HR: 0830h
AN: B31E-0344    [PDF]
TI: A New Perspective on the Temperature-Dependence of Stable Isotopes in Modern Precipitation
AU: * Kohn, M J
EM: mjk@geol.sc.edu
AF: University of South Carolina, Department of Geological Sciences EWS 617; 701 Sumter St., Columbia, SC 29208 United States
AU: Welker, J M
EM: jwelker@nrel.colostate.edu
AF: Colorado State University, Natural Resource Ecology Laboratory, Fort Collins, CO 80523 United States
AB: Distillation of water vapor from the atmosphere is primarily temperature-dependent, as illustrated by the well-known correlation between the stable isotope composition of precipitation and temperature. However, reevaluation of modern precipitation, temperature, and isotope data indicates major errors in the assignment of temperature dependencies. This has a profound impact on use of stable isotopes of precipitation for ecosystem studies, models of atmospheric circulation, and continental paleoclimate investigations. Past analysis has used the mean surface temperature over the time interval of sample collection (e.g., mean weekly, monthly, or annual temperature) to calculate temperature coefficients, but different approaches at mid-latitudes give different temperature coefficients ($\Delta\delta^{18}$O/$\Delta$T). (a) Spatial correlations among geographically distinct sites yield $\sim$0.55$\permil$%/$\deg$C; (b) Seasonal variations at single sites yield $\sim$0.2-0.4$\permil$/$\deg$C; and (c) 12 month running averages yield$\sim$0.5-1$\permil$/$\deg$C. These disparities result because there are systematic differences in temperature during precipitation events vs. time-averaged surface temperature means. Correction for this bias using hourly weather and monthly isotope data from US sites reconciles disparate temperature-dependence estimators for modern precipitation, and yields a consistent $\sim$0.55$\permil$/$\deg$C for all three approaches. Revised temperature coefficients based on surface observations are also commensurate with coefficients obtained using cloud base temperatures. Revised values are within the range of theoretical distillation models $\sim$0.5-0.7$\permil$/$\deg$C, and provide a consistent basis for investigating atmospheric processes and isotopic response to climate change.
DE: 0330 Geochemical cycles
DE: 1040 Isotopic composition/chemistry
DE: 1610 Atmosphere (0315, 0325)
DE: 1806 Chemistry of fresh water
DE: 1854 Precipitation (3354)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting