Results from Siple Dome
The Severinghaus lab has analyzed Siple Dome firn air, shallow ice, and parts of the deep ice core for nitrogen isotopes, argon isotopes, oxygen isotopes of O2, and krypton, xenon, neon, and oxygen/nitrogen ratios. The firn air studies were carried out in 1996-1997 and 1997-1998 in order to interpret the forthcoming deep gas record and to test the integrity of the air entrapment process, as well as to obtain samples of the early 20th century atmosphere for collaborative studies of atmospheric evolution (Butler et al., 2000). The deep ice core analyses so far have covered the interval 650-750 m, which spans the last deglaciation from about 22-10 kyr BP (Severinghaus et al, submitted). Major findings to date include:
- discovery of a step-like decrease in reconstructed firn thickness at 19.4 kyr BP, that has been interpreted as the response to an abrupt warming event;
- a small but statistically significant (p<0.001) step-like increase in d15Nexcess (a parameter sensitive to temperature change) that confirms the reality of the abrupt warming event
- a unique reduction in firn diffusive column thickness to essentially zero that may represent a several hundred-to-1000-yr interval of ablation, at 15.3 kyr BP.
- calibration of the thermal diffusion temperature change indicator using seasonal changes in firn air from Siple Dome (Severinghaus et al., 2001).
Future plans include measurement of Krypton/Nitrogen ratios in an attempt to reconstruct the atmospheric history of this ratio through the last glacial cycle, which may place constraints on the amount and timing of deep-ocean temperature change because of the contrasting solubilities of these gases.
Butler, J.H., Battle, M., Bender, M.L., Montzka, S.A., Clarke, A.D., Saltzman, E.S., Sucher, C.M., Severinghaus, J.P., and Elkins, J.W., A record of atmospheric halocarbons during the twentieth century from polar firn air, Nature 399, 749-755 (1999).
Severinghaus, J.P., Luz, B., Caillon, N., A Method for Precise Measurement of Argon 40/36 and Krypton/Argon Ratios in Trapped Air in Polar Ice with Applications to Past Firn Thickness and Abrupt Climate Change, submitted to Geochim. Cosmochim. Acta.
Severinghaus, J. P., Grachev, A., Battle, M., Thermal fractionation of air in polar firn by seasonal temperature gradients, Geochemistry, Geophysics, Geosystems 2, paper number 2000GC000146 (electronic journal) (2001).