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Rosen, JL, Brook EJ, Severinghaus JP, Blunier T, Mitchell LE, Lee JE, Edwards JS, Gkinis V.  2014.  An ice core record of near-synchronous global climate changes at the Bolling transition. Nature Geoscience. 7:459-463.   10.1038/ngeo2147   AbstractWebsite

The abrupt warming that initiated the Bolling-Allerod interstadial was the penultimate warming in a series of climate variations known as Dansgaard-Oeschger events. Despite the clear expression of this transition in numerous palaeoclimate records, the relative timing of climate shifts in different regions of the world and their causes are subject to debate. Here we explore the phasing of global climate change at the onset of the Bolling-Allerod using air preserved in bubbles in the North Greenland Eemian ice core. Specifically, we measured methane concentrations, which act as a proxy for low-latitude climate, and the N-15/N-14 ratio of N-2, which reflects Greenland surface temperature, over the same interval of time. We use an atmospheric box model and a firn air model to account for potential uncertainties in the data, and find that changes in Greenland temperature and atmospheric methane emissions at the Bolling onset occurred essentially synchronously, with temperature leading by 4.5(-24)(+21) years. We cannot exclude the possibility that tropical climate could iag changing methane concentrations by up to several decades, if the initial methane rise came from boreal sources alone. However, because even boreal methane-producing regions lie far from Greenland, we conclude that the mechanism that drove abrupt change at this time must be capable of rapidly transmitting climate changes across the globe.

Schaefer, H, Whiticar MJ, Brook EJ, Petrenko VV, Ferretti DF, Severinghaus JP.  2006.  Ice record of delta C-13 for atmospheric CH4 across the Younger Dryas-Preboreal transition. Science. 313:1109-1112.   10.1126/science.1126562   AbstractWebsite

We report atmospheric methane carbon isotope ratios (delta(CH4)-C-13) from the Western Greenland ice margin spanning the Younger Dryas - to - Preboreal (YD-PB) transition. Over the recorded similar to 800 years, delta(CH4)-C-13 was around - 46 per mil (parts per thousand); that is, similar to 1 parts per thousand higher than in the modern atmosphere and similar to 5.5 parts per thousand higher than would be expected from budgets without C-13-rich anthropogenic emissions. This requires higher natural C-13-rich emissions or stronger sink fractionation than conventionally assumed. Constant delta(CH4)-C-13 during the rise in methane concentration at the YD-PB transition is consistent with additional emissions from tropical wetlands, or aerobic plant CH4 production, or with a multisource scenario. A marine clathrate source is unlikely.

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.

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.

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.

Birner, B, Buizert C, Wagner TJW, Severinghaus JP.  2018.  The influence of layering and barometric pumping on firn air transport in a 2-D model. Cryosphere. 12:2021-2037.   10.5194/tc-12-2021-2018   AbstractWebsite

Ancient air trapped in ice core bubbles has been paramount to developing our understanding of past climate and atmospheric composition. Before air bubbles become isolated in ice, the atmospheric signal is altered in the firn column by transport processes such as advection and diffusion. However, the influence of low-permeability layers and barometric pumping (driven by surface pressure variability) on firn air transport is not well understood and is not readily captured in conventional one-dimensional (1-D) firn air models. Here we present a two-dimensional (2-D) trace gas advection-diffusion-dispersion model that accounts for discontinuous horizontal layers of reduced permeability. We find that layering or barometric pumping individually yields too small a reduction in gravitational settling to match observations. In contrast, when both effects are active, the model's gravitational fractionation is suppressed as observed. Layering focuses airflows in certain regions in the 2-D model, which acts to amplify the dispersive mixing resulting from barometric pumping. Hence, the representation of both factors is needed to obtain a realistic emergence of the lock-in zone. In contrast to expectations, we find that the addition of barometric pumping in the layered 2-D model does not substantially change the differential kinetic fractionation of fast-and slow-diffusing trace gases. Like 1-D models, the 2-D model substantially underestimates the amount of differential kinetic fractionation seen in actual observations, suggesting that further subgrid-scale processes may be missing in the current generation of firn air transport models. However, we find robust scaling relationships between kinetic isotope fractionation of different noble gas isotope and elemental ratios. These relationships may be used to correct for kinetic fractionation in future high-precision ice core studies and can amount to a bias of up to 0.45 degrees C in noble-gas-based mean ocean temperature reconstructions at WAIS Divide, Antarctica.

Schilt, A, Brook EJ, Bauska TK, Baggenstos D, Fischer H, Joos F, Petrenko VV, Schaefer H, Schmitt J, Severinghaus JP, Spahni R, Stocker TF.  2014.  Isotopic constraints on marine and terrestrial N2O emissions during the last deglaciation. Nature. 516:234-+.   10.1038/nature13971   AbstractWebsite

Nitrous oxide (N2O) is an important greenhouse gas and ozone-depleting substance that has anthropogenic as well as natural marine and terrestrial sources(1). The tropospheric N2O concentrations have varied substantially in the past in concert with changing climate on glacial-interglacial and millennial timescales(2-8). It is not well understood, however, how N2O emissions from marine and terrestrial sources change in response to varying environmental conditions. The distinct isotopic compositions of marine and terrestrial N2O sources can help disentangle the relative changes in marine and terrestrial N2O emissions during past climate variations(4,9,10). Here we present N2O concentration and isotopic data for the last deglaciation, from 16,000 to 10,000 years before present, retrieved from air bubbles trapped in polar ice at Taylor Glacier, Antarctica. With the help of our data and a box model of the N2O cycle, we find a 30 per cent increase in total N2O emissions from the late glacial to the interglacial, with terrestrial and marine emissions contributing equally to the overall increase and generally evolving in parallel over the last deglaciation, even though there is no a priori connection between the drivers of the two sources. However, we find that terrestrial emissions dominated on centennial timescales, consistent with a state-of-the-art dynamic global vegetation and land surface process model that suggests that during the last deglaciation emission changes were strongly influenced by temperature and precipitation patterns over land surfaces. The results improve our understanding of the drivers of natural N2O emissions and are consistent with the idea that natural N2O emissions will probably increase in response to anthropogenic warming(11).

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.