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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.

Rasmussen, SO, Abbott PM, Blunier T, Bourne AJ, Brook E, Buchardt SL, Buizert C, Chappellaz J, Clausen HB, Cook E, Dahl-Jensen D, Davies SM, Guillevic M, Kipfstuhl S, Laepple T, Seierstad IK, Severinghaus JP, Steffensen JP, Stowasser C, Svensson A, Vallelonga P, Vinther BM, Wilhelms F, Winstrup M.  2013.  A first chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core. Climate of the Past. 9:2713-2730.   10.5194/cp-9-2713-2013   AbstractWebsite

A stratigraphy-based chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core has been derived by transferring the annual layer counted Greenland Ice Core Chronology 2005 (GICC05) and its model extension (GICC05modelext) from the NGRIP core to the NEEM core using 787 match points of mainly volcanic origin identified in the electrical conductivity measurement (ECM) and dielectrical profiling (DEP) records. Tephra horizons found in both the NEEM and NGRIP ice cores are used to test the matching based on ECM and DEP and provide five additional horizons used for the timescale transfer. A thinning function reflecting the accumulated strain along the core has been determined using a Dansgaard-Johnsen flow model and an isotope-dependent accumulation rate parameterization. Flow parameters are determined from Monte Carlo analysis constrained by the observed depth-age horizons. In order to construct a chronology for the gas phase, the ice age-gas age difference (Delta age) has been reconstructed using a coupled firn densification-heat diffusion model. Temperature and accumulation inputs to the Delta age model, initially derived from the water isotope proxies, have been adjusted to optimize the fit to timing constraints from delta N-15 of nitrogen and high-resolution methane data during the abrupt onset of Greenland interstadials. The ice and gas chronologies and the corresponding thinning function represent the first chronology for the NEEM core, named GICC05modelext-NEEM-1. Based on both the flow and firn modelling results, the accumulation history for the NEEM site has been reconstructed. Together, the timescale and accumulation reconstruction provide the necessary basis for further analysis of the records from NEEM.

Kobashi, T, Severinghaus JP, Kawamura K.  2008.  Argon and nitrogen isotopes of trapped air in the GISP2 ice core during the Holocene epoch (0-11,500 B.P.): Methodology and implications for gas loss processes. Geochimica Et Cosmochimica Acta. 72:4675-4686.   10.1016/j.gca.2008.07.006   AbstractWebsite

Argon and nitrogen isotopes of air in polar ice cores provide constraints on past temperature and firn thickness, with relevance to past climate. We developed a method to simultaneously measure nitrogen and argon isotopes in trapped air from the same sample of polar ice. This method reduces the time required for analysis, allowing large numbers of measurements. We applied this method to the entire Holocene sequence of the GISP2 ice core (82.37-1692.22 m) with a 10-20 year sampling interval (670 depths). delta(40)Ar and delta(15)N show elevated values in the oldest part of the dataset, consistent with a thicker firn layer and increased temperature gradient in the firn due to the legacy of the abrupt warming at the end of the Younger Dryas interval and the gradual warming during the Preboreal interval (11.5-10.0 ka). The Preboreal Oscillation and the 8.2k event are clearly recorded. The data show remarkable stability after the 8.2k event. Available data suggests that post-coring gas loss involves two distinct types of fractionation. First, smaller molecules with less than a certain threshold size leak through the ice lattice with little isotopic fractionation. Second, gas composition changes via gas loss through microcracks, which induces isotopic fractionation. These two gas loss processes can explain most trends in our data and in other ice core records. (C) 2008 Elsevier Ltd. All rights reserved.

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.

Grachev, AM, Severinghaus JP.  2005.  A revised +10 +/- 4 degrees C magnitude of the abrupt change in Greenland temperature at the Younger Dryas termination using published GISP2 gas isotope data and air thermal diffusion constants. Quaternary Science Reviews. 24:513-519.   10.1016/j.quascirev.2004.10.016   AbstractWebsite

We revisit the portion of (Nature 391 (1998) 141) devoted to the abrupt temperature increase reconstruction at the Younger Dryas/Preboreal transition. The original estimate of + 5 to + 10 degrees C abrupt warming is revised to + 10 +/- 4 degrees C. The gas isotope data from the original work were employed, combined with recently measured precise air thermal diffusion constants (Geochim. Cosmochim. Acta 67 (2003a) 345; J. Phys. Chem. 23A (2003b) 4636). The new constants allow a robust interpretation of the gas isotope signal in terms of temperature change. This was not possible at the time of the original work, when no air constants were available. Three quasi-independent approaches employed in this work all give the same result of a + 10 degrees C warming in several decades or less. The new result provides a firm target for climate models that attempt to predict future climates. (c) 2005 Elsevier Ltd. All rights reserved.

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.

