GRIP Methane Data REFERENCES: Blunier, T., J.A. Chappellaz, J. Schwander, B. Stauffer, and D. Raynaud. 1995. Variations in atmospheric methane concentration during the Holocene epoch. Nature 374:46-49. Blunier, T., J.A. Chappellaz, J. Schwander, J.M. Barnola, T. Desperts, B. Stauffer, and D. Raynaud. 1993. Atmospheric methane record from a Greenland ice core over the last 1000 years. Geophysical Research Letters 20:2219 2222. chappellaz, J.A., T. Blunier, D. Raynaud, J.M. Barnola, J. Schwander, and B. Stauffer. 1993. Synchronous changes in atmospheric CH4 and Greenland climate between 40 and 8 kyr B.P. Nature 366:443-445. 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. Raynaud, D., J.A. Chappellaz, J.M. Barnola, Y.S. Korotkevich, and C. Lorius. 1988. Climatic and CH4 cycle implications of glacial-interglacial CH4 change in the Vostok ice core. Nature 333:655-657. DATA DESCRIPTION: The file contains two sets of five columns: Column 1: Depth (meters below the surface). Column 2: Mean age of the trapped air (calendar years before 1950). Column 3: CH4 concentration (ppbv, part per billion by volume). Column 4: Minimum CH4 concentration (two sigma experimental, ppbv). Column 5: Maximum CH4 concentration (two sigma experimental, ppbv). The depth of the sample corresponds to the depth below the surface of summer 1989. One sample for CH4 measurements corresponds to about 4 cm of core for LGGE and 2 cm for Bern, i.e. 2 cm (or 1 cm) on each side of the depth given in column 2. The provided dating corresponds to the original publications. The porous firn layer on the surface of the ice sheet is continuously exchanging air with the overlying atmosphere, the firn-air is a mixture of air masses that were last in contact with the atmosphere at different times. Air, which gets enclosed in bubbles, typically about 70m below the surface at Summit, thus has a mean age which is younger than the age of the surrounding ice. Consequently two time scales 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 the GRIP glaciological model (Dansgaard et al., 1993; Johnsen et al., 1992). For the Holocene period the time scale was modified such that the best fit on Holocene stratigraphic markers was obtained (S. Johnsen, personal communication). For the dating of the air a semi-empirical model of firn densification was used to determine the age difference between the trapped air and the surrounding ice (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 (Dansgaard et al., 1993; Johnsen et al., 1992). 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. Note that minor changes in the timescale may occur due to an update of the ice timescale or improved modelling for the age difference between gas and ice. The measurements presented have been performed in Bern with a dry extraction technique (Blunier et al., 1993) and in Grenoble with a melting-refreezing method (Chappellaz et al., 1990b; Chappellaz et al., 1993; Raynaud et al., 1988). In both laboratories the extracted air is analysed by gas chromatography. For the wet extractions a correction is made based on series of blanks of the extraction system, amounting to 18±26 ppbv. The minimum and maximum CH4 concentrations correspond to the experimental extrema taking into account the gas-chromatographic calibration uncertainty and the contamination uncertainty (both at the 95% confidence level). The analytical precisions, estimated from blank measurements with gas free ice, are ±20 and ±37 ppbv (2 Sigma) for the dry and wet extractions