Publications

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2018
Bauska, TK, Brook EJ, Marcott SA, Baggenstos D, Shackleton S, Severinghaus JP, Petrenko VV.  2018.  Controls on millennial-scale atmospheric CO2 variability during the last glacial period. Geophysical Research Letters. 45:7731-7740.   10.1029/2018gl077881   AbstractWebsite

Changes in atmospheric CO2 on millennial-to-centennial timescales are key components of past climate variability during the last glacial and deglacial periods (70-10 ka), yet the sources and mechanisms responsible for the CO2 fluctuations remain largely obscure. Here we report the C-13/C-12 ratio of atmospheric CO2 during a key interval of the last glacial period at submillennial resolution, with coeval histories of atmospheric CO2, CH4, and N2O concentrations. The carbon isotope data suggest that the millennial-scale CO2 variability in Marine Isotope Stage 3 is driven largely by changes in the organic carbon cycle, most likely by sequestration of respired carbon in the deep ocean. Centennial-scale CO2 variations, distinguished by carbon isotope signatures, are associated with both abrupt hydrological change in the tropics (e.g., Heinrich events) and rapid increases in Northern Hemisphere temperature (Dansgaard-Oeschger events). These events can be linked to modes of variability during the last deglaciation, thus suggesting that drivers of millennial and centennial CO2 variability during both periods are intimately linked to abrupt climate variability. Plain Language Summary Ice cores provide unique records of variations in atmospheric CO2 prior to the instrumental era. While it is clear that changes in atmospheric CO2 played a significant role in driving past climate change, it is unclear what in turn drove changes in atmospheric CO2. Here we investigate enigmatic changes in atmospheric CO2 levels during an interval of the last glacial period (similar to 50,000 to 35,000 years ago) that are associated with abrupt changes in polar climate. To determine the sources and sinks for atmospheric CO2, we measured the stable isotopes of carbon in CO2 and found that the primary source of carbon to the atmosphere was an organic carbon reservoir. Most likely, this carbon was sourced from a deep ocean reservoir that waxed and waned following changes in either the productivity of the surface ocean or stratification of the deep ocean. We also found that atmospheric CO2 can change on the centennial timescale during abrupt climate transitions in the Northern Hemisphere. This observation adds to a growing body of evidence that abrupt changes in atmospheric CO2 are an important component of past carbon cycle variability.

2016
Bauska, TK, Baggenstos D, Brook EJ, Mix AC, Marcott SA, Petrenko VV, Schaefer H, Severinghaus JP, Lee JE.  2016.  Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation. Proceedings of the National Academy of Sciences of the United States of America. 113:3465-3470.   10.1073/pnas.1513868113   AbstractWebsite

An understanding of the mechanisms that control CO2 change during glacial-interglacial cycles remains elusive. Here we help to constrain changing sources with a high-precision, high-resolution deglacial record of the stable isotopic composition of carbon in CO2 (delta C-13-CO2) in air extracted from ice samples from Taylor Glacier, Antarctica. During the initial rise in atmospheric CO2 from 17.6 to 15.5 ka, these data demarcate a decrease in delta C-13-CO2, likely due to a weakened oceanic biological pump. From 15.5 to 11.5 ka, the continued atmospheric CO2 rise of 40 ppm is associated with small changes in delta C-13-CO2, consistent with a nearly equal contribution from a further weakening of the biological pump and rising ocean temperature. These two trends, related to marine sources, are punctuated at 16.3 and 12.9 ka with abrupt, century-scale perturbations in delta C-13-CO2 that suggest rapid oxidation of organic land carbon or enhanced air-sea gas exchange in the Southern Ocean. Additional century-scale increases in atmospheric CO2 coincident with increases in atmospheric CH4 and Northern Hemisphere temperature at the onset of the Bolling (14.6-14.3 ka) and Holocene (11.6-11.4 ka) intervals are associated with small changes in delta C-13-CO2, suggesting a combination of sources that included rising surface ocean temperature.

