Publications

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2015
Mitchell, LE, Buizert C, Brook EJ, Breton DJ, Fegyveresi J, Baggenstos D, Orsi A, Severinghaus J, Alley RB, Albert M, Rhodes RH, McConnell JR, Sigl M, Maselli O, Gregory S, Ahn J.  2015.  Observing and modeling the influence of layering on bubble trapping in polar firn. Journal of Geophysical Research-Atmospheres. 120:2558-2574.   10.1002/2014jd022766   AbstractWebsite

Interpretation of ice core trace gas records depends on an accurate understanding of the processes that smooth the atmospheric signal in the firn. Much work has been done to understand the processes affecting air transport in the open pores of the firn, but a paucity of data from air trapped in bubbles in the firn-ice transition region has limited the ability to constrain the effect of bubble closure processes. Here we present high-resolution measurements of firn density, methane concentrations, nitrogen isotopes, and total air content that show layering in the firn-ice transition region at the West Antarctic Ice Sheet (WAIS) Divide ice core site. Using the notion that bubble trapping is a stochastic process, we derive a new parameterization for closed porosity that incorporates the effects of layering in a steady state firn modeling approach. We include the process of bubble trapping into an open-porosity firn air transport model and obtain a good fit to the firn core data. We find that layering broadens the depth range over which bubbles are trapped, widens the modeled gas age distribution of air in closed bubbles, reduces the mean gas age of air in closed bubbles, and introduces stratigraphic irregularities in the gas age scale that have a peak-to-peak variability of 10 years at WAIS Divide. For a more complete understanding of gas occlusion and its impact on ice core records, we suggest that this experiment be repeated at sites climatically different from WAIS Divide, for example, on the East Antarctic plateau.

2013
Fudge, TJ, Steig EJ, Markle BR, Schoenemann SW, Ding QH, Taylor KC, McConnell JR, Brook EJ, Sowers T, White JWC, Alley RB, Cheng H, Clow GD, Cole-Dai J, Conway H, Cuffey KM, Edwards JS, Edwards RL, Edwards R, Fegyveresi JM, Ferris D, Fitzpatrick JJ, Johnson J, Hargreaves G, Lee JE, Maselli OJ, Mason W, McGwire KC, Mitchell LE, Mortensen N, Neff P, Orsi AJ, Popp TJ, Schauer AJ, Severinghaus JP, Sigl M, Spencer MK, Vaughn BH, Voigt DE, Waddington ED, Wang XF, Wong GJ, Members WDP.  2013.  Onset of deglacial warming in West Antarctica driven by local orbital forcing. Nature. 500:440-+.   10.1038/nature12376   AbstractWebsite

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate(1,2). Hypotheses for a Northern Hemisphere trigger, through oceanic redistributions of heat, are based in part on the abrupt onset of warming seen in East Antarctic ice cores and dated to 18,000 years ago, which is several thousand years after high-latitude Northern Hemisphere summer insolation intensity began increasing from its minimum, approximately 24,000 years ago(3,4). An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently(2,5). Here we present results from a new, annually resolved ice-core record from West Antarctica that reconciles these two views. The records show that 18,000 years ago snow accumulation in West Antarctica began increasing, coincident with increasing carbon dioxide concentrations, warming in East Antarctica and cooling in the Northern Hemisphere(6) associated with an abrupt decrease in Atlantic meridional overturning circulation(7). However, significant warming in West Antarctica began at least 2,000 years earlier. Circum-Antarctic sea-ice decline, driven by increasing local insolation, is the likely cause of this warming. The marine-influenced West Antarctic records suggest a more active role for the Southern Ocean in the onset of deglaciation than is inferred from ice cores in the East Antarctic interior, which are largely isolated from sea-ice changes.

