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2018
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.

Bereiter, B, Shackleton S, Baggenstos D, Kawamura K, Severinghaus J.  2018.  Mean global ocean temperatures during the last glacial transition. Nature. 553:39-+.   10.1038/nature25152   AbstractWebsite

Little is known about the ocean temperature's long-term response to climate perturbations owing to limited observations and a lack of robust reconstructions. Although most of the anthropogenic heat added to the climate system has been taken up by the ocean up until now, its role in a century and beyond is uncertain. Here, using noble gases trapped in ice cores, we show that the mean global ocean temperature increased by 2.57 +/- 0.24 degrees Celsius over the last glacial transition (20,000 to 10,000 years ago). Our reconstruction provides unprecedented precision and temporal resolution for the integrated global ocean, in contrast to the depth-, region-, organism-and season-specific estimates provided by other methods. We find that the mean global ocean temperature is closely correlated with Antarctic temperature and has no lead or lag with atmospheric CO2, thereby confirming the important role of Southern Hemisphere climate in global climate trends. We also reveal an enigmatic 700-year warming during the early Younger Dryas period (about 12,000 years ago) that surpasses estimates of modern ocean heat uptake.

2017
Baggenstos, D, Bauska TK, Severinghaus JP, Lee JE, Schaefer H, Buizert C, Brook EJ, Shackleton S, Petrenko VV.  2017.  Atmospheric gas records from Taylor Glacier, Antarctica, reveal ancient ice with ages spanning the entire last glacial cycle. Climate of the Past. 13:943-958.   10.5194/cp-13-943-2017   AbstractWebsite

Old ice for paleo-environmental studies, traditionally accessed through deep core drilling on domes and ridges on the large ice sheets, can also be retrieved at the surface from ice sheet margins and blue ice areas. The practically unlimited amount of ice available at these sites satisfies a need in the community for studies of trace components requiring large sample volumes. For margin sites to be useful as ancient ice archives, the ice stratigraphy needs to be understood and age models need to be established. We present measurements of trapped gases in ice from Taylor Glacier, Antarctica, to date the ice and assess the completeness of the stratigraphic section. Using delta O-18 of O-2 and methane concentrations, we unambiguously identify ice from the last glacial cycle, covering every climate interval from the early Holocene to the penultimate interglacial. A high-resolution transect reveals the last deglaciation and the Last Glacial Maximum (LGM) in detail. We observe large-scale deformation in the form of folding, but individual stratigraphic layers do not appear to have undergone irregular thinning. Rather, it appears that the entire LGM-deglaciation sequence has been transported from the interior of the ice sheet to the surface of Taylor Glacier relatively undisturbed. We present an age model that builds the foundation for gas studies on Taylor Glacier. A comparison with the Taylor Dome ice core confirms that the section we studied on Taylor Glacier is better suited for paleo-climate reconstructions of the LGM due to higher accumulation rates.

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.

2015
Orsi, AJ, Kawamura K, Fegyveresi JM, Headly MA, Alley RB, Severinghaus JP.  2015.  Differentiating bubble-free layers from melt layers in ice cores using noble gases. Journal of Glaciology. 61:585-594.   10.3189/2015JoG14J237   AbstractWebsite

Melt layers are clear indicators of extreme summer warmth on polar ice caps. The visual identification of refrozen meltwater as clear bubble-free layers cannot be used to study some past warm periods, because, in deeper ice, bubbles are lost to clathrate formation. We present here a reliable method to detect melt events, based on the analysis of Kr/Ar and Xe/Ar ratios in ice cores, and apply it to the detection of melt in clathrate ice from the Eemian at NEEM, Greenland. Additionally, melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases, or by altering gas transport in the firn, which affects the gas chronology. We find that the easily visible 1 mm thick bubble-free layers in the WAIS Divide ice core do not contain sufficient melt to alter the gas composition in the core, and do not cause artifacts or discontinuities in the gas chronology. The presence of these layers during winter, and the absence of anomalies in soluble gases, suggests that these layers can be formed by processes other than refreezing of meltwater. Consequently, the absence of bubbles in thin crusts is not in itself proof of a melt event.