2011
Ritz, SP, Stocker TF, Severinghaus JP.  2011.  Noble gases as proxies of mean ocean temperature: sensitivity studies using a climate model of reduced complexity. Quaternary Science Reviews. 30:3728-3741.   10.1016/j.quascirev.2011.09.021   AbstractWebsite

Past global mean ocean temperature may be reconstructed from measurements of atmospheric noble gas concentrations in ice core bubbles. Assuming conservation of noble gases in the atmosphere-ocean system, the total concentration within the ocean mostly depends on solubility which itself is temperature dependent. Therefore, the colder the ocean, the more gas can be dissolved and the less remains in the atmosphere. Here, the characteristics of this novel paleoclimatic proxy are explored by implementing krypton, xenon, argon, and N(2) into a reduced-complexity climate model. The relationship between noble gas concentrations and global mean ocean temperature is investigated and their sensitivities to changes in ocean volume, ocean salinity, sea-level pressure and geothermal heat flux are quantified. We conclude that atmospheric noble gas concentrations are suitable proxies of global mean ocean temperature. Changes in ocean volume need to be considered when reconstructing ocean temperatures from noble gases. Calibration curves are provided to translate ice-core measurements of krypton, xenon, and argon into a global mean ocean temperature change. Simulated noble gas-to-nitrogen ratios for the last glacial maximum are delta Kr(atm) = -1.10 parts per thousand, delta Xe(atm) = -3.25 parts per thousand, and delta Ar(atm) = -0.29 parts per thousand. The uncertainty of the krypton calibration curve due to uncertainties of the ocean saturation concentrations is estimated to be +/- 0.3 degrees C. An additional 0.3 C uncertainty must be added for the last deglaciation and up to +/- 0.4 degrees C for earlier transitions due to age-scale uncertainties in the sea-level reconstructions. Finally, the fingerprint of idealized Dansgaard-Oeschger events in the atmospheric krypton-to-nitrogen ratio is presented. A delta Kr(atm) change of up to 0.34 parts per thousand is simulated for a 2 kyr Dansgaard-Oeschger event, and a change of up to 0.48 parts per thousand is simulated for a 4 kyr event. (C) 2011 Elsevier Ltd. All rights reserved.

2006
Schaefer, H, Whiticar MJ, Brook EJ, Petrenko VV, Ferretti DF, Severinghaus JP.  2006.  Ice record of delta C-13 for atmospheric CH4 across the Younger Dryas-Preboreal transition. Science. 313:1109-1112.   10.1126/science.1126562   AbstractWebsite

We report atmospheric methane carbon isotope ratios (delta(CH4)-C-13) from the Western Greenland ice margin spanning the Younger Dryas - to - Preboreal (YD-PB) transition. Over the recorded similar to 800 years, delta(CH4)-C-13 was around - 46 per mil (parts per thousand); that is, similar to 1 parts per thousand higher than in the modern atmosphere and similar to 5.5 parts per thousand higher than would be expected from budgets without C-13-rich anthropogenic emissions. This requires higher natural C-13-rich emissions or stronger sink fractionation than conventionally assumed. Constant delta(CH4)-C-13 during the rise in methane concentration at the YD-PB transition is consistent with additional emissions from tropical wetlands, or aerobic plant CH4 production, or with a multisource scenario. A marine clathrate source is unlikely.

2003
Caillon, N, Severinghaus JP, Jouzel J, Barnola JM, Kang JC, Lipenkov VY.  2003.  Timing of atmospheric CO2 and Antarctic temperature changes across termination III. Science. 299:1728-1731.   10.1126/science.1078758   AbstractWebsite

The analysis of air bubbles from ice cores has yielded a precise record of atmospheric greenhouse gas concentrations, but the timing of changes in these gases with respect to temperature is not accurately known because of uncertainty in the gas age-ice age difference. We have measured the isotopic composition of argon in air bubbles in the Vostok core during Termination III (similar to240,000 years before the present). This record most likely reflects the temperature and accumulation change, although the mechanism remains unclear. The sequence of events during Termination III suggests that the CO2 increase tagged Antarctic deglacial warming by 800 +/- 200 years and preceded the Northern Hemisphere deglaciation.