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On the Origin and Timing of Rapid Changes in Atmospheric GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 14, NO. 2, PAGES 559-572, JUNE 2000 On the origin and timing of rapid changesin atmospheric methane during the last glacial period EdwardJ. Brook,• SusanHarder, • Jeff Sevennghaus, ' • Eric J. Ste•g,. 3and Cara M. Sucher•'5 Abstract. We presenthigh resolution records of atmosphericmethane from the GISP2 (GreenlandIce SheetProject 2) ice corefor fourrapid climate transitions that occurred during the past50 ka: theend of theYounger Dryas at 11.8ka, thebeginning of theBolling-Aller0d period at 14.8ka, thebeginning of interstadial8 at 38.2 ka, andthe beginning of intersradial12 at 45.5 ka. Duringthese events, atmospheric methane concentrations increased by 200-300ppb over time periodsof 100-300years, significantly more slowly than associated temperature and snow accumulationchanges recorded in the ice corerecord. We suggestthat the slowerrise in methane concentrationmay reflect the timescale of terrestrialecosystem response to rapidclimate change. We find noevidence for rapid,massive methane emissions that might be associatedwith large- scaledecomposition of methanehydrates in sediments.With additionalresults from the Taylor DomeIce Core(Antarctica) we alsoreconstruct changes in the interpolarmethane gradient (an indicatorof thegeographical distribution of methanesources) associated with someof therapid changesin atmosphericmethane. The resultsindicate that the rise in methaneat thebeginning of theB011ing-Aller0d period and the laterrise at theend of theYounger Dryas were driven by increasesin bothtropical and boreal methane sources. During the Younger Dryas (a 1.3 ka cold periodduring the last deglaciation)the relativecontribution from borealsources was reduced relativeto the early andmiddle Holocene periods. 1. Introduction associatedwith orbitalforcing. A 20,000 year periodicityin the Vostok record was attributed to the effects of solar insolation Studies of ice cores from Greenland and Antarctica reveal that variationson the tropical monsooncycle [Chappellaz et al., significant changes in atmospheric methane concentration 1990; Petit-Maireet al., 1991], or alternatively,to temperature occurredwith a range of periodsover the past400,000+ years and precipitationvariations in high latitude wetland regions [Rasmussen and Khalil, 1984; Khalil and Rasmussen, 1987; [Crowley,1991]. More recently,higher-resolution records have Raynaud et al., 1988; Stauffer et al., 1988; Chappellazet al., becomeavailable from coresthrough the rapidly accumulating 1990, 1993a, 1997; Etheridge et al., 1992; Jouzel et al., 1993; Greenland Ice Sheet. Methane records from the Greenland Ice Blunier et al., 1993, 1995; Brook et al., 1996a, 1999; Petit et al., SheetProject 2 (GISP2) andGreenland Ice CoreProgram (GRIP) 1999]. Wetlandsare the major naturalmethane source [Fung et ice cores support the general patterns revealed at Vostok al. , 1991; Chappellaz et al., 1993b; Hein et al., 1997], and [Chappellazet al., 1993a;Brook et al., 1996a]. They alsoreveal researchin modem wetlands suggeststhat methane emissions that atmospheric methane levels varied significantly on depend on temperature,hydrologic balance, and possibly net millennial timescalesduring the last glacial period. The ecosystemproduction [Whiting and Chanton, 1993; Bubier and millennialvariations were coeval with millennial-scalewarmings Moore, 1994; Schlesinger, 1996]. Warm, wet, and highly and coolings(interstadial events) inferred from oxygenisotope productive conditions are associatedwith higher methane recordsfrom central Greenlandice cores,proxies for surface emissions. As a result, ice core methane records have been temperature[Johnsen et al., 1993; Grootes et al., 1993]. The ice interpreted as indicators of terrestrial climatic conditions on a core methanerecord therefore indicates a close relationship variety of timescales. betweeninterstadial climate and changesin terrestrialmethane Long records from the Vostok ice core indicate that emissions[Chappellaz et al., 1993a]. This conclusionaugments atmospheric methane concentrations varied on timescales a largebody of evidencesuggesting a correlationbetween rapid climate change in Greenland ice core records and terrestrial •Departmentof Geology, Washington State University, Vancouver. climatechange in a variety of locationsaround the world [e.g., 2ScrippsInstitution of Oceanography,University of California,San Grimm et al., 1993; Broecker et al., 1998; Gasse and Van Diego, La Jolla. Campo;1998, Grigg and Whitlock,1988; Allen et al., 1999]. 