The paleoenvironmental and astrobiological significance of tufas in the Santa Cruz region

Abstract

Tufas—carbonates that form in ambient-temperature, freshwater settings—are valuable paleoenvironmental proxies of their depositional conditions. In this study, we use tufa deposits from the Santa Cruz, CA region to (i) model regional paleohydroclimate and (ii) investigate controls of biosignature formation at multiple spatial scales. Previously undescribed inactive, spring-associated tufa mounds (~2 m wide and each over 1 m thick) from Henry Cowell State Park and Pogonip Park were investigated in this study. Henry Cowell additionally hosts carpeted, fluvial tufa adjacent to an active tufa-depositing spring. Infrared stimulated luminescence (IRSL) dating was used on cores collected from the two inactive mounds to determine the timing of carbonate growth. Radiocarbon (14C) dating was carried out on charcoal fragments from the fluvial carbonates at Henry Cowell State Park. IRSL and 14C dates of samples collected from the two sites agree with the age models from other proxies of past pluvial periods in the region (~16 to 5 ka). A piece of charcoal revealed the fluvial carbonates are relatively younger in age (853 ± .04 years BP) and likely represent recent or modern flow conditions. Decimeter-scale samples collected from Henry Cowell State Park were used for petrographic analysis and biosignature investigation. Microscopic investigations using light microscopy revealed specific biological and mineralogical controls on the meso (cm-scale) to macroscopic (m-scale) morphologies. Distinct light and dark calcite bands (~0.1-0.5 mm in thickness) were identified in all samples collected from Henry Cowell State Park suggesting seasonal growth patterns. The presence of the calcite microstructure of Oocardium stratum, a freshwater algae that is endemic to tufa environments at both sites, implies the inactive mounds formed under high stream velocities and are of meteoric origin. In the fluvial carbonates, decimeter-scale tufa mounds grow concentrically and vertically and display smaller domes (several cm in diameter), most of which contain smaller protuberances (several mm in diameter)—a cauliflower-like fabric. The mm-scale protuberances are internally laminated making them a clear astrobiological target due to their stromatolitic morphology. Diatoms were also found in the crust of samples collected from both perched and fluvial carbonates, further indicating biotic influence on tufa formation at the mm- to micro scales. As the first multiscale and chronological analysis of tufas in the Santa Cruz region, this work lays the groundwork for future analyses that will further corroborate their paleoenvironmental and astrobiological significance.