Robin W. Renaut

The Microbial Role in Hot Spring Silicification

Abstract Recent experimental studies indicate that microorganisms play a passive role in silicification. The organic functional groups that comprise the outer cell surfaces simply serve as heterogeneous nucleation sites for the adsorption of polymeric and/or colloidal silica, and because different microorganisms have different cell ultrastructural chemistry, species-specific patterns of silicification arise. Despite their templating role, they do not appear to increase the kinetics of silicification, and at the very most, they contribute only marginally to the magnitude of silicification. Instead, silicification is due to the polymerization of silicasupersaturated hydrothermal fluids upon discharge at the surface of the hot spring. Microorganisms do, however, impart an influence on the fabric of the siliceous sinters that form around hot spring vents. Different microorganisms have different growth patterns, that in turn, affect the style of laminations, the primary porosity of the sinter and the distribution of later-stage diagenetic cementation.

Taphonomy of Silicified Filamentous Microbes in Modern Geothermal Sinters—Implications for Identification

Abstract Silicified microbes found on the discharge aprons around modern geysers and hot springs commonly appear to be preserved superbly. This can be attributed to their rapid silicification, which often begins while they are alive. In geological terms, therefore, they are silicified instantaneously. Thus, it might be expected that these microbes should be good replicas of the living organisms and, therefore, easy to identify in terms of extant taxa. Silicified microbes found on the discharge aprons around geysers and hot springs of North Island, New Zealand, are preserved through replacement and/or encrustation. Organic matter is typically absent, and examples of sheaths being partly replaced or coated by other minerals, such as iron oxide, have not yet been recognized. Accordingly, the cellular-level information needed for microbe identification must be gleaned from features preserved in the silica. Unfortunately, the silicification processes commonly destroy and/or disguise most of the taxonomic features that are necessary for reliable identification in terms of extant taxa. Silicification may, for example, obscure the presence of a sheath and/or significantly alter the size of the microbes. The loss and/or modification of such important taxonomic features means that the identification of silicified microbes is fraught with problems and must be approached with caution.

Vertical zonation of biota in microstromatolites associated with hot springs, North Island, New Zealand

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.

Biogenicity of gold- and silver-bearing siliceous sinters forming in hot (75°C) anaerobic spring-waters of Champagne Pool, Waiotapu, North Island, New Zealand

Champagne Pool, a large hot spring at Waiotapu in North Island, New Zealand, is rimmed by a subaerial sinter dam and a shallow subaqueous shelf that is composed of orange sinter rich in metallic sulphides. Orange siliceous flocs, also rich in sulphides, are in constant circulation in the spring pool and form loose sediment on the shelf. The orange sinters and flocs are rich in As, Sb, Tl, and Hg, and have high concentrations of Au (>100 ppm) and Ag (>330 ppm). Most metallic sulphides are amorphous and disseminated throughout the sinter, instead of forming distinct mineral phases. The shelf sinters are domal and resemble stromatolites. The neutral chloride waters (pH 5.5; temperature 75°C), however, are virtually anaerobic. Examination of the sinters by scanning electron microscopy confirms that they are laminated and contain an abundant, low-diversity assemblage of filamentous, bacilliform, and coccoid microbes. The flocs are similarly composed of very small, silicified filaments. Silicification involved replacement of the cell walls and extensive encrustation by opal-A. Based on size and morphology, these microbes are probably anaerobic bacteria or archaea. By providing substrates for nucleation of the silica, the microbes are indirectly contributing to the formation of the gold-bearing sinters.

Primary silica oncoids from Orakeikorako Hot Springs, North Island, New Zealand

Oncoids, formed entirely of amorphous silica (opal-A), are common on the central to distal parts of Golden Fleece Terrace at the Orakeikorako geothermal site, North Island, New Zealand. The discoid oncoids, which grew in shallow (1 cm) pools bounded by low rimstone dams, are up to 16 mm long, 14 mm wide, and 7 mm high. The core (nucleus) and cortex are formed of alternating porous and non-porous laminae. Porous laminae are composed of complex, but loosely interwoven, meshworks of silicified filamentous cyanobacteria that have relatively little silica precipitated between them. The non-porous laminae have a radial fabric in which filamentous microbes have their long axes perpendicular to the underlying growth surface. Pore spaces between the filaments are partly filled with amorphous silica cement. The morphology of the microbes in these oncoids is well preserved because silica precipitation occurred while the microbes were alive or very soon after death. Early silica precipitation allowed the differences between porous and non-porous laminae to be preserved. The alternation between porous and non-porous laminae may reflect times when microbial growth outpaced silica precipitation and vice versa. Such changes probably reflect variations in the discharge patterns and/or the temperature of the spring waters that flowed across the microterraces during oncoid growth. Periods of rapid silica precipitation may have been a factor that caused microbial filaments to adopt the radial attitude that characterizes the oncoids.

