Carbon Isotopes Characterize Rapid Changes in Atmospheric Carbon Dioxide During the Last Deglaciation
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Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation Thomas K. Bauskaa,b,1, Daniel Baggenstosc, Edward J. Brooka, Alan C. Mixa, Shaun A. Marcottd, Vasilii V. Petrenkoe, Hinrich Schaeferf, Jeffrey P. Severinghausc, and James E. Leea aCollege of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331; bDepartment of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, United Kingdom; cScripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093; dDepartment of Geoscience, University of Wisconsin—Madison, Madison, WI 53706; eDepartment of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627; and fClimate and Atmosphere Center, National Institute of Water and Atmospheric Research Ltd, Wellington, New Zealand 6023 Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved February 2, 2016 (received for review July 17, 2015) An understanding of the mechanisms that control CO2 change during the terrestrial biosphere. A combined record including higher- 13 glacial–interglacial cycles remains elusive. Here we help to constrain precision EDC and Talos Dome data (11) documented a δ C-CO2 δ13 changing sources with a high-precision, high-resolution deglacial re- decrease beginning near 17.5 ka. This shift in C-CO2 was inter- 13 cord of the stable isotopic composition of carbon in CO2 (δ C-CO2)in preted to indicate that some process in the Southern Ocean (SO), air extracted from ice samples from Taylor Glacier, Antarctica. During possibly changes in upwelling, drove the initial CO2 rise. This pre- vious work did not resolve high-frequency variability in the δ13C-CO the initial rise in atmospheric CO2 from 17.6 to 15.5 ka, these data 2 13 records that may be essential for discerning mechanisms of change. demarcate a decrease in δ C-CO2, likely due to a weakened oceanic biological pump. From 15.5 to 11.5 ka, the continued atmospheric Here we use an analytical method (12) that employs dual-inlet CO rise of 40 ppm is associated with small changes in δ13C-CO , isotope ratio mass-spectrometry to obtain precision approaching that 2 2 of modern atmospheric measurements [∼0.02‰ 1-sigma pooled SD consistent with a nearly equal contribution from a further weak- based on replicate analysis compared with ∼0.05–0.11‰ for pre- ening of the biological pump and rising ocean temperature. These vious studies (9–11)]. We extracted atmospheric gases from large two trends, related to marine sources, are punctuated at 16.3 and (400–500 g) samples taken from surface outcrops of ancient ice at EARTH, ATMOSPHERIC, δ13 AND PLANETARY SCIENCES 12.9 ka with abrupt, century-scale perturbations in C-CO2 that sug- Taylor Glacier, Antarctica, at an average temporal resolution of – gest rapid oxidation of organic land carbon or enhanced air sea gas 165 y between 20 and 10 ka, and subcentury resolution during rapid exchange in the Southern Ocean. Additional century-scale increases in change events. This resolution allows us to delineate isotopic fin- atmospheric CO2 coincident with increases in atmospheric CH4 and gerprints of rapid shifts in CO that were previously impossible to – 2 Northern Hemisphere temperature at the onset of the Bølling (14.6 resolve. Our study complements recent precise observations of CO2 14.3 ka) and Holocene (11.6–11.4 ka) intervals are associated with concentration variations during the last deglaciation, which revealed δ13 small changes in C-CO2, suggesting a combination of sources that abrupt centennial-scale changes (13) (Fig. 2). included rising surface ocean temperature. During the initial 35-ppm CO2 rise from 17.6 to 15.5 ka, we find a 13 0.3‰ decrease in δ C-CO2 that is interrupted by a sharp mini- ice cores | paleoclimate | carbon cycle | atmospheric CO2 | last deglaciation mum coincident with rapid increases in CO2 and CH4 around 16.3 ka (13, 14) (Fig. 2). The 16.3-ka feature in the CO2 and CH4 concentration records, which corresponds to a 0.1‰ negative ex- ver thirty years ago ice cores provided the first clear evi- cursion in δ13C-CO , has been plausibly tied to the timing of dence that atmospheric CO increased by about 75 ppm as 2 O 2 Heinrich event 1 (13, 14) and signals a mode switch in the deglacial Earth transitioned from a glacial to an interglacial state (1, 2). After decades of research, the underlying mechanisms that drive Significance glacial–interglacial CO2 cycles are still unclear. A tentative consen- sus has formed that the deglaciation is characterized by a net transfer of carbon from the ocean to the atmosphere and terrestrial Antarctic ice cores provide a precise, well-dated history of in- biosphere, through