HOT SPRING DEPOSITS: PROXIMATE BIOTIC AND ABIOTIC PRECIPITATES

Carbonates, primarily aragonite, constitute the predominant precipitate near the orifices of Artesia and Young Hopeful geysers, whereas Si-, Fe-, and Mn-oxides become more abundant constituents downflow. Waters issue from the springs at approximately 90^oC and a pH of 7.7. As the water flows distally, temperature decreases, CO₂ evolves, and pH increases.

Aragonite forms pseudohexagonal crystals up to a few mm long, oriented perpendicularly to planar substrates as well as hemispheres composed of radiating splays. The splays are commonly 100 to 200 microns in diameter composed of pseudohexagonal crystals generally less than 10 microns wide. The radiating splays nucleate on, and partly incorporate, clumps of coccoid bacteria 10 to 20 microns in diameter. In contrast, there is no indication of bacterial fossils nor their former presence distally along the crystals. That is, the radiating splays originated where bacteria overcame the inhibition for crystal nucleation and once that was achieved continued crystal development was abiotic. On discontinuity surfaces, supersaturation of the waters has also produced planar two-dimensional aragonite splays in which the individual crystals display a bizarre saw-toothed pattern somewhat similar to that of skeletal crystals. Opaline silica commonly envelops the carbonate.

Intimately intermixed with the carbonate, Mn-oxides comprise coatings on cyanobacterial filaments, dense laminae, as well as small oncoids. Predominantly amorphous diaphanous micron-sized curved sheets of Mn-oxides form from these spring waters that contain less than 0.2 ppm Mn. The Mn-oxide sheets contain a plethora of rod-shaped bacteria and probably represent replaced biofilm. Distally in the hot spring deposit, Mn-oxides form rosettes that display crystalline forms 10 to 20 microns in diameter which are devoid of any biotic fossils, i.e., abiotic crystals.

These deposits indicate the intimate association of biotic and abiotic precipitates.