P53D-01
On the Evolution of Mineral Biosignatures
Earth's near-surface mineralogy has diversified over geologic time as a consequence of three primary
mechanisms: (1) the progressive separation and concentration of the elements from their original relatively
uniform distribution in the pre-solar nebula; (2) an increase in range of intensive variables such as pressure,
temperature, and the activities of H2O, CO2, and O2; and (3) the generation of far-from-
equilibrium conditions by living systems.
Following planetary accretion and differentiation, the initial mineral evolution of Earth's crust depended on a
sequence of geochemical and petrologic processes, including volcanism and degassing, fractional
crystallization, crystal settling, assimilation reactions, regional and contact metamorphism, plate tectonics,
and associated large-scale fluid-rock interactions. These processes produced the first
continents with their associated granitoids and pegmatites, hydrothermal ore deposits, metamorphic terrains,
evaporites, and zones of surface weathering, and resulted in an estimated 1500 different mineral species.
Biological processes began to affect Earth's surface mineralogy by the Eoarchean Era (3.85 to 3.6 Ga),
when large-scale surface mineral deposits, including banded iron formations, were precipitated under the
influences of changing atmospheric and ocean chemistry. The Paleoproterozoic "Great Oxidation Event" (2.2
to 2.0 Ga), when atmospheric oxygen may have risen to greater than one percent of modern levels, and the
Neoproterozoic increase in atmospheric oxygen, which followed several major glaciation events, ultimately
gave rise to multicellular life and skeletal biomineralization and irreversibly transformed Earth's surface
mineralogy. Biochemical processes may thus be responsible, directly or indirectly, for most of Earth's 4300
known mineral species.
The sequential evolution of Earth's mineralogy from chondritic simplicity to Phanerozoic complexity introduces
the dimension of geologic time to mineralogy and thus provides a dynamic alternate approach to framing,
and to teaching, the mineral sciences.
http://hazen.ciw.edu/research/mineral_evolution
P53D-02 INVITED
Calibration of Biosignatures in Microbial Ca:Mg Carbonates: Fossilized Evidence for Ancient Life
Ca carbonate minerals, such as calcite or aragonite, are known to precipitate by both abiologically and biologically produced processes. The abundant sedimentary mineral dolomite (CaMgCO3), however, precipitates exclusively as a microbial induced product under Earth surface conditions. Thus, the study of microbial dolomite precipitation in natural environments and laboratory culture experiments provides the potential to calibrate and evaluate the range of microbial biosignatures that may become fossilized in the carbonate rock record. Structural biosignatures associated with microbial dolomite, which are microscopically observable, include characteristic shapes, such as dumbbell, spheroid or cruciform structures, and pervasive exopolymeric substance (EPS) matrices within which the mineral nucleates. The occurrence of these biostructures fossilized in geologic dolomite samples provides unambiguous evidence for the past presence of microorganisms. Identification of comparable biostructures associated with other more ambiguous biominerals may be significant physical evidence for the activity of microorganisms in a variety of terrestrial and planetary environments. Furthermore, the study of microbial dolomite precipitation provides valuable information on relevant environmental conditions that can be extrapolated to interpret paleoenvironments. In particular, microbial dolomite forms under hypersaline conditions often associated with a range of anaerobic microbial processes, such as bacterial sulfate reduction and/or anaerobic methane oxidation. The interactions of these complex microbial communities lead to the incorporation of characteristic carbon-isotope signatures reflecting the various metabolisms involved in the biomineralization. Additionally, the specific organic functional groups of metabolically produced organic molecules, which are included in modern and ancient biominerals, can be quantitatively compared using Electron Energy Loss Spectroscopy (EELS). Finally, combining these recently calibrated microbial biosignatures with the ultimate classic macroscopic evidence for microbial life, i.e. stromatolites, provides a powerful approach to corroborate the biogenicity of this earliest life form.
P53D-03
Sources and Contributions of Oxygen During Microbial Pyrite Oxidation: the Triple Oxygen Isotopes of Sulfate
The triple isotopes of oxygen (Δ17O' = δ17O'-0.528 × δ18O' using logarithmic deltas) can trace the oxygen sources of sulfate produced during sulfide oxidation, an important biogeochemical process on Earth's surface and possibly also on Mars [1]. δ18OSO4 compositions are determined by the isotopic selectivity of the mechanism(s) responsible for their changes, and the δ18O value of the reactants (O2 vs. H2O). The relative proportional importance and contribution of each of those sources and mechanisms, as well as their associated isotopic fractionations, are not well understood. We are investigating the use of Δ 17O as a quantitative and qualitative tracer for the different processes and oxygen sources involved in sulfate production. Δ17O signatures are distinct fingerprints of these reservoirs, independent of fractionation factors that can be ambiguous. We conducted controlled abiotic and biotic (Acidithiobacillus ferrooxidans, A.f.) laboratory experiments in which water was spiked with 18O, allowing us to quantify the sources of sulfate oxygen and therefore the processes attending sulfate formation. Results of this Δ17O tracer study show that A.f. microbes initiate pyrite S-oxidation within hours of exposure, and that sulfate is produced from ~90% atmospheric oxygen. This initial lag-phase (< 3 days) is characterized by subtle and multiple changes in oxygen source and contribution that is likely due to the adjustment of the microbial metabolism from S to Fe2+-oxidation. A more detailed understanding of the microbial mechanisms and behavior in the initial lag-phase will aid in the understanding of the ecological conditions required for microbial populations to establish and survive. An exponential phase of growth, facilitated by microbial Fe2+-oxidation, follows. The source of sulfate rapidly switches to abiotic sulfide oxidation during exponential growth and the source of oxygen switches from atmospheric O2 to nearly ~100% water. Pending acquisition of complimentary chemistry data (in progress), we interpret our isotope data to indicate that the biotic fractionation factor ε18OSO4-O2 of at least ~ -25 to - 35‰ is augmented by microbially induced kinetic fractionation; it is larger than expected based on published equilibrium values [2,3,4]. Our inferred ε18OSO4-H2O of at least ~+10‰ is similar to some reported values. These new insights into the close links between microbial life cycle and sources of sulfate oxygen during sulfide oxidation, and their oxygen isotopic expressions, will help elucidate the role of microbial oxidation in natural systems. If microbial populations in natural systems remain in a perpetual lag-phase due to constrains of chemistry, atmospheric oxygen will imprint its isotopic signature onto sulfate deposits. Ultimately, such data could be used as biosignatures on Early Earth or Mars. [1] Brunner and Coleman (2008) EPSL 270, 63-72. [2] Balci et al. (2007) GCA 71, 3796-3811. [3] Pisapia et al. (2007) GCA 71, 2474-2490. [4] Taylor et al. (1984) GCA 48, 2669-2678.
P53D-04
Pyrrhotite: an Iron Sulfide Mineral Formed During Growth of Sulfate-Reducing Bacteria at a Hematite Surface
Many bacteria are capable of respiring on sulfate and other oxidized forms of sulfur under anaerobic conditions. The hydrogen sulfide that is formed during dissimilatory sulfate reduction (DSR) readily reacts with metals in the surrounding environment to form insoluble metal sulfides. Iron oxides are common substrata for colonization by sulfate-reducing bacteria (SRB) in sedimentary aquatic systems as well as in subsurface environments. While numerous studies have characterized iron sulfides formed during dissimilatory sulfate reduction by suspended populations of these bacteria in the presence of soluble iron, not much is known about those formed in the presence of biofilm populations associated with solid phase iron, particularly crystalline forms such as hematite. Under the latter conditions, we have observed the formation of the iron sulfide pyrrhotite, typically present in very low abundance in sediments and ore deposits compared to pyrite. The formation of pyrrhotite over pyrite is favored at low redox potential and sulfide activity, conditions we hypothesize are achieved at an iron oxide surface colonized by biofilm-forming SRB. Higher levels of hydrogenase activity by hematite surface-associated SRB than suspended cell populations likely promotes the low redox potential that favors pyrrhotite formation. The tendency for SRB in nature to associate with mineral particle surfaces, including iron oxides, suggests that some pyrrotite may have originated through biotic reactions. A comparison of the fine structure of pyrrhotite formed through these biotic processes with that formed under abiotic processes may reveal differences that provide a signature for biotically-derived pyrrhotite in the biosphere.
