Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene

TLDR

A rapid −2 to −3‰ shift in the ocean/atmosphere δ13C is difficult to explain given the reservoir’s large mass and conventional hypotheses. The study aims to incorporate the fate of methane from oceanic hydrates into models of the LPTM climate and paleoceanography. Warming bottom waters from 11 to 15°C over ~10,000 years would alter sediment thermal gradients and trigger dissociation of intermediate‑depth oceanic methane hydrates. The LPTM experienced >4°C bottom‑water warming, a −2 to −3‰ δ13C excursion, and the thermal dissociation of methane hydrates released 1.1–2.1×10^18 g of carbon (δ13C ≈ −60‰), sufficient to explain the isotope shift.

Abstract

Isotopic records across the “Latest Paleocene Thermal Maximum“ (LPTM) indicate that bottom water temperature increased by more than 4°C during a brief time interval (<10 4 years) of the latest Paleocene (∼55.6 Ma). There also was a coeval −2 to −3‰ excursion in the δ 13 C of the ocean/atmosphere inorganic carbon reservoir. Given the large mass of this reservoir, a rapid δ 13 C shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere. However, a direct consequence of warming bottom water temperature from 11 to 15°C over 10 4 years would be a significant change in sediment thermal gradients and dissociation of oceanic CH 4 hydrate at locations with intermediate water depths. In terms of the present‐day oceanic CH 4 hydrate reservoir, thermal dissociation of oceanic CH 4 hydrate during the LPTM could have released greater than 1.1 to 2.1 × 10 18 g of carbon with a δ 13 C of approximately −60‰. The release and subsequent oxidation of this amount of carbon is sufficient to explain a −2 to −3‰ excursion in δ 13 C across the LPTM. Fate of CH 4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the LPTM.