GRIP total air content REFERENCES: Raynaud, D., J.A. Chappellaz, C. Ritz, and P. Martinerie. 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. Lipenkov, B., F. Candaudap, J. Ravoire, E. Dulac, and D. Raynaud. 1995. A new device for the measurement of air content in polar ice. Journal of Glaciology 41:423-429. Schwander, J., J.M. Barnola, C. Andrie, M.C. Leuenberger, A. Ludin, D. Raynaud, and B. Stauffer. 1993. The age of the air in the firn and the ice at Summit, Greenland. Journal of Geophysical Research 98:2831-2838. Chappellaz, J.A. 1990. Etude du methane atmospherique au cours du dernier cycle climatique a partir d l'analyse de l'air piege dans la glace antarctique, PhD thesis, University of Grenoble, 214pp. DATA DESCRIPTION: This file contains four columns for the data down to 2280 m of depth: Column 1: Depth (meters below the surface) Column 2: Air content (cubic centimeters per gram of ice) Column 3: Code for the analytical method used: 1 for the chromatographic method , 2 for the barometric method Column 4: Age of the gas (calendar years before 1950) Note: as the ice below 2780 meters depth is believed to be stratigraphically disturbed (Chappellaz et al., 1997), no age is given for the corresponding samples. The depth corresponds to the depth below the surface of Summer 1989. As air samples getting enclosed in ice bubbles have an age which is younger than the age of the surrounding ice, there are two time scales which have to be known: 1) a time scale for the ice and 2) a time scale for the air. The time scale for the ice is provided by a glaciological model forced with the GRIP borehole temperature profile (Johnsen et al., 1995a). The dating of the air is obtained with a semi-empirical model of firn densification (Barnola et al., 1991). In addition, the depth at which the air becomes trapped at the firn-ice transition is made dependent on the firn temperature. The temperature and the accumulation rate, both needed by the model, are determined from the isotopic profile along the GRIP core (Johnsen et al., 1995a). Today's age difference amounts to about 210 yr (Schwander et al., 1993). The accuracy of the densification model strongly depends on the temperature and accumulation rate introduced. Therefore, minor changes in the timescale may occur, due to an update of the ice timescale or improved modelling of the age difference between air and ice. As the ice below 2780 m of depth is believed to be stratigraphically disturbed (Chappellaz et al., 1997), no age is given for the corresponding samples. Two experimental methods were used to measure the air content. Method 1 is a by-product of CH4 measurements (Chappellaz, 1990), and the air-content samples measured with that method are the same as for the GRIP CH4 measurements of LGGE presented elsewhere in the CD-Rom. Method 2 is a method specifically developed for accurate measurements of the air content in the ice (Lipenkov et al., 1995). Samples below 2780 m of depth were all analysed with method 1. In both methods, the extraction of trapped gases is made by melting and refreezing of the ice sample under vacuum ; the gas sample is then expanded in a vacuum