Buizert, C, Adrian B, Ahn J, Albert M, Alley RB, Baggenstos D, Bauska TK, Bay RC, Bencivengo BB, Bentley CR, Brook EJ, Chellman NJ, Clow GD, Cole-Dai J, Conway H, Cravens E, Cuffey KM, Dunbar NW, Edwards JS, Fegyveresi JM, Ferris DG, Fitzpatrick JJ, Fudge TJ, Gibson CJ, Gkinis V, Goetz JJ, Gregory S, Hargreaves GM, Iverson N, Johnson JA, Jones TR, Kalk ML, Kippenhan MJ, Koffman BG, Kreutz K, Kuhl TW, Lebar DA, Lee JE, Marcott SA, Markle BR, Maselli OJ, McConnell JR, McGwire KC, Mitchell LE, Mortensen NB, Neff PD, Nishiizumi K, Nunn RM, Orsi AJ, Pasteris DR, Pedro JB, Pettit EC, Price PB, Priscu JC, Rhodes RH, Rosen JL, Schauer AJ, Schoenemann SW, Sendelbach PJ, Severinghaus JP, Shturmakov AJ, Sigl M, Slawny KR, Souney JM, Sowers TA, Spencer MK, Steig EJ, Taylor KC, Twickler MS, Vaughn BH, Voigt DE, Waddington ED, Welten KC, Wendricks AW, White JWC, Winstrup M, Wong GJ, Woodruff TE, Members WDP.  2015.  Precise interpolar phasing of abrupt climate change during the last ice age. Nature. 520:661-U169.   10.1038/nature14401   AbstractWebsite

The last glacial period exhibited abrupt Dansgaard-Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeodimate archives'. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard-Oeschger cycle and vice versa''', suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw(4-6). Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events'. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision''''". Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 +/- 92 years (2 sigma a) for DansgaardOeschger events, including the Bolling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 +/- 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard-Oeschger dynamics.

Buizert, C, Cuffey KM, Severinghaus JP, Baggenstos D, Fudge TJ, Steig EJ, Markle BR, Winstrup M, Rhodes RH, Brook EJ, Sowers TA, Clow GD, Cheng H, Edwards RL, Sigl M, McConnell JR, Taylor KC.  2015.  The WAIS Divide deep ice core WD2014 chronology - Part 1: Methane synchronization (68-31 kaBP) and the gas age-ice age difference. Climate of the Past. 11:153-173.   10.5194/cp-11-153-2015   AbstractWebsite

The West Antarctic Ice Sheet Divide (WAIS Divide, WD) ice core is a newly drilled, high-accumulation deep ice core that provides Antarctic climate records of the past similar to 68 ka at unprecedented temporal resolution. The upper 2850m (back to 31.2 ka BP) have been dated using annual-layer counting. Here we present a chronology for the deep part of the core (67.8-31.2 ka BP), which is based on stratigraphic matching to annual-layer-counted Greenland ice cores using globally well-mixed atmospheric methane. We calculate the WD gas age-ice age difference (Delta age) using a combination of firn densification modeling, ice-flow modeling, and a data set of delta N-15-N-2, a proxy for past firn column thickness. The largest Delta age at WD occurs during the Last Glacial Maximum, and is 525 +/- 120 years. Internally consistent solutions can be found only when assuming little to no influence of impurity content on densification rates, contrary to a recently proposed hypothesis. We synchronize the WD chronology to a linearly scaled version of the layer-counted Greenland Ice Core Chronology (GICC05), which brings the age of Dansgaard-Oeschger (DO) events into agreement with the U = Th absolutely dated Hulu Cave speleothem record. The small Delta age at WD provides valuable opportunities to investigate the timing of atmospheric greenhouse gas variations relative to Antarctic climate, as well as the interhemispheric phasing of the "bipolar seesaw".

2014
Seierstad, IK, Abbott PM, Bigler M, Blunier T, Bourne AJ, Brook E, Buchardt SL, Buizert C, Clausen HB, Cook E, Dahl-Jensen D, Davies SM, Guillevic M, Johnsen SJ, Pedersen DS, Popp TJ, Rasmussen SO, Severinghaus JP, Svensson A, Vinther BM.  2014.  Consistently dated records from the Greenland GRIP, GISP2 and NGRIP ice cores for the past 104 ka reveal regional millennial-scale delta O-18 gradients with possible Heinrich event imprint. Quaternary Science Reviews. 106:29-46.   10.1016/j.quascirev.2014.10.032   AbstractWebsite

