Hidden Shallow-Shelf Deoxygenation Preceding OAE 2 Revealed by Mo-Lybdenum Isotope Records

Cretaceous Oceanic Anoxic Event 2 (OAE 2) represents a global episode of marine deoxygenation. Recent geochemical data, e.g., Tl and Ba isotopes, indicate that the initial sea-water deoxygenation preceded the OAE 2 positive carbon isotope excursion (CIE). Unravelling the processes and mechanisms of this initial deoxygenation is crucial for understanding ocean redox dynamics related to global warming. In this paper, we present iron speciation and molybdenum isotope data of shelf deposits from the Qiangdong section in southern Tibet to trace redox changes across OAE 2. Iron speciation data reveal that the depositional site remained overall oxic to oscillatory anoxic conditions. Free hydrogen sulfide was absent in the water column, suggesting that Mo-isotope signals in our record were governed primarily by changes in the burial flux of iron manganese oxides rather than sulfide scavenging. A positive shift of ~1.3‰ in δ98Mo values occurred, with its onset about 107 kyr prior to the OAE2 CIE. This δ98Mo change resulted from a ~80% decline in the burial of iron–manganese oxides, indicating a substantial deoxygenation. This deoxygenation interval was contemporaneous with regional methane seepages. Based on Earth system model simulations, enhanced methane dissociation (3 to 5 times present day levels) in shallow marine environments could have led to a notable decline in dissolved oxygen content—an effect comparable in magnitude to oxygen saturation decline driven by seawater warming. As such, we suggest that methane release served as an important driver of deoxygenation prior to OAE 2.