Geochemical evidence for gas hydrate in sediment near the Chile Triple Juction

Coring at ODP Sites 859, 860, and 861 near the Chile Triple Junction failed to recover anticipated gas hydrate that was inferred to be present from two lines of geophysical evidence: pre-cruise observation of a weak to strong bottom simulating reflector (BSR) marking the predicted base of the gas-hydrate stability zone, and post-cruise interpretation of the velocity and resistivity logs at Site 859 that suggests the presence of gas hydrate. In contrast to other gas-hydrate occurrences observed during previous DSDP and ODP drilling, this Chile Margin sediment is very low in contents of total organic carbon (TOC < 0.5%) and residual methane (C₁), that are inconsistent with an in-situ source of methane for gas-hydrate formation. However, methane/ethane (C₁/C₂) ratios (>200) and δ¹³ C, values (<-60‰) show little evidence that the methane came from deeper thermogenic sources. Pore-fluid freshening does, however, suggest that gas hydrate is present, disseminated thinly and heterogeneously throughout the stability zone, and occupies less than 25% of the available pore space. The environment of the gas hydrate in sediment near the Chile Triple Junction has unique characteristics relative to known gas-hydrate occurrences elsewhere