Abstracts of GRIP papers on gas measurements

This file contains abstracts of GRIP papers whose primary content concerns gas measurements (excluding studies of clathrates and of basal ice). Papers are listed alphabetically by first author. You can go straight to the abstract you want.

Anklin, M., Barnola, J.-M., Schwander, J., Stauffer, B. & Raynaud, D. 1995. Processes affecting the CO2 concentrations measured in Greenland ice. Tellus, 47B, 461-470.

Corresponding author: Martin Anklin, (now at) Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85721, USA.

Detailed CO2 measurements on ice cores from Greenland and Antarctica show different mean CO2 concentrations for samples at the same gas age. The deviation between Antarctic and Greenland CO2 records raises up to 20 ppmv during the last millennium. Based on the present knowledge of the global carbon cycle we can exclude such a high mean interhemispheric difference of the CO2 concentration between high northern and southern latitudes. Diffusive mixing of the air in the firn smoothes out short term variations of the atmospheric CO2 concen- tration. Nevertheless, we observe short term CO2 variations in Greenland ice in the range of 10-20 ppmv, which cannot represent atmospheric CO2 variations. Due to the low temperature at Summit, meltlayers can be excluded for most of the ice and they cannot account for the frequent anomalous short term CO2 variations and the elevated mean CO2 concentration in the Greenland ice. In this work we give some clues, that in situ production of CO2 in Greenland ice could build up excess CO2 after pore close off. Possible chemical reactions are the oxidation of organic carbon and the reaction between acidity and carbonate. We conclude that the carbonate-acidity reaction is the most probable process to explain the excess CO2 in the bubbles. The reaction could take place in very small liquid-like veins in cold ice, where the mobility of impurities is higher than in the ice lattice. At present, there exists no technique to measure the carbonate concentration in the ice directly. However, a comparison of CO2 analyses performed with a dry- and a wet-extraction technique allows to estimate the carbonate content of the ice. This estimate indicates a carbonate concentration in Greenland ice of about 0.4 +/- 0.2 micromol/l and a much lower concentration in Antarctic ice.

Anklin, M., Schwander, J., Stauffer, B., Tschumi, J., Fuchs, A., Barnola, J.M. & Raynaud, D. 1997. CO2 record between 40 and 8 kyr BP from the Greenland Ice Core Project ice core. Journal of Geophysical Research, 102, 26539-26545.

Corresponding author: Martin Anklin, (now at) Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85721, USA.

CO2 ice-core records show an increase in the atmospheric concentration of 80-100 parts per million by volume (ppmv) from the last glacial maximum (LGM) to the early Holocene. We present CO2 measurements performed on an ice core from central Greenland, drilled during the Greenland Ice Core Project (GRIP). This CO2 profile from GRIP confirms the most prominent CO2 increase from the LGM, with a mean concentration of 200 ppmv, to the early Holocene with concentrations between 290 and 310 ppmv. Some structures of the new CO2 record are similar to those previously obtained from the Dye 3 ice core (Greenland), which indicated a dilemma between Greenland and Antarctic CO2 records [Oeschger et al., 1988]. Both Greenland cores show high CO2 values for rather mild climatic periods during the last glaciation, whereas CO2 records from Antarctica do not show such high CO2 variations during the glaciation and, furthermore, the CO2 values in the early Holocene are about 20-30 ppmv higher in the GRIP record than in Antarctic records. There is some evidence that the difference could be due to chemical reactions between impurities in the ice leading to an increase of the CO2 concentration under certain conditions. If in situ processes can change the CO2 concentration in the air bubbles, the question arises about how reliably do CO2 records from ice cores reflect the atmospheric composition at the time of ice formation. The discrepancies between the CO2 profiles from Greenland and Antarctica can be explained by in situ production of excess CO2 due to interactions between carbonate and acidic species. Since the carbonate concentration in Antarctic ice is much lower than in Greenland ice, CO2 records from Antarctica are much less affected by such in situ-produced CO2.

Barnola, J.M., Anklin, M., Porcheron, J., Raynaud, D., Schwander, J. & Stauffer, B. 1995. CO2 evolution during the last millenium as recorded by Antarctic and Greenland ice. Tellus, 47B, 264-272.

Corresponding author: Jean-Marc Barnola, Laboratoire de Glaciologie et Geophysique de l'Environnement, BP96, 38402 St. Martin d'Heres Cedex, France.

