nai

Executive Summary

The signatures and environments of life

The primary focus of the Wisconsin Astrobiology Research Consortium (WARC) is determining the signatures and environments of life. Emphasis is placed on the isotopic compositions of major elements (C, O, S, Mg, Ca, and Fe) that are recorded in biogenic or other low-temperature minerals because the “footprint” of these signatures should be relatively large in the ancient terrestrial record or terrestrial-like planets such as Mars. Moreover, the isotopic compositions of the major elements in minerals such as oxides, sulfide/sulfate, and carbonates should be relatively immune to degradation by cosmic radiation or metamorphism. Four research investigations are underway: 1) an assessment of the inventory of organics in a planetary body, including acquisition and modification during biogenic and abiogenic processes; 2) a large experimental program aimed at development of biosignatures, paleoenvironmental proxies, and pathway tracers for aqueous-mineral systems; 3) testing covariations of biosignatures and paleoenvironmental indicators in natural terrestrial systems; and 4) development of in situ instrumentation to detect isotopic biosignatures on planetary missions. Education and Public Outreach activities are run through the Geology Museum at the University of Wisconsin — Madison, and additional activities are pursued by NASA-JPL staff, including participation in the Solar System Ambassadors and Solar System Educators programs. In Year 1, WARC membership included 12 co-investigators, three staff, eight post-docs, six graduate students, two undergraduate students, and 31 collaborators at other institutions.

Research

WARC began its tenure with the NASA Astrobiology Institute in Fall 2007, at an initial funding level of 27%. To accommodate this slow start, pursuit of Investigations 1 and 4 was delayed until Year 2. In Year 1 we focused on several components of Investigation 2, including studies of nano-structures of minerals, biological production of carbonates, microbial pyrite oxidation, and iron isotope fractionations in biologic and abiologic systems. In addition, several components of Investigation 3 were pursued, including field-based studies of pyrite oxidation, a modern site of bacterial iron reduction that is an analog to Precambrian banded iron formation genesis, and development of methods for in situ isotopic analysis of O, Si, Li, and Fe by ion microprobe.

1) Nano-structured minerals as tracers of microbial activities

The structure and morphology of minerals at the macroscopic scale form the very basis for mineral definitions, and hence are not commonly variable for a given mineral. At the nanometer scale, however, a much wider variety of structures are found, and an increasing body of evidence suggests that biogenic and non-biogenic morphologies may be distinct, pointing to a possibly unique control in mineral assembly by organic compounds that are produced by microbes. Initial work as concentrated on characterizing weathering products, including paleosols and weathering zones in basalt. Long nano-fibers are found for both carbonates and oxides, and abiologic assembly of these oriented fibers seems unlikely. This work has been pursued by Co-I Prof. Huifang Xu, along with UW-Madison graduate students Emily Freeman and Fangfu Zhang, post-doc Hiromi Konishi, undergraduate student Jason Huberty, and collaborator Tianhu Chen (Hefei University of Technology, China).

2) Biogenic formation of high-magnesium calcite in sulfide-rich systems

Thermodynamically stable calcite (CaCO₃) should contain very little magnesium at room temperature, and yet high-magnesium calcites are common in marine environments, where biogenic formation is associated with high extents of magnesium incorporation in the calcite crystal lattice. The association of high-magnesium calcite and bacterial sulfate reduction in modern marine sediments has led to the proposal that dissolved sulfide may represent the catalyst for Mg incorporation. Co-I Prof. Huifang Xu, along with UW-Madison graduate students Emily Freeman and Fangfu Zhang, post-doc Hiromi Konishi, undergraduate student Jason Huberty, and collaborator Tianhu Chen (Hefei University of Technology, China) are testing the hypothesis that dissolved sulfide lowers the dehydration energy of the Mg-water complex, enhancing incorporation into calcite. So far they have shown experimentally that up to 30 mole % MgCO₃ may be incorporated into calcite in the presence of pyrite powder or aqueous sulfide.

