ABSTRACT

The prevailing view is that tufa deposition at Mono Lake, east central California, is dominated by inorganic processes. This paper asserts that algae are important contributors to the formation of lithoid tufa at the lake, and probably also of other varieties.

Calcium carbonate as calcite, aragonite, and high-magnesium calcite has been deposited at Mono Lake in the form of pinnacled masses of tufa. These structures formed (and are still forming) about the orifices of springs issuing from beneath the highly-alkaline waters of this lake. The lake is a desiccated remnant of a formerly much larger Pleistocene water body. Lowering of the lake level has exposed many pinnacles, two of which were examined for this study. One of these pinnacles, approximately 1 meter in height, issues spring water through its sides and from a summit crater. The other structure is a dry pinnacle (no summit or internal spring) and is approximately 61 meters high and 11.5 meters in diameter.

A calcareous algal mat (1-3 mm thick) is attached to the sides of the summit-spring pinnacle beneath water cascading from its summit or exuding through its flanks. The algal mat is partially embedded in a thin (1-5 mm) layer of relatively dense lithoid tufa. The dense lithoid tufa sheathes a more porous lithoid tufa which forms the bulk of the summit-spring pinnacle. The mat is calcareous owing to an abundance of microcrystalline and pelletal calcite immeshed between the thalli of the filamentous algae (mostly blue-greens) which constitute the mat. Organic-rich microcrystalline calcite bearing abundant remains of filamentous algae form the underlying layer of dense lithoid tufa.

Porous lithoid tufa, which forms the bulk of most of the pinnacles near the present shoreline of the lake, contains about equal proportions of organic-rich microcrystalline calcite or aragonite and coarser sparry calcite. In the summit-spring pinnacle the fine-crystalline carbonate is calcite, but it is aragonite in the porous lithoid tufa of the dry pinnacle. The microcrystalline carbonate of both pinnacles contains remains of filamentous algae; loosely packed groups of spherical bodies, which are probably coccoid algal cells, are also found in the porous lithoid tufa of the summit-spring pinnacle.

Primarily because algae are intimately associated with freshly-deposited tufa, and because organic-rich micro-crystalline calcite and aragonite of older deposits of lithoid tufa are rife with algal remains, precipitation of lithoid tufa at Mono Lake is regarded as probably botanically induced. Most likely precipitation results from the photosynthetic withdrawl of carbon dioxide, which lowers the solubility of calcium carbonate in close proximity to the plants. About two-thirds of the sparry calcite has formed in close association with the microcrystalline carbonate and is also regarded as organically deposited. Some of the spar, however, has been deposited as a cavity lining or filling and therefore is probably of inorganic origin. The occurrence of high-magnesium calcite in nodose tu a of the dry pinnacle implies that this unusual and somewhat uncommon variety was organically precipitated.