CAUSES AND EFFECTS OF THE APTIAN (~117 MA) METHANE RELEASE

In 2001, we reported a negative excursion in early-Aptian atmospheric d¹³CO₂ (D=-3.6 to -6.5 ‰), based on d¹³C 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 d¹³C of terrestrial organic matter (n=153) and land-plant isolates (n=33) of D=-2.3 and -2.9 ‰, respectively. The shift was observed within palynological Zone I, which is temporally well-correlated with our previous work. Using an empirical relationship between d¹³C_plant and d¹³CO₂, we calculated Dd¹³CO₂=-2.1 to -2.6 ‰ during the early Aptian from the Arundel Clay shift. Given the probable composition of the early Cretaceous atmosphere, mass balance calculations favor a methane hydrate release as the cause of this excursion. In consideration of a mechanism for methane release, we calculated changes in global subduction indicated by the well-established and rapid 2-fold increase in seafloor production that was unique within the early Aptian compared to the last 144 million years. We show that increased frictional interaction between overriding and subducting plates caused uplift and compression sufficient to continuously destabilize a portion of the probable methane hydrate reservoir, thus creating a perturbation in the C-isotope record of the Aptian atmosphere, as reflected in the d¹³C of terrestrial photosynthesizers. The Aptian methane release is a new example of mechanistic coupling between major tectonic events and the global biosphere.