Stromatolites Forming in Acidic Hot-Spring Waters, North Island, New Zealand

Abstract Small spicular, columnar, and blade-shaped stromatolites are common features of hot spring and geyser systems in the Taupo Volcanic Zone of North Island, New Zealand. These organosedimentary structures can grow in many settings, where water temperature and pH vary widely. Although most common in neutral and alkaline waters, stromatolites also are forming in several acid-sulfate spring and geyser systems in the Waiotapu geothermal area and at Lake Rotokawa. Growth of these stromatolites is partly mediated by a biota that is dominated by fungi and locally, diatoms. When stromatolites from acidic thermal waters are compared with those from neutral and alkali waters, significant differences in their biota and mineralogy are evident. The biota in stromatolites from the neutral and alkali waters is dominated by prokaryotic bacteria (including cyanobacteria), whereas stromatolites from acidic waters are dominated by eukaryotic fungi and to a lesser extent, diatoms. Stromatolites in the neutral and alkali thermal waters are formed almost entirely of opaline silica, with calcite laminae present in a few localities. Although stromatolites in the acidic systems also are composed mainly of opaline silica, they contain substantial amounts of kaolinite and, locally, sulfur and/or jarosite. In ancient thermal deposits it may be possible to distinguish stromatolites that grew in acidic waters from those that formed in neutral and alkali systems by considering their preserved biota and mineralogy.

Sublacustrine precipitation of hydrothermal silica in rift lakes: evidence from Lake Baringo, central Kenya Rift Valley

Abstract Many lakes in volcanic regions are fed by hot springs that, in some basins, can contribute a large percentage of the annual recharge, especially during times of aridity. It is important to recognize any contemporary hydrothermal contribution in paleoenvironmental reconstruction of lake basins because recharge from thermal waters can potentially confuse paleoclimatic signals preserved in the lacustrine sedimentary record. Hot spring deposits (travertine, sinter) provide the most tangible evidence for thermal recharge to lakes. Although subaerial spring deposits have been widely studied, lacustrine thermal spring deposits, especially sublacustrine siliceous sinters, remain poorly known. Detailed field, petrographic and scanning electron microscope (SEM) studies have been made of fossil sublacustrine sinter exposed at Soro hot springs along the northeastern shoreline of Ol Kokwe, a volcanic island in Lake Baringo, Kenya. Modern hot springs at Soro, which discharge Na–HCO3–Cl waters from a deep reservoir (∼180 °C ), have thin (1–10 mm), friable microbial silica crusts around their subaerial vents, but thicker (>1 cm) sinter deposits are not forming. The fossil sinter, which is present as intergranular cements and crusts in littoral conglomerates and sandstones, is composed mainly of opaline silica (opal-A). Three types of fossil sinter are recognized: (1) massive structureless silica, which fills intergranular pores and forms crusts up to 5 cm thick; (2) pore-lining silica, some of which is isopachous, and (3) laminated silica crusts, which formed mainly on the upper surfaces of detrital particles. All three types contain well-preserved diatoms including lacustrine planktonic forms. Microbial remains, mainly filamentous and coccoid bacteria (including cyanobacteria) and extracellular polymeric gels, are locally abundant in the opaline silica, together with detrital clays and thin laminae composed of authigenic chlorite (?). Most of the hydrothermal silica precipitated when the thermal springs were submerged by fresh lake water. Silica precipitated upon rapid cooling of thermal (∼90 °C ) waters at and just below their interface with the overlying cooler (∼25 °C ) lake waters. Microbial mats locally acted as a filter that limited mixing and rapid dilution of the thermal fluids. Some of the silica originally may have been soft and partly gelatinous. Planktonic diatoms and detrital clay rained down, then became incorporated in the amorphous silica. Following a fall in lake level, the opal-A lithified and partly altered to cristobalite (opal-C) and chalcedony. The lowest fossil sinters were later encrusted by calcite stromatolites, with calcite and quartz forming late pore-filling cements. The age of the sublacustrine sinters is unknown, but some of the deposits could date back to the late Pleistocene. Similar conglomerates cemented by hydrothermal silica are present along fault lines at neighbouring Lake Bogoria. Such rocks may provide evidence for deep, hot fluid recharge to lakes when encountered in the geological record.

