Clusters of microstromatolites, up to 10 mm high with a basal diameter of 4 mm, grow on twigs and small islands in shallow hot-spring waters around Champagne Pool and on Primrose Terrace at Waiotapu, New Zealand. Similar microstromatolites are present in the ephemeral, acidic outflow waters from Inferno Crater at Waimangu. These microstromatolites, which form in thermally stressed environments, are composed of amorphous silica and, in some specimens, minor amounts of antimony. Calcite, sulfur, and gypsum crystals are present in some microstromatolite surfaces. The microstromatolites are characterized by vertically stacked zones, commonly only 1-2 mm high, that are defined principally by the distribution of microbes that grow on the external surface of these structures. Microorganisms preserved in the microstromatolites include bacteria, cyanobacteria, diatoms, tasmanitids (Inferno Crater only), amoebae (Inferno Crater only), and soil mites (Champagne Pool only). These microbes thrived even though the substrates, in the case of Champagne Pool, were only a few millimeters above water with a temperature (76 degrees C) that is much higher than their respective maximum tolerance temperatures. The vertical distribution of the microbes in the microstromatolites is probably controlled by a temperature gradient that is a function of the interaction of the steam rising from the spring water and the ambient air temperature. This study shows that the use of microbes as indicators of water paleotemperature in old hot-spring deposits must be treated with caution. Microbes with low maximum tolerance temperatures can survive, thrive, and be fossilized only a few millimeters above waters that are too hot for their survival.

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