We undertook an interlaboratory comparison of techniques used to extract and analyze trapped gase... more We undertook an interlaboratory comparison of techniques used to extract and analyze trapped gases in ice cores. The intercomparison included analyses of standard reference gases and samples of ice from the Greenland Ice Sheet Project 2 (GISP2) site. Concentrations of CO2, CH4, the •5180 of 02, the •515N of N2, and the O2/N2, and Ar/N2 ratios were measured in air standards and ice core sampries. The standard reference scales for CO2 and CH 4 were consistent at the +2% level. The •502/N2 and •5180 of O2 measurements showed substantial deviations between the two laboratories able to measure these ratios. The deviations are probably related to errors associated with calibration of the working standards. The •sAr/N2 and •515N of N2 measurements were consistent. Five laboratories analyzed the CH4 concentration in a 4.2-m section of the GISP2 ice core. The average of 20 discrete CH 4 measurements was 748+10 parts per billion by volume (ppbv). The standard deviation of these measurements w...
Dominic A. Winski, Tyler J. Fudge, David G. Ferris, Erich C. Osterberg, John M. 4 Fegyveresi, Jih... more Dominic A. Winski, Tyler J. Fudge, David G. Ferris, Erich C. Osterberg, John M. 4 Fegyveresi, Jihong Cole-Dai, Zayta Thundercloud, Thomas S. Cox, Karl J. Kreutz, 5 Nikolas Ortman, Christo Buizert, Jenna Epifanio, Edward J. Brook, Ross Beaudette, 6 Jeff Severinghaus, Todd Sowers, Eric J. Steig, Emma C. Kahle, Tyler R. Jones, 7 Valerie Morris, Murat Aydin, Melinda R. Nicewonger, Kimberly A. Casey, 8 Richard B. Alley, Edwin D. Waddington, Nels A. Iverson, Nelia W. Dunbar , Ryan 9 C. Bay, Joseph M. Souney, Michael Sigl, Joseph R. McConnell 10
Accurate estimates of the past extent of the Greenland ice sheet provide critical constraints for... more Accurate estimates of the past extent of the Greenland ice sheet provide critical constraints for ice sheet models used to determine Greenland’s response to climate forcing and contribution to global sea level. Here we use a continuous ice core dust record from the Renland ice cap on the east coast of Greenland to constrain the timing of changes to the ice sheet margin and relative sea level over the last glacial cycle. During the Holocene and the previous interglacial period (Eemian) the dust record was dominated by coarse particles consistent with rock samples from central East Greenland. From the coarse particle concentration record we infer the East Greenland ice sheet margin advanced from 113.4 ± 0.4 to 111.0 ± 0.4 ka BP during the glacial onset and retreated from 12.1 ± 0.1 to 9.0 ± 0.1 ka BP during the last deglaciation. These findings constrain the possible response of the Greenland ice sheet to climate forcings.
The South Pole Ice Core (SPICEcore) was drilled in 2014-2016 to provide a detailed multi-proxy ar... more The South Pole Ice Core (SPICEcore) was drilled in 2014-2016 to provide a detailed multi-proxy archive of paleoclimate conditions in East Antarctica during the Holocene and late Pleistocene. Interpretation of these records requires an accurate depthage relationship. Here, we present the SP19 timescale for the age of the ice of SPICEcore. SP19 is synchronized to the WD2014 chronology from the West Antarctic Ice Sheet Divide (WAIS Divide) ice core using stratigraphic matching of 251 volcanic events. These events indicate an age of 54,302 +/-519 years BP (before the year 1950) at the bottom of SPICEcore. Annual layers identified in sodium and magnesium ions to 11,341 BP were used to interpolate between stratigraphic volcanic tie points, yielding an annually-resolved chronology through the Holocene. Estimated timescale uncertainty during the Holocene is less than 18 years relative to WD2014, with the exception of the interval between 1800 to 3100 BP when uncertainty estimates reach +/-25 years due to widely spaced volcanic tie points. Prior to the Holocene, uncertainties remain within 124 years relative to WD2014. Results show an average Holocene accumulation rate of 7.4 cm/yr (water equivalent). The time variability of accumulation rate is consistent with expectations for steady-state ice flow through the modern spatial pattern of accumulation rate. Time variations in nitrate concentration, nitrate seasonal amplitude, and δ 15 N of N 2 in turn are as expected for the accumulation-rate variations. The highly variable yet wellconstrained Holocene accumulation history at the site can help improve scientific understanding of deposition-sensitive climate proxies such as δ 15 N of N 2 and photolyzed chemical compounds.
