Excess barite accumulation during the Paleocene-Eocene Thermal Maximum: Massive input of dissolved barium from seafloor gas hydrate reservoirs

A -2% to -3% excursion characterizes carbon isotope records across the Paleocene-Eocene Thermal Maximum (PETM), probably signifying transfer of ∼2000 gigatons of CH4 carbon from seafloor gas hydrate reservoirs to the ocean and atmosphere. During at least 30 k.y. of this carbon cycle perturbation, euhedral barite apparently accumulated in deep-sea sediment faster than before or after. This excess burial of "biogenic barite" has been argued to reflect elevated global primary productivity in surface waters, an interpretation in disagreement with microfossil assemblages. In this paper, we develop a numerical model for the marine Ba cycle to explore possible mechanisms for widespread barite accumulation. This model immediately highlights a fundamental problem with the productivity explanation: Ba has a short residence time in the ocean (∼8000 yr) so that excess Ba output on a global scale over periods >1000 yr signifies excess Ba input. However, a greater Ba input from conventional sources, hydrothermal waters and rivers, seems untenable. Assuming that available Ba records represent increased global output, we suggest an unconventional explanation for widespread barite accumulation. Prior to the PETM, gas hydrate reservoirs stored enormous masses of CH₄ and dissolved Ba²⁺. During the PETM, substantial quantities of gas hydrate converted to free CH₄ gas, increasing pore pressures, and releasing significant amounts of CH4 and dissolved Ba2+ to intermediate waters of the ocean. Consequently, dissolved Ba²⁺ concentrations in the deep ocean rose, a smaller fraction of sinking barite particles dissolved, and "biogenic barite" accumulation increased. The model satisfactorily explains available records but forces us to link components of the marine CH₄ and Ba cycles, an endeavor with few constraints.