Export 14 results:
Sort by: Author Title Type [ Year  (Desc)]
Bereiter, B, Kawamura K, Severinghaus JP.  2018.  New methods for measuring atmospheric heavy noble gas isotope and elemental ratios in ice core samples. Rapid Communications in Mass Spectrometry. 32:801-814.   10.1002/rcm.8099   AbstractWebsite

RationaleThe global ocean constitutes the largest heat buffer in the global climate system, but little is known about its past changes. The isotopic and elemental ratios of heavy noble gases (krypton and xenon), together with argon and nitrogen in trapped air from ice cores, can be used to reconstruct past mean ocean temperatures (MOTs). Here we introduce two successively developed methods to measure these parameters with a sufficient precision to provide new constraints on past changes in MOT. MethodsThe air from an 800-g ice sample - containing roughly 80mL STP air - is extracted and processed to be analyzed on two independent dual-inlet isotope ratio mass spectrometers. The primary isotope ratios (N-15, Ar-40 and Kr-86 values) are obtained with precisions in the range of 1 per meg (0.001) per mass unit. The three elemental ratio values Kr/N-2, Xe/N-2 and Xe/Kr are obtained using sequential (non-simultaneous) peak-jumping, reaching precisions in the range of 0.1-0.3. ResultsThe latest version of the method achieves a 30% to 50% better precision on the elemental ratios and a twofold better sample throughput than the previous one. The method development uncovered an unexpected source of artefactual gas fractionation in a closed system that is caused by adiabatic cooling and warming of gases (termed adiabatic fractionation) - a potential source of measurement artifacts in other methods. ConclusionsThe precisions of the three elemental ratios Kr/N-2, Xe/N-2 and Xe/Kr - which all contain the same MOT information - suggest smaller uncertainties for reconstructed MOTs (+/- 0.3-0.1 degrees C) than previous studies have attained. Due to different sensitivities of the noble gases to changes in MOT, Xe/N-2 provides the best constraints on the MOT under the given precisions followed by Xe/Kr, and Kr/N-2; however, using all of them helps to detect methodological artifacts and issues with ice quality.

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.

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.

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.

Orsi, AJ, Cornuelle BD, Severinghaus JP.  2012.  Little Ice Age cold interval in West Antarctica: Evidence from borehole temperature at the West Antarctic Ice Sheet (WAIS) Divide. Geophysical Research Letters. 39   10.1029/2012gl051260   AbstractWebsite

The largest climate anomaly of the last 1000 years in the Northern Hemisphere was the Little Ice Age (LIA) from 1400-1850 C. E., but little is known about the signature of this event in the Southern Hemisphere, especially in Antarctica. We present temperature data from a 300 m borehole at the West Antarctic Ice Sheet (WAIS) Divide. Results show that WAIS Divide was colder than the last 1000-year average from 1300 to 1800 C.E. The temperature in the time period 1400-1800 C.E. was on average 0.52 +/- 0.28 degrees C colder than the last 100-year average. This amplitude is about half of that seen at Greenland Summit (GRIP). This result is consistent with the idea that the LIA was a global event, probably caused by a change in solar and volcanic forcing, and was not simply a seesaw-type redistribution of heat between the hemispheres as would be predicted by some ocean-circulation hypotheses. The difference in the magnitude of the LIA between Greenland and West Antarctica suggests that the feedbacks amplifying the radiative forcing may not operate in the same way in both regions. Citation: Orsi, A. J., B. D. Cornuelle, and J. P. Severinghaus (2012), Little Ice Age cold interval in West Antarctica: Evidence from borehole temperature at the West Antarctic Ice Sheet (WAIS) Divide, Geophys. Res. Lett., 39, L09710, doi: 10.1029/2012GL051260.

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.

