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INTRODUCTIONStromatolites may serve as important proxies for early life on Earth. Many modern siliceous hot spring deposits, or sinters, have laminated growth structures that characterise such stromatolites (e.g., Walter et al. 1972; Doemel and Brock 1974; Renaut et al. 1998; Jones et al. 2000, 2005; Konhauser et al. 2001; Campbell et al. 2002; Guidry and Chafetz 2003; Handley et al. 2005). Sinters are often colonised by a range of (hyper)thermophilic microbiota that can become silicified and incorporated into these deposits (e.g., Inagaki et al. 2001; Blank et al. 2002; Walker et al. 2005). The taxonomic identities of these microbes can vary within and between different geothermal settings, and are a result of the numerous niches and (micro)habitats encountered in these localities (e.g., Schinteie 2005; Pancost et al. 2005, 2006). Microbes inhabiting thermal environments are often placed at the most deeply rooted parts of the universal tree of life (e.g., Barns et al. 1996; Stetter 1996; Pace 1997; Hugenholtz et al. 1998). Hence, the mineralisation of sinters and the phylogeny and ecology of hot spring microorganisms are central themes in studies concerning the origin of life, astrobiology and mineral-microbe interactions (e.g., Walter and Des Marais 1993; Henley 1996; Farmer and Des Marais 1999; Farmer 2000; Handley et al. 2005). Indeed, hot spring deposits, like sinters, may have provided surfaces that concentrated organic chemical constituents, thereby contributing to the formation of biological membranes important for the origin of life (Henley 1996). Studies concerned with the formation of sinter and the distribution of their associated microbiota require an understanding of the different litho- and biofacies occurring at hot spring sites (Farmer 2000). The deposition of actively forming sinters and the occurrence of specific microorganisms are affected by shifting environmental parameters that result in zonations around hot spring effluents with respect to sinter morphology, texture, and microbial species composition. In particular, deposits of sinter and their microbiota can act as sensitive indicators of pH-temperature conditions (e.g., Brock 1978; Cassie and Cooper 1989; Cady and Farmer 1996; Lowe et al. 2001; Jones et al. 2000; Jones and Renaut 2003; Lynne and Campbell 2003, Rodgers et al. 2004) and the hydrodynamics of spring discharge (e.g., Jones and Renaut 1997; Braunstein and Lowe 2001; Lowe et al. 2001; Guidry and Chafetz 2003; Lowe and Braunstein 2003). From observed variations in the texture and distribution of sinters, and the species composition or cell morphology of the associated microbiota, facies models have been constructed to characterise surficial deposits from alkali-chloride hot springs (e.g., Walter 1976; Cady and Farmer 1996; Farmer 2000; Campbell et al. 2001; Guidry and Chafetz 2003; Lowe and Braunstein 2003). While ancient sinters from extinct hot springs may retain some primary textural characteristics (e.g., Rice et al. 1995; Walter et al. 1996; Trewin et al. 2003; Campbell et al. 2001, 2004), their palaeoenvironmental signatures commonly are obscured by the loss of fine-scale microstructure due to diagenetic overprinting (Cady and Farmer 1996) or differential preservation (Guidry and Chafetz 2003). In addition, interpreting ancient sinter facies requires an understanding of the relative contributions of abiotic and biotic factors in the formation of a variety of sinter textures. However, studies of modern, actively forming sinters allow for such shortcomings in palaeoenvironmental reconstruction to be addressed (Farmer 1999). While most investigations are of sinters deposited from thermal waters of near neutral pH, very few (e.g., Jones et al. 1999, 2000; Mountain et al. 2003; Rodgers et al. 2004) have addressed sinters formed from highly acidic (pH ~3 or lower) hot spring waters. Such studies could potentially enable the recognition of extinct hot spring systems and their deposits, even where there is no longer any evidence of the original discharging waters. Indeed, acid hot spring deposits may be the most appropriate terrestrial analogues for the recognition of extraterrestrial hydrothermal deposits. Acidic hydrothermal fluids and solphatara-like environments may occur on Mars (e.g., Farmer 1996, 2000; McCollom and Hynek 2005). In addition, numerous landforms and sedimentary features observed on Mars appear to have equivalents in acid environments on Earth (Benison and Laclair 2002; Laclair and Benison 2002; Bullock 2005). Chemical and mineralogical data obtained from previous Mars missions also paint a picture of acid alteration (e.g., Benison and Laclair 2002; Kerr 2004; Bullock 2005; McCollom and Hynek 2005). This study aims to further contribute to the understanding of acid-derived sinters. We investigated microfacies (i.e., facies changes over centimetres or less) of siliceous stromatolitic sinters formed in acid-sulphate-chloride spring outflows (91-30°C, pH 2.1-2.3) located on the floodplain of Parariki Stream, Rotokawa Geothermal Field, New Zealand. This field provides a rare setting where acid fluids rich in silica deposit sinter. The deposits follow strong environmental gradients that result in distinctive morphological, textural, and microbial characteristics. Molecular, DNA-based surveys at the site have shown that microbial communities vary markedly with respect to species composition between different microfacies (Schinteie 2005). Sequences of 16S ribosomal RNA (rRNA) genes were extracted from individual sinters and were related to numerous archaeal, bacterial, and eukaryotic thermoacidophiles. The results of this molecular survey are the subject of a different contribution that closely ties in with the results outlined herein. We employed an integrative approach with the application of: (1) detailed mapping of sinter occurrences with respect to temperature, hydrodynamics of spring discharge, and sinter formation rates; (2) X-ray powder diffractometry (XRPD) to characterise sinter mineralogy; (3) thermal analysis for investigating water content of the sinters; (4) thin-section petrography for recognising broad sinter textures; and (5) Scanning Electron Microscopy (SEM) to resolve textural micro-structures and mineral-microbe relationships. Treatment of fresh sinter samples with glutaraldehyde slowed the deterioration of associated microbial communities, lessened the risk of contamination by post-collection microbial overgrowth, and better illuminated the role of microorganisms in acid-derived sinter texture formation (cf. Cady and Farmer 1996; Lynne and Campbell 2003; Handley et al. 2005). |
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