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Project Reports

• New Frontiers in Micro-Analysis of Isotopic Compositions of Natural Materials: Development of O, Si, and Li Isotopes

  The isotope ratios of oxygen and silicon are a sensitive monitor of sedimentary and hydrothermal processes for deposition of banded iron formation. Our focus on banded iron formations reflects the importance these unusual units have in biogeochemical cycling in the early Earth. In particular, we are examining the deposition of microlaminated sections of the Dales Gorge member of the Brockman Iron Formation from the Hamersley basin, which have alternatively been interpreted to represent annual varves in chemical precipitates from seawater, or variations in a sub-surface hydrothermal system. These conflicting models have relevance for interpreting the role of microbial life in precipitation of the Fe-oxides. In addition, δ18O and δ³⁴S values from coexisting minerals can be used to estimate the temperature of the Archean ocean, or of the hydrothermal system. δ⁷Li and δ18O values in zircons allow these tests to be applied to magmas that may have assimilated sedimentary materials, and, in the case of Jack Hills samples from SW Australia, provide a record of the earliest Earth (4.4 to 4.0 Ga), before the formation of all known rocks.

  ROADMAP OBJECTIVES: 4.1 7.1

• Nano-Structured Minerals as Tracers of Microbial Activities

  Biologically produced mineral formation at the nanometer scale often produces unique morphologies and compositions relative to abiologic imineral formation. For example, calcite nano-fibers in arid and semi-arid soils are demonstrably products of microbial activities, where bioorganics derived from microorganism have controlled growth of nano-fibrous calcite. Other examples of bioorganic control on nano minerals include ttitanium-free magnetite / maghemite nano-fibers that are closely associated with Fe-bearing smectite in a weathered basaltic glass, and it is possible that these minerals were produced by bacterial dissimilatory iron reduction of ferrihydrite nano-crystals through topotactic transformation to magnetite. Both calcite nano-fibers and Ti-free maghemite nano-crystals are likely biosignatures in dry soils and weathered basalt tuff deposits, raising the possibility that such features may be found in sedimentary rocks on other planetary bodies.

  ROADMAP OBJECTIVES: 7.1

• Production of Mixed Cation Carbonates in Abiologic and Biologic Systems

  Carbonate minerals commonly occur in terrestrial environments and they are found in extraterrestrial materials, such as meteorites and interplanetary dust particles. On earth, carbonates hold one of the earliest records of seawater chemistry. Carbonate minerals that form in thermodynamic equilibrium (abiotic formation) are constrained in Ca-Mg-Fe composition by the solvi that limit solid-solution to the magnesite-siderite join, the dolomite-ankerite join, and the calcite end member. However, it is now known that microorganisms may produce Ca-Mg-Fe compositions that lie between these solvi, and are therefore out of thermodynamic equilibrium, and such compositions may represent a biosignature for microbially mediated formation. The goal of this project is to understand the mechanisms by which carbonate minerals of unusual chemistry form at relatively low temperature so that we may better understand the processes that may form these minerals in the early Earth, on Mars, and other planetary bodies of the Solar System.

  ROADMAP OBJECTIVES: 4.1 7.1

• Iron Isotope Biosignatures: Laboratory Studies and Modern Environments

  This project seeks to define the biotic and abiotic mechanisms of Fe isotope fractionation in modern sedimentary environments and laboratory model systems. Such information is required to evaluate the potential role of Fe isotopes in understanding the evolution of microbial redox metabolism on the early Earth, and to evaluate their ability to serve as biosignatures of microbial metabolism on other planets (e.g. Mars).

  ROADMAP OBJECTIVES: 4.1 6.1 7.1

• Biogenic Formation of High-Magnesium Calcite in Sulfide-Rich Systems

  Calcite minerals that contain high proportions of magnsium are well-known biogneic minerals in modern marine environments, reflecting metastable incorporation of Mg in the calcite lattice. In the modern oceans, calcite may form in the presence of sulfide in hydrothermal systems, or, in the ancient Earth, high-magnesium calcite may have formed in the presence of high ambient sulfide (produced by bacterial sulfate reduction or hydrothermal systems), and this project is aimed at understanding the mechamisms involved in producing high-magnesium calcite in the presence of high dissolved sulfide.

  ROADMAP OBJECTIVES: 4.1 6.1 7.1

• Microbial Pyrite Oxidation in Nature and the Lab: Sulfate Mineral Biosignature Investigation

  In the laboratory the project aims to culture bacteria that oxidize iron and sulfur under various conditions (a range of temperatures and different degrees of acidity). The goal is to develop criteria to distinguish minerals that formed as indirect result of bacterial activity from those that form with no biological association. It s anticipated that such distinctions may involve subtle, but distinctive differences in chemical and isotopic compositions, including variations in the ratios of the naturally occurring stable (non-radioactive) isotopes of iron, sulfur and oxygen. Analyses will be made of similar minerals, formed naturally in an acid mine drainage area of SW Spain, that may be an analog for processes that are believed to have happened on the surface of Mars four billion years ago.

  ROADMAP OBJECTIVES: 2.1 4.1 7.1

• New Frontiers in Micro-Analysis of Isotopic Compositions of Natural Materials: Development of Fe Isotopes

  The isotopic composition of iron is an excellent signature of past redox processes and the presence of a hydrologic cycle. Moreover, Fe isotope compositions promise to be a unique marker for dissimilatory iron reduction by bacteria, making this isotope system an excellent biosignature. Our goal is to develop analytical methods to make precise Fe isotope analysis on individual Fe-bearing minerals at a 10 micron diameter spot resolution. This technology will allow assessment of the Fe isotope heterogeneity within an individual Fe-bearing mineral and between different mineral grains. Documentation of such inter- and intra-mineral variations is critical to establishing if the Fe isotope variations measured in ancient rocks is a primary signature indicative of the environment in which the rock formed, or if it is a result of later metamorphic or diagenetic processes. Moreover, performing such spot mineral analyses will allow one to correlate Fe isotope variations that are associated with petrographic and mineral/chemical variations that cannot easily be done using conventional techniques.

  ROADMAP OBJECTIVES: 4.1 7.2