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Battle, MO, Severinghaus JP, Sofen ED, Plotkin D, Orsi AJ, Aydin M, Montzka SA, Sowers T, Tans PP.  2011.  Controls on the movement and composition of firn air at the West Antarctic Ice Sheet Divide. Atmospheric Chemistry and Physics. 11:11007-11021.   10.5194/acp-11-11007-2011   AbstractWebsite

We sampled interstitial air from the perennial snowpack (firn) at a site near the West Antarctic Ice Sheet Divide (WAIS-D) and analyzed the air samples for a wide variety of gas species and their isotopes. We find limited convective influence (1.4-5.2 m, depending on detection method) in the shallow firn, gravitational enrichment of heavy species throughout the diffusive column in general agreement with theoretical expectations, a similar to 10 m thick lock-in zone beginning at similar to 67 m, and a total firn thickness consistent with predictions of Kaspers et al. (2004). Our modeling work shows that the air has an age spread (spectral width) of 4.8 yr for CO2 at the firn-ice transition. We also find that advection of firn air due to the 22 cm yr(-1) ice-equivalent accumulation rate has a minor impact on firn air composition, causing changes that are comparable to other modeling uncertainties and intrinsic sample variability. Furthermore, estimates of 1 age (the gas age/ice age difference) at WAIS-D appear to be largely unaffected by bubble closure above the lock-in zone. Within the lock-in zone, small gas species and their isotopes show evidence of size-dependent fractionation due to permeation through the ice lattice with a size threshold of 0.36 nm, as at other sites. We also see an unequivocal and unprecedented signal of oxygen isotope fractionation within the lock-in zone, which we interpret as the mass-dependent expression of a size-dependent fractionation process.

Cuffey, KM, Conway H, Gades AM, Hallet B, Lorrain R, Severinghaus JP, Steig EJ, Vaughn B, White JWC.  2000.  Entrainment at cold glacier beds. Geology. 28:351-354.   10.1130/0091-7613(2000)028<0351:eacgb>;2   AbstractWebsite

Here we present measurements of the gas content and isotopic composition of debris-rich basal layers of a polar glacier, Meserve Glacier, Antarctica, which has a basal temperature of -17 degrees C. These measurements show that debris entrainment has occurred without alteration of the glacial ice, and provide the most direct evidence to date that active entrainment occurs at the beds of cold glaciers, without bulk freezing of water. Entrainment at subfreezing temperatures may have formed the U-shaped trough containing Meserve Glacier. In addition to possibly allowing some cold-based glaciers to be important geomorphic agents, entrainment at subfreezing temperatures provides a general mechanism for formation of the dirty basal layers of polar glaciers and ice sheets, which are theologically distinct and can limit the time span of ice-core analyses. Furthermore, accumulating evidence suggests that geomorphologists should abandon the assumption that cold-based glaciers do not slide and abrade their beds.

Kawamura, K, Severinghaus JP, Albert MR, Courville ZR, Fahnestock MA, Scambos T, Shields E, Shuman CA.  2013.  Kinetic fractionation of gases by deep air convection in polar firn. Atmospheric Chemistry and Physics. 13:11141-11155.   10.5194/acp-13-11141-2013   AbstractWebsite

A previously unrecognized type of gas fractionation occurs in firn air columns subjected to intense convection. It is a form of kinetic fractionation that depends on the fact that different gases have different molecular diffusivities. Convective mixing continually disturbs diffusive equilibrium, and gases diffuse back toward diffusive equilibrium under the influence of gravity and thermal gradients. In near-surface firn where convection and diffusion compete as gas transport mechanisms, slow-diffusing gases such as krypton (Kr) and xenon (Xe) are more heavily impacted by convection than fast diffusing gases such as nitrogen (N-2) and argon (Ar), and the signals are preserved in deep firn and ice. We show a simple theory that predicts this kinetic effect, and the theory is confirmed by observations using a newly-developed Kr and Xe stable isotope system in air samples from the Megadunes field site on the East Antarctic plateau. Numerical simulations confirm the effect's magnitude at this site. A main purpose of this work is to support the development of a proxy indicator of past convection in firn, for use in ice-core gas records. To this aim, we also show with the simulations that the magnitude of the kinetic effect is fairly insensitive to the exact profile of convective strength, if the overall thickness of the convective zone is kept constant. These results suggest that it may be feasible to test for the existence of an extremely deep (similar to 30-40 m) convective zone, which has been hypothesized for glacial maxima, by future ice-core measurements.

