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Severinghaus, JP, Albert MR, Courville ZR, Fahnestock MA, Kawamura K, Montzka SA, Muhle J, Scambos TA, Shields E, Shuman CA, Suwa M, Tans P, Weiss RF.  2010.  Deep air convection in the firn at a zero-accumulation site, central Antarctica. Earth and Planetary Science Letters. 293:359-367.   10.1016/j.epsl.2010.03.003   AbstractWebsite

Ice cores provide unique archives of past atmospheres and climate, but interpretation of trapped-gas records and their climatic significance has been hampered by a poor knowledge of the prevalence of air convection in the firn layer on top of polar ice sheets. In particular, the phasing of greenhouse gases and climate from ice cores has been obscured by a discrepancy between empirical and model-based estimates of the age difference between trapped gases and enclosing ice, which may be due to air convection. Here we show that deep air convection (>23 m) occurs at a windy, near-zero-accumulation rate site in central Antarctica known informally as the Megadunes site (80.77914 degrees S, 124.48796 degrees E). Deep convection is evident in depth profiles of air withdrawn from the firn layer, in the observed pattern of the nitrogen isotope ratio (15)N/(14)N, the argon isotope ratio (40)Ar/(36)Ar, and in the mixing ratios of the anthropogenic halocarbons methyl chloroform (CH(3)CCl(3)) and HFC-134a (CH(2)FCF(3)). Transport parameters (diffusivities) were inferred and air was dated using measured carbon dioxide (CO(2)) and methane (CH(4)) mixing ratios, by comparing with the Law Dome atmospheric record, which shows that these are the oldest firn air samples ever recovered (CO(2) mean age = 1863 AD). The low accumulation rate and the consequent intense metamorphism of the firn (due to prolonged exposure to seasonal temperature cycling) likely contribute to deep air convection via large grain size and vertical cracks that act as conduits for vigorous air motion. The Megadunes site provides a possible modern analog for the glacial conditions in the Vostok, Dome Fuji, and Dome C ice core records and a possible explanation for lower-than-expected (15)N/(14)N ratios in trapped air bubbles at these times. A general conclusion is that very low accumulation rate causes deep air convection via its effect on firn structural characteristics. (C) 2010 Elsevier B.V. All rights reserved.

Cuffey, KM, Clow GD, Steig EJ, Buizert C, Fudge TJ, Koutnik M, Waddington ED, Alley RB, Severinghaus JP.  2016.  Deglacial temperature history of West Antarctica. Proceedings of the National Academy of Sciences of the United States of America. 113:14249-14254.   10.1073/pnas.1609132113   AbstractWebsite

The most recent glacial to interglacial transition constitutes a remarkable natural experiment for learning how Earth's climate responds to various forcings, including a rise in atmospheric CO2. This transition has left a direct thermal remnant in the polar ice sheets, where the exceptional purity and continual accumulation of ice permit analyses not possible in other settings. For Antarctica, the deglacial warming has previously been constrained only by the water isotopic composition in ice cores, without an absolute thermometric assessment of the isotopes' sensitivity to temperature. To overcome this limitation, we measured temperatures in a deep borehole and analyzed them together with ice-core data to reconstruct the surface temperature history of West Antarctica. The deglacial warming was 11.3 +/- 1.8 degrees C, approximately two to three times the global average, in agreement with theoretical expectations for Antarctic amplification of planetary temperature changes. Consistent with evidence from glacier retreat in Southern Hemisphere mountain ranges, the Antarctic warming was mostly completed by 15 kyBP, several millennia earlier than in the Northern Hemisphere. These results constrain the role of variable oceanic heat transport between hemispheres during deglaciation and quantitatively bound the direct influence of global climate forcings on Antarctic temperature. Although climate models perform well on average in this context, some recent syntheses of deglacial climate history have underestimated Antarctic warming and the models with lowest sensitivity can be discounted.

Grachev, AM, Severinghaus JP.  2003.  Determining the thermal diffusion factor for Ar-40/Ar-36 in air to aid paleoreconstruction of abrupt climate change. Journal of Physical Chemistry A. 107:4636-4642.   10.1021/jp027817u   AbstractWebsite

The thermal diffusion factor (alpha(T)) of Ar-40/Ar-36 in air has been measured in the laboratory for the first time. The mean values of alpha(T) x 10(3) that we find at -30.0 degreesC are 9.85 +/- 0.04 for air and 11.25 +/- 0.03 for pure argon. The latter value is more precise than the data found in the literature. The temperature dependence of the thermal diffusion factor in air in the range -60 to -10 degreesC can be described by an empirical equation alpha(T) x 10(3) = 26.08 - 3952/ (+/-1%), where is the effective average temperature. Results of this study are valuable for reconstruction of magnitudes of abrupt climate change events recorded in Greenland ice cores. For one abrupt warming event similar to15,000 years ago, near the end of the last glacial period, these results yield a warming of 11 +/- 3 degreesC over several decades or less. Theoretical calculations are not yet able to provide the needed accuracy, and the experimental results for the thermal diffusion factor in air should be used for paleoenvironmental studies.

