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Baggenstos, D, Severinghaus JP, Mulvaney R, McConnell JR, Sigl M, Maselli O, Petit JR, Grente B, Steig EJ.  2018.  A horizontal ice core from Taylor Glacier, its implications for Antarctic climate history, and an improved Taylor Dome ice core time scale. Paleoceanography and Paleoclimatology. 33:778-794.   10.1029/2017pa003297   AbstractWebsite

Ice core records from Antarctica show mostly synchronous temperature variations during the last deglacial transition, an indication that the climate of the entire continent reacted as one unit to the global changes. However, a record from the Taylor Dome ice core in the Ross Sea sector of East Antarctica has been suggested to show a rapid warming, similar in style and synchronous with the Oldest Dryas-Bolling warming in Greenland. Since publication of the Taylor Dome record, a number of lines of evidence have suggested that this interpretation is incorrect and reflects errors in the underlying time scale. The issues raised regarding the dating of Taylor Dome currently linger unresolved, and the original time scale remains the de facto chronology. We present new water isotope and chemistry data from nearby Taylor Glacier to resolve the confusion surrounding the Taylor Dome time scale. We find that the Taylor Glacier record is incompatible with the original interpretation of the Taylor Dome ice core, showing that the warming in the area was gradual and started at similar to 18 ka BP (before 1950) as seen in other East Antarctic ice cores. We build a consistent, up-to-date Taylor Dome chronology from 0 to 60 ka BP by combining new and old age markers based on synchronization to other ice core records. The most notable feature of the new TD2015 time scale is a gas age-ice age difference of up to 12,000 years during the Last Glacial Maximum, by far the largest ever observed.

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Huber, C, Beyerle U, Leuenberger M, Schwander J, Kipfer R, Spahni R, Severinghaus JP, Weiler K.  2006.  Evidence for molecular size dependent gas fractionation in firn air derived from noble gases, oxygen, and nitrogen measurements. Earth and Planetary Science Letters. 243:61-73.   10.1016/j.epsl.2005.12.036   AbstractWebsite

We present elemental and isotopic measurements of noble gases (He, Ne, Ar, Kr, and Xe), oxygen and nitrogen of firn air from two sites. The first set of samples was taken in 1998 at the summit of the Devon Ice Cap in the eastern part of Devon Island. The second set was taken in 2001 at NGRIP location (North Greenland). He and Ne are heavily enriched relative to Ar with respect to the atmosphere in the air near the close-off depth at around 50-70 in. The enrichment increases with depth and reaches the maximum value in the deepest samples just above the zone of impermeable ice where no free air could be extracted anymore. Similarly, elemental ratios of O(2)/N(2), O(2)/Ar and Ar/N(2) are increasing with depth. In contrast but in line with expectations, isotopic ratios of (15)N/(14)N, (18)O/(16)O, and (36)Ar/(40)Ar show no significant enrichment near the close-off depth. The observed isotopic ratios in the firn air column can be explained within the uncertainty ranges by the well-known processes of gravitational enrichment and thermal diffusion. To explain the elemental ratios, however, an additional fractionation process during bubble inclusion has to be considered. We implemented this additional process into our firn air model. The fractionation factors were found by fitting model profiles to the data. We found a very similar close-off fractionation behavior for the different molecules at both sites. For smaller gas species (mainly He and Ne) the fractionation factors are linearly correlated to the molecule size, whereas for diameters greater than about 3.6 A the fractionation seems to be significantly smaller or even negligible. An explanation for this size dependent fractionation process could be gas diffusion through the ice lattice. At Devon Island the enrichment at the bottom of the firn air column is about four times higher compared to NGRIP. We explain this by lower firn diffusivity at Devon Island, most probably due to melt layers, resulting in significantly reduced back diffusion of the excess gas near the close-off depth. The results of this study considerably increase the understanding of the processes occurring during air bubble inclusion near the close-off depth in firn and can help to improve the interpretation of direct firn air measurements, as well as air bubble measurements in ice cores, which are used in numerous studies as paleo proxies. (c) 2006 Elsevier B.V. All rights reserved.

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Severinghaus, JP, Battle MO.  2006.  Fractionation of gases in polar lee during bubble close-off: New constraints from firn air Ne, Kr and Xe observations. Earth and Planetary Science Letters. 244:474-500.   10.1016/j.epsl.2006.01.032   AbstractWebsite

Gas ratios in air withdrawn from polar firn (snowpack) show systematic enrichments of Ne/N(2), O(2)/N(2) and Ar/N(2), in the firn-ice transition region where bubbles are closing off. Air from the bubbles in polar ice is correspondingly depleted in these ratios, after accounting for gravitational effects. Gas in the bubbles becomes fractionated during the process of bubble close-off and fractionation may continue as ice cores are stored prior to analysis. We present results from firn air studies at South Pole and Siple Dome, Antarctica, which add Ne, Kr and Xe measurements to the suite of observations. Ne, O(2) and Ar appear to be preferentially excluded from the shrinking and occluding bubbles, and these gases therefore accumulate in the residual firn air, creating a progressive enrichment with time (and depth) in firn air. Early sealing of gases by thin horizontal impermeable layers into a nondiffusive zone or "lock-in zone" greatly enhances this enrichment. A simple model of the bubble close-off fractionation and lock-in zone enrichment fits the data adequately. The model presumes that fractionation is caused by selective permeation of gas through the ice lattice from slightly overpressured bubbles. The effect appears to be size-dependent, because Ne, 02 and Ar have smaller effective molecular diameters than N(2), and fractionation increases strongly with decreasing size. Ne is fractionated 34 2 times more than 0, in South Pole firn air and reaches an enrichment of 90 parts per thousand in the deepest sample. The large atoms Kr and Xe do not appear to be fractionated by this process, despite the large size difference between the two gases, suggesting a threshold atomic diameter of similar to 3.6 angstrom above which the probability becomes very small that the gas will escape from the bubble. These findings have implications for ice core and firn air studies that use gas ratios to infer paleotemperature, chronology and past atmospheric composition. (c) 2006 Elsevier B.V.. All rights reserved.