Publications

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2011
Brook, EJ, Severinghaus JP.  2011.  Methane and megafauna. Nature Geoscience. 4:271-272.   10.1038/ngeo1140   AbstractWebsite
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2006
Buerki, PR, Jackson BC, Schilling T, Rufer T, Severinghaus JP.  2006.  Improved helium exchange gas cryostat and sample tube designs for automated gas sampling and cryopumping. Geochemistry Geophysics Geosystems. 7   10.1029/2006gc001341   AbstractWebsite

[ 1] In order to eliminate the use of liquid helium for the extraction of atmospheric gases from polar ice cores, two units of a redesigned top load helium exchange gas cryostat were built and tested. The cryostats feature the shortest and largest diameter sample wells built to date, a base temperature below 7 Kelvin, and a sample well without baffles. The cryostats allowed shortening the length and thus increasing the gas pressure inside our sample tubes by 58% and increasing the amount of sample ending up in the mass spectrometer by 4.4%. The cryostats can either be used as mobile stand-alone units for manual gas processing lines or integrated into a fully automated vacuum extraction and gas analysis line. For the latter application the cryostat was equipped with a custom-designed automated changeover system.

2005
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.

2004
Landais, A, Caillon N, Severinghaus J, Barnola JM, Goujon C, Jouzel J, Masson-Delmotte V.  2004.  Isotopic measurements of air trapped in ice to quantify temperature changes. Comptes Rendus Geoscience. 336:963-970.   10.1016/j.crte.2004.03.013   AbstractWebsite

Isotopic measurements of air trapped in ice to quantify temperature changes. Isotopic measurements in polar ice core have shown a succession of rapid warming periods during the last glacial period over Greenland. However, this method underestimates the surface temperature variations. A new method based on gas thermal diffusion in the firn manages to quantify surface temperature variations through associated isotopic fractionations. We developed a method to extract air from the ice and to perform isotopic measurements to reduce analytical uncertainties to 0.006 and 0.020parts per thousand for delta(15)N and delta(40)Ar. It led to a 16 +/- 1.5degreesC surface temperature variation during a rapid warming (-70000 yr). (C) 2004 Academie des sciences. Publie par Elsevier SAS. Tous droits reserves.

2003
Grachev, AM, Severinghaus JP.  2003.  Laboratory determination of thermal diffusion constants for N-29(2)/N-28(2) in air at temperatures from-60 to 0 degrees C for reconstruction of magnitudes of abrupt climate changes using the ice core fossil-air paleothermometer. Geochimica Et Cosmochimica Acta. 67:345-360.   10.1016/s0016-7037(02)01115-8   AbstractWebsite

Rapid temperature change causes fractionation of isotopic gaseous species in air in firn (snow) by thermal diffusion, producing a signal that is preserved in trapped air bubbles as the snow forms ice. Using a model of heat penetration and gas diffusion in the firn, as well as the values of appropriate thermal diffusion constants, it is possible to reconstruct the magnitude of a particular paleoclimate change. Isotopic nitrogen in air serves as a convenient tracer for such paleoreconstruction, because the ratio N-29(2)/N-28(2) has stayed extremely constant in the atmosphere for greater than or equal to10(6) years. However, prior to this work no data were available for thermal diffusion of N-29(2)/N-28(2) in air, but only in pure N-2. We devised a laboratory experiment allowing fractionation of gases by thermal diffusion in a small, tightly controlled temperature difference. A mass spectrometer was employed in measuring the resulting fractionations yielding measurement precision greater than was attainable by earlier thermal diffusion investigators. Our laboratory experiments indicate that the value of the thermal diffusion sensitivity (Omega) for N-29(2)/N-28(2) in air is +(14.7 +/- 0.5) X 10(-3) per mil/degreesC when the average temperature is -30.0degreesC. The corresponding value for N-29(2)/N-28(2) in pure N-2 that we find is +(15.3 +/- 0.4) X 10(-3) per mil/degreesC at -30.6degreesC, in agreement with the previously available literature data within their large range of uncertainty. We find that an empirical equation, Omega = (8.656/T-K - 1232/T-K(2)) +/- 3% per mil/degreesC, describes the slight variation of the sensitivity values for N-29(2)/N-28(2) in air with temperature in the range of -60 to 0degreesC. A separate set of experiments also described in this paper rules out adsorption as a candidate for producing additional temperature change-driven fractionation of N-29(2)/N-28(2) in the firn air. The combined newly obtained data constitute a calibration of the fossil-air paleothermometer with respect to isotopic nitrogen and will serve to improve the estimates of the magnitudes of past abrupt climate changes recorded in ice cores. Copyright (C) 2003 Elsevier Science Ltd.

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.

2001
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.