Use of correlation and stepwise regression to evaluate physical controls on the stable isotope values of Panamanian rain and surface waters

To investigate the climatic controls on the stable isotope composition of surface waters from Panama, we measured delta O-18 and delta D on river and lake waters collected from throughout the isthmus. We statistically analyzed the isotope data using correlation (n = 162 samples) and multiple stepwise regression (n = 148 samples). Variables are delta O-18, delta D, deuterium-excess (d(x)), latitude, longitude, sample elevation, stream head elevation, median stream elevation above sample site, distance from the Caribbean, stream length above sampling site, estimated mean annual precipitation, and pH. Rainfall isotope data from Panama City were analyzed for temporal variability. Temporally, rainfall delta O-18 values are negatively correlated with precipitation amount. Spatially, surface water delta O-18 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 +/- 0.09) x delta O-18 x (10.1 +/- 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 +/- 0.07) x delta O-18 + (5.5 +/- 0.3), and delta D = (8.2 +/- 0.60) x delta O-18 + (10.6 +/- 3.3). We conclude that the dominant control on rain and surface water 6180 values associated with the Central American monsoon are temporal and spatial amount effects. Surface water delta O-18 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 O-18 values to various physical parameters and explains 74 parts per thousand 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 O-18 proxy, and will benefit ongoing paleoclimatic research in humid tropical regions. (c) 2005 Elsevier B.V. All rights reserved.