Modulation of Gas Hydrate Stability by Bottom Water Temperature The Santa Barbara Basin

Greenland ice-core data has revealed that rapid increase in atmospheric methane coincided with an abrupt warming event at the end of Younger Dryas glacial episode (at ca. 11.6 k.y. B.P.) and occurred over relatively short interval of a few decades (Severinghaus et al., 1998). High-resolution nitrogen- and argon-isotope data from the Boiling transition (from stadial to interstadial) recorded in the Greenland ice also shows that methane concentrations rose over a period of about 50 years, but began increasing 20 to 30 years after the onset of rapid warming (Severinghaus and Brook, 1999). Additional sources of methane have often been attributed to the spread of wetlands during the wet and warm periods (e.g., Blunier et al., 1995).

Kennett et al. (2000) argue that gas-hydrate dissociation is a more likely source of such rapid increases in methane at the onset of interstadials. In a detailed study of the Ô13C record from sediments of the Santa Barbara Basin, off California, these authors, for the first time, implicate temperature increase in the upper intermediate waters (400-1000 m), rather than reduced hydrostatic pressure, as the potential cause for hydrate dissociation. This record displays millennial-scale Dansgaard-Oeschger (D-O) cycles over the past 60 ky that are synchronous with rapid warmings and can be linked to warmings in the ice records from Greenland. These can also be tied to variations in bottom-water ventilation and temperature and changes in benthic foram assemblages. The record also implies that the warming of the surface water and atmosphere lagged behind the warming of intermediate waters.

Kennett et al. (2000) maintain that the energy needed for these rapid warmings could have come from dissociation of methane hydrates. Relatively large excursions of 613C (up to -5%o) in benthic foraminifera are associated with the D-0 events. Though the planktonic-foram 813C record remained relative static, during several brief intervals the planktonics also show notable negative shifts in values (up to -3%o), implying that the entire water column may have experienced rapid 612C enrichment during these intervals. They conclude that the large negative excursions in benthic 513C during interstadials most likely represent injections of biogenic methane in the system from clathrate sources and are consistent with poor basin ventilation, low oxygen levels, low faunal diversity and occurrence of lamination in these intervals.

Figure 2 summarizes the Kennett et al. (2000) model of changes in the methane flux controlled by gas-hydrate instability that is modulated by increase in bottom-water temperature in the Santa Barbara Basin. They argue that temperature increases in the bottom water of 2 - 3.5° C from stadials to interstadials are large enough to cause the dissociation and deroofing of gas hydrates, leading to increased upward flux of methane. Hydrate dissociation probably occurred close to the beginning of interstadials, when bottom water had warmed up but the sea level was still relatively low. Simultaneous large negative shifts of 813C in both the benthics and planktonics probably represent brief but massive and localized injections of methane from hydrate sources. Kennett et al. (2000) identify at least four such episodes of massive methane emissions when the entire water column suffered prominent negative 613C shifts.

The Kennett et al. (2000) data suggests a time lag between the warming of bottom waters and warming of the sea surface and the atmosphere. Thus, this model would imply that hydrate dissociation, which would be a consequence of bottom-water warming, could lead to catastrophic injections of methane into the atmosphere and accelerated greenhouse warming. Other data (e.g., Severinghaus et al., 1998; Severinghaus and Brook, 1999) suggest that warming occurs a few decades before the peak in atmospheric methane values, implying that warming causes methane increase and may not be a consequence of it. Thus, some authors (e.g., Raynaud et al., 1998; Blunier, 2000; Brook et al., 2000) are not entirely convinced of an active role for hydrate methane in stimulating rapid climate change.

Figure 2. The Kennett et al. model of the influence of changes in bottom-water temperature on methane flux in Santa Barbara Basin. Warmer intermediate waters that can destabilize the hydrate and increase the methane flux to the ocean water and atmosphere (A) characterize the interstadials. During these intervals the sulfate reduction zone (S.R.Z.) is narrow (B). During the stadials (C) the intermediate waters are cooler and the hydrate is stable, with low upward methane flux. The oxygen content of the water increases while S.R.Z. expands (D). (After Kennett et al., 2000, reproduced by permission).

Figure 2. The Kennett et al. model of the influence of changes in bottom-water temperature on methane flux in Santa Barbara Basin. Warmer intermediate waters that can destabilize the hydrate and increase the methane flux to the ocean water and atmosphere (A) characterize the interstadials. During these intervals the sulfate reduction zone (S.R.Z.) is narrow (B). During the stadials (C) the intermediate waters are cooler and the hydrate is stable, with low upward methane flux. The oxygen content of the water increases while S.R.Z. expands (D). (After Kennett et al., 2000, reproduced by permission).

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