MICROBIAL FABRICS, OXYGEN ISOTOPES AND PALEOTEMPERATURE FRAMEWORK OF PLIO-PLEISTOCENE HOT-SPRING DEPOSITS, DEATH VALLEY

Deducing the role that hydrothermal systems have played in the history of our biosphere depends to a large extent on the recognition and interpretation of hot-spring deposits in ancient rock record. Here we compare the temperature distributions of fabric-based microfacies of modern travertine-precipitating hot-springs at Angel Terrace, Yellowstone National Park, with the inferred isotopic paleotemperature distribution of similar fabrics/microfacies observed in Plio-Pleistocene travertine sinters from the Furnace Creek area of Death Valley, CA. Although microbial mats and biofilms have been shown to strongly biomediate the fabrics of modern travertine sinters over a broad range of temperature, previous work has shown that their carbon-oxygen isotope systematics is dominated by inorganic, Rayleigh-type fractionation processes (e.g. temperature decline, CO2 degassing and evaporation). We have asked the question: In addition to fabric-based biological information, do ancient travertine sinters also preserve paleotemperature information in their oxygen isotope record, particularly after significant diagenesis? To answer this question, we studied five distinctive fabric types in the Death Valley travertine spring deposits- primarily microbial biofabrics that have been previously described from modern travertine springs of Yellowstone and which have known temperature ranges. For the Death Valley examples, fabric/microfacies distributions were mapped in outcrops and sampled for petrofabrics (thin section), oxygen and carbon isotopes. To estimate the formation temperature of each of the Death Valley fabric types, we applied the empirical equation of Kim and O'Neil (1997). Estimates of the Delta O-18 of the source water were obtained from previously published D/H values that had been extracted from fluid inclusions preserved in co-existing subsurface sinters (Winograd et al. 1985). Results showed that although isotopic signatures of the Death Valley sinters were reset to somewhat lower paleotemperature values during diagenetic recrystallization, relative temperature relationships were retained. This discovery supports our conclusion that biofabric types can be useful proxies for recognizing relative paleotemperature trends in ancient sinters.