P53D-05
NanoSIMS Analysis of Sulfur Isotopes in Sulfides from ca. 2.7 Ga BIFs and Cherts as Tracers of Microbial Activity
Archean hydrothermal environments often host various types of sedimentary rocks that include banded iron formations (BIFs) and cherts. Modern microbial communities that inhabit these ecosystems are diverse and tend to play a strong role in sulfur cycling. The sulfur isotopic composition of sulfide minerals in ancient BIFs and cherts from volcano-sedimentary environments thus potentially preserves a record of microbial sulfur metabolism. In addition, sulfur isotopic compositions may contain information on the redox state of the atmosphere if the sulfur cycled in it and was eventually transferred to marine sediments before its crystallization into sulfide. We used the nanoSIMS 50L to measure in situ the isotopic composition of sulfides from ca. 2.7 Ga BIFs and cherts from the Abitibi terrain in Quebec and Ontario and from the Bababudan/Sandur area in Karnataka, India. Using a beamsize of 15 X 15 microns, the 2-sigma reproducibility on standards was ± 1.0 and ± 0.3 permil for δ34S and Δ33S values, respectively, and we obtained mass dependent fractionation slopes of 0.5167 and 0.5172. Important instrumental mass fractionation effects were observed for analyses intentionally targeting areas with topographic defects, poor polish, and re-analysis of old craters. So far, our survey of 2.7 Ga Abitibi and Bababudan/Sandur BIF and chert sulfides has revealed a range of about 15 permil in δ34S (between +6 and -9 permil) and mostly positive Δ33S values up to +2.9 permil. These data are consistent with a generally anoxic Neoarchean atmosphere and possibly with some microbial sulfur processing in the original environment.
P53D-06
Iron isotope fractionation by microbial iron reduction in modern chemically precipitated sediments
Laboratory experiments have demonstrated that dissimilatory microbial iron oxide reduction (DIR) can produce Fe(II) phases that have low 56Fe/54Fe ratios similar to those found in Neoarchean and Paleoproterozoic banded iron formations (BIFs) and shales. Direct application of these experiments to BIF formation has been hindered by the lack of Fe isotope data from modern environments that are analogous to BIFs. Here we report Fe inventories and isotopic compositions for chemically precipitated sediments in the Spring Creek Arm of Keswick Reservoir (SCAKR) downstream of the Iron Mountain acid mine drainage site in northern California, USA. The high concentration of reactive Fe(III) (ca. 50-100 mmol of amorphous Fe(III) oxyhydroxides per liter of bulk sediment) allows dissimilatory iron-reducing bacteria (DIRB) to predominate over dissimilatory sulfate-reducing bacteria in sediment carbon metabolism, making the SCAKR a better analog for BIFs compared to modern marine environments. DIR has generated millimolar concentrations of aqueous Fe(II) (Fe(II)aq) in SCAKR sediments. The Fe(II)aq has lower 56Fe/54Fe values than bulk HCl-extractable Fe; δ56Fe values for bulk HCl-extractable Fe fall within the range previously defined for mafic- to intermediate-composition terrestrial igneous rocks, modern marine sediments, suspended river sediments, Proterozoic-Phanerozoic shales, loess, aerosols, and soils. After removal of pore fluid, sediment was reacted for 1 hr with 0.1M HCl to isolate solid-phase Fe(II) (Fe(II)s), which was likely a mixture of sorbed Fe(II) and amorphous surface-precipitated Fe(II) hydroxide. Subsequent 24-hr extraction with 0.5M HCl recovered amorphous Fe(III) oxide (Fe(III)am). Sediment incubation experiments with SCAKR sediment produced trends in in Fe isotopic fractionations between Fe(II)aq, Fe(II)s, and Fe(III)am analogous to those observed in situ. Collectively the data suggest an equilibrium 56Fe/54Fe isotope fractionation between Fe(II)aq and Fe(III)am on the order of -2 per mil, and a smaller but significant fractionation between Fe(II)aq and Fe(II)s of -0.4 per mil. Our results illustrate how DIR could have produced large quantities of mobile Fe with low-δ56Fe values during early sediment diagenesis in Neoarchean and Paleoproterozic BIFs.
