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Centennial-Scale Changes in the Global Carbon Cycle During the Last Deglaciation

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Centennial-Scale Changes in the Global Carbon Cycle During the Last Deglaciation LETTER doi:10.1038/nature13799 Centennial-scale changes in the global carbon cycle during the last deglaciation Shaun A. Marcott1,2, Thomas K. Bauska1, Christo Buizert1, Eric J. Steig3, Julia L. Rosen1, Kurt M. Cuffey4, T. J. Fudge3, Jeffery P. Severinghaus5, Jinho Ahn6, Michael L. Kalk1, Joseph R. McConnell7, Todd Sowers8, Kendrick C. Taylor7, James W. C. White9 & Edward J. Brook1 Global climate and the concentration of atmospheric carbon diox- analogue to central Greenlandic deep ice cores, with a substantially 1,2 ide (CO2) are correlatedover recent glacial cycles . The combination better-dated gas chronology during the glacial period, and is able to of processes responsible for a rise in atmospheric CO2 at the last resolve atmospheric CO2 at sub-centennial resolution. glacial termination1,3 (23,000 to 9,000 years ago), however, remains We exploit the unique aspects of WDC to reconstruct atmospheric 1–3 uncertain . Establishing the timing and rate of CO2 changes in the CO2 and CH4 concentrations at high resolution over the last deglacia- past provides critical insight into the mechanisms that influence the tion. The atmospheric CH4 record is important in this context because carbon cycle and helps put present and future anthropogenic emis- CH4 varies in phase with rapid Greenlandic climate changes during the sions in context. Here we present CO2 and methane (CH4) records of deglaciation and therefore provides a means of synchronizing our Ant- 12 the last deglaciation from a new high-accumulation West Antarctic arctic record with Northern Hemisphere climate . Atmospheric CH4 ice core with unprecedented temporal resolution and precise chro- is primarily produced by land-based sources, principally tropical and nology. We show that although low-frequency CO2 variations parallel borealwetlands13 where emissions are driven by changes intemperature changes in Antarctic temperature, abrupt CO changes occur that have 2 and hydrology, and CH4 therefore serves as an indicator of processes in a clear relationship with abrupt climate changes in the Northern Hemi- the terrestrial biosphere. sphere. A significant proportion of the direct radiative forcing assoc- The WDC deglacial CO2 record is characterized by a long-term iated with the rise in atmospheric CO2 occurred in three sudden steps, ,80 p.p.m. increase, similar to other cores9,14, which begins at 18.1 kyr each of 10 to 15 parts per million. Every step took place in less than ago, occurs in several discrete steps throughout the deglacial transition, two centuries and was followed by no notable change in atmospheric and ends 7 kyr later, during the early Holocene epoch. Given the precise CO2 for about 1,000 to 1,500 years. Slow, millennial-scale ventilation WDC chronology, the phasing of CO2 changes with respect toAntarctic of Southern Ocean CO2-rich, deep-ocean water masses is thought to and global temperature2 can be more precisely evaluated. The timing of have been fundamental to the rise in atmospheric CO2 associated the initial deglacial CO rise leads the initial rise in global temperature2 with the glacial termination4, given the strong covariance of CO levels 2 2 (17.2 kyr ago) by several centuries, and continues to lead across the entire and Antarctic temperatures5. Our data establish a contribution from termination (Extended Data Fig. 7). The timing of the initial CO rise is an abrupt, centennial-scale mode of CO variability that is not directly 2 2 consistent with the newest gas chronology of EPICA Dome C5 (EDC), related to Antarctic temperature. We suggest that processes operating but more highly resolved and precise given the smaller gas-age uncer- on centennial timescales, probably involving the Atlantic meridional tainty in WDC than in EDC (Extended Data 4). The WDC methane overturning circulation, seem to be influencing global carbon-cycle record shows in detail the abrupt changes at the onset of the Bølling– dynamics and are at present not widely considered in Earth system models. Allerød and Younger Dryas stadials and the start of the Holocene. We Ice cores from Greenland6,7 provide unique records of rapid climate observe a smaller, but prominent, methane excursion at 16.3 kyr ago events of the past 120 kyr. However, because of relatively high concen- not previously reported from other records (Fig. 1). Our record also