Journal of Contemporary Water Research & Education, 2018
New techniques are needed to distinguish between leakage of methane (CH 4) into surface waters fr... more New techniques are needed to distinguish between leakage of methane (CH 4) into surface waters from gas wells and natural sources. Here, scientists worked with >50 citizen scientists in a hydrocarbon-rich basin (Pennsylvania, U.S.A.) to measure methane concentrations ([CH 4 ]) in streams. These measurements were combined with published observations to form a reconnaissance dataset. The dataset was then used to categorize sites as background or as impacted by other sources of gas. For 479 samples at 131 sites, 470 were supersaturated with respect to the atmosphere (>0.08 µg/L). Sites with the lowest concentrations generally were located in low-productivity, sandstone-underlain upland streams, while other streams contained CH 4 from sources in addition to atmospheric. The median of 63 sites not located near wetland habitats and not affected by known thermogenic influxes yielded an estimate of background [CH 4 ] in the streams, 0.5 µg/L. The highest individual measurements (~70 µg/L) in the stream dataset were observed in one site near a wetland and one site near a putatively leaking gas well. Inspection of the dataset revealed that values of [CH 4 ] above a threshold for non-wetland sites, 4 μg/L, signals gas is likely deriving from sources such as leaking gas wells, shallow organic-rich shales, coal, or landfills. Using historical and local volunteer knowledge, we discovered 12 non-wetland sites above the threshold that are potentially contaminated by such sources. Although sources of CH 4 cannot be proven from such surveys of [CH 4 ], stream sampling with nonscientists nonetheless allows discovery of sites of potential contamination that can be further investigated.
We report results from intercomparison exercises between laboratories that conduct measurements o... more We report results from intercomparison exercises between laboratories that conduct measurements of stable carbon and hydrogen isotope ratios of atmospheric CH<sub>4</sub> (δ<sup>13</sup>C-CH<sub>4</sub> and…
structural characterization of CO-inhibited forms of CpI, as well as that of CpII and other Fe-on... more structural characterization of CO-inhibited forms of CpI, as well as that of CpII and other Fe-only and NiFe hydrogenases, may contribute te additional insights to themechaqistic details of reversible hydrogen oxidation catalyzed by the hydrogenase enzymes. References and Notes Microbiol. Rev. 88, 109 (1992).
During the last glacial period, the North Atlantic region experienced a series of Dansgaard-Oesch... more During the last glacial period, the North Atlantic region experienced a series of Dansgaard-Oeschger cycles in which climate abruptly alternated between warm and cold periods. Corresponding variations in Antarctic surface temperature were out of phase with their Northern Hemisphere counterparts. The temperature relationship between the hemispheres is commonly attributed to an interhemispheric redistribution of heat by the ocean overturning circulation. Changes in ocean heat transport should be accompanied by changes in atmospheric circulation to satisfy global energy budget constraints. Although changes in tropical atmospheric circulation linked to abrupt events in the Northern Hemisphere are well documented, evidence for predicted changes in the Southern Hemisphere's atmospheric circulation during Dansgaard-Oeschger cycles is lacking. Here we use a high-resolution deuterium-excess record from West Antarctica to show that the latitude of the mean moisture source for Antarctic precipitation changed in phase with abrupt shifts in Northern Hemisphere climate, and significantly before Antarctic temperature change. This provides direct evidence that Southern Hemisphere mid-latitude storm tracks shifted within decades of abrupt changes in the North Atlantic, in parallel with meridional migrations of the intertropical convergence zone. We conclude that both oceanic and atmospheric processes, operating on di erent timescales, link the hemispheres during abrupt climate change.
