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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.

Goodge, JW, Severinghaus JP.  2016.  Rapid Access Ice Drill: a new tool for exploration of the deep Antarctic ice sheets and subglacial geology. Journal of Glaciology. 62:1049-1064.   10.1017/jog.2016.97   AbstractWebsite

A new Rapid Access Ice Drill (RAID) will penetrate the Antarctic ice sheets in order to create borehole observatories and take cores in deep ice, the glacial bed and bedrock below. RAID is a mobile drilling system to make multiple long, narrow boreholes in a single field season in Antarctica. RAID is based on a mineral exploration-type rotary rock-coring system using threaded drill pipe to cut through ice using reverse circulation of a non-freezing fluid for pressure-compensation, maintenance of temperature and removal of ice cuttings. Near the bottom of the ice sheet, a wireline latching assembly will enable rapid coring of ice, the glacial bed and bedrock below. Once complete, boreholes will be kept open with fluid, capped and available for future down-hole measurement of temperature gradient, heat flow, ice chronology and ice deformation. RAID is designed to penetrate up to 3300 m of ice and take cores in <200 hours, allowing completion of a borehole and coring in similar to 10 d at each site. Together, the rapid drilling capability and mobility of the system, along with ice-penetrating imaging methods, will provide a unique 3-D picture of interior and subglacial features of the Antarctic ice sheets.

Buizert, C, Baggenstos D, Jiang W, Purtschert R, Petrenko VV, Lu ZT, Muller P, Kuhl T, Lee J, Severinghaus JP, Brook EJ.  2014.  Radiometric Kr-81 dating identifies 120,000-year-old ice at Taylor Glacier, Antarctica. Proceedings of the National Academy of Sciences of the United States of America. 111:6876-6881.   10.1073/pnas.1320329111   AbstractWebsite

We present successful Kr-81-Kr radiometric dating of ancient polar ice. Krypton was extracted from the air bubbles in four similar to 350-kg polar ice samples from Taylor Glacier in the McMurdo Dry Valleys, Antarctica, and dated using Atom Trap Trace Analysis (ATTA). The Kr-81 radiometric ages agree with independent age estimates obtained from stratigraphic dating techniques with a mean absolute age offset of 6 +/- 2.5 ka. Our experimental methods and sampling strategy are validated by (i) Kr-85 and Ar-39 analyses that show the samples to be free of modern air contamination and (ii) air content measurements that show the ice did not experience gas loss. We estimate the error in the Kr-81 ages due to past geomagnetic variability to be below 3 ka. We show that ice from the previous interglacial period (Marine Isotope Stage 5e, 130-115 ka before present) can be found in abundance near the surface of Taylor Glacier. Our study paves the way for reliable radiometric dating of ancient ice in blue ice areas and margin sites where large samples are available, greatly enhancing their scientific value as archives of old ice and meteorites. At present, ATTA Kr-81 analysis requires a 40-80-kg ice sample; as sample requirements continue to decrease, Kr-81 dating of ice cores is a future possibility.

Buizert, C, Petrenko VV, Kavanaugh JL, Cuffey KM, Lifton NA, Brook EJ, Severinghaus JP.  2012.  In situ cosmogenic radiocarbon production and 2-D ice flow line modeling for an Antarctic blue ice area. Journal of Geophysical Research-Earth Surface. 117   10.1029/2011jf002086   AbstractWebsite

Radiocarbon measurements at ice margin sites and blue ice areas can potentially be used for ice dating, ablation rate estimates and paleoclimatic reconstructions. Part of the measured signal comes from in situ cosmogenic C-14 production in ice, and this component must be well understood before useful information can be extracted from C-14 data. We combine cosmic ray scaling and production estimates with a two-dimensional ice flow line model to study cosmogenic C-14 production at Taylor Glacier, Antarctica. We find (1) that C-14 production through thermal neutron capture by nitrogen in air bubbles is negligible; (2) that including ice flow patterns caused by basal topography can lead to a surface C-14 activity that differs by up to 25% from the activity calculated using an ablation-only approximation, which is used in all prior work; and (3) that at high ablation margin sites, solar modulation of the cosmic ray flux may change the strength of the dominant spallogenic production by up to 10%. As part of this effort we model two-dimensional ice flow along the central flow line of Taylor Glacier. We present two methods for parameterizing vertical strain rates, and assess which method is more reliable for Taylor Glacier. Finally, we present a sensitivity study from which we conclude that uncertainties in published cosmogenic production rates are the largest source of potential error. The results presented here can inform ongoing and future C-14 and ice flow studies at ice margin sites, including important paleoclimatic applications such as the reconstruction of paleoatmospheric C-14 content of methane.

