstand.