Coast Range, where a community of novel bacteria is associated with the precipitation of Mg-Ca carbonate cements. The carbonates may serve as a biosignature that could be used in the search for evidence of life on Mars.
Introduction 39A critical challenge facing the search for life in the solar system is the identification of unambiguous evidence of life (cf., Beaty et al., 2005). The presence of microbial life on Earth or an extraterrestrial planet does not ensure our ability to detect it. Evidence of life must be distinctive from a landscape created by abiotic processes (cf., Dietrich and Perron, 2006). The presence of water is deemed to be one of the key requirements for identifying an environment capable of hosting life on Mars (e.g., Knoll and Grotzinger, 2006). The goal of this study is to identify possible biosignatures from a Martian analog environment, namely, alkaline springs associated with ophiolites, sections of ocean crust and upper mantle that have been obducted onto continental crust, experiencing varying degrees of hydrothermal alteration in the process.Serpentinization, the reaction of water with olivine-and pyroxene-rich rocks common in mafic and ultramafic rocks to form serpentine, also produces heat and hydrogen gas that can sustain subsurface, chemosynthetic ecosystems, and also results in the formation of Mg-rich alkaline fluids. These fluids, when mixed with seawater (as seen at Lost City; Kelley et al., 2005) or emanating as surface waters (e.g., as described comprehensively by Pentecost, 2005) can produce substantial volumes of secondary carbonate deposits (e.g., Surour and Arafa, 1997). Alkaline springs associated with mafic and ultramafic rocks are model settings in which to identify possible mechanisms of biosignature formation because these compositions of rocks have persisted throughout all of the Earth's history. More importantly, low-temperature aqueous alteration processes (such as serpentinization) associated with mafic and ultramafic rocks on Earth are thought to be geologically similar to those occurring on Mars (e.g., Boston et al., 1992; Ming et al., 2006;Wyatt and McSween, 2006). 3 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 Interaction between reducing rocks (e.g., unweathered basalts and ultramafic rocks) and water results in an exothermic reaction that also produces hydrogen and methane, both potential energy sources for chemosynthetic microorganisms (Kelley et al., 2005;Sleep et al., 2004). Unfortunately, more detailed characterization of these systems is often limited by their relative inaccessibility -whether in the deep-sea hydrothermal environments or deep within the continental crust. More accessible systems are offered by ophiolite terranes, sections of oceanic crust and upper mantle that have been obducted onto land and which include both basaltic and ultramafic rocks. Similar rock types are (and were) abundant on planetary bodies -the crusts of differentiated bodies (such as Earth, Mars,...