line. In method 1, the air content measurement is obtained by injection of the air sample in a gas chromatograph and measurement of the dry air peak. In method 2, it is obtained by accurate pressure and temperature measurements in a calibrated volume of the vacuum line. This second technique has been proved to be accurate within +/- 0.6% and its average precision to be +/- 0.5% (Lipenkov et al., 1995). Method 1 is less accurate than method 2, due to an unknown gas temperature during the gas expansion. Its precision is better than +/- 5%. The results from the two methods on the GRIP core are in good agreement. The air content record of the GRIP core down to 2280 m of depth is interpreted in terms of past changes in the surface elevation of the GRIP site area and in past atmospheric pressure patterns. DATA: Depth (m) Total Air Method Gas Age Midpoint (ml/g) (yrs BP) 122.07 0.0861 1 167 144.03 0.0845 1 246 144.07 0.09 1 246 165.58 0.0899 1 377 189.23 0.0902 1 472 209.58 0.0931 1 558 231.58 0.0891 1 674 275.98 0.0937 1 893 276.03 0.0955 1 893 320.07 0.0906 1 1101 341.58 0.0918 1 1227 341.63 0.0917 1 1227 363.58 0.0885 1 1328 385.03 0.09 1 1434 407.58 0.0889 1 1560 429.58 0.0898 1 1683 451.58 0.0963 1 1780 473.58 0.0899 1 1905 495.58 0.088 1 2013 495.62 0.0902 1 2013 517.58 0.0918 1 2116 539.58 0.0872 1 2251 561.58 0.0881 1 2377 561.62 0.0876 1 2377 579.17 0.0944 1 2468 605.58 0.089 1 2626 627.58 0.0883 1 2737 650.06 0.0943 1 2865 671.58 0.0884 1 2990 694.07 0.0929 1 3129 730.42 0.0884 1 3354 762.87 0.0836 1 3550 794.72 0.0908 1 3741 826.12 0.0902 1 3947 856.37 0.0868 1 4139 887.05 0.0884 1 4340 887.09 0.0907 1 4340 887.13 0.0862 1 4340 915.61 0.0904 1 4542 943.78 0.0873 1 4722 973.18 0.0822 1 4945 973.32 0.0877 1 4946 1000.47 0.0892 1 5145 1000.51 0.0857 1 5145 1030.17 0.0879 1 5348 1055.69 0.0906 1 5540 1082.09 0.0857 1 5753 1107.72 0.0892 1 5940 1133.14 0.087 1 6156 1133.53 0.0885 1 6159 1158.25 0.0876 1 6356 1181.97 0.0892 1 6549 1206.72 0.0877 1 6747 1230.92 0.0866 1 6963 1244.06 0.089 1 7079 1254.93 0.0835 1 7182 1255.07 0.0868 1 7183 1267.24 0.0875 1 7290 1278.73 0.0911 1 7384 1290.92 0.0924 1 7496 1302.43 0.0878 1 7611 1313.84 0.085 1 7717 1324.37 0.0853 1 7803 1331.82 0.0853 1 7887 1344.48 0.0852 2 7998 1354 0.0874 1 8081 1354.04 0.0886 1 8081 1354.08 0.0853 1 8082 1375.57 0.0845 1 8304 1397.82 0.083 1 8540 1399.57 0.0847 2 8553 1399.61 0.0821 2 8553 1419.57 0.0845 1 8774 1442.83 0.0801 2 9014 1442.89 0.0803 2 9014 1454.48 0.0859 2 9117 1454.54 0.085 2 9117 1463.82 0.084 1 9236 1474.57 0.0814 1 9351 1474.61 0.0854 1 9352 1485.57 0.0831 1 9474 1496.57 0.0825 1 9595 1508.08 0.0914 1 9726 1510.04 0.084 2 9750 1510.09 0.0824 2 9750 1519.06 0.0828 2 9855 1519.08 0.083 1 9855 1524.11 0.0821 1 9906 1529.57 0.0828 1 9963 1529.58 0.0851 2 9963 1529.6 0.0872 2 9963 1540.9 0.0826 1 10102 1540.92 0.0847 2 10102 1540.94 0.0826 2 10103 1551.57 0.0812 1 10238 1551.61 0.0836 1 10239 1562.57 0.0809 1 10376 1562.61 0.0808 1 10376 1564.49 0.0846 2 10397 1564.54 0.0823 2 10398 1573.57 0.0855 1 10533 1584.57 0.0842 1 10661 1595.57 0.0835 1 10838 1595.61 0.0832 1 10839 1606.57 0.085 1 10985 1606.77 0.0867 1 10989 1617.57 0.0801 1 