We present a synchronization of the NGRIP, GRIP and GISP2 ice cores onto a master chronology extending back to 104 ka before present, providing a consistent chronological framework for these three Greenland records. The synchronization aligns distinct peaks in volcanic proxy records and other impurity records (chemo-stratigraphic matching) and assumes that these layers of elevated impurity content represent the same, instantaneous event in the past at all three sites. More than 900 marker horizons between the three cores have been identified and our matching is independently confirmed by 24 new and previously identified volcanic ash (tephra) tie-points. Using the reference horizons, we transfer the widely used Greenland ice-core chronology, GICC05modelext, to the two Summit cores, GRIP and GISP2. Furthermore, we provide gas chronologies for the Summit cores that are consistent with the GICC05modelext timescale by utilizing both existing and new gas data (CH4 concentration and delta N-15 of N-2). We infer that the accumulation contrast between the stadial and interstadial phases of the glacial period was -10% greater at Summit compared to at NGRIP. The delta O-18 temperature-proxy records from NGRIP, GRIP, and GISP2 are generally very similar and display synchronous behaviour at climate transitions. The 1180 differences between Summit and NGRIP, however, changed slowly over the Last Glacial Interglacial cycle and also underwent abrupt millennial-to-centennial-scale variations. We suggest that this observed latitudinal delta O-18 gradient in Greenland during the glacial period is the result of 1) relatively higher degree of precipitation with a Pacific signature at NGRIP, 2) increased summer bias in precipitation at Summit, and 3) enhanced Rayleigh distillation due to an increased source-to-site distance and a potentially larger source-to-site temperature gradient. We propose that these processes are governed by changes in the North American Ice Sheet (NAIS) volume and North Atlantic sea-ice extent and/or sea-surface temperatures (SST) on orbital timescales, and that changing sea-ice extent and SSTs are the driving mechanisms on shorter timescales. Finally, we observe that maxima in the Summit NGRIP delta O-18 difference are roughly coincident with prominent Heinrich events. This suggests that the climatic reorganization that takes place during stadials with Heinrich events, possibly driven by a southward expansion of sea ice and low SSTs in the North Atlantic, are recorded in the ice-core records. (C) 2014 Elsevier Ltd. All rights reserved.

Marcott, SA, Bauska TK, Buizert C, Steig EJ, Rosen JL, Cuffey KM, Fudge TJ, Severinghaus JP, Ahn J, Kalk ML, McConnell JR, Sowers T, Taylor KC, White JWC, Brook EJ.  2014.  Centennial-scale changes in the global carbon cycle during the last deglaciation. Nature. 514:616-+.   10.1038/nature13799   AbstractWebsite

Global climate and the concentration of atmospheric carbon dioxide (CO2) are correlated over recent glacial cycles(1,2). The combination of processes responsible for a rise in atmospheric CO2 at the last glacial termination(1,3) (23,000 to 9,000 years ago), however, remains uncertain(1-3). Establishing the timing and rate of CO2 changes in the past provides critical insight into the mechanisms that influence the carbon cycle and helps put present and future anthropogenic emissions in context. Here we present CO2 and methane (CH4) records of the last deglaciation from a new high-accumulation West Antarctic ice core with unprecedented temporal resolution and precise chronology. We show that although low-frequency CO2 variations parallel changes in Antarctic temperature, abrupt CO2 changes occur that have a clear relationship with abrupt climate changes in the Northern Hemisphere. A significant proportion of the direct radiative forcing associated with the rise in atmospheric CO2 occurred in three sudden steps, each of 10 to 15 parts per million. Every step took place in less than two centuries and was followed by no notable change in atmospheric CO2 for about 1,000 to 1,500 years. Slow, millennial-scale ventilation of Southern Ocean CO2-rich, deep-ocean water masses is thought to have been fundamental to the rise in atmospheric CO2 associated with the glacial termination(4), given the strong covariance of CO2 levels and Antarctic temperatures(5). Our data establish a contribution from an abrupt, centennial-scale mode of CO2 variability that is not directly related to Antarctic temperature. We suggest that processes operating on centennial timescales, probably involving the Atlantic meridional overturning circulation, seem to be influencing global carbon-cycle dynamics and are at present not widely considered in Earth system models.

