A new view on the evolution of seawater molybdenum inventories before and during the Cretaceous Oceanic Anoxic Event 2

Extreme periods of global warming in Earth's history have been strongly associated with declining oxygen concentrations in the oceans, a scenario not unlike the evolution observed in the oceans in recent times. One of the most intense Phanerozoic deoxygenation events (so called Ocean Anoxic Events (OAE)) is the late Cretaceous OAE2 approximately 94 Ma ago. Although several studies have investigated the evolution of redox sensitive geochemical proxies during the OAE2 event, geochemical records that span the time period leading up to OAE2, in particular for molybdenum (Mo) and its isotopes are rare. Here, we investigate Mo cycling in the Tarfaya upwelling system in the Cretaceous proto-North Atlantic Ocean before and during OAE 2 throughout a 5 Ma record. The observed changes in sedimentary Mo isotope compositions can be explained by a mode of Mo cycling similar to that in modern oxygen minimum zones. Based on this interpretation we estimate the pre-OAE2 Mo isotope composition of seawater to be 1.6 parts per thousand indicating that late Cretaceous pre-OAE2 seawater had a lighter Mo isotope composition than the modern ocean. We suggest that Mo burial in oxic sediments was likely diminished under conditions of globally reduced oxygen concentrations. This allows us to model the Mo isotope composition of late Cretaceous pre-OAE2 seawater without lowering of the seawater Mo inventory (e.g., -55% oxic sink, + 40% anoxic sink). Previous estimations of the Mo seawater isotope composition during OAE2 are close to the signal observed pre-OAE here. The Mo isotope variation associated with the onset of OAE2 is therefore small, which is unexpected because strong expansion of anoxic sedimentation in OAE2 is indicated by e.g. sulfur isotopes. A further decrease of Mn burial rates in the open ocean during OAE2 could buffer the Mo seawater isotope signal during OAE2, to account for the small offset. Our findings emphasize that changes to the relative size of not only the anoxic sink but also the oxic sink are important considerations when interpreting paleo-Mo isotope data.

Automated summary

Research has found that the Mo isotope composition in seawater before the late Cretaceous OAE2 was lighter than in the modern ocean, likely due to decreased Mo burial in oxic sediments under globally reduced oxygen concentrations. During OAE2, a further decrease in Mn burial rates in the open ocean could buffer the Mo seawater isotope signal, resulting in a small offset. Changes to the relative size of both the anoxic and oxic sinks are important considerations when interpreting paleo-Mo isotope data.