The West Antarctic Ice Sheet Divide (WAIS Divide, WD) ice core is a newly drilled, high-accumulat... more 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 ∼ 68 ka at unprecedented temporal resolution. The upper 2850 m (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 (age) using a combination of firn densification modeling, ice-flow modeling, and a data set of δ 15 N-N 2 , a proxy for past firn column thickness. The largest 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 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".
Stable carbon isotope analysis of methane (δ 13 C of CH 4) on atmospheric samples is one key meth... more Stable carbon isotope analysis of methane (δ 13 C of CH 4) on atmospheric samples is one key method to constrain the current and past atmospheric CH 4 budget. A frequently applied measurement technique is gas chromatography isotope ratio mass spectrometry coupled to a combustion-preconcentration unit. This report shows that the atmospheric trace gas krypton can severely interfere during the mass spectrometric measurement leading to significant biases in δ 13 C of CH 4 if krypton is not sufficiently separated during the analysis. The effect comes about by the lateral tailing of the peak of doubly charged 86 Kr in the neighbouring m/z, 44, 45, and 46 Faraday cups. Accordingly, the introduced bias is dependent on the chromatographic separation, the Kr to CH 4 mixing ratio in the sample, the mass spectrometer source tuning as well as the detector configuration and can amount to up to several permil in δ 13 C. Apart from technical solutions to avoid this interference we present correction routines to a posteriori remove the bias.
Paleoatmospheric records of trace-gas concentrations recovered from ice cores provide important s... more Paleoatmospheric records of trace-gas concentrations recovered from ice cores provide important sources of information on many biogeochemical cycles involving carbon, nitrogen, and oxygen. Here, we present a 106,000-year record of atmospheric nitrous oxide (N 2 O) along with corresponding isotopic records spanning the last 30,000 years, which together suggest minimal changes in the ratio of marine to terrestrial N 2 O production. During the last glacial termination, both marine and oceanic N 2 O emissions increased by 40 ± 8%. We speculate that our records do not support those hypotheses that invoke enhanced export production to explain low carbon dioxide values during glacial periods.
One explanation for the abrupt increases in atmospheric CH 4 , that occurred repeatedly during th... more One explanation for the abrupt increases in atmospheric CH 4 , that occurred repeatedly during the last glacial cycle involves clathrate destabalization events. Because marine clathrates have a distinct deuterium/hydrogen (D/H) isotope ratio, any such destabilization event should cause the D/H ratio of atmospheric CH 4 (δD CH4 ) to increase. Analyses of air trapped in the ice from the second Greenland ice sheet project show stable and/or decreasing δD CH4 values during the end of the Younger and Older Dryas periods and one stadial period, suggesting that marine clathrates were stable during these abrupt warming episodes. Elevated glacial δD CH4 values may be the result of a lower ratio of net to gross wetland CH 4 emissions and an increase in petroleum-based emissions.
Records of the 13 C/ 12 C (d 13 CH 4) and the D/H (dD CH4) ratio of atmospheric methane were reco... more Records of the 13 C/ 12 C (d 13 CH 4) and the D/H (dD CH4) ratio of atmospheric methane were recovered from the GISP II ice core covering the last 11,000 years. All totaled, 76 samples were analyzed for d 13 CH 4 and 65 adjacent samples for dD CH4 between 86 and 1696 m below surface (mbs) providing a temporal resolution that is better than one pair of isotope samples every 200 years. The d 13 CH 4 record exhibits a decreasing trend throughout the Holocene beginning at À46.4& at 11,000 years BP (BP defined as 1950 AD ¼ 11 ka), and decreasing to À48.4& at 1 ka. The 2& d 13 CH 4 drop is likely to be a combination of increased CH 4 emissions from Arctic lake ecosystems and an increase in the ratio of C 3 /C 4 plants in wetlands where CH 4 is emitted. The C 3 /C 4 ratio increase is the result of increasing CO 2 values throughout the Holocene combined with the activation of high NH ecosystems that are predominantly C 3 type. The dD CH4 record over the early-mid Holocene shows a slightly decreasing trend that would be predicted by increased CH 4 emissions from Arctic lakes. Between 4 ka and 1 ka, dD CH4 values increase by w20& while d 13 CH 4 values remain effectively constant. There are at least two plausible explanations for this 20& dD CH4 shift. First, a dramatic shift in CH 4 emissions from higher latitudes to the tropics could account for the observed shift though the lack of a corresponding d 13 CH 4 shift is difficult to reconcile. Secondly, a gradual release of marine clathrates with enriched dD CH4 values explains both the dD CH4 and d 13 CH 4 records over this period.
