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Glacial/Interglacial Wetland, Biomass Burning, and Geologic Methane

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Glacial/Interglacial Wetland, Biomass Burning, and Geologic Methane Glacial/interglacial wetland, biomass burning, and PNAS PLUS geologic methane emissions constrained by dual stable isotopic CH4 ice core records Michael Bocka,b,1, Jochen Schmitta,b, Jonas Becka,b, Barbara Setha,b, Jérôme Chappellazc,d,e,f, and Hubertus Fischera,b,1 aClimate and Environmental Physics, Physics Institute, University of Bern, 3012 Bern, Switzerland; bOeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland; cCNRS, IGE (Institut des Géosciences de l’Environnement), F-38000 Grenoble, France; dUniversity of Grenoble Alpes, IGE, F-38000 Grenoble, France; eIRD (Institut de Recherche pour le Développement), IGE, F-38000 Grenoble, France; and fGrenoble INP (Institut National Polytechnique), IGE, F-38000 Grenoble, France Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved June 1, 2017 (received for review August 20, 2016) 13 2 Atmospheric methane (CH4) records reconstructed from polar ice depleted in both stable isotopologues ( Cand HorD;“light” cores represent an integrated view on processes predominantly tak- isotopic sources) compared with the source mix. However, there are ing place in the terrestrial biogeosphere. Here, we present dual stable two important natural sources relatively enriched in 13CandD 13 isotopic methane records [δ CH4 and δD(CH4)] from four Antarctic ice (“heavy” sources): biomass burning (BB), an important and cli- cores, which provide improved constraints on past changes in natural matically variable process (22–26), and “geologic” emissions of old methane sources. Our isotope data show that tropical wetlands methane (GEMs; called GEM by, for example, refs. 27–29). and seasonally inundated floodplains are most likely the controlling Accordingly, the isotopic fingerprint of methane has been suc- sources of atmospheric methane variations for the current and two cessfully used to shed light on relative source mix changes (30–33). older interglacials and their preceding glacial maxima. The changes in However, biogenic sources (such as wetlands and BB) may change these sources are steered by variations in temperature, precipitation, their isotopic signatures in parallel to changing climate and envi- and the water table as modulated by insolation, (local) sea level, and ronmental conditions (25, 34). For the last glacial, Möller et al. (25) δ 13 13 monsoon intensity. Based on our D(CH4) constraint, it seems that showed that the C content of CH4 (δ CH4)ishigheratlowCO2 geologic emissions of methane may play a steady but only minor role levels and concluded that biome changes play a major role in this in atmospheric CH changes and that the glacial budget is not dom- EARTH, ATMOSPHERIC, 4 isotopic change. The deuterium content of CH4 [δD(CH4)] is also a inated by these sources. Superimposed on the glacial/interglacial function of the isotopic signature of the water used for methano- AND PLANETARY SCIENCES variations is a marked difference in both isotope records, with sys- genesis (31, 35–38). However, the net impact of global meteoric tematically higher values during the last 25,000 y compared with water isotope changes caused by the transfer of water from the ocean older time periods. This shift cannot be explained by climatic changes. to ice sheets is only of secondary importance (refs. 31 and 36 and Rather, our isotopic methane budget points to a marked increase in 13 references therein). Both δ CH4 and δD(CH4) are influenced by at fire activity, possibly caused by biome changes and accumulation of least one other factor, namely the ratio of net to gross CH4 pro- fuel related to the late Pleistocene megafauna extinction, which took duction in wetlands (i.e., the fraction of CH4 consumed by meth- place in the course of the last glacial. anotrophs before emission to the atmosphere). This process varies as a function of the water table and site conditions, leading to changes atmosphere | methane | megafauna | ice core | stable isotopes in strength and isotopic composition of CH4 emissions (31, 39). ast variations of the atmospheric methane mixing ratio Significance P([CH4]) have been documented using ice cores on various – timescales (1 5). Compared with preindustrial levels of 715 parts Polar ice is a unique archive of past atmosphere. Here, we pre- per billion (ppb) (6, 7), anthropogenic emissions have led to a sent methane stable isotope records (used as source fingerprint) 2.5-fold increase since about the year 1800 CE as documented by for the current and two past interglacials and their preceding the National Oceanic and Atmospheric Administration Earth Sys- glacial maxima. Our data are used to constrain global emissions tem Research Laboratory. The lowest values of around 360 ppb are of methane. Tropical wetlands and floodplains seem to be the measured in ice cores during glacial