Earth Pages News has commented several times on developments in the connection between ocean currents and climate, over the last 3 years. The subject has many aspects, and these have been bundled and brought up to date in one of a series of review articles on the relationship between climate and the hydrological cycle in Nature’s occasional Insight series (Rahmstorf, S. 2002. Ocean circulation and climate during the last 120,000 years. Nature, v. 419, p. 207-214). Stefan Rahmsdorf covers the evidence to date that implicates changes in deep circulation in rapid and dramatic climate shifts, such as changed air temperatures over the Greenland ice cap and iceberg armadas in the North Atlantic. Another review outlines the longer-term perspective of links between atmosphere, oceans, ice sheets, solid-Earth processes and astronomical forcing in shifts of climate and sea level over the last 3 Ma. Central to this linked system is the transfer of tens of millions of cubic kilometres of water from tropics to poles, and from ice sheets to sea levels (Lambeck, K. et al. 2002. Links between climate and sea levels for the past three million years. Nature, v. 419, p. 199-206).
Alaskan source proposed for end-Palaeocene warming
Between 58 and 52 Ma, around the Palaeocene-Eocene boundary, Earth’s climate bucked the long-term cooling trend during the Cenozoic, by warming considerably. Since the warming lasted for so long, it seems likely to have been caused by an enhanced atmospheric “greenhouse” gases rather than by either astronomical or oceanic causes. Carbon isotope data around the P-E boundary can be interpreted in terms of massive releases of biogenic methane, perhaps from gas hydrates on the sea floor. However, such releases are likely to have been sudden, and a more continual release of “greenhouse” gases fits the record better; but that begs the questions where and how? Catastrophic methane release has been invoked for the dramatic rise in deep-ocean and high-latitude temperatures within 10 thousand years exactly at the P-E boundary.
Lengthy climatic warming can stem from increased volcanism and sea-floor spreading, but there is scanty evidence for either during this period. Another possibility is production of gases as a result of tectonic activity, either by involvement of carbonate sediments in metamorphism, which releases CO2, or “stewing” organic matter in thick sedimentary sequences. Candidates for the last are the thick accretionary prisms at Pacific destructive margins, an especially appropriate example being that of the Gulf of Alaska which grew rapidly during this period (Hudson, T.I. & Magoon, I.B. 2002. Tectonic controls on greenhouse gas flux to the Paleogene atmosphere from the Gulf of Alaska accretionary prism. Geology, v. 30, p. 547-550). Oceanic and continental margin sediments scraped off descending oceanic lithosphere contain buried organic matter. Increased heat flow, perhaps associated with rising magmas, can cause organic debris to break down to hydrocarbons. Over-maturation results in the formation of methane, potentially in vast volumes, that can leak continually to the atmosphere. Methane rapidly oxidizes to CO2, decreasing the warming effect, but able to linger for considerable periods. Hudson and Magoo calculate such enormous releases, that even disputes over the amount of accreted sediment in the Gulf of Alaska do little to rule out its being a major source for climatically implicated gases. This first suggestion of a role for accretionary prisms in climate change may spur studies of such processes elsewhere, in an attempt to remove much of the load from the BLAG hypothesis that involves metamorphic release of CO2 in a difficult to verify process of lithospheric flatus.
See also: Clift, P. & Bice, K. 2002. Baked Alaska. Science, v. 419, p.129-130