Proceedings of the National Academy of Sciences, 1997
Air trapped in glacial ice offers a means of reconstructing variations in the concentrations of a... more Air trapped in glacial ice offers a means of reconstructing variations in the concentrations of atmospheric gases over time scales ranging from anthropogenic (last 200 yr) to glacial/interglacial (hundreds of thousands of years). In this paper, we review the glaciological processes by which air is trapped in the ice and discuss processes that fractionate gases in ice cores relative to the contemporaneous atmosphere. We then summarize concentration–time records for CO 2 and CH 4 over the last 200 yr. Finally, we summarize concentration–time records for CO 2 and CH 4 during the last two glacial–interglacial cycles, and their relation to records of global climate change.
Proceedings of the National Academy of Sciences, 2004
Our work was motivated by discoveries of prokaryotic communities that survive with little nutrien... more Our work was motivated by discoveries of prokaryotic communities that survive with little nutrient in ice and permafrost, with implications for past or present microbial life in Martian permafrost and Europan ice. We compared the temperature dependence of metabolic rates of microbial communities in permafrost, ice, snow, clouds, oceans, lakes, marine and freshwater sediments, and subsurface aquifer sediments. Metabolic rates per cell fall into three groupings: ( i ) a rate, μ g ( T ), for growth, measured in the laboratory at in situ temperatures with minimal disturbance of the medium; ( ii ) a rate, μ m ( T ), sufficient for maintenance of functions but for a nutrient level too low for growth; and ( iii ) a rate, μ s ( T ), for survival of communities imprisoned in deep glacial ice, subsurface sediment, or ocean sediment, in which they can repair macromolecular damage but are probably largely dormant. The three groups have metabolic rates consistent with a single activation energy ...
We undertook an interlaboratory comparison of techniques used to extract and analyze trapped gase... more We undertook an interlaboratory comparison of techniques used to extract and analyze trapped gases in ice cores. The intercomparison included analyses of standard reference gases and samples of ice from the Greenland Ice Sheet Project 2 (GISP2) site. Concentrations of CO2, CH4, the •5180 of 02, the •515N of N2, and the O2/N2, and Ar/N2 ratios were measured in air standards and ice core sampries. The standard reference scales for CO2 and CH 4 were consistent at the +2% level. The •502/N2 and •5180 of O2 measurements showed substantial deviations between the two laboratories able to measure these ratios. The deviations are probably related to errors associated with calibration of the working standards. The •sAr/N2 and •515N of N2 measurements were consistent. Five laboratories analyzed the CH4 concentration in a 4.2-m section of the GISP2 ice core. The average of 20 discrete CH 4 measurements was 748+10 parts per billion by volume (ppbv). The standard deviation of these measurements w...
Dominic A. Winski, Tyler J. Fudge, David G. Ferris, Erich C. Osterberg, John M. 4 Fegyveresi, Jih... more Dominic A. Winski, Tyler J. Fudge, David G. Ferris, Erich C. Osterberg, John M. 4 Fegyveresi, Jihong Cole-Dai, Zayta Thundercloud, Thomas S. Cox, Karl J. Kreutz, 5 Nikolas Ortman, Christo Buizert, Jenna Epifanio, Edward J. Brook, Ross Beaudette, 6 Jeff Severinghaus, Todd Sowers, Eric J. Steig, Emma C. Kahle, Tyler R. Jones, 7 Valerie Morris, Murat Aydin, Melinda R. Nicewonger, Kimberly A. Casey, 8 Richard B. Alley, Edwin D. Waddington, Nels A. Iverson, Nelia W. Dunbar , Ryan 9 C. Bay, Joseph M. Souney, Michael Sigl, Joseph R. McConnell 10
Accurate estimates of the past extent of the Greenland ice sheet provide critical constraints for... more Accurate estimates of the past extent of the Greenland ice sheet provide critical constraints for ice sheet models used to determine Greenland’s response to climate forcing and contribution to global sea level. Here we use a continuous ice core dust record from the Renland ice cap on the east coast of Greenland to constrain the timing of changes to the ice sheet margin and relative sea level over the last glacial cycle. During the Holocene and the previous interglacial period (Eemian) the dust record was dominated by coarse particles consistent with rock samples from central East Greenland. From the coarse particle concentration record we infer the East Greenland ice sheet margin advanced from 113.4 ± 0.4 to 111.0 ± 0.4 ka BP during the glacial onset and retreated from 12.1 ± 0.1 to 9.0 ± 0.1 ka BP during the last deglaciation. These findings constrain the possible response of the Greenland ice sheet to climate forcings.