Severinghaus, JP, Grachev A, Luz B, Caillon N.  2003.  A method for precise measurement of argon 40/36 and krypton/argon ratios in trapped air in polar ice with applications to past firn thickness and abrupt climate change in Greenland and at Siple Dome, Antarctica. Geochimica Et Cosmochimica Acta. 67:325-343.   10.1016/s0016-7037(02)00965-1   AbstractWebsite

We describe a method for measuring the (40)Ar/(36)Ar ratio and the (84)Kr/(36)Ar ratio in air from bubbles trapped in ice cores. These ratios can provide constraints on the past thickness of the firn layer at the ice core site and on the magnitude of past rapid temperature variations when combined with measured (15)N/(14)N. Both variables contribute to paleoclimatic studies and ultimately to the understanding of the controls on Earth's climate. The overall precision of the (40)Ar/(36)Ar method (1 standard error of the mean) is 0.012parts per thousand for a sample analyzed in duplicate, corresponding to +/-0.6 in in reconstructed firn thickness. We use conventional dynamic isotope ratio mass spectrometry with minor modifications and special gas handling techniques designed to avoid fractionation. About 100 g of ice is used for a duplicate pair of analyses. An example of the technique applied to the GISP2 ice core yields an estimate of 11 +/- 3K of abrupt warming at the end of the last glacial period 15,000 years ago. The krypton/argon ratio can provide a diagnostic of argon leakage out of the bubbles, which may happen (naturally) during bubble close-off or (artifactually) if samples are warmed near the freezing point during core retrieval or storage. Argon leakage may fractionate the remaining (40)Ar/(36)Ar ratio by +0.007parts per thousand per parts per thousand change in (84)Kr/(36)Ar, introducing a possible bias in reconstructed firn thickness of about +2 in if thermal diffusion is not accounted for or +6 in if thermal diffusion effects are quantified with measured (15)N/(14)N. Reproducibility of (84)Kr/(36)Ar measured in air is about +/-0.2parts per thousand (1 standard error of the mean) but is about +/-1parts per thousand for ice core samples. Ice core samples are systematically enriched in (84)Kr/(36)Ar relative to atmosphere by similar to5parts per thousand, probably reflecting preferential size-dependent exclusion of the smaller argon atom during bubble entrapment. Recent results from the Siple Dome ice core reveal two climate events during the last deglaciation, including an 18-in reduction in firn thickness associated with an abrupt warming at sometime between 18 and 22 kyr BP and a partial or total removal of the firn during an ablation event at 15.3 kyr BP. Copyright (C) 2003 Elsevier Science Ltd.

Severinghaus, JP, Grachev A, Battle M.  2001.  Thermal fractionation of air in polar firn by seasonal temperature gradients. Geochemistry Geophysics Geosystems. 2   10.1029/2000GC000146   AbstractWebsite

Air withdrawn from the top 5-15 m of the polar snowpack (fim) shows anomalous enrichment of heavy gases during summer, including inert gases. Following earlier work, we ascribe this to thermal diffusion, the tendency of a gas mixture to separate in a temperature gradient, with heavier molecules migrating toward colder regions. Summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion. Here we explore and quantify this process further in order to (1) correct for bias caused by thermal diffusion in firn air and ice core air isotope records, (2) help calibrate a new technique for measuring temperature change in ice core gas records based on thermal diffusion [Severinghaus et al., 1998], and (3) address whether air in polar snow convects during winter and, if so, whether it creates a rectification of seasonality that could bias the ice core record. We sampled air at 2-m-depth intervals from the top 15 m of the firn at two Antarctic sites, Siple Dome and South Pole, including a winter sampling at the pole. We analyzed (15)N/(14)N, (40)Ar/(36)Ar, (40)Ar/(38)Ar, (18)O/(16)O of O(2), O(2)/N(2), (84)Kr/(36)Ar, and (132)Xe/(36)Ar. The results show the expected pattern of fractionation and match a gas diffusion model based on first principles to within 30%. Although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision, relative values of different gas pairs may. At Siple Dome, delta (40)Ar/4 is 66 +/- 2% as sensitive to thermal diffusion as delta (15)N, in agreement with laboratory calibration; delta (18)O/2 is 83 +/- 3%, and delta (84)Kr/48 is 33 +/- 3% as sensitive as delta (15)N. The corresponding figures for summer South Pole are 64 +/- 2%, 81 +/- 3%, and 34 +/- 3%. Accounting for atmospheric change, the figure for deltaO(2)/N(2)/4 is 90 +/- 3% at Siple Dome. Winter South Pole shows a strong depletion of heavy gases as expected. However, the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities (except for Kr, constant at 34 +/- 4%), suggesting the action of some other process that is not currently understood. No evidence for wintertime convection or a rectifier effect is seen.