Buizert, C, Gkinis V, Severinghaus JP, He F, Lecavalier BS, Kindler P, Leuenberger M, Carlson AE, Vinther B, Masson-Delmotte V, White JWC, Liu ZY, Otto-Bliesner B, Brook EJ.  2014.  Greenland temperature response to climate forcing during the last deglaciation. Science. 345:1177-1180.   10.1126/science.1254961   AbstractWebsite

Greenland ice core water isotopic composition (delta O-18) provides detailed evidence for abrupt climate changes but is by itself insufficient for quantitative reconstruction of past temperatures and their spatial patterns. We investigate Greenland temperature evolution during the last deglaciation using independent reconstructions from three ice cores and simulations with a coupled ocean-atmosphere climate model. Contrary to the traditional delta O-18 interpretation, the Younger Dryas period was 4.5 degrees +/- 2 degrees C warmer than the Oldest Dryas, due to increased carbon dioxide forcing and summer insolation. The magnitude of abrupt temperature changes is larger in central Greenland (9 degrees to 14 degrees C) than in the northwest (5 degrees to 9 degrees C), fingerprinting a North Atlantic origin. Simulated changes in temperature seasonality closely track changes in the Atlantic overturning strength and support the hypothesis that abrupt climate change is mostly a winter phenomenon.

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.

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.

2013
Fudge, TJ, Steig EJ, Markle BR, Schoenemann SW, Ding QH, Taylor KC, McConnell JR, Brook EJ, Sowers T, White JWC, Alley RB, Cheng H, Clow GD, Cole-Dai J, Conway H, Cuffey KM, Edwards JS, Edwards RL, Edwards R, Fegyveresi JM, Ferris D, Fitzpatrick JJ, Johnson J, Hargreaves G, Lee JE, Maselli OJ, Mason W, McGwire KC, Mitchell LE, Mortensen N, Neff P, Orsi AJ, Popp TJ, Schauer AJ, Severinghaus JP, Sigl M, Spencer MK, Vaughn BH, Voigt DE, Waddington ED, Wang XF, Wong GJ, Members WDP.  2013.  Onset of deglacial warming in West Antarctica driven by local orbital forcing. Nature. 500:440-+.   10.1038/nature12376   AbstractWebsite

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate(1,2). Hypotheses for a Northern Hemisphere trigger, through oceanic redistributions of heat, are based in part on the abrupt onset of warming seen in East Antarctic ice cores and dated to 18,000 years ago, which is several thousand years after high-latitude Northern Hemisphere summer insolation intensity began increasing from its minimum, approximately 24,000 years ago(3,4). An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently(2,5). Here we present results from a new, annually resolved ice-core record from West Antarctica that reconciles these two views. The records show that 18,000 years ago snow accumulation in West Antarctica began increasing, coincident with increasing carbon dioxide concentrations, warming in East Antarctica and cooling in the Northern Hemisphere(6) associated with an abrupt decrease in Atlantic meridional overturning circulation(7). However, significant warming in West Antarctica began at least 2,000 years earlier. Circum-Antarctic sea-ice decline, driven by increasing local insolation, is the likely cause of this warming. The marine-influenced West Antarctic records suggest a more active role for the Southern Ocean in the onset of deglaciation than is inferred from ice cores in the East Antarctic interior, which are largely isolated from sea-ice changes.

Petrenko, VV, Severinghaus JP, Smith AM, Riedel K, Baggenstos D, Harth C, Orsi A, Hua Q, Franz P, Takeshita Y, Brailsford GW, Weiss RF, Buizert C, Dickson A, Schaefer H.  2013.  High-precision C-14 measurements demonstrate production of in situ cosmogenic (CH4)-C-14 and rapid loss of in situ cosmogenic (CO)-C-14 in shallow Greenland firn. Earth and Planetary Science Letters. 365:190-197.   10.1016/j.epsl.2013.01.032   AbstractWebsite

