Glacial/interglacial wetland, biomass burning, and geologic methane emissions constrained by dual stable isotopic CH ₄ ice core records

Abstract

Atmospheric methane (CH 4 ) records reconstructed from polar ice cores represent an integrated view on processes predominantly taking place in the terrestrial biogeosphere. Here, we present dual stable isotopic methane records [δ 13 CH 4 and δD(CH 4 )] from four Antarctic ice cores, which provide improved constraints on past changes in natural methane sources. Our isotope data show that tropical wetlands and seasonally inundated floodplains are most likely the controlling sources of atmospheric methane variations for the current and two older interglacials and their preceding glacial maxima. The changes in these sources are steered by variations in temperature, precipitation, and the water table as modulated by insolation, (local) sea level, and monsoon intensity. Based on our δD(CH 4 ) constraint, it seems that geologic emissions of methane may play a steady but only minor role in atmospheric CH 4 changes and that the glacial budget is not dominated by these sources. Superimposed on the glacial/interglacial variations is a marked difference in both isotope records, with systematically higher values during the last 25,000 y compared with older time periods. This shift cannot be explained by climatic changes. Rather, our isotopic methane budget points to a marked increase in fire activity, possibly caused by biome changes and accumulation of fuel related to the late Pleistocene megafauna extinction, which took place in the course of the last glacial.atmosphere | methane | megafauna | ice core | stable isotopes

Bayesian Analysis of the Glacial‐Interglacial Methane Increase Constrained by Stable Isotopes and Earth System Modeling

“…We used a probabilistic approach to show that both biomass burning and geologic sources of methane may have played a role in setting the observed enrichment of atmospheric CH 4 (Bock et al, ). The latter is supported by the proposed sea level control on marine seepage of CH 4 (Etiope et al, ; Luyendyk et al, ).…”

Interlaboratory comparison of <i>δ</i><sup>13</sup>C and <i>δ</i>D measurements of atmospheric CH<sub>4</sub> for combined use of data sets from different laboratories

“…The University of Bern (UB) makes δ 13 C-CH 4 measurements from ice cores using a GC-IRMS system with an overall reproducibility of 0.15 ‰ Bock et al, 2017). The UB measurements are referenced to a whole-air working standard with a CH 4 mole fraction of 1508.2 ppb and an assigned δ 13 C-CH 4 value of −47.34 ± 0.02 ‰ (named "Boulder, CA08289" in Schmitt et al, 2014).…”

“…3.10). UB also measures δD-CH 4 for ice core samples (Bock et al, 2010(Bock et al, , 2017. The overall measurement precision for ice core sample (including extraction of air from an ice sample) was evaluated to be 2.3 ‰.…”

Bipolar carbon and hydrogen isotope constraints on the Holocene methane budget

“…The stable isotope data (δD-CH 4 and δ 13 C-CH 4 ) presented in this study were measured at the University of Bern on samples from the NGRIP (North Greenland Ice Core Project) ice core from Greenland. Antarctic samples from the EDML (EPICA Dronning Maud Land) and the TALDICE (TALos Dome Ice CorE) ice cores were analysed for δD-CH 4 and δ 13 C-CH 4 , respectively (Bock et al, 2017). Note that for both parameters the samples from Greenland and Antarctica were measured during the same time interval with the same measurement system by the same operators and in randomised order.…”

“…2.4.1). (Cantrell et al, 1990;Tyler et al, 1994;Brenninkmeijer et al, 1995;Irion et al, 1996;Gierczak et al,1997;Quay et al, 1999;Feilberg et al, 2005), the north-south ratio of the individual sink processes (ratio n/s), and the parameters for each hemisphere used in our CH 4 box model inversion. Note that the sizes of the two well-mixed boxes are not equal; the sizes are defined by the global mean annual latitude of the inter-tropical convergence zone (ITCZ) ϕ ITCZ = 5 • N , which acts as a barrier to the hemispheric exchange of air masses.…”