HR: 09:00h
AN: PP21A-03    [Abstracts]
TI: A New Look at the Global Distribution of Isotopes in Precipitation
AU: * Posmentier, E S
EM: eric.s.posmentier@dartmouth.edu
AF: Dartmouth College, HB 6105, Hanover, NH 03755 United States
AU: Faiia, A M
EM: anthony.m.faiia@dartmouth.edu
AF: Dartmouth College, HB 6105, Hanover, NH 03755 United States
AU: Feng, X
EM: xiahong.feng@dartmouth.edu
AF: Dartmouth College, HB 6105, Hanover, NH 03755 United States
AB: The hydrogen and oxygen isotopic ratios of meteoric water, as preserved in geologic materials, are widely used to reconstruct past climatic conditions. Correlation between temperature and isotopic composition of precipitation for temperate regions has been well documented, and modern day correlations are used in interpreting paleo-isotopic records, although temperature is not the direct causative factor in isotopic distribution. Semi-tropical and tropical locations demonstrate correlation between amount of precipitation and isotopic value. In this report, we propose that the parameter (E-P)/P, where E is evaporation and P is precipitation, explains the annual mean isotopic distribution in both temperate and tropical environments. This parameter is also shown to correlate with the seasonal distribution of isotopic ratios in precipitation globally. Global data was downloaded from the GNIP (Global Network for Isotopes in Precipitation) database (http://isohis.iaea.org). This set is comprised of monthly hydrogen and oxygen isotopic compositions, amount of precipitation, and temperature for over 500 stations throughout the world. The monthly oxygen isotopic data were plotted against station latitude and a spline function fitted for each month. The resulting distributions show a local minimum in isotopic values in the tropics, which migrates seasonally following the ITCZ. On either side of the minimum, two asymmetric maxima occur which change in position and amplitude throughout the year. We use a simple one-dimensional model to predict the seasonal isotopic distribution. The model divides the world into 5-degree latitudinal zones. Seasonally varying evaporation and precipitation are specified for each zone from marine climatologic records, and the model uses the mass balance equations to calculate the import or export of water vapor and the isotopic value of precipitation falling in each zone. The results show that the major variations in isotopic value of precipitation at all latitudes can be explained by this simple model, the precipitation δ¹⁸O being positively correlated with (E-P)/P. When (E-P)/P is more negative, more of the precipitated vapor must be "imported" ± from a distance and is thus isotopically depleted, and when (E-P)/P is positive, the precipitated vapor is the early fraction of the locally evaporated water to precipitate and is isotopically enriched.
DE: 1040 Isotopic composition/chemistry
DE: 3210 Modeling
DE: 3309 Climatology (1620)
DE: 3319 General circulation
DE: 3354 Precipitation (1854)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2005 Joint Assembly