Measurements of radiocarbon (C-14) in carbon dioxide (CO2), methane (CH4) and carbon monoxide (CO) from glacial ice are potentially useful for absolute dating of ice cores, studies of the past atmospheric CH4 budget and for reconstructing the past cosmic ray flux and solar activity. Interpretation of C-14 signals in ice is complicated by the fact that the two major C-14 components-trapped atmospheric and in situ cosmogenic-are present in a combined form, as well as by a very limited understanding of the in situ component. This study measured (CH4)-C-14 and (CO)-C-14 content in glacial firn with unprecedented precision to advance understanding of the in situ C-14 component. (CH4)-C-14 and (CO)-C-14 were melt-extracted on site at Summit, Greenland from three very large (similar to 1000 kg each) replicate samples of firn that spanned a depth range of 3.6-5.6 m. Non-cosmogenic C-14 contributions were carefully characterized through simulated extractions and a suite of supporting measurements. In situ cosmogenic (CO)-C-14 was quantified to better than +/- 0.6 molecules g(-1) ice, improving on the precision of the best prior ice (CO)-C-14 measurements by an order of magnitude. The (CO)-C-14 measurements indicate that most (>99%) of the in situ cosmogenic C-14 is rapidly lost from shallow Summit firn to the atmosphere. Despite this rapid C-14 loss, our measurements successfully quantified (CH4)-C-14 in the retained fraction of cosmogenic C-14 (to +/- 0.01 molecules g(-1) ice or better), and demonstrate for the first time that a significant amount of (CH4)-C-14 is produced by cosmic rays in natural ice. This conclusion increases the confidence in the results of an earlier study that used measurements of (CH4)-C-14 in glacial ice to show that wetlands were the likely main driver of the large and rapid atmospheric CH4 increase approximately 1 1.6 kyr ago. (C) 2013 Elsevier B.V. All rights reserved.

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.

2012
Buizert, C, Martinerie P, Petrenko VV, Severinghaus JP, Trudinger CM, Witrant E, Rosen JL, Orsi AJ, Rubino M, Etheridge DM, Steele LP, Hogan C, Laube JC, Sturges WT, Levchenko VA, Smith AM, Levin I, Conway TJ, Dlugokencky EJ, Lang PM, Kawamura K, Jenk TM, White JWC, Sowers T, Schwander J, Blunier T.  2012.  Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland. Atmospheric Chemistry and Physics. 12:4259-4277.   10.5194/acp-12-4259-2012   AbstractWebsite

Air was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1 sigma Gaussian distribution. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age- gas age difference (Delta age) at the firn-ice transition is calculated to be 182(-9)(+3) yr. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore, diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records.

2011
Kobashi, T, Kawamura K, Severinghaus JP, Barnola JM, Nakaegawa T, Vinther BM, Johnsen SJ, Box JE.  2011.  High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core. Geophysical Research Letters. 38   10.1029/2011gl049444   AbstractWebsite

Greenland recently incurred record high temperatures and ice loss by melting, adding to concerns that anthropogenic warming is impacting the Greenland ice sheet and in turn accelerating global sea-level rise. Yet, it remains imprecisely known for Greenland how much warming is caused by increasing atmospheric greenhouse gases versus natural variability. To address this need, we reconstruct Greenland surface snow temperature variability over the past 4000 years at the GISP2 site (near the Summit of the Greenland ice sheet; hereafter referred to as Greenland temperature) with a new method that utilises argon and nitrogen isotopic ratios from occluded air bubbles. The estimated average Greenland snow temperature over the past 4000 years was -30.7 degrees C with a standard deviation of 1.0 degrees C and exhibited a long-term decrease of roughly 1.5 degrees C, which is consistent with earlier studies. The current decadal average surface temperature (2001-2010) at the GISP2 site is -29.9 degrees C. The record indicates that warmer temperatures were the norm in the earlier part of the past 4000 years, including century-long intervals nearly 1 C warmer than the present decade (20012010). Therefore, we conclude that the current decadal mean temperature in Greenland has not exceeded the envelope of natural variability over the past 4000 years, a period that seems to include part of the Holocene Thermal Maximum. Notwithstanding this conclusion, climate models project that if anthropogenic greenhouse gas emissions continue, the Greenland temperature would exceed the natural variability of the past 4000 years sometime before the year 2100. Citation: Kobashi, T., K. Kawamura, J. P. Severinghaus, J.-M. Barnola, T. Nakaegawa, B. M. Vinther, S. J. Johnsen, and J. E. Box (2011), High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core, Geophys. Res. Lett., 38, L21501, doi:10.1029/2011GL049444.

