Abstract
We model numerically regions of the Canadian continental shelves during successive glacio-eustatic cycles to illustrate past, current and future marine gas hydrate (GH) stability and instability. These models indicated that the marine GH resource has dynamic features and the formation age and resource volumes depend on the dynamics of the ocean-atmosphere system as it responds to both natural (glacial-interglacial) and anthropogenic (climate change) forcing. Our models focus on the interval beginning three million years ago (i.e., Late Pliocene-Holocene). They continue through the current interglacial and they are projected to its anticipated natural end. During the current interglacial the gas hydrate stability zone (GHSZ) thickness in each region responded uniquely as a function of changes in water depth and sea bottom temperature influenced by ocean currents. In general, the GHSZ in the deeper parts of the Pacific and Atlantic margins (≥1316 m) thinned primarily due to increased water bottom temperatures. The GHSZ is highly variable in the shallower settings on the same margins (~400–500 m). On the Pacific Margin shallow GH dissociated completely prior to nine thousand years ago but the effects of subsequent sea level rise reestablished a persistent, thin GHSZ. On the Atlantic Margin Scotian Shelf the warm Gulf Stream caused GHSZ to disappear completely, whereas in shallow water depths offshore Labrador the combination of the cool Labrador Current and sea level rise increased the GHSZ. If future ocean bottom temperatures remain constant, these general characteristics will persist until the current interglacial ends. If the sea bottom warms, possibly in response to global climate change, there could be a significant reduction to complete loss of GH stability, especially on the shallow parts of the continental shelf. The interglacial GH thinning rates constrain rates at which carbon can be transferred between the GH reservoir and the atmosphere-ocean system. Marine GH can destabilize much more quickly than sub-permafrost terrestrial GHs and this combined with the immense marine GH reservoir suggests that GH have the potential to affect the climate-ocean system. Our models show that GH stability reacts quickly to water column pressure effects but slowly to sea bottom temperature changes. Therefore it is likely that marine GH destabilization was rapid and progressive in response to the pressure effects of glacial eustatic sea level fall. This suggests against a catastrophic GH auto-cyclic control on glacial-interglacial climate intervals. It is computationally possible but, unfortunately in no way verifiably, to analyze the interactions and impacts that marine GHs had prior to the current interglacial because of uncertainties in temperature and pressure history constraints. Thus we have the capability, but no confidence that we can contribute currently to questions regarding the relationships among climate, glacio-eustatic sea level fluctuations and marine GH stability without improved local temperature and water column histories. We infer that the possibility for a GH control on climate or oceanic cycles is speculative, but qualitatively contrary to our model results.
Highlights
Marine Gas Hydrates as a Possible Mechanism of Climate and Oceanic ChangeMarine methane gas hydrate (GH) dissociation (Figure 1) on continental shelves due to glacio-eustatic reduction of water column pressure is proposed as an auto-cyclic mechanism controlling climate during glacial-interglacial intervals
The model result suggests that a relict GH layer from the end of the last glacial persists at a depth below the sea flow approximately twice that expected for newly formed gas hydrates
Our previous GH stability simulations in Beaufort Mackenzie region and the Canadian Arctic Archipelago [6], began in the Tertiary, but the results showed that the consideration of a long history (14.0 to 1.0 million years (Myrs) ago) had an insignificant effect on the Holocene results, such that the uncertainties attending possible alternative early temperature histories did not result in discernable variations in the current depth to the base of GHs, which is the only observable test of model validity
Summary
Marine Gas Hydrates as a Possible Mechanism of Climate and Oceanic Change. Marine methane gas hydrate (GH) dissociation (Figure 1) on continental shelves due to glacio-eustatic reduction of water column pressure is proposed as an auto-cyclic mechanism controlling climate during glacial-interglacial intervals. Some infer that episodic atmospheric methane flux, caused by massive to “catastrophic” marine GH destabilization driven by sea level lowering in response to the sequestering of water in continental ice sheets triggered, or is a significant cause of glacial-interglacial transitions, coined the “clathrate gun” hypothesis [2]. Recent review of the hypothesis on degassing of onshore/offshore permafrost and marine GHs [5] has concluded that “catastrophic”, widespread dissociation of methane GHs will not be triggered by continued climate warming at contemporary rates (0.2 °C per decade) over timescales of a few hundred years. Extensive marine methane oxidation is a potential source oceanic anoxia
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