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3.4 Geochemical context for early life (House, Kaplan, Kavner, Manning, Schauble, Venkatesan, Young)

3.4.1 Chemical feedbacks

	The development of life on the early Earth provides clues to possible evolution of life elsewhere. Artwork courtesy of JPL/NASA.

Hydrothermal systems in the early Earth were likely hosted by olvine-rich ultramafic igneous rocks. In such systems, the most important silicate-hydrolysis reaction is the conversion of the mineral olivine to serpentine and magnetite (serpentinization). Because this reaction produces large amounts of H₂ gas, CH4 gas, and basic solutions (e.g., Janecky and Seyfried 1986; Coveney et al. 1987; Abrajano et al. 1988; Rona et al. 1992; Charlou and Donval 1993; Kelley 1996; O’Hanley 1996; Kelley et al. 2002), sites of active serpentinization should have been favorable environments for chemotrophic organisms on the early Earth. Manning and coworkers are testing this hypothesis by conducting experiments on serpentinization in the presence of microorganisms.

Manning and House will use an experimental program that will investigate model ecosystems involving primitive microorganisms, olivine, and hot seawater. Preliminary experiments have been conducted using the hyperthermophilic methanogen Methanopyrus kandleri in 100°C solutions with olivine, serpentine, and magnetite. M. kandleri is used because its molecular phylogeny suggests minimal evolution away from the hypothesized universal ancestor of life on Earth, it thrives at 100°C in oceanic hydrothermal vents, and its metabolism utilizes H₂ during chemoautotrophic production of CH4. M. kandleri also facilitates reduction of aqueous Fe(III)-bearing organic compounds, which could release O₂ and create positive feedback for additional olivine-sourced Fe(II) oxidation.

Results at 100°C, 3 bar H₂+CO₂, show that M. kandleri is readily cultured during hydrolysis of olivine (Herrera et al. 2003). Concentrations of Fe, Mg, and Si in solution were higher in the presence of M. kandleri than in abiotic systems, implying enhanced olivine dissolution rates and strong chemical feedback. The olivine-rich ultramafic-hosted hydrothermal systems that were abundant in the early Earth are favorable environments for this microorganism. Further experiments are planned, including those involving isotopically-doped Mg and Fe for tracing the exchange of Mg and Fe between phases in the system using the isotopes of these elements using multiple-collector inductively coupled plasma-source mass spectrometry (MC-ICPMS, see next section).

3.4.2 Abiotic pathways to complex organics

The prebiotic Earth contained abundant organic matter that was produced by abiotic chemical pathways. The standard model for the origin of life hypothesizes that the earliest organisms arose from this complex, abiotic, organic brew. Careful assessment of this hypothesis relies on two factors: (1) completeness of the inventory of the prebiotic organic mix, and (2) development of appropriate tests to assess the contributions of different pathways. We are conducting studies in both these areas.

To address the completeness of the hypothesized organic inventory, we are exploring alternative pathways for abiotic organic production. Numerous mechanisms have been proposed for abiotic organic production, including electrical discharge, cometary and meteorite delivery, and production in early surficial hydrothermal systems. However, a major additional pathway in the early Earth that has not been considered is the generation of methane, n-alkanes, and potentially simple organic acids and N-bearing compounds at elevated pressure and temperature during tectonic burial of nascent hydrated ultramafic plates in Hadean time. Substantial amounts of organic compounds are produced by this process in the modern Earth. The larger volumes of ultramafic material at the surface of the early Earth motivate chemical reactive flow modeling to investigate organic production by this mechanism.

3.4.3 Role of metals in early biology

A second line of investigation involves using intermediate-weight stable isotopes (e.g., isotopes of Mg, Fe, Cu, and Si) to investigate the role of metals in the early organic chemistry of the Earth. The first application is as tracers of biologically and non-biologically mediated reactions between rocks and waters relevant to organic synthesis. Manning and Young will investigate the partitioning of the stable isotopes in laboratory simulations of water-rock reactions in the presence and absence of microbes to establish isotopic criteria for distinguishing biologically from non-biologically mediated reactions.

