Experimental studies on New Zealand hot spring sinters: rates of growth and textural development

To study the rate of growth of sinters in New Zealand hot springs, field experiments were conducted in seven geothermal areas. At Wairakei, fan-shaped subaqueous deposits of amorphous silica grow rapidly (350 kg a(-1) m(-2) of drain wall) and are composed of silicified filaments with subordinate bacillus and spirillium-shaped organisms. Characterization of bacteria revealed isolates sharing > 97% 16S rRNA gene sequence homologies affiliated with Thermus, Meiothermus,Bacillus, Tepidomonas, Thermomonas, Porphyrobacter, Thermonema, and, Hydrogenophilus spp., as well as previously uncultured bacteria. At Rotokawa, microstromatolites have a slow growth rate (0.004 mm day(-1)) that is attributed to low pH, capillary rise, and evaporation. At Champagne Pool, sinter growth (0.023 mm day(-1)) is dominated by wave action building alternating microbe-rich and microbe-poor layers. Silica sinter was not observed at Waikite, where slides developed a layer of calcite (0.026 mm day(-1)). Sinter growth at Ngatamariki (0.016 mm day(-1)) forms by capillary rise, evaporation, and diffusion and at Tokaanu, subaqueous growth is slow (0.002 mm day(-1)) and contains silicified microbes. Textures at Orakei Korako indicate similar mechanisms to Ngatamariki, except that growth is more rapid (0.023 mm day(-1)) due to a splash contribution. Silica and calcite saturation indices adequately explain the growth of the sinters and calcite, indicating that microbes are not inducing precipitation where it should not occur. The rate of precipitation is correlated with silica supersaturation, but pH effects can alter this relationship. The degree of preservation of microbial material is explained by the effect of Ostwald ripening on silica spherules. Subaqueous growth allows coarsening of spherules and poor preservation of smaller microbes while subaerial nucleation is rapid, Ostwald ripening is inactive, and better preservation can be expected.