METHANE HYDRATES IN LATE QUATERNARY CLIMATE CHANGE

The methane (CH4) hydrate reservoir has been largely ignored as a component of climate change in the late Quaternary (since ~800 ka) in spite of having experienced the largest known sea level and upper ocean temperature changes of the late Neogene – conditions conducive to instability of marine methane hydrates. Geological evidence is growing for widespread late Quaternary instability of the gas hydrate reservoir and associated episodic transfer of CH4 into the ocean/atmosphere system with potential for greenhouse climatic forcing. The remarkable similarity of atmospheric CH4 and temperature variation recorded in ice cores suggests a critical role of CH4 in Quaternary climate change. Unlike the prevailing interpretation that continental wetlands were the principal source of rapid atmospheric CH4 increases during the late Quaternary, we suggest a marine sedimentary methane hydrate source. Negligible wetland ecosystems existed during the last glacial episode due to 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 could not account for the abrupt atmospheric CH4 increases 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, CH4 emissions resulting from episodic instability of the hydrate reservoir contributed significantly to the distinctive behavior of late Quaternary climate change, including the rapidity, asymmetry, and magnitude of warming events on orbital (Milankovitch) and millennial time scales. CH4 releases to the atmosphere provided a crucial trigger for abrupt warmings, reinforced by other greenhouse gases, especially water vapor. 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. The timing of this activity was not random, but appears to have been focussed at intervals of major climatic warming.