Biosignature preservation in Miocene evaporite and hot spring deposits of the southeast Cady Mountains, California, U.S.A.
The reliable detection of biosignatures in ancient geological materials is a developing field, with applications to studies of life on Earth and the exploration for life on other planets. In this study, we examined Miocene-aged evaporites and hot spring travertines of the Hector Fm. of the southern Cady Mts. of southeastern California. The paleoenvironments studied represent a broad range of temperature, pH and salinity, for comparing modes of biosignature capture and preservation.
Reconstruction of the depositional setting for the Hector Fm. evaporites suggests they represent a perennial, hypersaline lake system, with fluctuating water levels. Primary evaporites were gypsum and anhydrite, which experienced partial replacement by celestine during K-metasomatism by low temperature hydrothermal fluids. Intense, late stage hydrothermal alteration by Fe-rich fluids altered units adjacent to faults. Beds affected by this event have distinctive trace elemental suites and isotopic values (mean δ 34S 8.4‰ and mean δ18O = 13.3‰).
Petrographic and geochemical analyses of Hector Fm. travertines show evidence for early diagenetic recrystallization and cementation, followed by later hydrothermal alteration adjacent to brecciated fault zones. Fluids changed the geochemistry of these samples, while leaving primary microfabrics intact. Samples adjacent to faults have lower isotopic values (mean δ 18O = 20.1‰; mean δ13C = −1.28‰) than samples away from faults, regardless of microfabric.
Biosignatures preserved in evaporite and travertine deposits include a range of morphological, mineralogical and organic compounds. Organic geochemical analysis of samples yielded an organic suite (n-alkanes, β-carotane, CPI >1, OEP >1 and possibly gammacerane) consistent with a hypersaline lacustrine environment and biological sources that included cyanobacterial, algae and higher plants.
Results suggest while that important morphological, organic, and mineral indicators for biological activity may persist through extensive post-depositional alteration, while isotopic and elemental signatures indicative of primary environments are easily altered. The study demonstrates that the integration of evidence over a range of spatial scales, from outcrop to microfabric, to geochemistry, provides important constraints for paleoenvironmental and fossil biosignature analysis.