Brook, EJ, Severinghaus JP.  2011.  Methane and megafauna. Nature Geoscience. 4:271-272.   10.1038/ngeo1140   AbstractWebsite
n/a
Battle, MO, Severinghaus JP, Sofen ED, Plotkin D, Orsi AJ, Aydin M, Montzka SA, Sowers T, Tans PP.  2011.  Controls on the movement and composition of firn air at the West Antarctic Ice Sheet Divide. Atmospheric Chemistry and Physics. 11:11007-11021.   10.5194/acp-11-11007-2011   AbstractWebsite

We sampled interstitial air from the perennial snowpack (firn) at a site near the West Antarctic Ice Sheet Divide (WAIS-D) and analyzed the air samples for a wide variety of gas species and their isotopes. We find limited convective influence (1.4-5.2 m, depending on detection method) in the shallow firn, gravitational enrichment of heavy species throughout the diffusive column in general agreement with theoretical expectations, a similar to 10 m thick lock-in zone beginning at similar to 67 m, and a total firn thickness consistent with predictions of Kaspers et al. (2004). Our modeling work shows that the air has an age spread (spectral width) of 4.8 yr for CO2 at the firn-ice transition. We also find that advection of firn air due to the 22 cm yr(-1) ice-equivalent accumulation rate has a minor impact on firn air composition, causing changes that are comparable to other modeling uncertainties and intrinsic sample variability. Furthermore, estimates of 1 age (the gas age/ice age difference) at WAIS-D appear to be largely unaffected by bubble closure above the lock-in zone. Within the lock-in zone, small gas species and their isotopes show evidence of size-dependent fractionation due to permeation through the ice lattice with a size threshold of 0.36 nm, as at other sites. We also see an unequivocal and unprecedented signal of oxygen isotope fractionation within the lock-in zone, which we interpret as the mass-dependent expression of a size-dependent fractionation process.

2010
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.

Kobashi, T, Severinghaus JP, Barnola JM, Kawamura K, Carter T, Nakaegawa T.  2010.  Persistent multi-decadal Greenland temperature fluctuation through the last millennium. Climatic Change. 100:733-756.   10.1007/s10584-009-9689-9   AbstractWebsite

Future Greenland temperature evolution will affect melting of the ice sheet and associated global sea-level change. Therefore, understanding Greenland temperature variability and its relation to global trends is critical. Here, we reconstruct the last 1,000 years of central Greenland surface temperature from isotopes of N(2) and Ar in air bubbles in an ice core. This technique provides constraints on decadal to centennial temperature fluctuations. We found that northern hemisphere temperature and Greenland temperature changed synchronously at periods of similar to 20 years and 40-100 years. This quasi-periodic multi-decadal temperature fluctuation persisted throughout the last millennium, and is likely to continue into the future.

2009
Grachev, AM, Brook EJ, Severinghaus JP, Pisias NG.  2009.  Relative timing and variability of atmospheric methane and GISP2 oxygen isotopes between 68 and 86 ka. Global Biogeochemical Cycles. 23   10.1029/2008gb003330   AbstractWebsite

The global biogeochemical cycle of methane has received wide attention because of methane's role as a greenhouse gas. Measurements of methane in air trapped in Greenland ice cores provide a high-resolution record of methane levels in the atmosphere over the past similar to 100 ka, providing clues about what controls the methane cycle on geologic timescales. Remarkable similarity between local temperature recorded in Greenland ice cores and changes in global methane concentrations has been noted in previous studies, with the inference that the local temperature variations have global significance, but the resolution of sampling and measurement precision limited fine-scale comparison of these variables. In this work a higher-precision (similar to 2 ppb) methane data set was obtained from the Greenland Ice Sheet Project 2 (GISP2) ice core for the time interval between 86 and 68 ka, encompassing three large abrupt warming events early in the last glacial period: Dansgaard-Oeschger (D-O) events 19, 20, and 21. The new data set consists of duplicate measurements at 158 depths, with average time resolution of 120 years. Such detailed measurements over D-O 21, the longest in Greenland records, have not yet been reported for other ice cores. The new data set documents short-term variability (similar to 20 ppb typical amplitude), which is remarkably persistent, and in many cases similar features are observed in the most detailed published delta(18)O(ice) record. High-precision GISP2 delta(15)N data show that changes in Greenland temperature are synchronous with the methane variations at the onset of D-O events 19, 20, and 21, supporting previous results from the Greenland Ice Core Project ice core for D-O 19 and 20. Cross-spectral analysis quantifies the extremely close similarity between the new methane record and the delta(18)O(ice) record. Because methane sources are widely distributed over the globe, this work further validates delta(18)O(ice) at Greenland summit as a geographically broad climate indicator on millennial to multicentennial timescales.

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.

2008
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.

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.