In order to study in detail the pre-industrial C02 level (back to about 900 AD) and its temporal variations, several ice cores from Greenland and Antarctica were analysed in two laboratories, and compared with previous records. The agreement between the two laboratories and between the different cores of the same hemisphere is good. However, the comparison of the northern hemisphere (Greenland) and southern hemisphere (Antarctica) records shows values systematically higher in the north than in the south, ranging from 20 ppmv at the turn of this millennium to nearly zero around the 18th century. Based on our present knowledge of the carbon cycle, an inter-hemispheric gradient of 20 ppmv is unrealistic. Thus, in the oldest part of the record, at least one profile should not represent the true atmospheric CO2 concentrations. A companion paper by Anklin et al. (submitted), discusses the possible processes which can alter the atmospheric CO2 once trapped in the ice. Due to the fact that the impurity content is one order of magnitude lower in the Antarctic than in the Greenland ice, we are much more confident in the Antarctic record. The new results from D47 and D57 (Adelie Land) presented in this paper, confirm the CO2 fluctuation of about 10 ppmv at the end of the 13th century, previously observed by Siegenthaler et al. (1988) on an ice core drilled at South Pole. This fluctuation corresponds to a small imbalance of the carbon cycle ( ~ 0.3 GT C/yr), but its duration led to a significant cumulative input into the atmosphere. The changes observed in the pre-industrial level are discussed in terms of climatic noise and variability.

Blunier, T., Chappellaz, J., Schwander, J., Stauffer, B. & Raynaud, D. 1995. Variations in atmospheric methane concentration during the Holocene epoch. Nature, 374, 46-49.

Corresponding author: Thomas Blunier, Physics Institute, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland.

Records of the variation in atmospheric methane concentration have been obtained from ice cores for the past 1,000 years and for the period 8,000--220,000 yr BP (refs 1-4), but data for the intervening period, spanning most of the present interglacial period (Holocene), are patchy (refs 5-7 and references therein). Here we present a continuous, high-resolution record of atmospheric methane from 8,000 to 1,000 yr BP, from the GRIP ice core in central Greenland. Unlike most other climate proxies from ice cores (such as oxygen isotope composition and electrical conductivity), methane concentrations show significant variations - up to 15% - during the Holocene. We have proposed thatt variations in the hydrological cycle at low latitudes are the dominant control on past levels of atmospheric methane. This is now supported by the observation that the lowest methane concentrations in our new record occur in the mid-Holocene when many tropical lakes dried up. The concentration increases during the Late Holocene, probably owing to an increasing contribution from northern wetlands.

Chappellaz, J., Blunier, T., Raynaud, D., Barnola, J.M., Schwander, J. & Stauffer, B. 1993. Synchronous changes in atmospheric CH4 and Greenland climate between 40 and 8 kyr BP. Nature, 366, 443-445.

Corresponding author: Jerome Chappellaz, Laboratoire de Glaciologie et Geophysique de l'Environnement, BP96, 38402 St. Martin d'Heres Cedex, France.

Ice-core reconstructions of atmospheric methane concentrations for the past 220 kyr have revealed large variations associated with different climatic periods. But the phase relationship between climate and methane has been uncertain because of dating uncertainties and the coarse sampling interval of available methane records. Here we present a high-resolution record of atmospheric methane from 40 to 8 kyr ago from the GRIP ice core in Greenland. Our improved resolution and dating allow us to conclude that the large changes in atmospheric methane concentration during the last deglaciation were in phase (+/-200 years) with the variations in Greenland climate. Our results confirm the previous observation that methane increased to Holocene levels when much of the Northern wetlands was still ice-covered, lending support to the suggestion that low-latitude wetlands were responsible for the observed changes. We observe oscillations in methane concentration associated with the warm periods (interstadials) that occurred throughout the glacial period, suggesting that the interstadials were at least hemispheric in their extent. We propose that variations in the hydrological cycle at low latitudes may be responsible for the variations in both methane and Greenland temperature during the interstadials.

Chappellaz, J., Blunier, T., Kints,S., Dallenbach,A., Barnola, J.M., Schwander, J., Raynaud,D. & Stauffer, B. 1997. Changes in the atmospheric CH4 gradient between Greenland and Antarctica during the Holocene Journal of Geophysical Research, 102, 15987-15997.

Corresponding author: Jerome Chappellaz, Laboratoire de Glaciologie et Geophysique de l'Environnement, BP96, 38402 St. Martin d'Heres Cedex, France.

High-resolution records of atmospheric methane over the last 11,500 years have been obtained from two Antarctic ice cores (D47 and Byrd) and a Greenland core (Greenland Ice Core Project). These cores show similar trapping conditions for trace gases in the ice combined with a comparable sampling resolution; this together with a good relative chronology, provided by unequivocal CH4 features, allows a direct comparison of the synchronized Greenland and Antarctic records, and it reveals significant changes in the interpolar difference of CH4 mixing ratio with time. On the average, over the full Holocene records, we find an interpolar difference of 447 ppbv. A minimum difference of 337 ppbv is observed from 7 to 5 kyr B.P. whereas the maximum gradient (503 ppbv) took place from 5 to 2.5 kyr B.P. A gradient of 444 ppbv is observed during the early Holocene (11.5 to 9.5 kyr B.P). We use a three-box model to translate the measured differences into quantitative contributions of methane sources in the tropics and the middle to high latitudes of the northern hemisphere. The model results support the previous interpretation that past natural CH4 sources mainly lay in tropical regions, but it also suggests that boreal regions provided a significant contribution to the CH4 budget especially at the start of the Holocene. The growing extent of peat bogs in boreal regions would also have counterbalanced the drying of the tropics over the second half of the Holocene. Finally, our model results suggest a large source increase in tropical regions from the late Holocene to the last millennium, which may partly be caused by anthropogenic emissions.