Actively growing siliceous oncoids in the Waiotapu geothermal area, North Island, New Zealand

Oncoids that are actively growing in some of the shallow-water pools around Champagne Pool, Waiotapu, New Zealand, are formed of amorphous silica (opal-A) with minor amounts of native sulphur. The growth of these oncoids is being mediated by a high-diversity microbial biota that achieves optimal growth in waters that have a temperature of 35–42°C and pH of 6.6–6.9. Although this biota is dominated by coccoid, bacilliform, and filamentous cyanobacteria, sulphur-oxidizing bacteria are common in the cortical laminae of some oncoids. In addition, diatoms and silicified pollen grains are present in some cortical laminae. The silicified microbes are superbly preserved with their three-dimensional form and some of their internal structures being apparent, including the sites of sulphur globules in the sulphur-oxidizing bacteria. The microbes probably acted as templates for silica precipitation and thereby mediated the growth of the oncoids. Some cortical laminae are formed entirely of silicified pollen grains that came from the Pinus radiata that grow in the exotic pine plantations around Champagne Pool. By using these laminae as a record of the annual pollination event (typically in September–November), it can be shown that many of the oncoids are <15 years old and that the cortical laminae grew at 0.2–1.0 mm (average 0.35 mm) per year. The average daily rate of silica precipitation (0.5–2.75 μm, average 1 μm) is high compared to the average size of the microbes (commonly < 1 μm in length and diameter). This rapid rate of silica precipitation accounts for the superb preservation of the microbes and is consistent with silicification of the microbes within days of their demise.

Silicified Microbes in a Geyser Mound: The Enigma of Low-Temperature Cyanobacteria in a High-Temperature Setting

Abstract A geyser mound, which has formed around the vent of an abandoned well-bore in the Tokaanu geothermal area (North Island, New Zealand) over the last 50 years, is formed primarily of white opal-A along with lesser amounts of black opal-A, reticulate, amorphous or poorly crystalline clay, halite, and scattered detrital quartz grains. Despite the boiling water ejected from the vent, at least 19 taxa of silicified microbes are present in the geyserite. Calothrix, Phormidium, and Synechococcus dominate. Other microbes, including local concentrations of diatoms, are widely scattered. The dominance of cyanobacteria is enigmatic given that these microbes typically are found in cooler water environments (< 60°C) on the medial to distal parts of geyser and hot-spring discharge aprons. The proximal zone around a geyser vent is environmentally unstable because it oscillates between periodic eruptions of boiling water and cooler phases in accord with the eruptive cycle of the geyser. Thus, the environmental niches on a geyser mound are temporally and spatially variable. The periodicity, duration, and magnitude of the eruptions, and the dispersal patterns of boiling water during eruptions control the distribution of the microenvironmental niches and their temporal changes. Dispersal and cooling of the ejected water will, for example, be controlled by factors such as height of the erupted water, ambient air temperature, and prevailing wind direction. Biotas compatible with relatively cool waters may thrive on geyser mounds because they inhabit areas that are not inundated constantly by boiling water. Their development and the sinter fabrics that they generate in the proximal zone of a geyser vent means that ancient deposits must be interpreted with caution.

Recent carbonate sedimentation and brine evolution in the saline lake basins of the Cariboo Plateau, British Columbia, Canada

There are more than 100 closed, saline lakes in the semiarid, intermontane plateaus of British Columbia. They range from shallow perennial lakes to ephemeral playas. Most are groundwater-fed and lie within glaciofluvial deposits and till. Some have permanent salts. Where underlain by basalts, sodium carbonate brines predominate. Magnesium sulphate brines occur where catchments lie within Paleozoic sedimentary rocks, metasediments and basic volcanics. A few sodium sulphate brines are also present.A reconnaissance study of the sediments and mineralogy of 21 lake basins has shown that carbonates, including extensive magnesite and hydromagnesite deposits, and several occurrences of protodolomite, are widely precipitated in lake basins of each brine type. Analyses of stream, spring, ground and lake waters from the Cariboo Plateau region demonstrate that carbonate precipitation probably constitutes the major chemical divide responsible for producing the two dominant types of brine.