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.

Petrenko, VV, Smith AM, Brook EJ, Lowe D, Riedel K, Brailsford G, Hua Q, Schaefer H, Reeh N, Weiss RF, Etheridge D, Severinghaus JP.  2009.  14C-CH4 Measurements in Greenland Ice: Investigating Last Glacial Termination CH4 Sources. Science. 324:506-508.   10.1126/science.1168909   AbstractWebsite

The cause of a large increase of atmospheric methane concentration during the Younger Dryas-Preboreal abrupt climatic transition (similar to 11,600 years ago) has been the subject of much debate. The carbon-14 (C-14) content of methane ((CH4)-C-14) should distinguish between wetland and clathrate contributions to this increase. We present measurements of (CH4)-C-14 in glacial ice, targeting this transition, performed by using ice samples obtained from an ablation site in west Greenland. Measured (CH4)-C-14 values were higher than predicted under any scenario. Sample (CH4)-C-14 appears to be elevated by direct cosmogenic C-14 production in ice. C-14 of CO was measured to better understand this process and correct the sample (CH4)-C-14. Corrected results suggest that wetland sources were likely responsible for the majority of the Younger Dryas-Preboreal CH4 rise.

Schaefer, H, Petrenko VV, Brook EJ, Severinghaus JP, Reeh N, Melton JR, Mitchell L.  2009.  Ice stratigraphy at the Pakitsoq ice margin, West Greenland, derived from gas records. Journal of Glaciology. 55:411-421.   10.3189/002214309788816704   AbstractWebsite

Horizontal ice-core sites, where ancient ice is exposed at the glacier surface, offer unique opportunities for paleo-studies of trace components requiring large sample volumes. Following previous work at the Pakitsoq ice margin in West Greenland, we use a combination of geochemical parameters measured in the ice matrix (delta(18)O(ice)) and air occlusions (delta(18)O(atm), delta(15)N of N(2) and methane concentration) to date ice layers from specific climatic intervals. The data presented here expand our understanding of the stratigraphy and three-dimensional structure of ice layers outcropping at Pakitsoq. Sections containing ice from every distinct climatic interval during Termination I, including Last Glacial Maximum, Bolling/Allerod, Younger Dryas and the early Holocene, are identified. In the early Holocene, we find evidence for climatic fluctuations similar to signals found in deep ice cores from Greenland. A second glacial-interglacial transition exposed at the extreme margin of the ice is identified as another outcrop of Termination I (rather than the onset of the Eemian interglacial as postulated in earlier work). Consequently, the main structural feature at Pakitsoq is a large-scale anticline with accordion-type folding in both exposed sequences of the glacial-Holocene transition, leading to multiple layer duplications and age reversals.

Kobashi, T, Severinghaus JP, Barnola JM.  2008.  4 +/- 1.5 degrees C abrupt warming 11,270 yr ago identified from trapped air in Greenland ice. Earth and Planetary Science Letters. 268:397-407.   10.1016/j.epsl.2008.01.032   AbstractWebsite

Nitrogen and argon isotopes in air trapped in a Greenland ice core (GISP2) show two prominent peaks in the interval 11,800-10,800 B.P., which indicate two large abrupt warming events. The first abrupt wanning (10 +/- 4 degrees C) is the widely documented event at the end of the Younger Dryas. Here, we report on the second abrupt warming (4 +/- 1.5 degrees C), which occurred at the end of a short lived cooler interval known as the Preboreal Oscillation (11,270 +/- 30 B.P.). A rapid snow accumulation increase suggests that the climatic transition may have occurred within a few years. The character of the Preboreal Oscillation and the subsequent abrupt warming is similar to the Dansgaard-Oeschger (D/O) events in the last glacial period, suggestive of a common mechanism, but different from another large climate change at 8,200 B.P., in which cooling was abrupt but subsequent warming was gradual. The large abrupt warming at 11,270 B.P. may be considered to be the final D/O event prior to the arrival of the present stable and warm epoch. (c) 2008 Elsevier B.V. All rights reserved.