Chappellaz, J., Brook, E., Blunier, T. & Malaize, B. 1997. CH4 and delta O-18 of O-2 records from Antarctic and Greenland ice: A clue for stratigraphic disturbance in the bottom part of the Greenland Ice Core Project and the Greenland Ice Sheet Project 2 ice cores. Journal of Geophysical Research, 102, 26547-26557.

Corresponding author: Jerome Chappellaz, Laboratoire de Glaciologie et Geophysique de l'Environnement, BP96, 38402 St. Martin d'Heres Cedex, France.

The suggestion of climatic instability during the last interglacial period (Eem), based on the bottom 10% of the Greenland Ice core Project (GRIP) isotopic profile, has been questioned because the bottom record from the neighboring Greenland Ice Sheet Project 2 (GISP2) core (28 km away) is strikingly different over the same interval and because records of the delta(18)O of atmospheric O-2 from both cores showed unexpected rapid fluctuations. Here we present detailed methane records from the Vostok (Antarctica), GRIP, and GISP2 cores over the relevant intervals. The GRIP and GISP2 data show rapid and large changes in methane concentration, which are correlative with variations of the delta(18)O of the ice, while the Vostok record shows no such variations. This discrepancy reinforces the suggestion that the bottom sections of the Greenland records are disturbed. By combining the methane data with measurements of delta(18)O of O-2 in the same samples, we attempt to constrain the nature of the stratigraphic disturbance and the age of the analyzed ice samples. Our results suggest that ice layers from part of the last interglacial period exist in the lower section of both ice cores and that some of the apparent climate instabilities in the GRIP core would be the result of a mixture of ice from the last interglacial with ice from the beginning of the last glaciation or from the penultimate glaciation.

Raynaud, D., Chappellaz, J., Ritz, C. & Martinerie, P. 1997. Air content along the Greenland Ice Core Project core: A record of surface climatic parameters and elevation in central Greenland. Journal of Geophysical Research, 102, 26607-26613.

Corresponding author: Dominique Raynaud, Laboratoire de Glaciologie et Geophysique de l'Environnement, BP96, 38402 St. Martin d'Heres Cedex, France.

We present here measurements of the air content of the ice, V, performed along the Greenland Ice Core Project (GRIP) ice core. The main features of the longterm trends are (1) a decrease of 13% between the last glacial maximum (LGM) and the earliest part of the Holocene, and (2) an increase of 8% during the Holocene. The results are discussed in terms of changes in atmospheric pressure, surface elevation and porosity at close-off. The V record contains a significant signal of past changes of surface elevation in qualitative agreement with ice sheet modeling simulations. It suggests a thickening of central Greenland during the transition from the LGM to the early Holocene, and a significant thinning through the Holocene period. It also stresses the large influence on past V variations of changes in ice porosity, which are not explained by the present-day spatial relationship with temperature and may reflect changes in other surface climatic parameters (like precipitation seasonality or wind stress). The potential role of temporal variations of atmospheric pressure patterns is also evaluated. Air content results in the GRIP ice older than 110 ka indicate values approximately in the same range as those observed during the last 40,000 years, with generally higher air content corresponding to isotopically warmer ice.

Schwander, J., Sowers, T., Barnola, J.-M., Blunier, T., Fuchs, A. & Malaize, B. 1997. Age scale of the air in the summit ice: implication for glacial-interglacial temperature change. Journal of Geophysical Research, 102, 19483-19493.

Corresponding author: Jakob Schwander, Physics Institute, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland.

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 d18O 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 d18O 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 d15N 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.

Sowers, T., Brook, E., Etheridge, D., Blunier, T., Fuchs, A., Leuenberger, M., Chappellaz, J., Barnola, J.M., Wahlen, M., Deck, B. & Weyhenmeyer, C. 1997. An interlaboratory comparison of techniques for extracting and analyzing trapped gases in ice cores. Journal of Geophysical Research, 102, 26527-26538.

Corresponding author: Todd Sowers, Department of Geosciences, Pennsylvania State University, University Park, PA 16802-2714, USA.

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 delta(18)O of O-2, the delta(15)N of N-2, and the O-2/N-2, and Ar/N-2 ratios were measured in air standards and ice core samples. The standard reference scales for CO2 and CH4 were consistent at the +/-2% level. The delta(O2)/N-2 and delta(18)O of O-2 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 delta Ar/N-2 and delta(15)N of N-2 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 CH4 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 CH4 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.