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Project Progress

Ohmoto Group

1. Identification of Mineralogical and Chemical Biosignatures in Ancient Rocks
a. Ferric oxides in paleosols and in submarine hydrothermal systems: Enrichment of of ferric-iron –rich minerals (e.g., goethite, hematite) in paleosols suggests the abundance of organic acids, and therefore the abundance of terrestrial biomass, necessary to mobilize Fe^III during soil formation under an oxygenated atmosphere. Enrichments of ferric oxides in submarine hydrothermal deposits (i.e., massive sulfide deposits and banded iron formations) and in associated rocks (e.g., submarine volcanic rocks) suggest O₂-rich ocean water and an aerobic world (Ohmoto et al., 2011; Ohmoto et al., in review).

b. Fe-poor carbonates: Primary carbonates with Fe/Ca atomic ratio <0.05 indicate the formation in Fe-poor water bodies. Such carbonates are found to constitute the 2.74 Ga Tumbiana stromatolites and 2.9 Ga Steep Rock stromatolites, suggesting that these stromatolites were formed by oxygenic photoautotrophs. If anoxygenic photoautotrophs had created the stromatolites, the stromatolites would have been Fe-rich (Ohmoto et al., in review).

c. Siderite in sedimentary rocks: Siderite crystals in Archean-aged sedimentary rocks, as well as those in younger rocks, appear from their textural, chemical and isotopic characteristics to have formed during the diagenesis of organic C-rich sediments by utilizing (i) Fe^II from hydrothermal fluids or Fe^III in sediments and (ii) CO₂/HCO₃- from both the normal seawater and from the decay of organic matter. Their δ¹³C values may distinguish the dominant microbes involved in the decomposition of organic matter, whether sulfate-reducing bacteria or methanogen (Ohmoto et al., 2004, Nature; Ohmoto et al., in review).

d. Detrital kerogen in sedimentary rocks. Kerogen in sedimentary rocks is typically decomposed during weathering, aided by anaerobic organisms, under an oxygenated atmosphere. However, anaerobic organisms do not decompose matured kerogen (with H/C atomic ratios less than ~0.5). Therefore, the absence of detrital kerogen in normal sedimentary rocks (i.e., sandstones, shales, chert, carbonate) would indicate the development of aerobic biomasses on land under an oxygenated atmosphere. In contrast, the ubiquitous occurrence of detrital kerogen in normal sedimentary rocks would indicate rock weathering under a reducing atmosphere (± anaerobic biomasses) (Ohmoto et al., in review; Ohmoto and House, in review).

e. Detrital pyrite, uraninite, and siderite: The presence of these minerals in rapidly deposited sedimentary rocks (e.g., glaciogenic conglomerates and sandstones) does not indicate weathering under a reducing atmosphere as suggested by many previous researchers, because these minerals are found in rocks of all geologic age. However, the absence of these minerals in normal sedimentary rocks is strong evidence for weathering under an oxygenated atmosphere (Ohmoto et al., in review).

f. Anomalous concentrations of Fe^III, S, C, U, Mo, Ce, Cr and Th in paleosols suggest the presence of an aeroboic terrestrial biosphere, because the mobility of these elements increases under an O₂-rich atmosphere, forming complexes with organic acids (Ohmoto et al., 2011; Ohmoto et al., in review).

g. Anomalous concentrations of Fe^III, S, C, U, Mo, Ce, and Cr in submarine hydrothermal deposits and submarine volcanic rocks are signatures of oceans rich in O₂, SO₄^2-, U, Mo, and Cr, and thus of an aerobic world (Ohmoto et al., 2011; Ohmoto et al., in prep.)

h. Anomalous isotopic compositions of Pb in submarine volcanic rocks are signatures of reaction with U-rich seawater, thus of an aerobic world (Ohmoto et al., 2012).

i. Anomalous isotopic compositions of S (AIF-S), or mass-independent fractionation of sulfur isotopes (MIF-S), in sedimentary rocks may be signatures of reactions between organic C-rich sediments and SO₄^2- rich seawater in hydrothermal environments (Watanabe et al., 2012; Ohmoto et al., 2011; Ohmot0 et al., in review), rather than signatures of a reducing atmosphere. This suggestion is based on the results of our laboratory investigations on S isotope fractionations during reactions between organic compounds and SO₄² (Watanabe et al., 2012) and SO₂ and activated C (Hamasaki, 2011) at elevated temperatures, as well as geochemical and mineralogical investigations of AIF-S bearing rocks (Ohmoto et al., in review).

2. Discovery of evidence for the development of an aerobic world by ~3.5 Ga: We have recognized the above mineralogical and geochemical lines of evidence for an aerobic world in a variety of rocks formed in the oceans and on land, between ~3.5 – 2.2 Ga in age, from Western Australia, South Africa, and Canada. These findings suggest that by 3.5 Ga cyanobacteria and other aerobic microbes (as well as anaerobic microbes) had already evolved, thus facilitating the oxygenation of both the atmosphere and oceans (except local anoxic basins) (Ohmoto et al., 2011; 2012; Ohmoto et al., in review). The results of computer simulations of phosphorous- and carbon geochemical cycles through the continental crust – oceans – atmosphere systems also suggest that the anaerobic world would have been replaced by an aerobic world within one hundred million years since the appearance of anaerobic and aerobic organisms >3.8 Ga ago. This is because the CO₂ content of the atmosphere and oceans in the anaerobic world would have been diminished to below 0.1 PAL, even with high volcanic CO₂ flux due to the inability of anaerobic organisms to completely recycle kerogen back to CO₂ during weathering (Ohmoto and House, in review).