2009
Severinghaus, JP, Beaudette R, Headly MA, Taylor K, Brook EJ.  2009.  Oxygen-18 of O2 Records the Impact of Abrupt Climate Change on the Terrestrial Biosphere. Science. 324:1431-1434.   10.1126/science.1169473   AbstractWebsite

Photosynthesis and respiration occur widely on Earth's surface, and the O-18/O-16 ratio of the oxygen produced and consumed varies with climatic conditions. As a consequence, the history of climate is reflected in the deviation of the O-18/O-16 of air (delta O-18(atm)) from seawater delta O-18 (known as the Dole effect). We report variations in delta O-18(atm) over the past 60,000 years related to Heinrich and Dansgaard-Oeschger events, two modes of abrupt climate change observed during the last ice age. Correlations with cave records support the hypothesis that the Dole effect is primarily governed by the strength of the Asian and North African monsoons and confirm that widespread changes in low-latitude terrestrial rainfall accompanied abrupt climate change. The rapid delta O-18(atm) changes can also be used to synchronize ice records by providing global time markers.

2004
Seibt, U, Brand WA, Heimann M, Lloyd J, Severinghaus JP, Wingate L.  2004.  Observations of O-2 : CO2 exchange ratios during ecosystem gas exchange. Global Biogeochemical Cycles. 18   10.1029/2004gb002242   AbstractWebsite

We determined O-2:CO2 exchange ratios of ecosystem fluxes during field campaigns in different forest ecosystems (Harvard Forest/United States, Griffin Forest/United Kingdom, Hainich/Germany). The exchange ratios of net assimilation observed in chamber experiments varied between 0.7 and 1.6, with averages of 1.1 to 1.2. A measurement of soil gas exchange yielded an exchange ratio of 0.94. On the other hand, the observed canopy air O-2:CO2 ratios, derived from the concurrent variations of O-2 and CO2 abundances in canopy air, were virtually indistinguishable from 1.0 over the full diurnal cycle. Simulations with a simple one-box model imply that the combined processes of assimilation, respiration, and turbulent exchange yield canopy air O-2:CO2 ratios that differ from the exchange ratios of the separate fluxes. In particular, the simulated canopy air O-2:CO2 ratios (1.01 to 1.12) were clearly lower than the exchange ratios of net turbulent fluxes between the ecosystem and the atmosphere (1.26 to 1.38). The simulated canopy air ratios were also sensitive to changes in the regional O-2:CO2 ratio of air above the canopy. Offsets between the various exchange ratios could thus arise if the component ecosystem fluxes have different diurnal cycles and distinct exchange ratios. Our results indicate that measurements of O-2 and CO2 abundances in canopy air may not be the appropriate method to determine O-2:CO2 exchange ratios of net ecosystem fluxes.

2000
Brook, EJ, Harder S, Severinghaus J, Steig EJ, Sucher CM.  2000.  On the origin and timing of rapid changes in atmospheric methane during the last glacial period. Global Biogeochemical Cycles. 14:559-572.   10.1029/1999gb001182   AbstractWebsite

We present high resolution records of atmospheric methane from the GISP2 (Greenland Ice Sheet Project 2) ice core for four rapid climate transitions that occurred during the past 50 ka: the end of the Younger Dryas at 11.8 ka, the beginning of the Bolling-Allerod period at 14.8 ka, the beginning of interstadial 8 at 38.2 ka, and the beginning of interstadial 12 at 45.5 ka. During these events, atmospheric methane concentrations increased by 200-300 ppb over time periods of 100-300 years, significantly more slowly than associated temperature and snow accumulation changes recorded in the ice core record. We suggest that the slower rise in methane concentration may reflect the timescale of terrestrial ecosystem response to rapid climate change. We find no evidence for rapid, massive methane emissions that might be associated with large-scale decomposition of methane hydrates in sediments. With additional results from the Taylor Dome Ice Core (Antarctica) we also reconstruct changes in the interpolar methane gradient tan indicator of the geographical distribution of methane sources) associated with some of the rapid changes in atmospheric methane. The results indicate that the rise in methane at the beginning of the Bolling-Allerod period and the later rise at the end of the Younger Dryas were driven by increases in both tropical and boreal methane sources. During the Younger Dryas (a 1.3 ka cold period during the last deglaciation) the relative contribution from boreal sources was reduced relative to the early and middle Holocene periods.