Siple Dome Ice Reveals Two Modes of Millennial CO2 Change During the Last Ice Age
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ARTICLE Received 4 Dec 2013 | Accepted 26 Mar 2014 | Published 29 Apr 2014 DOI: 10.1038/ncomms4723 OPEN Siple Dome ice reveals two modes of millennial CO2 change during the last ice age Jinho Ahn1 & Edward J. Brook2 Reconstruction of atmospheric CO2 during times of past abrupt climate change may help us better understand climate-carbon cycle feedbacks. Previous ice core studies reveal simultaneous increases in atmospheric CO2 and Antarctic temperature during times when Greenland and the northern hemisphere experienced very long, cold stadial conditions during the last ice age. Whether this relationship extends to all of the numerous stadial events in the Greenland ice core record has not been clear. Here we present a high-resolution record of atmospheric CO2 from the Siple Dome ice core, Antarctica for part of the last ice age. We find that CO2 does not significantly change during the short Greenlandic stadial events, implying that the climate system perturbation that produced the short stadials was not strong enough to substantially alter the carbon cycle. 1 School of Earth and Environmental Sciences, Seoul National University, Seoul 151742, Korea. 2 College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA. Correspondence and requests for materials should be addressed to J.A. (email: [email protected]). NATURE COMMUNICATIONS | 5:3723 | DOI: 10.1038/ncomms4723 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4723 ce core records from Greenland reveal a detailed history of the last ice age, marine sediment records indicate shoaled abrupt climate change during the last glacial period. Warm AMOC18 and increased opal flux14 in the Southern Ocean, Iand cold periods (interstadial and stadial, respectively) although the chronology of the proxy for the latter is not well repeated on millennial timescales but rapid switches between constrained. The shoaling of AMOC likely coincided with the two happened in decades1–4. Antarctic ice core records, reduction in AMOC strength that might have caused reduction however, reveal gradual warming during Geenlandic stadials and in northward oceanic heat transport, the gradual warming in 5,6 cooling during interstadials . The out-of-phase interhemispheric Antarctica and CO2 outgassing from the Southern ocean during climate relationship is usually referred to as the ‘bipolar seesaw’7 the long stadials10,14, analogous to the early stage of the last 14,19,20 and the most popular hypothesis for the control mechanism deglacial Antarctic warming and CO2 increase . includes reorganization of ocean-atmosphere circulation and High-resolution records from the EPICA Dronning Maud change in meridional heat transport, possibly caused by fresh Land (EDML) ice core in east Antarctica show the ‘bipolar water input into the North Atlantic8–10. seesaw’ operated not only during the major long stadials but also Reconstruction of atmospheric CO2 during abrupt climate during other short ones, and that stadial duration is positively change events may help us better understand climate-carbon correlated with the magnitude of the temperature increase in cycle feedbacks and provide data for testing carbon cycle models Antarctica. These observations support the hypothesis of under variety of boundary conditions. Existing Antarctic ice core reduction in AMOC during both long and short stadials21, records show CO2 increases during long Greenlandic stadials, although there is no clear marine evidence of AMOC reduction which are also accompanied by major Antarctic warmings11,12. during each of the short stadials, perhaps owing to insufficient 6 Ventilation of CO2-rich deep water in the Southern Ocean may data resolution and/or chronology . have controlled ocean-atmosphere carbon exchange and therefore By analogy to the relationship between CO2 and major 13 atmospheric CO2 concentration . Marine sediment records from Antarctic warmings/long Greenlandic stadials, we might expect 14 the Southern Ocean indicate increased opal flux and reduced small CO2 increases during the small Antarctic warmings/short 15 stratification during the Younger Dryas event and the long Greenlandic stadials. However, atmospheric CO2 change during stadial preceding the Bølling-Allerød event14,15, both times of the short stadials is not well resolved in existing ice core records 11,12,22 rising CO2, and can be interpreted as a record of increased owing to low temporal data resolution (280–570 years ). 