Petrenko, VV, Smith AM, Brailsford G, Riedel K, Hua Q, Lowe D, Severinghaus JP, Levchenko V, Bromley T, Moss R, Muhle J, Brook EJ.  2008.  A new method for analyzing (14)C of methane in ancient air extracted from glacial ice. Radiocarbon. 50:53-73. AbstractWebsite

We present a new method developed for measuring radiocarbon of methane ((14)CH(4)) in ancient air samples extracted from glacial ice and dating 11,000-15,000 calendar years before present. The small size (similar to 20 mu g CH(4) carbon), low CH(4) concentrations ([CH(4)], 400-800 parts per billion [ppb]), high carbon monoxide concentrations ([CO]), and low (14)C activity of the samples created unusually high risks of contamination by extraneous carbon. Up to 2500 ppb CO in the air samples was quantitatively removed using the Sofnocat reagent. (14)C procedural blanks were greatly reduced through the construction of a new CH(4) conversion line utilizing platinized quartz wool for CH(4) combustion and the use of an ultra-high-purity iron catalyst for graphitization. The amount and (14)C activity of extraneous carbon added in the new CH(4) conversion line were determined to be 0.23 +/- 0.16 pg and 23.57 +/- 16.22 pMC, respectively. The amount of modern (100 pMC) carbon added during the graphitization step has been reduced to 0.03 mu g. The overall procedural blank for all stages of sample handling was 0.75 0.38 pMC for similar to 20-mu g, (14)C-free air samples with [CH(4)] of 500 ppb. Duration of the graphitization reactions for small (<25 mu g C) samples was greatly reduced and reaction yields improved through more efficient water vapor trapping and the use of a new iron catalyst with higher surface area. (14)C corrections for each step of sample handling have been determined. The resulting overall (14)CH(4) uncertainties for the ancient air samples are similar to 1.0 pMC.

Aciego, SM, Cuffey KM, Kavanaugh JL, Morse DL, Severinghaus JP.  2007.  Pleistocene ice and paleo-strain rates at Taylor Glacier, Antarctica. Quaternary Research. 68:303-313.   10.1016/j.yqres.2007.07.013   AbstractWebsite

Ice exposed in ablation zones of ice sheets can be a valuable source of samples for paleoclimate studies and information about long-term ice dynamics. We report a 28-km long stable isotope sampling transect along a flowline on lower Taylor Glacier, Antarctica, and show that ice from the last glacial period is exposed here over tens of kilometers. Gas isotope analyses on a small number of samples confirm our age hypothesis. These chronostratigraphic data contain information about past ice dynamics and in particular should be sensitive to the longitudinal strain rate on the north flank of Taylor Dome, averaged over millennia. The imprint of climatic changes on ice dynamics may be discernible in these data. (c) 2007 University of Washington. All rights reserved.

Taylor, KC, White JWC, Severinghaus JP, Brook EJ, Mayewski PA, Alley RB, Steig EJ, Spencer MK, Meyerson E, Meese DA, Lamorey GW, Grachev A, Gow AJ, Barnett BA.  2004.  Abrupt climate change around 22 ka on the Siple Coast of Antarctica. Quaternary Science Reviews. 23:7-15.   10.1016/j.quascirev.2003.09.004   AbstractWebsite

A new ice core from Siple Dome, Antarctica suggests the surface temperature increased by similar to6degreesC in just several decades at approximately 22 ka BP. This abrupt change did not occur 500 kin away in the Byrd ice core, or in climate proxy records in the Siple Dome core indicative of the mid-latitude Pacific. This demonstrates there was significant spatial heterogeneity in the response of the Antarctic climate during the last deglaciation and draws attention to unexplained mechanisms of abrupt climate change in Antarctica. (C) 2003 Elsevier Ltd. All rights reserved.

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.

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.

Severinghaus, JP, Brook EJ.  1999.  Abrupt climate change at the end of the last glacial period inferred from trapped air in polar ice. Science. 286:930-934.   10.1126/science.286.5441.930   AbstractWebsite

The last glacial period was terminated by an abrupt warming event in the North Atlantic similar to 15,000 years before the present, and warming events of similar age have been reported from Low Latitudes. Understanding the mechanism of this termination requires that the precise relative timing of abrupt climate warming in the tropics versus the North Atlantic be known. Nitrogen and argon isotopes in trapped air in Greenland ice show that the Greenland Summit warmed 9 +/- 3 degrees C over a period of several decades, beginning 14,672 years ago. Atmospheric methane concentrations rose abruptly over a similar to 50-year period and began their increase 20 to 30 years after the onset of the abrupt Greenland warming. These data suggest that tropical climate became warmer or wetter (or both) similar to 20 to 80 years after the onset of Greenland warming, supporting a North Atlantic rather than a tropical trigger for the climate event.