3Departmentof Earthand EnvironmentalScience, University of Pennsylvania,Philadelphia. Gaps in our understanding of millennial-scale shifts in nGraduateSchool of Oceanography,University of RhodeIsland, methaneconcentration include incomplete knowledge of (1) the Narragansett,Rhode Island. detailed timing and magnitude of the rapid concentration SNowat NOAA Office of Global Programs,Silver Spring, Maryland. changes,(2) the possiblerole of massive releasesof clathrate- boundmethane from marinesediments and permafrost,and (3) Copyright2000 by the AmericanGeophysical Union. the methanesource locations and how they changedwith time. Paper number 1999GB001182. In this paper we examine these issuesusing methanerecords 0886-6236/00/1999GB001182512.00 covering the past 50,000 years from two ice cores, the GISP2 559 560 BROOK ET AL: RAPID METHANE VARIATIONS DURING THE LAST GLACIAL PERIOD core from centralGreenland and the Taylor Dome core from the wasnormally analyzed twice. The loop pressuresfor the first and Ross Sea sector of Antarctica. We use the results to constrain the secondmeasurements were 40-50 and 15-20 torr, respectively. rate and magnitudeof methaneconcentration change associated Methane concentrationswere quantified by measuringpeak with rapid climate shifts during the last glacial period and areasusing ELAB (OMS Tech, Miami, Florida) chromatographic deglaciation.We alsodetermine the interpolarmethane gradient, softwareand computerinterface. Our working standardwas a a possibleproxy for the geographicaldistribution of methane high-pressurecylinder of syntheticair preparedby Scott-Marin sources[Rasmussen and Khalil, 1984], at selectedtime intervals, SpecialtyGases. This standardhas a methaneconcentration of andexamine the implicationsof changesin the gradient. 962+ 6 ppb(mean + 2 ti/nesSE, whereSE is standarderror), determinedby comparisonto two similar cylinders that were 2. Samples and Analytical Procedures calibrated at the NOAA/Climate Monitoring and Diagnostics Laboratory,which have methaneconcentrations of 1488+3 ppb 2.1. Ice Core Samples and 1824+3 ppb. We calibratedthe instrumentdaily with six to eightinjections of the workingstandard at a rangeof sampleloop The GISP2 ice core was recovered in 1993 at 72ø36'N, pressuresbetween 10 and 60 torr. The averagedaily precisionof 38ø30'Win centralGreenland. We have extendedour original these runs of the calibration standard was + 9 ppb (2 x SE, GISP2 data set [Brooket al., 1996a]by analyzingsamples from n-347). 129 additionaldepths in the upper2438 m, which representsthe Contaminationin our sampleprocessing was evaluatedin two past 50 ka. We focused on abrupt transitionsin the record, ways. First, we ran a total analytical blank, as follows. An ice includingthe terminationof the YoungerDryas periodat •-11.8 sample analyzed the previous day was melted and refrozen 3 ka, the beginningof the B011ing-Aller0dperiod at --14.5 ka, and times in a vacuum vessel. After each refreezing step any the onsetsof interstadials8 and 12, rapid warming eventsthat remainingair in the vesselwas pumpedout. The ice samplewas occurredat-- 38.2 and45.5 ka in the GISP2 isotoperecord. The removedfrom the vessel, and the edgeswere trimmed with the GISP2 methane data set over the 0-50 ka interval now includes band saw normally used to cut ice samples. The ice cube was 289 sampledepths. The Taylor Dome ice core was recoveredin then placedin a secondvessel, and the ambientair was pumped 1994 at 77ø48'S,159ø53'E on Taylor Dome, a smallice domein out while maintainingthe vesselat -30øC. The vesselwas then the Ross Sea sector of Antarctica. The Taylor Dome cooled to -80øC. One hundred torr of standard air was introduced paleoclimaterecord extends to greaterthan 130 ka at a depthof into the vacuum line and sample vessel, the vessel valve was 554 m [Grootesand Steig, 1994;Steig, 1996; Steiget al., 2000] closed, and the remainder of the line was evacuated. One hundred but we focushere on methanemeasurements of 137 samplesin torr in the samplevessel resulted in a pressureof 40-50 torr in the upper 445 m (0-50 ka), with high resolutionmeasurements the GC sampleloop when the gaswas expanded,a value similar over the time intervals listed above. Data are available from the to that obtainedwhen analyzingmost ice samples. This "blank National Oceanic and Atmospheric Administration (NOAA) sample"was then melted and refrozen. We analyzedthe air in GeophysicalData Center(http://www.ngdc.noaa.gov/) or from the headspace and determined the blank as the difference the authors. between the expected value of the standardand the measured value. Blanksmeasured in this way over a 27 monthtime period 2.2. Analytical Methods had a meanof 16 + 4 ppb (mean + 2 x SE, n-63). Ice samplesweighing--35 g were placedin cubicalstainless In an attempt to locate the source of contamination, we steel vacuum vessels with volumes of- 40 cm 3. The vessels performedan additionalblank experiment. After analysisof an have Nupro SS-4H stainlesssteel bellows valves and were sealed ice core
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