The other line of study focuses on isotope fractionation attending fundamental physicochemical processes, with the goal of understanding the extent to which stable isotopes of transition metals and alkali metals might be used as biomarkers. In particular, transition metal isotope fractionation attending redox reactions will be monitored using electrochemical techniques. Kavner and Young plan to use potentiostatic techniques to study the fractionation of iron, nickel, and chromium isotopes during redox processes relevant to organometallic chemistry.

Studies by Kavner and Young will focus not only on the magnitude of the partitioning of the isotopes, but also on the "slopes" defined by three-isotope systems (§ 3.6.2). The slopes, under favorable circumstances, can be used to distinguish equilibrium steps from kinetic steps in a fractionation process. The expected outcome will be a quantitative tie between the driving potential for chemical oxidation/reduction processes, and corresponding isotope fractionation signatures. This will help elucidate the mechanism by which microorganisms generate specific isotope signatures and allow us to isolate discrete fractionating steps in both biological and non-biological redox systems. The results can be used for comparisons with fractionation in more complicated systems.

3.4.4 Equilibrium Fe isotope fractionation between Fe²⁺ and Fe³⁺ - an experimental approach

Mass-dependent fractionations of the stable isotopes of iron have recently been discovered, and it is observed that large isotopic fractionations are largely restricted to precipitates formed in low-temperature natural and laboratory environments. This property of the Fe-isotope system suggests that it may be ideally suited for the identification of ancient low-temperature environments on Earth and other planets. Major unanswered questions remain, however, regarding the causes of environmental Fe-isotope fractionations and the possibility of using observed signatures in rock samples to unequivocally identify ancient biological activity.

There is evidence to suggest that the largest fractionations (1‰ to 3‰) typically occur when iron is partially oxidized or reduced in the presence of liquid water. These fractionations are preserved in the rock record when the different oxidation states (Fe³⁺ and Fe²⁺, typically) are separated, for instance by precipitation of Fe³⁺O(OH). Fe-redox transformations are commonly mediated by microorganisms, and it has been suggested that the biological redox fractionation (~1.5‰) is characteristically smaller than the inorganic fractionation (~3‰). Significant uncertainty in the equilibrium organic fractionation persists, however, due to the difficulty of separating Fe³⁺ and Fe²⁺ reversibly. One way to avoid this problem is to allow an easily separable phase (an immiscible organic solvent such as diethyl ether) to equilibrate with mixed solutions with a range of Fe³⁺/Fe²⁺ ratios. Such a technique can, in principle, be applied to many exchange-labile chemical species in aqueous solutions.

Schauble and Young intend to use fundamental laboratory studies to develop techniques for determining Fe-isotope fractionations between different species in aqueous solutions. The project will examine the potential of using an immiscible organic solvent phase (diethyl ether) as a rapidly equilibrating and easily separable reservoir of dissolved iron. In Fe³⁺-Fe²⁺-HNO₃-HCl solutions, [FeCl₄]^- is the only ether-soluble species. It is expected, therefore, that the isotopic partitioning behavior of iron in the ether phase will be nearly constant over a range of aqueous Cl– activities, Fe³⁺/Fe²⁺ ratios, and pH. Measured fractionations between the iron dissolved in this organic reservoir and aqueous solutions of varying chemistry (for instance, a series of increasing Fe²⁺/Fe³⁺ ratios) will reflect changing isotopic behavior in the aqueous phase, allowing indirect measurement of fractionations between ether-solvated [FeCl₄]– and aqueous [Fe(H₂O)₆]³⁺, Fe²⁺ (i.e. [Fe(H₂O)₆]²⁺), Fe3+-chloro complexes such as [Fe(H₂O)₅Cl]²⁺, and Fe³⁺-hydroxyl complexes. Relatively fast equilibration (~minutes to hours) between ether and aqueous phases is expected, based on bulk iron partitioning experiments from the literature.

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