respectively (Blunier et al., 1993). Replicate measurements on the same depth levels are consistent with the estimated analytical precision. The records from the two labs (Blunier et al., 1995) reveal very similar trends, but showed a systematic shift of 30 ppbv. In the original publication the results were thus arbitrarily corrected by -15 and +15 ppbv for Bern and Grenoble respectively. New analyses revealed that the entire 30 ppbv shift was due to a drift of the Bern standard gas in one container. In the dataset included here, we thus decrease the GRIP results of Blunier et al. (1995) by 15 ppbv. Note that the GRIP data presented in Chappellaz et al. (1993) remain unaffected by this correction and that the Bern standard drift affects only the Bern results published in Blunier et al. (1995). In addition, one sample at 1398-m of depth (CH4 concentration of 742 ppbv) is removed from the Chappellaz et al (1993) series, as it was shown recently to have been contaminated by the drilling fluid (Chappellaz et al., 1997). Concentrations measured by dry extraction were significantly higher than concentrations measured by wet extraction in the depth range 1220-1320 m. We attributed this to effects resulting from the air-bubble to air-hydrate transformation which takes place in that depth range. These data are not included here. DATA: Results from LGGE Results from Blunier et al. Depth (m) Gas Age CH4 Min Max Depth (m) Gas Age CH4 Min Max Midpoint (yrs BP) (ppbv) (ppbv) (ppbv) Midpoint (yrs BP) (ppbv) (ppbv) (ppbv) 122.07 175 744 708 780 111.11 129 796 776 816 144.07 267 702 670 734 132.56 221 710 690 730 165.58 371 710 677 743 132.56 221 724 704 744 189.23 483 708 676 740 154.56 319 713 693 733 209.58 564 707 671 743 176.56 425 723 703 743 231.58 682 695 658 732 198.56 519 698 678 718 275.98 893 701 667 735 220.56 628 717 697 737 276.03 892 699 661 737 242.56 730 717 697 737 320.07 1113 673 632 714 276.07 893 702 682 722 341.58 1214 691 654 728 276.07 893 709 689 729 341.63 1214 672 635 709 276.09 893 715 695 735 363.58 1326 688 650 726 276.09 893 697 677 717 385.03 1424 666 632 700 309.09 1050 670 650 690 407.58 1561 664 629 699 309.09 1050 687 667 707 429.58 1666 675 638 712 331.09 1171 657 637 677 451.58 1777 679 640 718 331.09 1171 647 627 667 473.58 1909 638 604 672 353.09 1281 673 653 693 495.58 2013 675 640 710 353.09 1281 664 644 684 495.62 2013 650 607 693 375.09 1379 676 656 696 517.58 2137 636 600 672 375.09 1379 696 676 716 539.58 2255 630 590 670 397.07 1511 663 643 683 561.58 2380 638 598 678 397.07 1511 665 645 685 579.17 2487 634 598 670 397.09 1511 654 634 674 605.58 2619 620 587 653 418.56 1609 655 635 675 627.58 2741 624 587 661 418.56 1609 656 636 676 650.06 2872 617 576 658 441.09 1716 663 643 683 671.58 2996 606 573 639 441.09 1716 660 640 680 694.07 3123 628 582 674 463.09 1852 666 646 686 730.42 3345 617 583 651 463.09 1852 686 666 706 762.87 3536 605 571 639 485.09 1962 661 641 681 794.72 3739 613 574 652 485.09 1962 650 630 670 826.12 3938 618 591 645 507.05 2079 640 620 660 856.37 4132 606 569 643 507.05 2079 645 625 665 887.13 4347 601 558 644 507.07 2079 646 626 666 887.09 4347 614 579 649 507.09 2079 637 617 657 915.61 4543 622 588 656 507.09 2079 655 635 675 943.78 4740 610 574 646 528.56 2209 642 622 662 973.32 4954 624 587 661 528.56 2209 637 617 657 1000.47 5148 573 543 603 551.07 2326 640 620 660 1030.17 5380 592 559 625 551.09 2326 638 618 658 1055.69 5564 594 555 633 575.84 2461 631 611 651 1082.09 5771 611 580 642 575.84 2461 