The South Pole Ice Core (SPICEcore) was drilled in 2014-2016 to provide a detailed multi-proxy ar... more The South Pole Ice Core (SPICEcore) was drilled in 2014-2016 to provide a detailed multi-proxy archive of paleoclimate conditions in East Antarctica during the Holocene and late Pleistocene. Interpretation of these records requires an accurate depthage relationship. Here, we present the SP19 timescale for the age of the ice of SPICEcore. SP19 is synchronized to the WD2014 chronology from the West Antarctic Ice Sheet Divide (WAIS Divide) ice core using stratigraphic matching of 251 volcanic events. These events indicate an age of 54,302 +/-519 years BP (before the year 1950) at the bottom of SPICEcore. Annual layers identified in sodium and magnesium ions to 11,341 BP were used to interpolate between stratigraphic volcanic tie points, yielding an annually-resolved chronology through the Holocene. Estimated timescale uncertainty during the Holocene is less than 18 years relative to WD2014, with the exception of the interval between 1800 to 3100 BP when uncertainty estimates reach +/-25 years due to widely spaced volcanic tie points. Prior to the Holocene, uncertainties remain within 124 years relative to WD2014. Results show an average Holocene accumulation rate of 7.4 cm/yr (water equivalent). The time variability of accumulation rate is consistent with expectations for steady-state ice flow through the modern spatial pattern of accumulation rate. Time variations in nitrate concentration, nitrate seasonal amplitude, and δ 15 N of N 2 in turn are as expected for the accumulation-rate variations. The highly variable yet wellconstrained Holocene accumulation history at the site can help improve scientific understanding of deposition-sensitive climate proxies such as δ 15 N of N 2 and photolyzed chemical compounds.
Journal of Contemporary Water Research & Education, 2018
New techniques are needed to distinguish between leakage of methane (CH 4) into surface waters fr... more New techniques are needed to distinguish between leakage of methane (CH 4) into surface waters from gas wells and natural sources. Here, scientists worked with >50 citizen scientists in a hydrocarbon-rich basin (Pennsylvania, U.S.A.) to measure methane concentrations ([CH 4 ]) in streams. These measurements were combined with published observations to form a reconnaissance dataset. The dataset was then used to categorize sites as background or as impacted by other sources of gas. For 479 samples at 131 sites, 470 were supersaturated with respect to the atmosphere (>0.08 µg/L). Sites with the lowest concentrations generally were located in low-productivity, sandstone-underlain upland streams, while other streams contained CH 4 from sources in addition to atmospheric. The median of 63 sites not located near wetland habitats and not affected by known thermogenic influxes yielded an estimate of background [CH 4 ] in the streams, 0.5 µg/L. The highest individual measurements (~70 µg/L) in the stream dataset were observed in one site near a wetland and one site near a putatively leaking gas well. Inspection of the dataset revealed that values of [CH 4 ] above a threshold for non-wetland sites, 4 μg/L, signals gas is likely deriving from sources such as leaking gas wells, shallow organic-rich shales, coal, or landfills. Using historical and local volunteer knowledge, we discovered 12 non-wetland sites above the threshold that are potentially contaminated by such sources. Although sources of CH 4 cannot be proven from such surveys of [CH 4 ], stream sampling with nonscientists nonetheless allows discovery of sites of potential contamination that can be further investigated.