trations of impurities8, Greenlandic ice cores do not provide reliable resolves the beginning of the deglacial methane rise at 17.8 kyr ago (Extended Data Table 1). atmospheric CO2 records. Antarctic ice cores, which contain an order- of-magnitude fewer impurities, provide reliable records8, but existing The WDC CO2 record demonstrates that CO2 varied in three dis- tinct modes during the deglaciation. The first mode is relatively grad- data from the deglacial period from Antarctica either provide only low 21 temporal resolution9 or are of relatively low precision10. ual change (,10 p.p.m. kyr ): such changes in CO2 began at 18.1 and The West Antarctic Ice Sheet Divide ice core (WDC) (79.467u S, 13.0 kyr ago and were broadly coincident with a reduction in the strength 112.085u W, 1,766 m above sea level) was drilled to a depth of 3,405 m of the Atlantic meridional overturning circulation (AMOC; Fig. 2i), a 2 2 in 2011 and spans the past ,68 kyr. At present, the site has a mean annual cold North Atlantic and warming in the Southern Hemisphere . The snow accumulation of 22 cm ice equivalent per year and a surface tem- second mode is rapid increase: 10–15 p.p.m. increases in CO2 occurred perature of 230 uC. Annual layer counting to 2,800 m depth (,30 kyr in three short (100–200 yr) intervals at 16.3, 14.8 and 11.7 kyr ago, the ago) provides a very accurate timescale for comparison with data from latter two at times of rapid resumption of the AMOC and warming in other archives11. The difference in age (Dage) between the ice and the the Northern Hemisphere. The rapid changes at 14.8 and 11.7 kyr ago gas trapped within it, which is critical for developing a gas-age chrono- were first noted at EDC9,14, but the magnitude, duration and timing are logy, is 205 6 10 yr at present and was 525 6 100 yr at the last glacial now more fully resolved because of the unique site conditions at WDC. maximum (LGM) (Extended Data Fig. 1). Given the high accumulation The third mode is no change in atmospheric CO2. These apparent pla- at the site, minimal smoothing due to gas transport and gradual occlu- teaus in the WDC CO2 record lasted for 1,000–1,500 yr and occurred sion, and precise chronological constraints, WDC is the best Antarctic directly after the rapid, century-scale increases. 1College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA. 2Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA. 3Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA. 4Department of Geography, University of California, Berkeley, California 94720, USA. 5Scripps Institution of Oceanography, University of California, San Diego, California 92037, USA. 6School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, South Korea. 7Desert Research Institute, Nevada System of Higher Education, Reno, Nevada 89512, USA. 8Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA. 9INSTAAR, University of Colorado, Boulder, Colorado 80309, USA. 616 | NATURE | VOL 514 | 30 OCTOBER 2014 ©2014 Macmillan Publishers Limited. All rights reserved LETTER RESEARCH e –34 Age (yr) –36 23,000 21,000 19,000 17,000 15,000 13,000 11,000 9,000 –38 Greenland 0.06 232 i –40 GGC5 (34˚ N) Pa/ O (‰) d 0.07 18 NGRIP 800 –42 230 δ –6.2 0.08 –44 h Th 700 EDC –46 CH –6.4 0.09 4 600 (p.p.b.) C (‰) 13 –6.6 –10 δ 0.5 500 δ g –9 18 –6.8 c –8 O (‰) ) WDC CH 4 400 Hulu (32˚ N) –2 –7 –0.5 f –9 280 –6 b –5 –8 O (‰) Greenhouse 260 CO –1.5 18 Alkenone (ng g gas forcing δ 700 e 2 –7 Direct radiative forcing (W m Borneo (4˚ N) 240 (p.p.m.) 600 –2.5 MD97-2120 (46˚ S) 500 220 400 a –32 WDC CO2 d ) 8 1.5 300 200 –1 2 1.2 200 –34 6 –1 ) 180 kyr 0.9 100 –2 –36 –2 4 0.6 TN57-13PC (53˚ S) O (‰) East Antarctica –38 Opal flux –6 18 δ18 2 E27-23 (60˚ S) WDC O (g cm δ 0.3 –40 –10 0 0 0 c IRD stack –42 0.1 Temp. anomaly (˚C) –14 280 0.2 LGM HS1 B/A YD Hol b IRD stack 0.3 260 23,000 21,000 19,000 17,000 15,000 13,000 11,000 9,000 0.4 240 Age (yr) 800 a (p.p.m.) 2 220 CH Figure 1 | Greenhouse gas and stable water isotope measurements from 700 11 CO WDC CO 2 4 Antarctica and Greenland. a, Oxygen isotopes from WDC (grey; black line is (p.p.b.) 200 11-point weighted average) and water-isotope-derived temperature composite 600 5 18 18 16 18 16 from East Antarctica (green). d O 5 ( O/ O)sample/( O/ O)VSMOW –1; 180 500 VSMOW, Vienna Standard Mean Ocean Water. b, Atmospheric CO2 WDC CH concentrations (this study; black line is 5-point weighted average). c, Direct 4 400 32 radiative forcing of CO2,CH4 and N2O (ref. 31) using a simplified expression . d, Atmospheric CH4 concentrations (this study and ref.
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