Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis
James P. Kennett, Dept. of Geol. Sciences and Marine Science Institute, University of California Santa Barbara
The methane (CH4) hydrate reservoir has largely been ignored as a component of climate change in the Quaternary. However, the late Quaternary experienced the largest known sea level and upper ocean temperature changes for much of the Cenozoic, conditions conducive to instability of the methane hydrate reservoir (clathrates and free gas). Geological evidence grows for widespread late Quaternary instability of the gas hydrate reservoir and associated episodic transfer of CH4 into the ocean/atmosphere system with potential resulting greenhouse climatic forcing. The remarkable similarity of atmospheric CH4 and temperature variation recorded in ice cores suggests a key role of CH4 in Quaternary climate change.
Unlike the prevailing interpretation for continental wetlands as the primary source for rapid atmospheric CH4 increases during the late Quaternary, we suggest a methane hydrate reservoir source. Geologic evidence indicates limited wetland production during the last glacial episode; the result of global aridity, low sea level, incised, well-flushed river systems and low water tables. Wetland ecosystems were insufficiently developed during the last glacial episode and are unlikely to account for the abrupt rises in atmospheric CH4 during glacial and stadial terminations. Instead, geologic evidence strongly suggests that the large modern wetland ecosystems developed almost exclusively during the Holocene.
According to the Clathrate Gun Hypothesis, episodic atmospheric CH4 emissions resulting from instability of the hydrate reservoir contributed significantly to the distinctive behavior of late Quaternary climate change on orbital (Milankovitch) and millennial time scales. Resulting CH4 releases to the atmosphere provided a crucial trigger for abrupt warmings, reinforced by other greenhouse gases, including water vapor. A growing body of evidence exists in support of major methane transfer from ocean (and lake) floor through the water column (and hence into the atmosphere) based on an increasing diversity of proxies. These inferred emissions often occurred during major climate transitions on orbital and millennial time scales.
Late Quaternary hydrate instability seems to have resulted from frequent, rapid upper intermediate water temperature oscillations on upper continental margins in the depth zone of potential hydrate instability. This instability is reflected by widespread development of slumps, debris flows, and pockmarks and associated mass sediment transport. Much of this activity was not random, but appears to have been focused at intervals of major climatic warming. Evidence for instability of the methane hydrate system during the late Quaternary implies concern for modern climate responses to global warming.