| Lithologic, faunal, seismic, and isotopic evidence from the Blake Nose (subtropical western North Atlantic) links a massive release of biogenic methane from marine gas hydrate reservoirs at ∼55.5 Ma to a dramatic warming of deep ocean and high-latitude surface waters, the largest perturbation in the combined ocean-atmosphere carbon cycle of the past 90 m.y., a mass extinction event in benthic faunas, and a radiation of mammalian orders. In order to evaluate the nature of the isotopic response during the latest Paleocene thermal maximum (LPTM) and carbon isotope excursion (CIE), I establish interspecies correction factors for benthic foraminifera often used to generate Paleocene-Eocene isotope records. I identify LPTM methane release sites along the eastern U.S. continental margin. These sites overlie a Mesozoic reef front that may have created gas overpressure zones, facilitating methane release.; I explore the feasibility of (and evidence for) two mechanisms for methane release, thermal dissociation and mechanical disruption. Current models of LPTM thermal dissociation rely on a change in deepwater source region(s) from high-latitude cool deepwaters to low-latitude, warm saline deepwaters (WSDW) to increase deepwater temperatures rapidly enough to trigger the massive thermal dissociation of gas hydrates. The heat flow model developed here places narrow constraints on the feasibility of thermal dissociation; isotopic records do not substantiate the necessary time lag between deepwater warming and methane release predicted by this model. Furthermore, δ13C comparisons and ocean circulation models do not support the requisite change to WSDW at this time.; In light of inconclusive evidence for the thermal dissociation of gas hydrates in the latest Paleocene, I evaluate erosion and mechanical disruption of sediments as an alternate causal triggering mechanism for the LPTM methane release. Previous studies document that Antarctic-source deepwater circulated around the western North Atlantic in the late Paleocene. This eroded the base of the U.S. continental slope, causing margin oversteepening, widespread slope failure, and the landward retreat of the slope. I propose that continental slope failure breached gas hydrate layers and opened conduits to free- and dissolved methane gas reservoirs below in the latest Paleocene, releasing methane and precipitating the LPTM. |