Orsi, AJ, Kawamura K, Fegyveresi JM, Headly MA, Alley RB, Severinghaus JP.  2015.  Differentiating bubble-free layers from melt layers in ice cores using noble gases. Journal of Glaciology. 61:585-594.   10.3189/2015JoG14J237   AbstractWebsite

Melt layers are clear indicators of extreme summer warmth on polar ice caps. The visual identification of refrozen meltwater as clear bubble-free layers cannot be used to study some past warm periods, because, in deeper ice, bubbles are lost to clathrate formation. We present here a reliable method to detect melt events, based on the analysis of Kr/Ar and Xe/Ar ratios in ice cores, and apply it to the detection of melt in clathrate ice from the Eemian at NEEM, Greenland. Additionally, melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases, or by altering gas transport in the firn, which affects the gas chronology. We find that the easily visible 1 mm thick bubble-free layers in the WAIS Divide ice core do not contain sufficient melt to alter the gas composition in the core, and do not cause artifacts or discontinuities in the gas chronology. The presence of these layers during winter, and the absence of anomalies in soluble gases, suggests that these layers can be formed by processes other than refreezing of meltwater. Consequently, the absence of bubbles in thin crusts is not in itself proof of a melt event.

Broecker, WS, Severinghaus JP.  1992.  Diminishing oxygen (News and Views). Nature. 358:710-711.   10.1038/358710a0   Abstract
Buizert, C, Severinghaus JP.  2016.  Dispersion in deep polar firn driven by synoptic-scale surface pressure variability. Cryosphere. 10:2099-2111.   10.5194/tc-10-2099-20160   AbstractWebsite

Commonly, three mechanisms of firn air transport are distinguished: molecular diffusion, advection, and near-surface convective mixing. Here we identify and describe a fourth mechanism, namely dispersion driven by synoptic-scale surface pressure variability (or barometric pumping). We use published gas chromatography experiments on firn samples to derive the along-flow dispersivity of firn, and combine this dispersivity with a dynamical air pressure propagation model forced by surface air pressure time series to estimate the magnitude of dispersive mixing in the firn. We show that dispersion dominates mixing within the firn lock-in zone. Trace gas concentrations measured in firn air samples from various polar sites confirm that dispersive mixing occurs. Including dispersive mixing in a firn air transport model suggests that our theoretical estimates have the correct order of magnitude, yet may overestimate the true dispersion. We further show that strong barometric pumping, such as at the Law Dome site, may reduce the gravitational enrichment of delta N-15-N-2 and other tracers below gravitational equilibrium, questioning the traditional definition of the lock-in depth as the depth where delta N-15 enrichment ceases. Last, we propose that Kr-86 excess may act as a proxy for past synoptic activity (or paleo-storminess) at the site.

Seltzer, AM, Buizert C, Baggenstos D, Brook EJ, Ahn J, Yang JW, Severinghaus JP.  2017.  Does delta O-18 of O-2 record meridional shifts in tropical rainfall? Climate of the Past. 13:1323-1338.   10.5194/cp-13-1323-2017   AbstractWebsite

Marine sediments, speleothems, paleo-lake elevations, and ice core methane and delta O-18 of O-2 (delta O-18(atm)) records provide ample evidence for repeated abrupt meridional shifts in tropical rainfall belts throughout the last glacial cycle. To improve understanding of the impact of abrupt events on the global terrestrial biosphere, we present composite records of delta O-18(atm) and inferred changes in fractionation by the global terrestrial biosphere (Delta epsilon(LAND)) from discrete gas measurements in the WAIS Divide (WD) and Siple Dome (SD) Antarctic ice cores. On the common WD timescale, it is evident that maxima in Delta epsilon(LAND) are synchronous with or shortly follow small-amplitude WD CH4 peaks that occur within Heinrich stadials 1, 2, 4, and 5 - periods of low atmospheric CH4 concentrations. These local CH4 maxima have been suggested as markers of abrupt climate responses to Heinrich events. Based on our analysis of the modern seasonal cycle of gross primary productivity (GPP)-weighted delta(OatmO)-O-18 of terrestrial precipitation (the source water for atmospheric O-2 production), we propose a simple mechanism by which Delta epsilon(LAND) tracks the centroid latitude of terrestrial oxygen production. As intense rainfall and oxygen production migrate northward, Delta epsilon(LAND) should decrease due to the underlying meridional gradient in rainfall delta O-18. A southward shift should increase Delta epsilon(LAND). Monsoon intensity also influences delta O-18 of precipitation, and although we cannot determine the relative contributions of the two mechanisms, both act in the same direction. Therefore, we suggest that abrupt increases in Delta epsilon(LAND) unambiguously imply a southward shift of tropical rainfall. The exact magnitude of this shift, however, remains under-constrained by Delta epsilon(LAND).