[B-PT04] Biotic History

[BPT04-P04] Decoupling ocean acidification from bottom water anoxia in the Cretaceous greenhouse

*Kazuyoshi Moriya¹ (1.Department of Earth Sciences, Faculty of Education and Integrated Arts and Sciences, Waseda University)

Keywords:Cretaceous, Ocean acidification, Oceanic anoxic event

The Cretaceous is known as one of the archetypal greenhouse intervals. During the mid-Cretaceous thermal maximum, Oceanic Anoxic Events (OAEs) repeatedly occurred. Especially at the Cenomanian/Turonian boundary (CTB), well-known OAE2, accompanied by massive deposition of organic carbon, positive shift in δ¹³C, deterioration of carbonate deposition, and mass extinction, has been reported. These events were triggered by the injection of volcanic CO₂ into the atmosphere, resulting in ocean acidification and bottom water anoxia.
On the other hand, while the deterioration of carbonate deposition has been reported at the Santonian/Campanian boundary (SCB), total organic carbon and δ¹³C of bulk organic compounds show no evidence of bottom water anoxia. This observation indicates that ocean acidification occurred in the upper water column without any sign of bottom water anoxia at the SCB. According to the reconstruction of atmospheric CO₂ concentration, pCO₂ at the SCB is significantly lower than that at the CTB. The difference between the carbonate and organic carbon cycle between CTB and SCB may indicate that the organic carbon cycle, primarily bottom water anoxia, occurred only at high pCO₂ intervals. In contrast, the carbonate carbon cycle, including ocean acidification, easily collapses with the wobble of pCO₂.