a combination of changes in ocean temperature, creasing atmospheric CO2 during the last glacial to interglacial nutrient utilization, circulation, and alkalinity. Partitioning these transition. However, the mechanisms that drive the increase re- changes in terms of magnitude and timing is challenging. Estimates main unclear. Here we reconstruct a key indicator of the sources of the glacial–interglacial carbon cycle budget are highly uncertain, of atmospheric CO2 by measuring the stable isotopic composition ranging from 20–30 ppm for the effect of rising ocean temperature, of CO2 in samples spanning the period from 22,000 to 11,000 5–55 ppm for ocean circulation changes, and 5–30 ppm for de- years ago from Taylor Glacier, Antarctica. Improvements in pre- cision and resolution allow us to fingerprint CO sources on the creasing iron fertilization (3, 4), with feedbacks from CaCO3 2 compensation accounting for up to 30 ppm (5, 6). centennial scale. The data reveal two intervals of rapid CO2 rise A precise history of the stable isotopic composition of atmo- that are plausibly driven by sources from land carbon (at 16.3 and 13 spheric carbon dioxide (δ C-CO2) can constrain key processes 12.9 ka) and two others that appear fundamentally different and controlling atmospheric CO2 (7, 8). A low-resolution record from likely reflect a combination of sources (at 14.6 and 11.5 ka). 13 the Taylor Dome ice core (9) identified a decrease in δ C-CO2 at the onset of the deglacial CO2 rise that was followed by increases in Author contributions: T.K.B., D.B., E.J.B., A.C.M., V.V.P., and J.P.S. designed research; T.K.B., D.B., 13 S.A.M., V.V.P., H.S., and J.E.L. performed research; T.K.B. and D.B. analyzed data; and both CO2 and δ C-CO2 (Fig. 1). A higher-resolution record from the European Project for Ice Coring in Antarctica Dome T.K.B. wrote the paper with input from all authors. C (EDC) ice core (10) provided additional support for the rapid The authors declare no conflict of interest. 13 δ C-CO2 decrease associated with the initial CO2 rise, and box This article is a PNAS Direct Submission. modeling indicated that this decrease was consistent with changes in Data deposition: Date are deposited at the National Climatic Data Center (NOAA), https:// marine productivity. The record also included other rapid changes www.ncdc.noaa.gov/paleo/study/19765 13 1 in δ C-CO2, albeit at low precision, supporting large variations of To whom correspondence should be addressed. Email: [email protected]. 13 organic carbon fluxes, notably a sharp increase in δ C-CO2 during This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. the Bølling–Allerød (BA) interval attributed to carbon uptake by 1073/pnas.1513868113/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1513868113 PNAS Early Edition | 1of6 Downloaded by guest on September 28, 2021 24 22 20 18 16 14 12 10 plausible signature of a glacial–interglacial weakening of the -6.0 marine biological pump by forcing a decrease in the strength of the Subantarctic biological pump from near full efficiency (PO4 = -6.2 −3 −3 0.2 mmol m ) to preindustrial level (PO4 = 1.4 mmol m ). The timing of this change is scaled to the decrease in dust delivery to -6.4 Antarctica (16). This leads to an increase in CO2 and decrease 13 of δ C-CO2; the relationship is characterized in our model with (VPDB) -6.6 2 y0 equal to −8.6‰. For comparison, factorial experiments with – -6.8 the Bern 3D model of the last glacial interglacial cycle isolate C - CO 13 the effect of iron fertilization in the model (17) with a resultant Keeling intercept of −9.6‰. Second, we vary the rate of SO -7.0 Taylor Glacier Data (this study) Schmitt et al., 2012: Taylor Dome (Smith et al., 1999) EDC Sublimation upwelling in our box model. Greater ocean ventilation raises EDC Cracker (Elsig et al., 2009; Schmitt et al., 2012) Talos Dome CO and lowers δ13C-CO resultinginaKeelingplotintercept -7.2 EDC Ball Mill (Lourantou et al., 2010) Spline Smoothed Compilation 2 2 of −8.4‰. Experiments with the Bern 3D model where wind 24 22 20 18 16 14 12 10 stress over the SO was varied (20–180%) show that atmospheric Age (ka) CO2 positively correlates with the rate of ocean overturning δ13 Fig. 1. Carbon isotope records during the last deglaciation. Taylor Glacier (18). The simulated CO2 and C-CO2 produce a Keeling plot 13 intercept between −7.6‰ [when the Atlantic Meridional Over- δ C-CO2 data from this study (red). Previous work from Taylor Dome (gray open circles) (9), Grenoble EDC data (open green squares) (10), Bern EDC data turning Circulation (AMOC) is in an “on” state] and −8.6‰ (when (orange circles) (11, 45), sublimation measurements from EDC (blue triangles), AMOC is in an “off” state).