HR: 0800h
AN: C51B-1036    [Abstracts]
TI: Stable isotope values of rainfall and surface waters from Panama and Costa Rica
AU: * Lachniet, M S
EM: matthew.lachniet@ccmail.nevada.edu
AF: University of Nevada Las Vegas, Department of Geoscience 4505 Maryland Parkway, Box 4010,, Las Vegas, NV 89154 United States
AU: Patterson, W P
EM: Bill.patterson@usask.ca
AF: University of Saskatchewan, Department of Geological Sciences 114 Science Place, Saskatoon, SK S7N 5E2 Canada
AB: Our understanding of stable isotope values of tropical rainfall is limited. In physiographically-diverse areas like Panama and Costa Rica, isotope data from a few GNIP stations at sea level is not directly applicable to sites at different elevations and locations. Thus, physical and climatic variables that control rainfall isotope values in areas with significant relief such as Central America are poorly known. Because of this problem, we utilize surface water samples as a rainfall proxy to investigate the climatic controls on the stable isotope composition of waters from Panama and Costa Rica. We measured $\delta^{18}$O and $\delta$D on ~270 river and lake waters collected from Lake Nicaragua in the west to the Darien of Panama in the east, on both the Caribbean and Pacific slopes and intervening Cordilleras. We statistically analyzed the isotope data from Panama using correlation and multiple stepwise regression against environmental and geographic variables. GNIP Isotope data from Panama City were analyzed for temporal variability. Temporally, rainfall $\delta^{18}$O values are negatively correlated with precipitation amount. Spatially, surface water $\delta^{18}$O values decrease with distance from the Caribbean, an indication of progressive rainout as air masses traverse the isthmus. The Panama surface water line is defined as $\delta$D = (7.6 $\pm$ 0.09) x $\delta^{18}$O + (10.1 $\pm$ 0.6), statistically identical to that from Costa Rica. Respective Panama meteoric water lines based on monthly and annual data are $\delta$D = (7.4 $\pm$ 0.07) x $\delta^{18}$O + (5.5 $\pm$ 0.3), and $\delta$D = (8.2 $\pm$ 0.60) x $\delta^{18}$O + (10.6 $\pm$ 3.3). We conclude that the dominant control on rain and surface water $\delta^{18}$O values associated with the Central American Monsoon are temporal and spatial amount effects. Surface water $\delta^{18}$O values also show significant correlations with drainage basin parameters such as median and stream head elevation, and latitude and longitude. An equation derived from stepwise multiple regression relates $\delta^{18}$O values to various physical parameters and explains 74% of the observed isotopic variation. Weaker correlations were found between deuterium excess and physiographic variables. Our results support the use of spatial sampling of surface waters for use as a rainfall $\delta^{18}$O proxy, and will benefit ongoing paleoclimatic research in humid tropical regions.
DE: 3344 Paleoclimatology
DE: 1704 Atmospheric sciences
DE: 1799 General or miscellaneous
DE: 1854 Precipitation (3354)
DE: 1040 Isotopic composition/chemistry
SC: Cryosphere [C]
MN: 2004 AGU Fall Meeting