Orsi, AJ, Kawamura K, Masson-Delmotte V, Fettweis X, Box JE, Dahl-Jensen D, Clow GD, Landais A, Severinghaus JP.  2017.  The recent warming trend in North Greenland. Geophysical Research Letters. 44:6235-6243.   10.1002/2016gl072212   AbstractWebsite

The Arctic is among the fastest warming regions on Earth, but it is also one with limited spatial coverage of multidecadal instrumental surface air temperature measurements. Consequently, atmospheric reanalyses are relatively unconstrained in this region, resulting in a large spread of estimated 30 year recent warming trends, which limits their use to investigate the mechanisms responsible for this trend. Here we present a surface temperature reconstruction over 1982-2011 at NEEM (North Greenland Eemian Ice Drilling Project, 51 degrees W, 77 degrees N), in North Greenland, based on the inversion of borehole temperature and inert gas isotope data. We find that NEEM has warmed by 2.7 +/- 0.33 degrees C over the past 30 years, from the long-term 1900-1970 average of -28.55 +/- 0.29 degrees C. The warming trend is principally caused by an increase in downward longwave heat flux. Atmospheric reanalyses underestimate this trend by 17%, underlining the need for more in situ observations to validate reanalyses.

Severinghaus, JP, Bender ML, Keeling RF, Broecker WS.  1996.  Fractionation of soil gases by diffusion of water vapor, gravitational settling, and thermal diffusion. Geochimica Et Cosmochimica Acta. 60:1005-1018.   10.1016/0016-7037(96)00011-7   AbstractWebsite

Air sampled from the moist unsaturated zone in a sand dune exhibits depletion in the heavy isotopes of N-2 and O-2. We propose that the depletion is caused by a diffusive flux of water vapor out of the dune, which sweeps out the other gases, forcing them to diffuse back into the dune. The heavy isotopes of N-2 and O-2 diffuse back more slowly, resulting in a steady-state depletion of the heavy isotopes in the dune interior. We predict the effect's magnitude with molecular diffusion theory and reproduce it in a laboratory simulation, finding good agreement between field, theory, and lab. The magnitude of the effect is governed by the ratio of the binary diffusivities against water vapor of a pair of gases, and increases similar to linearly with the difference between the water vapor mole fraction of the site and the advectively mixed reservoir with which it is in diffusive contact (in most cases the atmosphere). The steady-state effect is given by delta(i) = [i/j/i(0)/j(0) - 1] 10(3) parts per thousand congruent to [(1 - x(H2O)/1 - x(H2O0))((Dj-H2O/Di-H2O)-1) -1] 10(3) parts per thousand, where delta(i) is the fractional deviation in permil of the gas i/gas j ratio from the advectively mixed reservoir, x(H2O) and x(H2O0) are respectively the mole fractions of water vapor at the site and in the advectively mixed reservoir, and D-i-H2O is the binary diffusion coefficient of gas i with water vapor. The effect is independent of scale at steady state, but approaches steady state with the time constant of diffusion set by the length scale. Exploiting the mechanism, we make an experimental estimate of the relative diffusivities of O-2 and N-2 against water vapor, finding that O-2 diffuses 3.6 +/- 0.3% faster than N-2 despite its greater mass. We also confirm in the study dune the presence of two additional known processes: gravitational fractionation, heretofore seen only in the unconsolidated firn of polar ice sheets, and thermal diffusion, well described in laboratory studies but not seen previously in nature. We predict that soil gases in general will exhibit the three effects described here, the water vapor flux fractionation effect, gravitational fractionation, and thermal diffusion. However, our analysis neglects Knudsen diffusion and thus may be inapplicable to fine-grained soils.