Transient modeling for ocean redox conditions during the mid-cretaceous Oceanic Anoxic Event 2

Abstract

Reconstructions of the mid-Cretaceous Oceanic Anoxic Event 2 (OAE2) indicate that the ocean redox state was temporally and spatially heterogeneous. Its evolution is considered to be induced by the variation of atmospheric carbon dioxide concentrations (pCO₂) due to the activities of large igneous provinces (LIPs) and associated changes in global meridional overturning circulation, nutrient input, and phosphorus recycling. However, their respective roles on the OAE2 are still under debate. Here, we conduct transient numerical simulations using an intermediate-complexity Earth system model with reconstructed pCO₂ values. In our simulations, bottom anoxia occurred in the equatorial Atlantic Ocean and the Tethyan rim before OAE2. This changed only slightly with varying pCO₂. In contrast, increasing nutrient concentrations due to increased continental weathering can lead to a marked increase in anoxic areas, which is further expanded by enhanced phosphorus recycling in the low-oxygen marine area. The North Atlantic, low-latitude South Atlantic, Southeast Pacific, and Western Tethys oceans are most prone to developing bottom water anoxia. The modeled anoxic areas exhibit distinctly different spatio-temporal patterns during OAE2 with varying weathering intensities. Comparison of the simulation results with newly assembled geological records suggests that anoxic bottom water only accounted for ∼20–40% of the global bottom area and that the continental phosphorus weathering intensity was increased by ∼40–60% compared to the pre-OAE2 level.