Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene

Abstract

Isotopic records across the “Latest Paleocene Thermal Maximum“ (LPTM) indicate that bottom water temperature increased by more than 4°C during a brief time interval (<104 years) of the latest Paleocene (∼55.6 Ma). There also was a coeval −2 to −3‰ excursion in the δ13C of the ocean/atmosphere inorganic carbon reservoir. Given the large mass of this reservoir, a rapid δ13C shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere. However, a direct consequence of warming bottom water temperature from 11 to 15°C over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic CH4 hydrate at locations with intermediate water depths. In terms of the present‐day oceanic CH4 hydrate reservoir, thermal dissociation of oceanic CH4 hydrate during the LPTM could have released greater than 1.1 to 2.1 × 1018 g of carbon with a δ13C of approximately −60‰. The release and subsequent oxidation of this amount of carbon is sufficient to explain a −2 to −3‰ excursion in δ13C across the LPTM. Fate of CH4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the LPTM.

Reconciliation of marine and terrestrial carbon isotope excursions based on changing atmospheric CO2 levels

“…We used the following mass balance equation modified from McInerney and Wing 11 to quantify DpCO 2 for each of the proposed sources (vertical, coloured lines in Fig. 3: green ¼ methane hydrate 14 , d 13 C source ¼ À 60%; purple ¼ thermogenic methane 54 or permafrost thawing 50 12,27,57). We calculated the value for CIE marine ( À 2.6%) as the average of the CIEs measured in benthic forams ( À 2.5 ± 1.0%, n ¼ 36), planktic forams ( À 2.7±1.0%, n ¼ 36) and bulk marine carbonate ( À 2.7±1.1%, n ¼ 33) as listed in Table 1 of McInerney and Wing 11 .…”

“…The magnitude of the carbon isotope excursion (CIE) and the amount of pCO 2 rise (DpCO 2 ) calculated for such events should relate to one another, and the source of the carbon input and changes in the global carbon cycle have been reconstructed upon this premise (for example, refs 5,9,11,12). Determination of the 'true' magnitude of a CIE (that is, the amount of the CIE caused only by the change in d 13 C CO2 ) is fundamental to calculating the amount of carbon added to the atmosphere at the event and improving our understanding of feedbacks in the climate system 13,14 .…”

Anoxia and high primary production in the Paleogene central Arctic Ocean: First detailed records from Lomonosov Ridge

“…The oldest one represents the PETM event already described in detail elsewhere [Pagani et al, 2006;Sluijs et al, 2006] and probably associated with a massive greenhouse gas input [Dickens et al, 1995]. A second similar event, although of smaller magnitude, occurs at about 368 mcd (early Eocene) and may correlate with the ''Elmo Event'' probably representing another global thermal maximum [Lourens et al, 2005].…”

Archaeal lipids in Mediterranean cold seeps: molecular proxies for anaerobic methane oxidation

“…Recent work has highlighted the potential climatic impact of methane (e.g., Dickens et al, 1995;Henriet and Meinert, 1998;Petit et al, 1999), prompting considerable interest in the controls on methane production and consumption. In marine sediments, anaerobic methane oxidation is the dominant pathway for methane consumption (Blair and Aller, 1995;Borowski et al, 1996;Burns, 1998;Iverson and Jørgensen, 1985;Reeburgh, 1980), and consequently, the flux of methane to the atmosphere from marine sediments is small compared to other sources (Reeburgh, 1996).…”