3) Production of mixed cation carbonates in abiologic and biologic systems

The constraints imposed by thermodynamics on the equilibrium compositions of Ca-Mg-Fe carbonates in low-temperature aqueous environments provides an important reference frame for interpreting unusual compositions that may reflect a biosignature. For example, natural iron-rich carbonates such as siderite may have calcium contents that exceed those permitted by equilibrium thermodynamics, suggesting a biological role in Ca incorporation. In addition, abiologic synthesis of pure magnesium carbonate has proven difficult, and it is possible that such compositions require a role for biology. Co-I Prof. Chris Romanek and post-doc Monica Sanchez Roman at the University of Georgia are leading the experimental work in this project, with an initial focus on inorganic free-drift experiments, inorganic chelation experiments, and microbial experiments. Results so far confirm the difficulty in producing non-equilibrium mixed cation carbonates through abiologic processes, but in the presence of oxalate (but in the absence of microbes), such compositions may be produced. Understanding the apparently important role of organic ligands during carbonate assembly is important, and parallel efforts by Co-I’s Prof. Nita Sahai and Prof. Huifang Xu on ab initio modeling and mineralogy, respectively, are aimed at determining the mechanisms that are involved at the mineral-fluid interface.

4) Microbial pyrite oxidation in nature and the lab: sulfate mineral biosignature investigation

It is well known that under low-pH conditions, microbial oxidation of pyrite is faster by several orders of magnitude than abiologic oxidation by O₂, providing strong circumstantial evidence that many sulfate minerals, the oxidized product of sulfide, may ultimately owe their origin to biological activity. Microbial oxidation of pyrite is common in “acid-mine” environments, such as Rio Tinto, Spain (Figure 1). It would be incorrect, however, to assume these observations rule out an important role for O₂, and using water and O₂ of variable ¹⁶O-¹⁷O-¹⁸O compositions, Co-I Max Coleman at JPL, along with JPL scientists

Randall Mielke and Karen Ziegler and UCLA collaborator Prof. Ed Young, have shown that bacteria use atmospheric O₂ to initiate oxidation of pyrite sulfur, followed by a shift to use of oxygen in water as sulfur oxidation proceeds. Initiation of oxidation of Fe²⁺ in pyrite to Fe³⁺ is accompanied by the change in oxygen sources during sulfur oxidation. The results of these experiments have important implications for the origin of sulfate minerals on other planets such as Mars because they place constraints on the ambient conditions required for various pathways of sulfate mineral formation.

5) Iron isotope biosignatures: Laboratory studies and modern environments

Iron is one of the most abundant redox-sensitive elements in the earth’s crust that is cycled by microbes, and the bonding changes that accompany changes in redox state (Fe²⁺ or Fe³⁺) impart a significant fractionation in the stable isotopes of Fe, commonly expressed as changes in the ⁵⁶Fe/⁵⁴Fe ratio. Oxidation increases the ⁵⁶Fe/⁵⁴Fe ratio, but the effect appears to be the same for abiologic oxidation by O₂ or anaerobic photosynthetic Fe²⁺ oxidation, and so Fe isotopes do not appear to be a useful biosignature for the oxidation pathway. Nevertheless, it seems clear that Fe isotope variations may only be produced on a planetary body that hosted a hydrologic cycle that included redox cycling of metals. There is now a wealth of evidence that reduction of Fe³⁺ to Fe²⁺, which produces decreases in ⁵⁶Fe/⁵⁴Fe ratios, is most commonly powered by microbial dissimilatory iron reduction (DIR). Laboratory work in Year 1 included biological DIR and abiological experiments in the presence of dissolved silica, intended to better reflect the Archean oceans on Earth. The ability of DIR to produce large quantities of low-⁵⁶Fe/⁵⁴Fe aqueous Fe²⁺ was confirmed, even in the presence of dissolved silica. Turning to a natural environment, a modern field site was studied (Keswick Reservoir, northern California, USA), and the results confirm that under conditions of high Fe3+ flux, and low rates of bacterial sulfate reduction, DIR is capable of producing large quantities of low-⁵⁶Fe/⁵⁴Fe aqueous Fe²⁺. Moreover, incubation experiments using the same sediment and microbial population from the field site produced the same Fe isotope fractionations as measured at the field site. This work provides very strong support for using Fe isotopes as a biosignature for tracing microbial iron reduction in Earth’s past. In environments where DIR was fueled by a supply of reactive Fe3+ and organic carbon, but under conditions of low bacterial sulfate reduction, the Fe isotope shifts to low ⁵⁶Fe/⁵⁴Fe ratios (negative δ⁵⁶Fe values) may be confidently ascribed to biological iron reduction between about 3,000 and 2,000 m.y. ago (Figure 2). In contrast, the rise in bacterial sulfate reduction after ~2,400 m.y. ago, as indicated in the increasing spread in δ³⁴S values, was accompanied by a decrease in the extent of Fe isotope fractionation (Figure 2), suggesting that dissolved sulfide produced by bacterial sulfate reduction titrated reactive Fe³⁺, preventing its use by DIR. Co-I’s Prof. Eric Roden, Prof. Clark Johnson, and Dr. Brian Beard are leading this initiative, and the field-based work has comprised the graduate thesis of George Tangalos. Co-I Prof. Huifang Xu has also been involved in the mineralogical work.

