Causes and Effects of the early Aptian ( ∼117 Ma) Methane Release

In 2001, we reported a negative excursion in early-Aptian atmospheric δ ¹³CO₂ (Δ = -3.6 to -6.5 ‰ ), based on δ ¹³C analyses of organic matter and land-plant isolates from coarsely-sampled Colombian estuarine and near-shore sediments. Here we present similar results for an Aptian section of the Arundel Clay (Potomac Group, central Maryland), which is well-known for its exceptional preservation of terrestrial plant materials. Sampling across 13 meters of sediment at ∼10-cm intervals revealed a clear shift in the δ ¹³C of terrestrial organic matter (n=153) and land-plant isolates (n=33) of Δ = -2.3 and -2.9 ‰ , respectively. The shift was observed within palynological Zone I, which is temporally well-correlated with our previous work. Given the probable composition of the early Cretaceous atmosphere, a methane hydrate release is the likely cause of this excursion; isotopic mass balance of our record in conjunction with the δ ¹³C_carbonate record of Menegatti et al., 1998 suggest a total methane hydrate C release = ∼ 1,100 Gt ( ∼10% of the modern reservoir) over a period of approximately 500 kyr. In consideration of a mechanism for early Aptian methane release, we calculated changes in global subduction rates during the Early Cretaceous from the classic high-resolution plate reconstructions established by Engebretson,1985. These reconstructions revealed a dramatic decrease in the motion of the Farallon plate toward the subduction zones of the North Pacific basin during the early Aptian, caused by a massive increase in frictional interaction (i.e., seismic coupling) between overriding and subducting plates stretching from northeast Asia, to Alaska, to British Columbia. Associated forces caused uplift and compression in continental margins sufficient to continuously destabilize a portion of the probable methane hydrate reservoir (evidence of this compression is also observed in the geologic record [Vaughan et al. 1995]). The methane hydrate release created a perturbation in early Aptian terrestrial and marine C reservoirs, as reflected in the δ ¹³C of terrestrial and marine organisms. We speculate further as to the influence of this methane release on the Early Cretaceous atmosphere as well as early Aptian OAE events. Marine oxidation of the CH₄ implied by the marine carbonate δ ¹³C record would consume = ∼ 5,500 Gt of O₂, enough to remove at least 60% of the O₂ dissolved in the below-surface ocean, perhaps leading to anoxic conditions at depth. Furthermore, terrestrial oxidation of released CH₄ to CO₂ would increase the atmospheric ratio of CO₂:O₂ from 0.007 to 0.008, perhaps facilitating the geographic expansion of the angiosperms (which have been hypothesized to out-compete gymnosperms near values of 0.01 [Kienast, 1991]). Over and above these speculations, the Aptian methane release is a new example of mechanistic coupling between major tectonic events and the global biosphere, as recorded in the stable isotope record.