Showing papers by "Todd Sowers published in 1997"

Holocene climatic instability: A prominent, widespread event 8200 yr ago

Abstract: The most prominent Holocene climatic event in Greenland ice-core proxies, with approximately half the amplitude of the Younger Dryas, occurred ∼8000 to 8400 yr ago. This Holocene event affected regions well beyond the North Atlantic basin, as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane. Widespread proxy records from the tropics to the north polar regions show a short-lived cool, dry, or windy event of similar age. The spatial pattern of terrestrial and marine changes is similar to that of the Younger Dryas event, suggesting a role for North Atlantic thermohaline circulation. Possible forcings identified thus far for this Holocene event are small, consistent with recent model results indicating high sensitivity and strong linkages in the climatic system.

Age scale of the air in the summit ice: Implication for glacial-interglacial temperature change

Abstract: The air occluded in ice sheets and glaciers has, in general, a younger age (defined as the time after its isolation from the atmosphere) than the surrounding ice matrix because snow is first transformed into open porous firn, in which the air can exchange with the atmosphere. Only at a certain depth (firn-ice transition) the pores are pinched off and the air is definitely isolated from the atmosphere. The firn-ice transition depth is at around 70 m under present climatic conditions at Summit, central Greenland. The air at this depth is roughly 10 years old due to diffusive mixing, whereas the ice is about 220 years old. This results in an age difference between the air and the ice of 210 years. This difference depends on temperature and accumulation rate and did thus not remain constant during the past. We used a dynamic firn densification model to calculate the firn-ice transition depth and the age of the ice at this depth and an air diffusion model to determine the age of the air at the transition. Past temperatures and accumulation rates have been deduced from the δ18O record using time independent functions. We present the results of model calculations of two paleotemperature scenarios yielding a record of the age difference between the air and the ice for the Greenland Ice Core Project (GRIP) and the Greenland Ice Sheet Project Two (GISP2) ice cores for the last 100,000 years. During the Holocene, the age difference stayed rather stable around 200 years, while it reached values up to 1400 years during the last glaciation for the colder scenario. The model results are compared with age differences obtained independently by matching corresponding climate events in the methane and δ18O records assuming a very small phase lag between variations in the Greenland surface temperature and the atmospheric methane. The past firn-ice transition depths are compared with diffusive column heights obtained from δ15N of N2 measurements. The results of this study corroborate the large temperature change of 20 to 25 K from the coldest glacial to Holocene climate found by evaluating borehole temperature profiles.

Gases in ice cores

Abstract: 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 CO2 and CH4 over the last 200 yr. Finally, we summarize concentration-time records for CO2 and CH4 during the last two glacial-interglacial cycles, and their relation to records of global climate change.

An interlaboratory comparison of techniques for extracting and analyzing trapped gases in ice cores

Abstract: 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. ThesAr/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 was close to the total analytical uncertainty associated with the measurements. In all cases, those laboratories employing a dry extraction technique determined higher CH 4 values than laboratories using a wet extraction technique. The origin of this difference is unclear but may involve uncertainties associated with blank corrections. Analyses of the CO2 concentration of trapped gases showed extreme variations which cannot be explained by analytical uncertainties alone. Three laboratories measured the (CO2) on 21 discrete depths yielding an average value of 283+13 parts per million by volume (ppmv). In this case, the standard deviation was roughly a factor of 2 greater than the analytical uncertainties. We believe the variability in the measured (CO2) results from impurities in the ice which may have compromised the (CO2) of trapped gases in Greenland ice.