6) New frontiers in micro-analysis of isotopic compositions of natural materials: Development of Fe isotopes

The importance of iron isotopes as a tracer of redox cycling on a planetary body, as well as an indicator of a hydrologic cycle, focuses attention on developing the capability for in situ isotopic measurements in samples, either remotely on another planet, or in Earth-based labs on samples that have been returned from planetary missions. Initial work on high-precision in situ Fe isotope analysis has been undertaken using the UW Madison CAMECA ims-1280 ion microprobe. Analysis spot sizes of 10 microns produce sufficient ion counts for iron oxides such as magnetite to produce internal precisions of 0.25 per mil (2SD) for ⁵⁶Fe/⁵⁴Fe ratios. Accuracy is strongly affected by variations in instrumental mass bias that accompany different samples in terms of mounting and compositional differences, and work is currently underway to understand such effects. In addition to magnetite, development of analytical protocols for analysis of pyrite and Fe-bearing carbonates is planned. Co-I Dr. Brian Beard has led this work, in collaboration with Dr. Noriko Kita and Co-I Prof.
John Valley, assisted by Jim Kern, Brian Hess, and Dr. John Fournelle, all at UW-Madison.

7) New frontiers in micro-analysis of isotopic compositions of natural materials: Development of O, Si, and Li isotopes

The isotopic compositions of Li, O, and Si may record fluid sources and weathering pathways in sedimentary environments, which bear on the environments that may support life. Analytical development work on in situ Li and O isotope analysis using the UW Madison CAMECA ims-1280 ion microprobe has shown that the world’s oldest zircons (4.0 to 4.4 b.y. old) record Li and O isotope compositions that indicate a hydrologic cycle very early in Earth’s history. Development work in Year 1 has also extended to in situ Si isotope measurements of quartz in cherts and banded iron formations, possibly allowing distinction between terrestrial weathering and hydrothermal sources of Si. Co-I Prof. John Valley has led this initiative, in collaboration with Dr. Noriko Kita, staff members Jim Kern, Brian Hess, Mike Spicuzza, and Dr. John Fournelle, post-docs Reinhard Kozdon, Philipp Heck, and Taka Ushikubo, and graduate student Jason Huberty, all at UW Madison.

Education and Public Outreach

The UW Madison Geology Museum launched its astrobiology E/PO efforts with a lecture and hands-on demonstration tour of ten elementary schools in southern Wisconsin, where the theme was “the smallness of microbes”. Over 500 elementary school children participated in these activities. In spring 2008, UW-Madison hosted the campus-wide “Science Expeditions”, which draws over 1500 people. As part of this event, the Geology Museum developed and hosted an interactive program titled “Mission Invisible: Discover Unseen Life on Earth”. In early summer 2008, astrobiology was the theme for the Geology Museum Open House, which included interactive activities on meteorites and impact craters, banded iron formations, stromatolites and earth’s oldest rocks, and extremophiles; about 300 people attended this event. Co-I Rich Slaughter, Assistant Geology Museum Director Brooke Norsted, and JPL-based Co-I Kay Ferrari participated in a three-day astrobiology workshop at Alder Planetarium in Chicago titled “Astrobiology for Museum Workers”, which included representatives from over 20 museums and planetariums across the U.S. Also in Year 1, Co-I Kay Ferrari continued her work in the Solar System Ambassadors (currently 523 members in all 50 states) and Solar System Educators (currently 64 members in 44 states) programs. Co-I Ferrari also began her work on “Astrobiology 101”, where she is coordinating this new astrobiology training program for Solar System Ambassadors, Solar System Educators, and Museum Alliance members.