Brook, EJ, Severinghaus JP.  2011.  Methane and megafauna. Nature Geoscience. 4:271-272.   10.1038/ngeo1140   AbstractWebsite
Hamme, RC, Severinghaus JP.  2007.  Trace gas disequilibria during deep-water formation. Deep-Sea Research Part I-Oceanographic Research Papers. 54:939-950.   10.1016/j.dsr.2007.03.008   AbstractWebsite

We present high-precision measurements by a new isotope dilution technique of a suite of inert gases in the North Pacific. Remarkably smooth gradients in Ar, Kr and Xe from near equilibrium in intermediate waters to several percent undersaturated in deep waters were observed. The general pattern in the deepest waters was that Ar, Kr and Xe were undersaturated (Ar least and Xe most), while N-2 was close to equilibrium, and Ne was supersaturated. We propose that this pattern was produced by the interaction between the different physical properties of the gases (solubility and the temperature dependence of solubility) with the rapid cooling and high wind speeds that characterize deep-water formation regions. In a simple model of deep-water formation by convection, the saturations of the more temperature-sensitive gases were quickly driven down by rapid cooling and could not reequilibrate with the atmosphere before the end of the winter. In contrast, the gas exchange rate of the more bubble-sensitive gases (Ne and N-2) was able to meet or exceed the drawdown by cooling. Our simple convection model demonstrates that the heavier noble gases (Ar, Kr and Xe) are sensitive on seasonal timescales to the competing effects of cooling and air-sea gas exchange that are also important to setting the concentration Of CO2 in newly formed waters. (c) 2007 Elsevier Ltd. All rights reserved.

Kawamura, K, Parrenin F, Lisiecki L, Uemura R, Vimeux F, Severinghaus JP, Hutterli MA, Nakazawa T, Aoki S, Jouzel J, Raymo ME, Matsumoto K, Nakata H, Motoyama H, Fujita S, Goto-Azuma K, Fujii Y, Watanabe O.  2007.  Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years. Nature. 448:912-U4.   10.1038/nature06015   AbstractWebsite

The Milankovitch theory of climate change proposes that glacial interglacial cycles are driven by changes in summer insolation at high northern latitudes(1). The timing of climate change in the Southern Hemisphere at glacial-interglacial transitions (which are known as terminations) relative to variations in summer insolation in the Northern Hemisphere is an important test of this hypothesis. So far, it has only been possible to apply this test to the most recent termination(2,3), because the dating uncertainty associated with older terminations is too large to allow phase relationships to be determined. Here we present a new chronology of Antarctic climate change over the past 360,000 years that is based on the ratio of oxygen to nitrogen molecules in air trapped in the Dome Fuji and Vostok ice cores(4,5). This ratio is a proxy for local summer insolation(5), and thus allows the chronology to be constructed by orbital tuning without the need to assume a lag between a climate record and an orbital parameter. The accuracy of the chronology allows us to examine the phase relationships between climate records from the ice cores(6-9) and changes in insolation. Our results indicate that orbital-scale Antarctic climate change lags Northern Hemisphere insolation by a few millennia, and that the increases in Antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of Northern Hemisphere summer insolation. These results support the Milankovitch theory that Northern Hemisphere summer insolation triggered the last four deglaciations(3,10,11).

Lee, JY, Marti K, Severinghaus JP, Kawamura K, Yoo HS, Lee JB, Kim JS.  2006.  A redetermination of the isotopic abundances of atmospheric Ar. Geochimica Et Cosmochimica Acta. 70:4507-4512.   10.1016/j.gca.2006.06.1563   AbstractWebsite

Atmospheric argon measured on a dynamically operated mass spectrometer with an ion source magnet, indicated systematically larger Ar-40/Ar-16 ratios compared to the generally accepted value of Nier [Nier A.O., 1950. A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium. Phys. Rev. 77, 789-793], 295.5 +/- 0.5, which has served as the standard for all isotopic measurements in geochemistry and cosmochemistry. Gravimetrically prepared mixtures of highly enriched Ar-36 and Ar-40 were utilized to redetermine the isotopic abundances of atmospheric Ar, using a dynamically operated isotope ratio mass spectrometer with minor modifications and special gas handling techniques to avoid fractionation. A new ratio Ar-40/Ar-36 = 298.56 +/- 0.31 was obtained with a precision of 0.1%, approximately 1% higher than the previously accepted value. Combined with the Ar-38/Ar-36 (0.1885 +/- 0.0003) measured with a VG5400 noble gas mass spectrometer in static operation, the percent abundances of Ar-36, Ar-38, and Ar-40 were determined to be 0.3336 +/- 0.0004, 0.0629 +/- 0.0001, and 99.6035 +/- 0.0004, respectively. We calculate an atomic mass of Ar of 39.9478 +/- 0.0002. Accurate Ar isotopic abundances are relevant in numerous applications, as the calibration of the mass spectrometer discrimination. (c) 2006 Elsevier Inc. 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.

