Constraining past global tropospheric methane budgets with carbon and hydrogen isotope ratios in ice

Abstract

Upon closer inspection, the classical view of the synchronous relationship between tropospheric methane mixing ratio and Greenland temperature observed in ice samples reveals clearly discernable variations in the magnitude of this response during the Late Pleistocene (!50 kyr BP). During the Holocene this relationship appears to decouple, indicating that other factors have modulated the methane budget in the past 10 kyr BP. The d 13 CH 4 and dD-CH 4 of tropospheric methane recorded in ice samples provide a useful constraint on the palaeomethane budget estimations. Anticipated changes in palaeoenvironmental conditions are recorded as changes in the isotope signals of the methane precursors, which are then translated into past global d 13 CH 4 and dD-CH 4 signatures. We present the first methane budgets for the late glacial period that are constrained by dual stable isotopes. The overall isotope variations indicate that the Younger Dryas ( YD) and Preindustrial Holocene have methane that is 13 C-and 2 H-enriched, relative to Modern. The shift is small for d 13 CH 4 (approx. 1‰) but greater for dD-CH 4 (approx. 9‰). The YD d 13 CH 4 -dD-CH 4 record shows a remarkable relationship between them from 12.15 to 11.52 kyr BP. The corresponding C-and H-isotope mass balances possibly indicate fluctuating emissions of thermogenic gas. This d 13 CH 4 -dD-CH 4 relationship breaks down during the YD-Preboreal transition. In both age cases, catastrophic releases of hydrates with Archaeal isotope signatures can be ruled out. Thermogenic clathrate releases are possible during the YD period, but so are conventional natural gas seepages.

Exploring the influence of ancient and historic megaherbivore extirpations on the global methane budget

“…Although the importance of methane as a greenhouse gas is undisputed (12), net radiative forcing from the 69.6 Tg CH 4 y −1 decrease estimated here is unclear. Methane has fluctuated tremendously over the late Quaternary in response to many factors and not always in concert with temperature; the dynamics of methane are notoriously hard to model (13,43,44). Methane is often cited among triggers or additive factors in climate fluctuation (13,43), but lead-lag relationships are unclear (44), and a general decoupling has been demonstrated within the Northern Hemisphere during the Holocene (44).…”

“…Methane has fluctuated tremendously over the late Quaternary in response to many factors and not always in concert with temperature; the dynamics of methane are notoriously hard to model (13,43,44). Methane is often cited among triggers or additive factors in climate fluctuation (13,43), but lead-lag relationships are unclear (44), and a general decoupling has been demonstrated within the Northern Hemisphere during the Holocene (44). A further complication is that our analyses suggest the decrease in global methane concentration at the YD was unique; comparison with other drops over the past 800 ka demonstrate that it was significantly more abrupt than others over the late Quaternary (SI Appendix, Table S3 and Figs.…”

The Role of Methane Emissions on Ancient and Present Climatic Changes

“…EPA, 2010a andWhiticar &Schaefer, 2007. The emissions from shales are not considered by EPA and other references quoted here, and not evaluated.…”

“…The emissions from shales are not considered by EPA and other references quoted here, and not evaluated. Also the fugitive emissions from industries, transport, aviation and other activities are not here considered explicitly and grouped into the total anthropogenic sources (Whiticar & Schaefer, 2007 From Lassey et al, 2007. (g) Mean value for anthropogenic emissions (Wuebbles & Hayhoe, 2002).…”

Critical issues in trace gas biogeochemistry and global change

“…In particular, the multiple isotope signals (hydrogen (dD-CH 4 ), carbon (d 13 CH 4 ) and radiogenic 14 CH 4 ) of gases in ice may provide clues to identify the processes underlying past changes in the tropospheric CH 4 budget (Schaefer et al 2006;Whiticar & Schaefer 2007).…”