Two Real World Examples 31 Eel River Basin

Brooks et al. (1991) reported the presence of gas hydrates (>99% CH4) in sediment cores collected near the crest of a large sedimentary ridge in the Eel River basin off the coast of northern California. When this site was revisited with the MBARI ROV Ventana in 1997 and in 1999, no gas hydrates were found in the shallow sediment cores collected but several active gas vents were observed on both occasions. A temperature increase of 0.4-0.5°C was observed for the bottom water between the time when Brooks et al. (1991) collected their samples and our investigations. In order to determine whether this small change in bottom water temperatures was the reason that no gas hydrates were found, despite the presence of abundant free methane gas, we plotted the relevant hydrographic data on the methane hydrate phase diagram (Fig. 7). Given sufficient methane concentrations, hydrates would be stable in the region to the left and below the stability line. The bottom-water temperature observed by

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-0.4 0 0.4 0.8 1.2 1.6 2.0 2.4 Temperature Difference [°C]

Brooks et al. (1991) when plotted at the appropriate temperature clearly falls within this region; whereas the temperature we observed does not. Thus, for a

Figure 7. P-T stability curve for methane hydrate in seawater from equation 7 (—) and selected prediction using Multiflash (+). Bottom water temperatures from by Brooks et al. (1991) (x) and the authors (o). The in situ temperature profile nearby the dive site (—•) and offset 1 °C (•••).

first cut at an explanation, it would appear that the simple thermodynamic stability of methane hydrates explains why no gas hydrates were collected. It is not possible to say at this early stage whether the warming we observed at this site since Brooks et al. (1991) collected their samples is due to global warming or whether a more localized explanation is possible. Even so, if we shift the observed temperature profile 1°C, the potential impact of even a modest amount of warming on the stability of gas hydrate deposits is evident. At present, the temperature profile crosses the stability curve for methane hydrate at 5.41 MPa, equivalent to 526 m depth - ten meters into the sediments near the crest of the ridge. When shifted, the temperature profile crosses the stability curve at 5.86 MPa, equivalent to 571 m. This is an increase of 45 m. If such a shift in seawater temperature were to occur instantaneously, there would be a lag time before the thermal effect penetrated deep into the sediments. However, as it does, any existing gas hydrates within this zone would eventually decompose, releasing methane gas and water. Whether this is sufficient to destabilize the sediments and cause seafloor collapse remains to be seen.

Figure 7. P-T stability curve for methane hydrate in seawater from equation 7 (—) and selected prediction using Multiflash (+). Bottom water temperatures from by Brooks et al. (1991) (x) and the authors (o). The in situ temperature profile nearby the dive site (—•) and offset 1 °C (•••).

Temperature [°C]

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