Caillon, N, Severinghaus JP, Barnola JM, Chappellaz J, Jouzel J, Parrenin F.  2001.  Estimation of temperature change and of gas age ice age difference, 108 kyr BP, at Vostok, Antarctica. Journal of Geophysical Research-Atmospheres. 106:31893-31901.   10.1029/2001jd900145   AbstractWebsite

Air trapped in ice core bubbles provides our primary source of information about past atmospheres. Air isotopic composition ((15)N/(14)N and (40)Ar/(36)Ar) permits an estimate of the temperature shifts associated with abrupt climate changes because of isotope fractionation occurring in response to temperature gradients in the snow layer on top of polar ice sheets. A rapid surface temperature change modifies temporarily the firn temperature gradient, which causes a detectable anomaly in the isotopic composition of nitrogen and argon. The location of this anomaly in depth characterizes the gas age - ice age difference (Deltaage) during an abrupt,Gwent by correlation with the deltaD (or 5180) anomaly in the ice. We focus this study on the marine isotope stage 5d/5c transition (108 kyr B.P.), a climate warming which was one of the most abrupt events in the Vostok (Antarctica) ice isotopic record [Petit et al., 1999]. A step-like decrease in delta(15)N and delta(40)Ar/4 from 0.49 to 0.47 parts per thousand (possibly a gravitational signal due to a change in firn thickness) is preceded by a small but detectable delta(15)N peak (possibly a thermal diffusion signal). We obtain an estimate of 5350 +/- 300 yr for Deltaage, close to the model estimate of 5000 years obtained using the Vostok glaciological timescale. Our results also suggest that the use of the present-day spatial isotope-temperature relationship slightly underestimates (but by no more than 20 +/- 15%) the Vostok temperature change from present day at that time, which is in contrast to the temperature estimate based on borehole temperature measurements in Vostok which suggests that Antarctic temperature changes are underestimated by up to 50%.

Brook, EJ, Harder S, Severinghaus J, Steig EJ, Sucher CM.  2000.  On the origin and timing of rapid changes in atmospheric methane during the last glacial period. Global Biogeochemical Cycles. 14:559-572.   10.1029/1999gb001182   AbstractWebsite

We present high resolution records of atmospheric methane from the GISP2 (Greenland Ice Sheet Project 2) ice core for four rapid climate transitions that occurred during the past 50 ka: the end of the Younger Dryas at 11.8 ka, the beginning of the Bolling-Allerod period at 14.8 ka, the beginning of interstadial 8 at 38.2 ka, and the beginning of interstadial 12 at 45.5 ka. During these events, atmospheric methane concentrations increased by 200-300 ppb over time periods of 100-300 years, significantly more slowly than associated temperature and snow accumulation changes recorded in the ice core record. We suggest that the slower rise in methane concentration may reflect the timescale of terrestrial ecosystem response to rapid climate change. We find no evidence for rapid, massive methane emissions that might be associated with large-scale decomposition of methane hydrates in sediments. With additional results from the Taylor Dome Ice Core (Antarctica) we also reconstruct changes in the interpolar methane gradient tan indicator of the geographical distribution of methane sources) associated with some of the rapid changes in atmospheric methane. The results indicate that the rise in methane at the beginning of the Bolling-Allerod period and the later rise at the end of the Younger Dryas were driven by increases in both tropical and boreal methane sources. During the Younger Dryas (a 1.3 ka cold period during the last deglaciation) the relative contribution from boreal sources was reduced relative to the early and middle Holocene periods.

Severinghaus, JP, Sowers T, Brook EJ, Alley RB, Bender ML.  1998.  Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice. Nature. 391:141-146.   10.1038/34346   AbstractWebsite

Rapid temperature change fractionates gas Isotopes in unconsolidated snow, producing a signal that is preserved in trapped air bubbles as the snow forms ice, The fractionation of nitrogen and argon isotopes at the end of the Younger Dryas cold interval, recorded in Greenland ice, demonstrates that warming at this time was abrupt. This warming coincides with the onset of a prominent rise in atmospheric methane concentration, indicating that the climate change was synchronous (within a few decades) over a region of at least hemispheric extent, and providing constraints on previously proposed mechanisms of climate change at this time, The depth of the nitrogen-isotope signal relative to the depth of the climate change recorded in the Ice matrix indicates that, during the Younger Dryas, the summit of Greenland was 15 +/- 3 degrees C colder than today.