Origins of tufa mounds in Searles Lake, California.

摘要

Over 500 tufa mounds, formed from spring vents in the lake bottom, occur in the dry bed of Searles Lake, southeastern California. The Pleistocene mounds range from minor features to 45 m in height.;Microscopically, the tufa deposits are predominantly composed of microbiologically produced or mediated constituents, e.g. spherulites, pisoids, peloids, and stromatolites. These biogenic features are fundamentally made-up of different calcified bacterial bodies, e.g., nano-spheres, micro-rods, rod-like crystals, and rosary-like chain-of-beads. All these observations indicate that formation of the tufa deposits is predominately a result of biogenic processes.;Low delta18O and delta13C values in facies P1 (the oldest) are attributed to rapid increases of the lake volume; whereas high delta18O and delta13C values from facies N are due to a subsequent decease of the lake volume caused by high evaporation and biogenic productivity. The relatively high delta 18O and delta13C values in facies LC are due to evaporation and fast CO2 degassing. Lastly, low delta18O and delta 13C values in samples from the P2 facies indicate rapid flooding during deposition. Covariance of delta18O and delta13C values occurs in the tufa deposits because of hydrological closure of the lake during tufa formation. For the different facies, however, variations in the covariant trends are due to the insensitivity of delta13C values to lake volume changes under conditions of hyper-alkalinity.;Four distinctive tufa facies include: (1) porous tufa deposits (P, including P1 (old) and P2 (young)) composed of highly irregular thin laminae, (2) nodular deposits (N) made-up of millimeter-size coated grains, (3) layered columnar tufa deposits (C), and (4) finely laminated crusts (LC). Tufa growth follows a sequence, P1 - N or C --- LC - P2, from the oldest to the youngest. The sequence records the climatic and hydrological history of Searles Lake during tufa formation.