We report results from intercomparison exercises between laboratories that conduct measurements o... more We report results from intercomparison exercises between laboratories that conduct measurements of stable carbon and hydrogen isotope ratios of atmospheric CH<sub>4</sub> (δ<sup>13</sup>C-CH<sub>4</sub> and…
structural characterization of CO-inhibited forms of CpI, as well as that of CpII and other Fe-on... more structural characterization of CO-inhibited forms of CpI, as well as that of CpII and other Fe-only and NiFe hydrogenases, may contribute te additional insights to themechaqistic details of reversible hydrogen oxidation catalyzed by the hydrogenase enzymes. References and Notes Microbiol. Rev. 88, 109 (1992).
During the last glacial period, the North Atlantic region experienced a series of Dansgaard-Oesch... more During the last glacial period, the North Atlantic region experienced a series of Dansgaard-Oeschger cycles in which climate abruptly alternated between warm and cold periods. Corresponding variations in Antarctic surface temperature were out of phase with their Northern Hemisphere counterparts. The temperature relationship between the hemispheres is commonly attributed to an interhemispheric redistribution of heat by the ocean overturning circulation. Changes in ocean heat transport should be accompanied by changes in atmospheric circulation to satisfy global energy budget constraints. Although changes in tropical atmospheric circulation linked to abrupt events in the Northern Hemisphere are well documented, evidence for predicted changes in the Southern Hemisphere's atmospheric circulation during Dansgaard-Oeschger cycles is lacking. Here we use a high-resolution deuterium-excess record from West Antarctica to show that the latitude of the mean moisture source for Antarctic precipitation changed in phase with abrupt shifts in Northern Hemisphere climate, and significantly before Antarctic temperature change. This provides direct evidence that Southern Hemisphere mid-latitude storm tracks shifted within decades of abrupt changes in the North Atlantic, in parallel with meridional migrations of the intertropical convergence zone. We conclude that both oceanic and atmospheric processes, operating on di erent timescales, link the hemispheres during abrupt climate change.
The West Antarctic Ice Sheet Divide (WAIS Divide, WD) ice core is a newly drilled, high-accumulat... more 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 ∼ 68 ka at unprecedented temporal resolution. The upper 2850 m (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 (age) using a combination of firn densification modeling, ice-flow modeling, and a data set of δ 15 N-N 2 , a proxy for past firn column thickness. The largest 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 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".
Stable carbon isotope analysis of methane (δ 13 C of CH 4) on atmospheric samples is one key meth... more Stable carbon isotope analysis of methane (δ 13 C of CH 4) on atmospheric samples is one key method to constrain the current and past atmospheric CH 4 budget. A frequently applied measurement technique is gas chromatography isotope ratio mass spectrometry coupled to a combustion-preconcentration unit. This report shows that the atmospheric trace gas krypton can severely interfere during the mass spectrometric measurement leading to significant biases in δ 13 C of CH 4 if krypton is not sufficiently separated during the analysis. The effect comes about by the lateral tailing of the peak of doubly charged 86 Kr in the neighbouring m/z, 44, 45, and 46 Faraday cups. Accordingly, the introduced bias is dependent on the chromatographic separation, the Kr to CH 4 mixing ratio in the sample, the mass spectrometer source tuning as well as the detector configuration and can amount to up to several permil in δ 13 C. Apart from technical solutions to avoid this interference we present correction routines to a posteriori remove the bias.
Paleoatmospheric records of trace-gas concentrations recovered from ice cores provide important s... more Paleoatmospheric records of trace-gas concentrations recovered from ice cores provide important sources of information on many biogeochemical cycles involving carbon, nitrogen, and oxygen. Here, we present a 106,000-year record of atmospheric nitrous oxide (N 2 O) along with corresponding isotopic records spanning the last 30,000 years, which together suggest minimal changes in the ratio of marine to terrestrial N 2 O production. During the last glacial termination, both marine and oceanic N 2 O emissions increased by 40 ± 8%. We speculate that our records do not support those hypotheses that invoke enhanced export production to explain low carbon dioxide values during glacial periods.
