REFINING THE CALIBRATION OF STABLE ISOTOPE PALEO-ELEVATION STUDIES: LESSONS AND STRATEGIES FROM MODERN PRECIPITATION PATTERNS

An examination of the patterns of oxygen and hydrogen isotope compositions of modern meteoric water yields a number of features that can be used to both refine the calibration of the elevation-isotopic ratio relationship and develop strategies for its application as a paleo-altimeter.

Within-basin heterogeneity in the isotopic composition of surface water / ground water is often a result of highlands surrounding a basin. The Rillito Wash collects runoff from the Santa Catalina Mountains and is the region within the Tucson basin with the most negative d¹⁸O values. Basinal rainfall averages –4.8 ‰ SMOW in the summer and –8.5 ‰ in the winter. The most negative groundwater in the Rillito wash is –14.5 ‰. Similar heterogeneities are seen modern surface waters of other basins and in inferred d¹⁸O values for Paleogene rivers in Montana and Wyoming. In paleo-elevation studies a comparison of samples collected proximal to highlands with distal samples may allow temperature uncertainty or variability to be better constrained in the calculation of the d¹⁸O or dD of ancient waters sourced at different elevations

Even if the isotopic composition of paleo-waters can be accurately calculated, the variance of modern measured isotope – elevation gradients introduces considerable uncertainty. Uncertainties can be reduced by classing localities with respect to continentality, latitude and aridity. One of the largest sources of variance in the isotope-elevation gradient is the effect of rain shadows. While both windward and leeward regions have negative gradients with increasing elevation, the slope of the relationship is usually very different.

Can this variability be modeled? Rayleigh fractionation models describe windward patterns well, but cannot explain leeward trends or the large variance seen in isotope-elevation gradients. Both the water balance of individual raindrops and models of rain generation based on cloud physics show that secondary interactions of raindrops with water vapor dominate the isotopic composition of water reaching the ground. The interaction of rain with low elevation moisture can explain the leeward isotope-elevation gradients. Using tritium as a tracer for the mixing of high and low elevation water vapor shows that secondary interactions can be very important.