BIOGEOCHEMISTRY OF CA-MG CARBONATE CEMENTS ASSOCIATED WITH OPHIOLITE-HOSTED COLD SPRINGS, COAST RANGE, CALIFORNIA, USA

We have studied the biogeochemistry of Ca-Mg carbonate cements precipitated along stream beds within the Del Puerto Ophiolite, approximately 100 km SE of San Francisco. Our investigation focuses on rock, water, and biological samples collected from an area a few hundred meters from Adobe Springs, near the confluence of Del Puerto and Adobe Creeks. Water in this setting has interacted with serpentinized ultramafic rocks to yield alkaline fluids (pH > 8.3) high in Mg (~120 mg/L) and bicarbonate (> 400 mg/L). Throughout the dry summer months, the only flows in Adobe Creek are those fed by springs, and surface flow is limited to a zone that extends a few hundred meters above Adobe Springs down to Del Puerto Creek. A thick accumulation of recently formed carbonate cements lines these creek beds, producing a conglomerate with creek sediments that range from submillimeter-sized grains to pebbles several cm in diameter. Waters at this site support varied microbial biota, including photosynthetic and cyanobacteria in seasonal mats, and algal communities. The mats and algae grow in direct contact with the carbonates and may be incorporated within these actively precipitating cements. Our goal is to characterize evidence of biosignature preservation within the cements using detailed petrographic, SEM, and chemical analysis. Such signatures could be used to imply the existence of life in analogous Martian environments.