14,15 upwelling and CO2 outgassing in the Southern Ocean . We investigate CO2 variations during the short Greenlandic During the same time intervals, the Atlantic meridional stadials, with a multi-decadal to centennial CO2 record with a overturning circulation (AMOC) was reduced and the Antarctic mean sampling resolution of 95 years, from the Siple Dome ice temperature gradually increased16,17. During the long stadials of core, Antarctica. Our new results cover the time interval of HS2 HS3 HS4 3 4 4.1 5 6 7 8 910 –36 NGRIP 2 a Warmer –38 δ Greenlandic climate –40 18 Colder O (per mi) –42 600 –44 b –46 500 Greenland CH4 composite EDML CH4 (p.p.b.) Siple Dome CH 230 4 400 4 CH 220 CO 300 2 210 (p.p.m.) Siple Dome CO (this study) 2 200 c –240 190 –250 –48 –260 –49 Siple Dome dD 180 O (per mil) Antarctic climate –270 O (per mil) –50 EDML d180 Warmer 18 18 δ δ –51 d –280 –52 –290 Colder EDML Siple 22 24 26 28 30 32 34 36 38 40 Age (ka) Figure 1 | High-resolution CO2 and climate records during abrupt climate change during the last ice age. (a) Greenlandic isotopic temperature record from the NGRIP ice core57. Black numbers represent Dansgaard-Oeschger events. HS stands for Heinrich stadial, indicating long stadials that include 21 24 21 Heinrich events. (b) Atmospheric CH4 records from Greenland (grey) , Siple Dome (red) and EDML (blue) ice cores. Black dots are new Siple Dome CH4 data (this study). Red triangles indicate age control points. (c) Atmospheric CO2 record from Siple Dome, Antarctica ice core (this study). Red line 23 represents 300-year running means of the CO2 record. Black dots are published records .(d) Antarctic temperature proxy records from Siple Dome (dark blue)24 and EDML (grey)21 ice cores. All the ages are synchronized on Greenland Ice Core Chronology 2005 (GICC05) timescale. Blue and pink boxes indicate time intervals of short and long stadials (Greenlandic cold spans), respectively. During those stadials, Antarctic temperature increased. 2 NATURE COMMUNICATIONS | 5:3723 | DOI: 10.1038/ncomms4723 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4723 ARTICLE Greenlandic abrupt climate events (Dansgaard-Oeschger or DO a 15 DO8 events) DO2–7 and our sampling resolution is sufficient to examine CO2 trends during the short stadials lasting for Talos DO8 800–1200 years. Combined with recently published high- 23 resolution data for DO8–10 from the same core , we con- 10 DO4 structed a complete high-resolution CO2 record from 22 to 41 ka. Results Talos DO12 5 Natural smoothing of gas records in the Siple Dome ice core. anomaly (p.p.m.) Snow accumulation rates at the Siple Dome are relatively high, and 2 therefore smoothing of gas records by diffusion and gradual bubble DO9 DO6 CO DO3 close-off in the firn (unconsolidated snow layer on the top of ice 0 DO5 DO7 sheet) is relatively small24. The high snow accumulation rates also DO4.1 result in a small uncertainty in the relative timing between gas ages and ice ages (Dage)25, allowing better comparison of gas records with temperature proxy records24. A firn densification model for 0.0 0.5 1.0 1.5 Siple Dome shows Dage of 500–1000 years during the 22–41-ka Greenlandic stadial duration (1000 years) period24. Because the width of the gas age distribution at half- height is typically about 10% of Dage25–28, we estimate the gas age b DO8 distribution of the Siple Dome record to be o100 years during the 14 time interval of study. The sharp increases in the Siple Dome CH4 record (Fig. 1) clearly confirm that the smoothing of gas records is 12 Warmer minimal on multi-centennial timescales and supports our 10 estimation of smoothing of the Siple Dome CO2 record. DO6 DO4.1 8 Colder DO5 DO7 DO4 Two modes of CO2 change during Greenlandic stadials.As 6 B shown in Fig. 1, we observe small CO2 variations of 5 p.p.m. on DO3 D change (per mil) 4 centennial timescales during the short stadials. A 300-year run- δ ning mean (red curve) removes these features (Fig. 1), illustrating 2 that CO2 change was negligible on multi-centennial timescales. We observe small decreases on longer timescales during most of 0 DO9 the short stadials (Fig. 1), but these are part of a long-term trend. 0.0 0.4 0.8 1.2 1.6 After detrending it becomes clear that the CO2 change associated with short stadials themselves is insignificant (Fig. 2a). In Greenlandic stadial duration (1000 years) contrast, we observe CO2 increases during the long stadials, confirming previous results from different Antarctic ice c 15 DO8 11,12,22,29 Heinrich (long) cores (Fig. 2a). The isotopic temperature proxy (dDice) stadials from the Siple Dome ice core shows small Antarctic warmings during most of the short stadials (Fig. 2b) and confirms previous 10 DO4 results from the EDML ice core21, implying that the small Antarctic warmings during the short stadials are not only local features but at least of larger regional extent because Siple Dome is located in the Pacific sector, while the EDML core is in the 5 Atlantic sector in Antarctica. Combining the Siple Dome CO2 anomaly (p.p.m.) and climate records, we plot the time evolution of CO2 versus the 2 temperature proxy (dD ) anomalies during Greenlandic stadials CO ice DO6 DO5 Non-Heinrich or Antarctic warmings, using the detrended Siple Dome CO2 and 0 DO3 DO7 (short) stadials temperature proxy records for short stadials (Fig.