Methane hydrate, an ice-like combination of methane and water found along the worlds continental margins, occurs in quantities that rival all known conventional hydrocarbon reservoirs (Kvenvolden and Lorenson 2001), and may be the most promising source of natural hydrocarbons for many countries including Chile. Methane hydrates may also be important in global climate. Recently, the “clathrate gun” hypothesis of climate change has identified methane hydrates as potentially playing a significant role in the rapid warming trends of the last 1 Ma (Kennet et al. 2002).

Of particular interest to the dynamics of the methane hydrate reservoir is the distribution of methane hydrate and gas at the very edge of the hydrate stability zone (HSZ); the point at the seafloor where pressure is just high enough, and temperature just low enough form or dissociate methane hydrate. Not only is the methane hydrate here the most susceptible to dissociation either by thermal or pressure perturbation, but it also may be released to the ocean without being impeded by overlying sediment. The HSZ edge along a section of the Chilean margin about 60 km northwest of Concepcion was drilled in 2002 during ocean drilling program (ODP) leg 202 (sites 1234; 1015m water depth, and 1235; 489m water depth, [preliminary report, 2002]). The drilling objectives were methane hydrates but the two fairly closely spaced sites serendipitously bracket the seafloor intersection of the HSZ. This area was later surveyed (spring 2003) with DTAGS (deep-towed reflection seismic) and piston cores. A weak bottom simulating reflector (BSR) can be seen in the seismic data, suggesting at least some gas accumulation at the base of the HSZ. However, the BSR appears to fade out well seaward of the theoretical intersection (based on temperatures and pressures recovered from CTD casts), where the HSZ is a few tens of meters thick.

The absence of a BSR intersecting the seafloor suggests there is not sufficient methane in the pore water to exceed saturation. This is consistent with the sulfate gradient obtained at ODP Site 1234, which puts the sulfate methane interface (SMI) at about 20m below seafloor. Methane is simply being consumed at this depth. The implications are 1) it may be possible to determine the methane flux near the HSZ edge remotely using an observed depth where the BSR vanishes, 2) methane released from by bottom water temperature increase or sealevel fall will be below the SMI, potentially affecting its transport to the water column, and its affect on sediment strength.

Kennet, J. P., K. G. Cannariato, I. L. Hendy, R. J. Behl, Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis, AGU, Washington, D. C., 2002.

Kvenvolden, K. A. and T. D. Lorenson, The Global occurance of natural gas hydrate, in Natural Gas Hydrates: Occurrence, Distribution, and Detection, C. K. Paull and W. P. Dillon, Eds., Geoph. Monogr. Ser. 124, 3-8, AGU, Washington, D. C., 2001.

Ocean Drilling Program Leg 202 preliminary report, on line publication at: http://odp.pangaea.de/publications/prelim/202_prel/202toc.html, 2002