Could changing ocean circulation have destabilized methane hydrate at the Paleocene/Eocene boundary?

Abstract

[1] During the Paleocene-Eocene Thermal Maximum (PETM, $55 Ma), marine and terrestrial carbon isotope values exhibit a negative shift of at least 2.5%, indicative of massive destabilization of marine methane hydrates, releasing $2000 gigatons of methane carbon. The cause of the hydrate destabilization is unknown but has been speculated to be warming due to a change from high-latitude to low-latitude deepwater formation. Here we present results from a numerical ocean model indicating that a sudden switch of deepwater formation from southern to northern high latitudes caused middepth and deepocean warming of 3°-5°C. The switch is caused by a slow increase in the intensity of the atmospheric hydrologic cycle, as expected under increasing temperatures and consistent with PETM sedimentary evidence. Deepened subtropical subduction prior to the thermohaline circulation switch causes warming of 1°-4°C in limited areas at thermocline through upper intermediate depths, which could destabilize methane hydrates gradually and at progressively greater depths. Warming accompanying a south-to-north switch in deepwater formation would produce sufficient warming to destabilize seafloor gas hydrates over most of the world ocean to a water depth of at least 1900 m.

Kick-starting ancient warming

“…Yet it is difficult to sustain a succession of such events through a positive feedback effect: it takes a millennium or more for the atmospheric warming from each methane pulse to return to deep ocean waters where it can mobilize the hydrates buried beneath the seabed [6][7][8] , far longer than the atmospheric residence time of the original methane. For seabed hydrates to be able to trigger the PETM, the initial catastrophic sediment slides must have been forced to recur much more rapidly, at intervals of about a decade, and this period of forced repetition must have been sustained over perhaps a millennium, until additional methane from mobilized hydrate could maintain the new warm climate 9 (see Supplementary Information).…”

Paleoenvironmental changes across the Cretaceous/Tertiary boundary at Flaxbourne River and Woodside Creek, eastern Marlborough, New Zealand

“…The regional context for Marlborough indicates that relatively minor climatic cooling may have enhanced coastal upwelling of cool southern sourced deep water through intensification of westerly winds and cyclonic circulation (Haq 1981;Huber & Sloan 2001;Bice & Marotzke 2002). It appears that phase 3 represents an interval of c. 450 000 yr in which the climate around New Zealand became progressively cooler.…”

Eos, Transactions, American Geophysical Union Volume 87, Number 17, 25 April 2006

“…During the PETM, reduced pole-to-equator sea surface temperature gradients or changes in continental freshwater runoff may have shifted the site of deep-water formation from a Southern Ocean locus to subtropical latitudes or to high latitudes in the Northern Hemisphere [e.g., Kennett and Stott, 1991;Bice and Marotzke, 2002;Nunes and Norris, 2006], introducing warm water to the deep sea and driving methane clathrate destabilization and further greenhouse warming [Bice and Marotzke, 2002].…”