631 611 651 1107.72 5965 610 573 647 595.09 2566 630 610 650 1133.53 6181 596 554 638 595.09 2566 626 606 646 1158.25 6386 632 595 669 617.09 2687 603 583 623 1181.97 6582 622 586 658 617.09 2687 616 596 636 1206.72 6789 626 594 658 638.56 2808 628 608 648 1230.92 7000 645 611 679 638.56 2808 636 616 656 1244.06 7124 661 629 693 661.09 2944 621 601 641 1255.07 7223 634 601 667 661.09 2944 619 599 639 1267.24 7320 628 594 662 683.05 3065 610 590 630 1278.73 7427 659 628 690 683.05 3065 602 582 622 1290.92 7530 652 620 684 746.34 3441 604 584 624 1302.43 7647 661 609 713 746.34 3441 609 589 629 1313.84 7746 644 607 681 779.37 3654 645 625 665 1324.37 7844 629 590 668 779.38 3654 640 620 660 1331.82 7915 644 608 680 779.4 3654 632 612 652 1354.08 8104 588 551 625 779.4 3654 641 621 661 1354.04 8104 588 543 633 812.92 3855 625 605 645 1375.57 8325 652 609 695 812.94 3855 619 599 639 1419.57 8762 690 651 729 812.94 3855 636 616 656 1463.82 9202 710 666 754 842.02 4045 611 591 631 1474.57 9308 693 656 730 842.03 4045 598 578 618 1485.57 9421 695 659 731 901.46 4437 609 589 629 1496.57 9528 715 676 754 901.46 4437 624 604 644 1508.08 9653 726 685 767 930.11 4643 627 607 647 1519.08 9773 712 676 748 930.11 4643 626 606 646 1529.57 9891 720 678 762 959.21 4848 602 582 622 1540.9 10013 727 684 770 959.21 4848 605 585 625 1551.57 10141 720 679 761 986.71 5053 590 570 610 1562.57 10296 744 708 780 986.71 5053 596 576 616 1573.57 10447 722 684 760 1041.12 5453 591 571 611 1584.57 10588 720 687 753 1041.14 5453 588 568 608 1595.57 10768 691 651 731 1068.66 5665 581 561 601 1606.57 10919 719 681 757 1068.66 5665 589 569 609 1617.57 11093 698 658 738 1095.04 5874 607 587 627 1625.82 11254 677 642 712 1095.04 5874 607 587 627 1635.33 11449 726 681 771 1121.46 6086 608 588 628 1645.23 11575 686 654 718 1121.46 6086 613 593 633 1656.07 11893 484 452 516 1170.39 6483 623 603 643 1667.07 12152 472 440 504 1170.39 6483 632 612 652 1667.11 12152 471 436 506 1195.14 6700 622 602 642 1678.07 12585 533 503 563 1195.14 6700 615 595 635 1689.07 12757 666 626 706 1219.34 6910 663 643 683 1700.07 13076 689 659 719 1219.34 6910 654 634 674 1711.21 13290 685 652 718 1722.07 13534 673 638 708 1733.07 13691 660 623 697 1744.07 13984 656 626 686 1755.78 14242 633 600 666 1766.34 14382 653 618 688 1777.07 14629 536 498 574 1788.07 14946 464 428 500 1799.07 15243 453 416 490 1810.07 15649 482 451 513 1821.07 15979 429 393 465 1832.23 16305 416 386 446 1843.07 16710 364 333 395 1854.57 17048 374 334 414 1865.07 17341 375 341 409 1876.07 17714 359 320 398 1887.23 18057 360 321 399 1898.07 18461 372 336 408 1909.07 18923 357 315 399 1920.08 19338 374 336 412 1931.07 19864 362 328 396 1942.07 20298 347 308 386 1953.62 20915 365 332 398 1964.07 21344 402 369 435 1975.07 21813 402 369 435 1986.23 22506 375 337 413 1997.07 23037 372 340 404 2008.07 23667 378 343 413 2018.68 24191 351 314 388 2030.07 24904 390 357 423 2041.62 25208 447 408 486 2052.19 25950 423 385 461 2063.58 26428 396 356 436 2074.03 27046 381 347 415 2085.62 27832 426 395 457 2090.57 28089 382 349 415 2095.52 28380 404 371 437 2107.07 29116 504 470 538 2118.07 29586 392 359 425 2129.07 30288 498 456 540 2140.07 30984 431 399 463 2151.32 31697 540 507 573 2162.1 32176 473 440 506 2172.52 32981 453 419 487 2184.07 33597 566 529 603 2195.62 34174 554 518 590 2206.62 34701 575 539 611 2217.24 35394 450 394 506 2227.52 36320 422 390 454 2239.07 37028 425 388 462 2250.07 37817 505 471 539 2260.62 38622 412 375 449 2272.17 39408 509 468 550