maxima, whereas for the last dominant sources of atmospheric methane changes, steered by eight interglacials, [CH4] typically ranged between 600 and 700 ppb past variations in sea level, monsoon intensity, temperature, and (4). Fast and significant stadial/interstadial [CH4] increases occurred the water table. In contrast, geologic emissions of methane are – within a few decades during glacials in parallel to Dansgaard stable over a wide range of climatic conditions. The long-term Oeschger events (8). Furthermore, [CH4] levels in the Northern shift seen in both isotopes for the last 25,000 y compared with Hemisphere are mostly higher than those in the Southern Hemi- older intervals is likely connected to changes in the terrestrial sphere (9). These data taken together with our understanding of the biosphere and fire regimes as a consequence of megafauna present day methane budget (10–15) show that wetlands and primarily extinction. those located in the tropics dominate natural CH4 emissions. On long timescales, wetland extent (16) and CH4 fluxes are also strongly Author contributions: M.B., J.S., and H.F. designed research; M.B., J.S., and B.S. performed influenced by the position of the Intertropical Convergence Zone research; M.B. and J.S. contributed new reagents/analytic tools; M.B., J.S., J.B., and H.F. (ITCZ) (17) and the global monsoon system (18, 19), which in turn, analyzed data; M.B. performed methane box modeling; J.B. performed methane box modeling and firn diffusion correction; M.B., J.S., J.B., and H.F. prepared the discussion is steered by changes in solar insolation. There is an ongoing debate of results; and M.B., J.S., J.C., and H.F. wrote the paper. on the relative contributions from tropical vs. boreal wetlands (ref. 9 The authors declare no conflict of interest. and references therein). The latter is possibly overestimated, because This article is a PNAS Direct Submission. other high-latitude sources, like thawing permafrost, thermokarst Data deposition: The ice core data reported in this paper are available at https://doi. lakes (20), or (cryospherically capped) geologic seeps (21), also pangaea.de/10.1594/PANGAEA.873918. contribute to this source from a hemispheric (ice core) point of view. 1To whom correspondence may be addressed. Email: [email protected] or hubertus. Additional constraints on the contribution of the various meth- [email protected]. ane sources can be derived from the carbon and hydrogen stable This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. isotopic signature of CH4 in ice cores. Microbially produced CH4 is 1073/pnas.1613883114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1613883114 PNAS Early Edition | 1of9 Downloaded by guest on September 26, 2021 In this contribution, we greatly extend the existing carbon and smoother MIS 2/1 and MIS 12/11.3 transitions. On the contrary, all δ13 – hydrogen isotopic information of CH4 from ice cores in terms of of the CH4 transitions investigated are gradual (Figs. S3 S5). temporal coverage, temporal resolution, accuracy, and precision. Within full glacial periods and interglacials, δD(CH4) shows Our data cover three interglacials and their preceding glacial rather small variations [standard deviation (SD) is typically terminations and glacial maxima. The interpretation of these around 5‰ (Table S1)] compared with the large leverage of data centers around the discussion of the source side of the sources and sinks, indicating little changes in the source com- δ13 methane cycle. This assumption is justified in the light of recent position. On the contrary, CH4 shows pronounced trends that work on sinks of methane (40–43), with net variations that are differ for all of the interglacials investigated. Most importantly, δ13 δ estimated to be relatively small (SI Text). Hence, dual stable interglacial CH4 is not correlated to [CH4]. Next, while for D δ13 isotope records of methane provide important insights into the (CH4) and CH4, the interglacial MIS 5.5 and MIS 11.3 mean suite of terrestrial and marine processes emitting methane and levels are comparable, a clear shift of the mean values for the their changes in the past. Holocene is evident (Fig. 1 and Fig. S2). A shift of similar size is also found for the mean level during the LGM compared with Ice Core Measurements MIS 6 and MIS 12 (Fig. 1 and Table S1). 13 δ13 We measured records of methane stable carbon (δ CH4) and Our data confirm earlier findings (25, 49) that CH4 is hydrogen [δD(CH )] isotopes from four Antarctic ice cores (Fig. evolving independently from [CH4] for large parts of the ice core 4 δ13 1 and Figs. S1 and S2–S5): i.e., the two European Project for Ice record. This feature is substantiated by new CH4 data from Coring in Antarctica (EPICA) ice cores from Dronning Maud EDC over the MIS 12/11.3 transition and the penultimate glacial Land (EDML) and from Dome Concordia (EDC), the Talos termination as well as for variations during MIS 11.3.
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