Lars Möller

Hydrogen Isotopes Preclude Marine Hydrate CH4 Emissions at the Onset of Dansgaard-Oeschger Events

Glacial Gas Intracellular bacterial pathogens, such as a series of sudden and large warming episodes, called Dansgaard-Oeschger events, interrupted the cold conditions of the last glacial period. Large increases in the concentration of atmospheric methane accompanied the events, whose causes have remained the object of much speculation. Bock et al. (p. 1686) report measurements of the hydrogen isotopic composition of methane recovered in the North Greenland Ice Core Project. The excess atmospheric methane accompanying two Dansgaard-Oeschger events did not come from marine clathrates; instead, the methane probably came from increased fluxes from boreal wetlands, another major source of methane. Catastrophic destabilization of marine methane clathrates did not trigger rapid warming episodes 39,000 and 35,000 years ago. The causes of past changes in the global methane cycle and especially the role of marine methane hydrate (clathrate) destabilization events are a matter of debate. Here we present evidence from the North Greenland Ice Core Project ice core based on the hydrogen isotopic composition of methane [δD(CH4)] that clathrates did not cause atmospheric methane concentration to rise at the onset of Dansgaard-Oeschger (DO) events 7 and 8. Box modeling supports boreal wetland emissions as the most likely explanation for the interstadial increase. Moreover, our data show that δD(CH4) dropped 500 years before the onset of DO 8, with CH4 concentration rising only slightly. This can be explained by an early climate response of boreal wetlands, which carry the strongly depleted isotopic signature of high-latitude precipitation at that time.

A gas chromatography/pyrolysis/isotope ratio mass spectrometry system for high‐precision δD measurements of atmospheric methane extracted from ice cores

Air enclosures in polar ice cores represent the only direct paleoatmospheric archive. Analysis of the entrapped air provides clues to the climate system of the past in decadal to centennial resolution. A wealth of information has been gained from measurements of concentrations of greenhouse gases; however, little is known about their isotopic composition. In particular, stable isotopologues (deltaD and delta(13)C) of methane (CH(4)) record valuable information on its global cycle as the different sources exhibit distinct carbon and hydrogen isotopic composition. However, CH(4) isotope analysis is limited by the large sample size required and the demanding analysis as high precision is required. Here we present a highly automated, high-precision online gas chromatography/pyrolysis/isotope ratio monitoring mass spectrometry (GC/P/irmMS) technique for the analysis of deltaD(CH(4)). It includes gas extraction from ice, preconcentration, gas chromatographic separation and pyrolysis of CH(4) from roughly 500 g of ice with CH(4) concentrations as low as 350 ppbv. Ice samples with approximately 40 mL air and only approximately 1 nmol CH(4) can be measured with a precision of 3.4 per thousand. The precision for 65 mL air samples with recent atmospheric concentration is 1.5 per thousand. The CH(4) concentration can be obtained along with isotope data which is crucial for reporting ice core data on matched time scales and enables us to detect flaws in the measurement procedure. Custom-made script-based processing of MS raw and peak data enhance the systems performance with respect to stability, peak size dependency, hence precision and accuracy and last but not least time requirement.