STABLE ISOTOPIC VARIABILITY WITHIN MODERN TRAVERTINES

Six hot and two ambient water travertine systems were sampled to deter mine the relationships between the stable isotopic composition of the travertines and the waters from which they were deposited. This was co nducted in order to evaluate the use of geochemical analyses of ancien t travertines for the interpretation of the composition of the waters from which they precipitated, climatic conditions at time of formation , etc. The waters displayed downflow trends of progressively higher (i ) delta C-13 values, in all 8 systems, and (ii) delta O-18 values, in all 6 hot water systems. Whereas the stable isotopic values of the min eral precipitates sometimes showed similar trends, the magnitude of th e downflow changes commonly was quite different than that exhibited by the water data. Additionally, different types of precipitates, which formed within centimeters of each other, commonly had different stable isotopic compositions, e.g., crusts which formed at the air/water int erface always had higher delta C-13 and delta O-18 values than constit uents which formed within the immediately subjacent water column. The lack of a simple relationship between stable isotopic composition of t he water and the precipitate is due to the fact that the stable isotop ic composition of the precipitates are controlled by a number of varia bles, including the water's composition, temperature, level of saturat ion, etc. And these variables can change dramatically within very shor t distances and at the same spot within very short time intervals. Thu s, as demonstrated by the stable isotope data, attempting to interpret the composition of the water from the composition of the deposit is a highly risky venture.