Field Work: As a part of Astrobiology 590 “Field course in Astrobiology”, offered during the Spring Semester, 2012, as a requirement for students in the dual title Ph.D. program in Astrobiology, Ohmoto and House led a group of 15 graduate students from Penn State and four undergraduate students from Howard and Lincoln Universities for a two-week long field trip to Ontario, Canada to study the following astrobiology-related geologic formations: 2.9 Ga stromatolites and paleosols at Steep Rock; 2.9 Ga molybdenum-gold mineralization at Marathon; 2.7 Ga red beds at Shewadevan; 2.7 Ga Algoma-type banded iron formation at Wawa; 2.7 Ga submarine volcanic rocks, banded iron formations, volcanogenic massive sulfide deposits, komatiite, and Cu-Ni-Au mineralization in the Timmins – Temagami district; 2.45 Ga Pronto paleosols and Ville Marie paleosols; 2.4-2.3 Ga uraniferous conglomerates, glaciogenic sediments and red bed sequence in the Elliot Lake area; 1.85 Ga Sudbury impact structure, oldest trace fossils(?), and massive Cu-Ni sulfide deposits associated with the Sudbury eruptive; ~1.85 Ga Gunflint Iron Formation, Sudbury-impact related tsunami deposits, black shales, and red beds in the Thunder Bay area Ontario; and ~1.85 Ga banded iron formation at Hibbing, Minnesota.

Kump Group

Below is a brief summary of important results from our research:

1. The chemocline community at Green Lake undergoes a seasonal transformation in both depth (deepening through the season) and in composition (from a dominance of purple sulfur bacteria to green sulfur bacteria). Even during the late fall, with a dominance of the GSB, there is no detectable isorenieratene in the water column. Thus, we still find (after the original work of Katja Meyer, reported last year) that the GSB at Green Lake do not produce this diagnostic biomarker for photic zone euxinia, so important in the geologic record.

2. We have reinterpreted the redox proxy record of the shallow-water Late Permian, previously thought to indicate shallow-water anoxia, as instead the result of input of terrigenous clastic material. This significantly alters our understanding of how widespread anoxia was during the event: it seems to be restricted to deeper water settings (not to areas where microbialites were being deposited). It is consistent with recent findings from the ASU group that anoxia might have been restricted in time to the extinction event and its aftermath, not extending back into the late Permian.

3. We are putting some bounds on the rates of carbon addition during the Permian extinction event by inverting the C isotope records using Earth system models of intermediate complexity. Rates of C addition are comparable to those during the PETM (ca. 0.5-1 Pg C per year).

Hedges Group
Timetrees are evolutionary trees (phylogenies) scaled to geologic time. The NAI research of Blair Hedges mainly concerns the refinement of the timetree of life. This information helps to relate chemical and geological biomarkers in the rock record to organisms that may have produced those biomarkers. In turn this provides a better understanding of biosphere evolution, and helps to identify possibly universal mechanisms and pathways applicable to life on other planets. In 2009, the Hedges lab produced 12 publications bearing on this goal, including an edited book covering all of life down to the family level (S. B. Hedges & S. Kumar, The Timetree of Life, Oxford U Press). The entire book is freely available as PDFs at www.timetree.org, and there have been 450 citations to the book (and chapters) thus far in the literature. Since that project, research in the Hedges Lab has included studies on the earliest divergences in life, using protein sequence data (ongoing), and selected major events in the history of life, including prokaryotes and eukaryotes. Now, there is more emphasis on species-level divergences as well. The web database and resource TimeTree (www.timetree.org) continues to be developed by Hedges and his collaborator Sudhir Kumar (ASU), and, in 2012, it holds divergence data from 2100 published studies, about 800 studies more than in 2011. It receives approximately 250,000 page views per year and is used in K-12 and university-level education, as well as research. The database is mobile-friendly and therefore can be used with any smartphone or tablet, including Androids and iPhones/iPads. In addition, dedicated free applications were developed for Apple iPhones and iPads, and these have been updated as necessary in 2012. All of these contain a detailed geological timescale using standardized colors for geologic periods and allow users to visualize all molecular data points in this reference scale, and explore more detailed information for each data point. Custom timetree visualization is currently in development for projected release in 2013.

Kasting Group
Most of my early Earth work during this period has been devoted to elucidating the redox budget of the early Earth and to determining its implications for the initial rise of atmospheric O₂ around 2.5 Ga. This work is described in two book chapters and one paper that is currently under review at Chemical Geology (see list below). Understanding redox budgets is critical for evaluating the possibility of so-called “false positives” for life on extrasolar planets, that is, for understanding whether the spectroscopic identification of O₂ or O₃ in a planet’s atmosphere can be considered robust evidence for life.