One explanation for the abrupt increases in atmospheric CH 4 , that occurred repeatedly during th... more One explanation for the abrupt increases in atmospheric CH 4 , that occurred repeatedly during the last glacial cycle involves clathrate destabalization events. Because marine clathrates have a distinct deuterium/hydrogen (D/H) isotope ratio, any such destabilization event should cause the D/H ratio of atmospheric CH 4 (δD CH4 ) to increase. Analyses of air trapped in the ice from the second Greenland ice sheet project show stable and/or decreasing δD CH4 values during the end of the Younger and Older Dryas periods and one stadial period, suggesting that marine clathrates were stable during these abrupt warming episodes. Elevated glacial δD CH4 values may be the result of a lower ratio of net to gross wetland CH 4 emissions and an increase in petroleum-based emissions.
Records of the 13 C/ 12 C (d 13 CH 4) and the D/H (dD CH4) ratio of atmospheric methane were reco... more Records of the 13 C/ 12 C (d 13 CH 4) and the D/H (dD CH4) ratio of atmospheric methane were recovered from the GISP II ice core covering the last 11,000 years. All totaled, 76 samples were analyzed for d 13 CH 4 and 65 adjacent samples for dD CH4 between 86 and 1696 m below surface (mbs) providing a temporal resolution that is better than one pair of isotope samples every 200 years. The d 13 CH 4 record exhibits a decreasing trend throughout the Holocene beginning at À46.4& at 11,000 years BP (BP defined as 1950 AD ¼ 11 ka), and decreasing to À48.4& at 1 ka. The 2& d 13 CH 4 drop is likely to be a combination of increased CH 4 emissions from Arctic lake ecosystems and an increase in the ratio of C 3 /C 4 plants in wetlands where CH 4 is emitted. The C 3 /C 4 ratio increase is the result of increasing CO 2 values throughout the Holocene combined with the activation of high NH ecosystems that are predominantly C 3 type. The dD CH4 record over the early-mid Holocene shows a slightly decreasing trend that would be predicted by increased CH 4 emissions from Arctic lakes. Between 4 ka and 1 ka, dD CH4 values increase by w20& while d 13 CH 4 values remain effectively constant. There are at least two plausible explanations for this 20& dD CH4 shift. First, a dramatic shift in CH 4 emissions from higher latitudes to the tropics could account for the observed shift though the lack of a corresponding d 13 CH 4 shift is difficult to reconcile. Secondly, a gradual release of marine clathrates with enriched dD CH4 values explains both the dD CH4 and d 13 CH 4 records over this period.
Proceedings of the National Academy of Sciences, 1997
Air trapped in glacial ice offers a means of reconstructing variations in the concentrations of a... more Air trapped in glacial ice offers a means of reconstructing variations in the concentrations of atmospheric gases over time scales ranging from anthropogenic (last 200 yr) to glacial/interglacial (hundreds of thousands of years). In this paper, we review the glaciological processes by which air is trapped in the ice and discuss processes that fractionate gases in ice cores relative to the contemporaneous atmosphere. We then summarize concentration–time records for CO 2 and CH 4 over the last 200 yr. Finally, we summarize concentration–time records for CO 2 and CH 4 during the last two glacial–interglacial cycles, and their relation to records of global climate change.
Proceedings of the National Academy of Sciences, 2004
Our work was motivated by discoveries of prokaryotic communities that survive with little nutrien... more Our work was motivated by discoveries of prokaryotic communities that survive with little nutrient in ice and permafrost, with implications for past or present microbial life in Martian permafrost and Europan ice. We compared the temperature dependence of metabolic rates of microbial communities in permafrost, ice, snow, clouds, oceans, lakes, marine and freshwater sediments, and subsurface aquifer sediments. Metabolic rates per cell fall into three groupings: ( i ) a rate, μ g ( T ), for growth, measured in the laboratory at in situ temperatures with minimal disturbance of the medium; ( ii ) a rate, μ m ( T ), sufficient for maintenance of functions but for a nutrient level too low for growth; and ( iii ) a rate, μ s ( T ), for survival of communities imprisoned in deep glacial ice, subsurface sediment, or ocean sediment, in which they can repair macromolecular damage but are probably largely dormant. The three groups have metabolic rates consistent with a single activation energy ...
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Papers by Todd Sowers