Barium isotopes constrain the triggering mechanism of the Cretaceous OAE 2 in the Neotethys Ocean

The Cenomanian–Turonian boundary oceanic anoxic event (known as OAE 2, occurring 94.5–93.9 million years ago) provides an opportunity to clarify climatic forcing on marine environmental perturbations. OAE 2 has been extensively studied regarding its triggering mechanism and rates of marine deoxygenation in the proto-North Atlantic, Western Interior Seaway, Pacific, and European pelagic shelf. However, the detailed timing of the onset of ocean deoxygenation and the triggering mechanism behind the organic carbon burial leading to OAE 2 in the Neotethys Ocean remains less well-constrained. Here, we fill this gap by presenting high-resolution barium isotope (δ¹³⁸Ba) data from the highly expanded Tibet OAE 2 section spanning the Cenomanian–Turonian boundary. We observed a large negative δ¹³⁸Ba excursion that correlates with the positive δ¹³C shift. The onset of the negative δ¹³⁸Ba excursion precedes that of δ¹³C by an estimated 400 kyr, indicating that ocean deoxygenation began 400 kyr before OAE 2 in the Neotethys Ocean. The δ¹³⁸Ba values of the studied carbonates are significantly lower than those of surface seawater observed in the modern Atlantic and Pacific oceans, suggesting that export productivity levels in the pre-OAE 2 Neotethys Ocean were substantially lower than those in modern oceans. We thus provide new evidence that the burial of organic carbon during OAE 2 led to the observed positive δ¹³C excursion driven by extensive shallow-water anoxia, even when considering contributions from changes in marine primary productivity.