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The Paleocene/Eocene Thermal Maximum (PETM): Triggering
mechanisms and climate feedback loops of methane release from gashydrates
By Eric Petermann
At the Paleocene/Eocene boundary (55 Ma BP) a general global warming trend was overruled by an outstanding 4-6 °C (Dickens et al. 1997; Zachos et al. 2001) short-term warming event. This information is derived from a -2 to -3δ negative δ18 O excursion accompanied by an -2.5 δ 13C excursion (Fig.1). Both isotope excursions occur simultaneously and reach their minimal values very rapidly within less than 10 ka. They return to initial values after ~200 ka. PETM climate conditions resulted in extinction of archaic mammals while modern mammalian ancestors have appeared as well as in extinction or temporal disappearance of many deep-sea species (Dickens et al. 1997).
δ18 O values of benthic foraminifera of all oceans and planktic foraminifera at high-latitude locations are marked by a sharp decrease (Fig.1) indicating a striking global temperature increase of the deep-sea and high-latitude surface water temperatures (Dickens et al. 1997). A simultaneous negative 13C excursion is documented in marine as well as in terrestrial environments all over the world. The carbon isotope excursion is attributed to the release of massive quantities of biogenic methane (13C = -60δ) adding large amounts of 12C to the inorganic carbon reservoir. Mass balance calculations suggest a transfer of 1400 to 2800 Gt (Dickens et al.1997) respectively 1500 to 2200 Gt (Katz et al. 2001) of CH4 to the ocean/atmosphere system to explain the negative carbon isotope excursion. By using borehole and seismic data potential methane release sites have been indentified at the western margin of the North Atlantic Basin (Katz et al. 2001). Most of the released methane was quickly oxidized to CO2. Dickens et al. (1997) calculated a resulting rise in atmospheric pCO2 by 70-160 ppm; subsequently strengthening the greenhouse effect.
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Fig.1: Oxygen and Carbon isotope values based on surface and thermocline dwelling as well as benthic foraminifera at the Paleocene/Eocene boundary from ODP Site 690 (southern Atlantic). (Thomas et al. 2002)
However, this rise in atmospheric pCO2 is insufficient to explain a warming of the whole ocean by 4-5°C. Therefore, it requires additional amplification due to climate feedback loops. Two major hypotheses for triggering the methane release exist: thermal dissociation and mechanical disruption. The thermal dissociation hypothesis is based on a rapid water temperature increase and the subsequent widespread dissociation of gashydrates. Instead of warming of the entire water body a pronounced water warming can be realized by a change in deep water source region. The heat propagates into the sediment, melts the gas hydrates and releases free methane gas bubbles. This process increases the sediment pore pressure, resulting in a destabilization of the sediment column and finally slope failure. The consequence of slope failure is the release of massive quantities of methane from the gas reservoir trapped below (Katz et al. 2001). A constraint on the thermal dissociation hypothesis as primary trigger provides a heat flow model by Katz et al. (2001). The model shows that the minimum time required for the heat propagating into the sediment and melting the gashydrates is calculated with 2-4 ka. Therefore, a rapid rise in water temperatures should precede the methane release, or in terms of isotope values, the δ18O decrease should precede the δ13C decrease. Though, the isotope values do not show any delay. However, a low sedimentation rate makes it difficult to extract a high-temporal resolution from the sediment cores.
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