Thibault de Garidel-Thoron, and three co-authors, reported in 2004 that past atmospheric methane-concentration oscillations recorded in polar ice cores vary together with rapid global climatic changes during the last glacial episode. In the “clathrate gun hypothesis,” massive releases of deep-sea methane from marine gas-hydrate dissociation led to these well known, global, abrupt warmings in the past. If evidence for such releases in the water column exists, however, the mechanism and eventual transfer to the atmosphere has not yet been documented clearly.
In this paper the authors describe a high-resolution marine-sediment record of stable carbon isotopic changes from the Papua Gulf, off Papua New Guinea, which exhibits two extremely depleted excursions (down to -9‰) at ≈39,000 and ≈55,000 years. Morphological, isotopic, and trace metal evidence dismisses authigenic calcite as the main source of depleted carbon. Massive methane release associated with deep-sea gas-hydrate dissociation is the most likely cause for such large depletions of δ13C. The absence of a δ13C gradient in the water column during these events implies that the methane rose through the entire water column, reaching the sea–air interface and thus the atmosphere. Foraminiferal δ18O composition suggests that the rise of the methane in the water column created an upwelling flow. These inferred emission events suggest that during the last glacial episode, this process was likely widespread, including tropical regions. Thus, the release of methane from the ocean floor into the atmosphere cannot be dismissed as a strong positive feedback in climate dynamics processes.
During the last glacial episode, rapid climatic changes recorded in ice-core records are closely associated with variations in concentration of atmospheric methane, a powerful greenhouse gas, showing a maximum amplitude of ≈200 parts per billion by volume. Until recently, the main source of atmospheric methane during the last glacial episode was attributed to low-latitude continental wetlands emissions, whereas the potential role of deep-sea and lake-gas hydrates was largely ignored. The accepted hypothesis of a strong link between climate and methane emissions from wetlands was called into question recently by an exhaustive review of the timing and extension in the geological record of these wetlands (4), as well as by numerical simulations of the global carbon cycle. Finally, two independent geochemical proxies, the isotopic composition of foraminifera and the molecular composition of the organic matter, shed light on the potential role of large methane releases from gas-hydrate dissociation as an important source in the oceanic global carbon cycle.
These two records from the Santa Barbara Basin have shown that these inferred methane releases occurred during the last glacial episode in response to a warming of the intermediate waters and thus presumably of the deep-sea sediments. These deep-sea methane emissions occurred synchronously with rapid climate warmings associated with atmospheric methane increases and led Kennett et al. to propose the “clathrate gun hypothesis,” which postulates that deep-sea methane hydrates played a significant role in late Quaternary climate changes. Thus far, evidence for such methane releases in the glacial world is limited to a few regions, and the mechanism by which the methane passed through the water column, and eventually to the atmosphere, has not yet been demonstrated. The authors documented evidence that two massive methane releases from clathrate dissociation, of amplitude two to three times larger than those inferred in the Santa Barbara Basin, occurred during the last glacial episode in the western Pacific.
Original working URL: https://www.pnas.org/content/101/25/9187#ref-4
See recent paper on the subject: https://www.i2mconsultants.com/downloads/corp/geosciences-09-00443.pdf
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