P53D-07
Aggregation Phenomena in Cyanobacterial Analogues of Ancient Stromatolites
If one is to understand and time the evolution of such fundamental processes as photosynthesis, it is imperative to recognize and interpret microbial fossils. This goal is challenging because many of the oldest putative fossils are only identified as biotic by a distinctive morphology. To this end, we examine the forces that shape modern, cone-forming cyanobacterial mats that are thought to grow in a manner similar to ancient structures called conical stromatolites. Here we show that the initial stages in the growth of a mat are shaped primarily by the diffusion of oxygen over macroscopic distances. We observe that cyanobacteria aggregate into 100-200 micrometer diameter clumps when oxygen is present, but these gliding bacteria migrate away from the clumps when oxygen is removed from the system. Not only does oxygen have the ability to induce clumping, but the diffusion of photosynthetically produced oxygen also determines the 200 micrometer - 1 mm spacing among the clumps arranged in a cm-scale lattice. Although many current models of stromatolite morphogenesis rely exclusively on bacterium-scale processes, our observations show that macroscale interactions control the early stages in the growth of modern cone- forming biofilms. Intriguingly, similarly sized and spaced clumps initiated the growth of conical stromatolite classified as Thyssagetes odontophyes, providing evidence that a metabolic waste product shaped some microbial mats in Early Mesoproterozoic.
P53D-08 INVITED
Interpreting Biosignatures in the Context of Marine Evaporitic Environments
A biosignature is an object, substance and/or pattern whose origin specifically requires a biological agent. The usefulness of a biosignature is determined, not only by the probability of life creating it, but also by the improbability of nonbiological processes producing it. So what sets life apart from the rest? Life as we know it is the harnessing of free energy to sustain and perpetuate, by molecular replication and evolution, a high density of information in the form of functional complex molecules and functionally-related larger structures. Accordingly, biosignatures can arise from key attributes such as converting solar to chemical energy, exploiting the versatility of organic chemistry to sustain metabolic processes and preserve information, and maintaining microenvironments that enhance these functions. The external environment affects such functions and so it must be defined in order to interpret effectively the biosignatures that emerge from them. Hypersaline benthic cyanobacterial communities at Guerrero Negro, Baja California Sur, Mexico provide illustrative examples of biosignatures research that is relevant to our early biosphere and to Mars. Where brines are undersaturated with respect to gypsum, filamentous cyanobacteria dominate over unicellular cyanobacteria and can trap and bind sedimentary grains more effectively, thus altering their response to sedimentary processes and creating laminated fabrics. Biofilms in gypsiferrous sediments also can alter the response of the clastic or crystal matrix to chemical and physical sedimentary processes such as erosion or precipitate accumulation. Gypsum precipitating within biofilms offers compelling evidence of biological influences on crystal textures and habits. Such gypsum exhibits dissolution textures, accessory mineral precipitation and unique crystal form aspect ratios. Irregular textures include conchoidal and globular features associated with both dissolution and nucleation that are likely affected by biofilm pore water compositions. The accessory phases forming in association with gypsum-hosted biofilms (Sē, Ca-carbonate, and Sr/Ca-sulfate) are known byproducts of bacterially mediated sulfate reduction. Light penetrates the relatively transparent gypsum to sustain discretely layered successions of orange-, green-, purple-, pink-, and black-pigmented endoevaporitic biofilms. Lipid biosignatures include carotenoids, tricyclic terpenoids, benzothiophenes, thiacycloalkanes and methylhopanoids. These represent the aggregate effects of light regimes and hypersaline conditions. Features that could be preserved over geological timescales therefore include sedimentary textures, minerals, crystal forms, and lipids. Collectively these features can serve both as biosignatures and paleoenvironmental indicators on early Earth and on Mars.