11164 1617.61 0.0814 1 11165 1619.49 0.0811 2 11182 1619.54 0.0819 2 11182 1625.82 0.0869 1 11347 1626.04 0.0904 1 11356 1635.33 0.0826 1 11480 1645.23 0.0853 1 11551 1656.07 0.0887 1 11850 1667.07 0.0907 1 11997 1667.11 0.09 1 11998 1674.68 0.0925 2 12294 1674.72 0.0908 2 12296 1678.07 0.0899 1 12718 1689.07 0.09 1 12794 1700.07 0.0893 1 13173 1711.21 0.089 1 13362 1722.07 0.0876 1 13596 1729.48 0.0892 2 13735 1729.51 0.0824 2 13735 1733.07 0.0917 1 13755 1744.07 0.0922 1 14058 1755.78 0.0888 1 14327 1766.34 0.0824 1 14437 1777.07 0.0874 1 14629 1784.72 0.0887 2 14816 1788.07 0.0927 1 14943 1799.07 0.0888 1 15236 1810.07 0.0907 1 15672 1821.07 0.0913 1 15910 1832.23 0.0919 1 16263 1839.72 0.0926 2 16649 1843.07 0.0923 1 16690 1854.57 0.0955 1 17106 1865.07 0.0983 1 17339 1876.07 0.0899 1 17723 1887.23 0.0933 1 18056 1894.48 0.0949 2 18284 1898.07 0.0931 1 18399 1909.07 0.0943 1 18942 1920.08 0.0968 1 19259 1931.07 0.0953 1 19789 1942.07 0.0973 1 20310 1949.49 0.0958 2 20715 1953.62 0.094 1 21116 1964.07 0.0919 1 21509 1975.07 0.0911 1 21687 1975.09 0.0989 2 21688 1975.23 0.0941 2 21690 1986.21 0.0936 2 22689 1986.22 0.0931 2 22689 1986.23 0.0921 1 22690 1997.07 0.0978 1 23160 2005.28 0.094 2 23548 2008.07 0.0933 1 23920 2018.68 0.094 1 24564 2030.07 0.0957 1 25519 2041.62 0.0921 1 25604 2052.19 0.0939 1 26548 2059.71 0.0919 2 26642 2059.74 0.0895 2 26641 2063.58 0.0939 1 26916 2074.03 0.0927 1 27500 2085.62 0.0901 1 28399 2090.57 0.0925 1 28724 2095.52 0.0945 1 28816 2107.07 0.0869 1 29983 2114.15 0.0917 2 30044 2114.19 0.092 2 30045 2118.07 0.0946 1 30211 2129.07 0.0864 1 31208 2140.07 0.0954 1 31704 2151.32 0.0909 1 32667 2162.1 0.0914 1 32950 2169.68 0.0972 2 33493 2169.73 0.0948 2 33496 2172.52 0.1006 1 33923 2184.07 0.093 1 34584 2195.62 0.0908 1 35162 2206.62 0.0844 1 35699 2217.24 0.0905 1 36068 2224.68 0.0905 2 36760 2224.73 0.0907 2 36764 2227.52 0.0928 1 37193 2239.07 0.0963 1 38005 2250.07 0.0852 1 38967 2260.62 0.093 1 39740 2272.17 0.0876 1 40587 2279.49 0.0908 2 40851 2279.51 0.0879 2 40851 2780.82 0.0908 1 2783.57 0.0892 1 2784.67 0.0922 1 2786.32 0.0903 1 2788.52 0.0921 1 2789.31 0.0924 1 2791.82 0.0885 1 2793.47 0.0896 1 2795.14 0.0933 1 2795.67 0.0895 1 2797.87 0.0901 1 2798.97 0.0885 1 2800.07 0.0856 1 2803.37 0.0905 1 2804.53 0.0912 1 2806.67 0.0918 1 2807.77 0.092 1 2809.42 0.0863 1 2810.97 0.0964 1 2811.01 0.0895 1 2811.07 0.0903 1 2813.82 0.0895 1 2815.51 0.0909 1 2817.14 0.0932 1 2819.32 0.0891 1 2821.52 0.0924 1 2822.54 0.0907 1 2822.58 0.0888 1 2824.27 0.0921 1 2826.74 0.0981 1 2828.12 0.0919 1 2829.77 0.09 1 2832.52 0.0891 1 2833.62 0.0944 1 2835.27 0.0887 1 2837.47 0.0871 1 2839.67 0.0829 1 2841.32 0.0897 1 2842.97 0.0885 1 2843.54 0.0852 1 2843.58 0.0891 1 2844.62 0.0877 1 2846.82 0.078 1 2847.32 0.0818 1 2847.37 0.0844 1 2847.96 0.0796 1 2849.57 0.0801 1 2850.67 0.0801 1 2852.95 0.0835 1 2853.97 0.0801 1 2855.36 0.0822 1 2855.62 0.0824 1 2857.51 0.0867 1 2859.48 0.0852 1 2861.67 0.0822 1 2862.77 0.0798 1 2863.87 0.0874 1 2866.07 0.0839 1 2866.11 0.0901 1 2871.02 0.087 1 2874.32 0.0873 1 2880.42 0.087 1 2883.76 0.0897 1 2891.37 0.0873 1 2900.72 0.0891 1 2907.95 0.0894 1 2909.6 0.0926 1