Investigation 2

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Investigation 2. Biosignatures: Developing the Tools for Detection of Ancient Life and Determining Paleoenvironments.

 The proxies that can be used to determine ancient microbial ecology require calibration via experimental studies that address biologic and abiologic processes, and deal with the realities of preservation and alteration in the rock record. Our goal is to develop a mechanistic understanding of the proxies that have been used to interpret ancient rocks, and to develop new proxies. We will focus our experimental studies on three mineral groups: clays, Fe-Si oxides, and carbonates.

2.1 Silicate Weathering and Clay Formation

2.1.1 Mg-Fe-Si isotope investigation of the isotopic exchange and fractionation between fluids and smectites: A tracer for paleoenvironments and weathering

Lead: Brian Beard, UW-Madison

Summary: Phyllosilicates on Noachian martian terranes provide important information on the habitability of early Mars, and such minerals provide one of the main records of continental weathering for the early Earth. An extensive experimental plan is proposed to understand the Mg-Fe-Si isotope fractionations associated with formation, dissolution, and cation exchange of smectite-group clays. This work will allow us to develop proxies to infer past environmental conditions such as fluid composition, water to rock ratio, temperature, and pH, which in turn will inform us about the habitability of early Mars and Earth.


Simplified clay cycle on Mars. From Tosca and Hurowitz (2011).

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2.1.2 Microbial redox transformation of Fe-bearing phyllosilicate minerals and implications for life detection on Mars and other rocky planets

Lead: Eric Roden, UW-Madison

Summary: This project will provide new insight into mineralogical and stable isotope changes that accompany microbial redox transformation of Fe in phyllosilicate phases. The results will expand current knowledge of the range of Fe(II)-bearing phyllosilicates that can provide energy for microbial chemolithotrophic metabolism, and provide fundamental experimental and analytical information that can be used to interpret phyllosilicate structure and redox speciation data in the context of potential biological activity. We will evaluate the extent to which biomolecules may be recovered from model Fe-reducing or oxidizing organisms from Fe-bearing minerals, providing a connection to efforts on biomolecule recovery and life detection.


Experiments using Fe(III)-Si gels that likely represented primary marine precipitates in the Archean oceans of the Earth. Left two bottles contained cultures of Desulfuromonas acetoxidans, which produced extensive reduced-Fe products. Right two bottles were abiologic controls, which showed no reduction.

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2.2 Iron-Silica Minerals: Oxides, Jaspers, and Cherts

2.2.1 Si, Fe, and Mo isotope fractionations during iron oxide/hydroxide and silica precipitation in Archean seawater

Lead: Clark Johnson, UW-Madison

Summary: The isotopic compositions of structural Si and Fe in mixed Fe-Si hydroxide precipitates that were ubiquitous in Precambrian oceans on Earth, or were likely hydrothermal precipitates on Mars, promise to provide unique insights into weathering (Si) and redox cycling (Fe). In addition to Si and Fe, we propose determine the sorption behavior of the redox-sensitive element Mo to the Fe-Si hydroxides that were important in the Precambrian oceans. The proposed experiments on Si, Fe, and Mo isotope fractionations in abiological systems form an essential reference point for interpreting potential differences in biological systems, as well as for interpretation of oxides, jaspers, and cherts in the ancient rock record.


Effect of Fe:Si ratio on Fe isotope fractionations between Fe(II)aq and Fe-Si hydroxides as function of Fe-Si bonding (Wu et al., 2012). Similar effects are anticipated for Si isotope fractionations.

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2.2.2 Production of Si, Fe, and Mo isotope fractionations by microbial dissimilatory Fe(III) reduction

Lead: Eric Roden, UW-Madison

Summary: Previous work on microbial dissimilatory iron reduction (DIR) of Fe-Si co-precipitates (Fe-Si CP) demonstrates that the end products are distinct from those produced by abiologic aqueous Fe(II) interaction with Fe-Si CPs, producing distinct Fe isotope fractionations. It remains unknown, however, if distinct Si and Mo isotope fractionations are produced by DIR relative to identical abiologic systems. For natural iron oxide deposits such as banded iron formations (BIFs), we hypothesize that DIR produces distinct Si and Mo isotope fractionations during release of sorbed Si and Mo relative to abiologic reactions.


(A) TEM image and SAED pattern of unreduced Fe(III)–Si coprecipitate. The material is amorphous as indicated by the SAED pattern. (B) HRTEM image and SAED pattern of reduced (20 mM acetate) Fe(III)–Si coprecipitate. The reduced material showed evidence of localized zones of partial transformation to a primitive smectite-like phase (indicated by the white arrows) that were detectable by SAED (rings 1, 2 and 3, corresponding to 1.5, 2.5 and 4.5 Angstrom diffraction lines respectively).

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2.3 Origin of Carbonates

2.3.1 Environmental proxies, stable isotope fractionations, and formation pathways

Leads: Chris Romanek, Furman University; Clark Johnson, UW-Madison; John Eiler, Caltech

Summary: A better understanding of the fundamental factors that control the geochemistry of carbonate minerals is required for geochemical codes to be accurately deciphered from ancient terrestrial carbonates and sedimentary deposits on Mars. This investigation describes a series of laboratory investigations where the pCO2, temperature, precipitation rate, and solution chemistry of fluids are independently controlled during the synthesis of carbonate minerals from solution. Despite extensive prior study of carbonates, our approach promises critical breakthroughs in the use of carbonates as paleoenvironmental proxies for ancient atmospheric CO2 and O2, paleotemperatures, and microbial redox cycling, key parameters for understanding the aqueous environments that were conducive to the origin and evolution of life.


Saturation state and precipitation rate data from Busenburg and Plummer (1986) demonstrate the dependence on reaction kinetics on pCO2 during the precipitation of calcite from solution.

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2.3.2 Carbonate-associated sulfate as a paleoproxy of ancient microbial diagenesis

Lead: Max Coleman, NASA-JPL

Summary: Siderite is a biosignature and paleoenvironmental proxy. We will extract carbonate associated sulfate, used mainly so far to infer the sulfur isotope composition of ancient oceans, to understand details of the diagenetic environment which forms a microbial ecosystem. We will examine the competition or cooperation of sulfate- and iron-reducing organisms by measuring the sulfate’s S and O isotopes to give the extent of reduction and to correct for contributions from oxidation of sulfides. We will apply the method to recent and then successively older sediments, back to the Proterozoic. In parallel we will culture sulfate- and iron-reducing bacteria together to simulate and further investigate the natural system.


Distinct three O isotope variations in microbially produced sulfate relative to the terrestrial fractionation line (Δ17O=0).

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2.3.2 Role of microbial polysaccharides in governing composition of Ca-Mg-carbonates at low temperature

Lead: Huifang Xu, UW-Madison

Summary: We propose that microbial polysaccharides excreted by anaerobic microbes can serve as natural catalysts for dolomite and even Mg-rich dolomite precipitation. The proposed studies will shed new light on the fundamental connection between anaerobic microorganisms and formations of sedimentary dolomite and other Ca-Mg-carbonates. These results will provide new insights for the role of microbes in Ca-Mg-carbonate formation on the early Earth, as well as Mars, including the carbonates found in the ALH84001 meteorite.


Compositions of synthetic Ca-Mg carbonates induced by EPS from FB and SRB. 100 mg/L of EPS was used for experiments. The Mg:Ca ratios were the initial ratios of the experimental solutions.

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