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2009 Annual Science Report

Pennsylvania State University Reporting  |  JUL 2008 – AUG 2009

Executive Summary

The Penn State Astrobiology Research Center (PSARC) has charted a new direction focused on the recognition and characterization of microbial life, past and present. At the core of Astrobiology and at the forefront of NASA goals is the construction of a fundamental scientific knowledge base that enables the recognition of signatures of life on the early Earth, in extreme environments, and in extraterrestrial settings. Our new work focuses on four major projects: (1) Developing New Biosignatures, (2) Biosignatures in relevant microbial ecosystems, (3) Biosignatures in ancient rocks, and (4) Biosignatures in extraterrestrial settings.

Developing New Biosignatures
The development and experimental testing of potential indicators of life is essential for providing a critical scientific basis for the exploration of life in the cosmos. In microbial cultures, potential new biosignatures can be found among isotopic ratios, elemental compositions, and chemical changes to the growth media. Additionally, life can be detected and investigated in natural systems by directing cutting-edge instrumentation towards the investigation of microbial cells, microbial fossils, and microbial geochemical products. Our efforts are focused on creating innovative approaches for the analyses of cells and other organic material, finding ways in which metal abundances and isotope systems reflect life, and developing creative approaches for using environmental DNA to study present and past life. For our various subprojects related to developing new biosignatures, we have made great progress this year.

Raman studies of preserved kerogen in Eocene plant fossils resulted in a paper in Organic Geochemisry (Czaja et al., 2009). Also, this year new Raman studies were initiated with various collaborators. For our work on weathering patterns as a biosignature, Hausrath et al. 2009 showed that citrate-mediated release of individual elements from granite and basalt correlated with the stability constant for the element-citrate complex. Our work on weathering continues including work on Svalbard samples. We are also starting work on biomineralization of jarosite, For PSARC efforts directed at isotopic biosignatures in minerals, Co-PIs Matt Fantle and Jenn Macalady worked on field sampling of minerals and ground waters in a sulfidic, carbonate-hosted cave microbial ecosystem. Our research using Methanosarcina to develop microbial biosignatures started up with two lines of inquire. First, we are looking at methyl-sulfide formation and second, we are investigating poly-phosphate production. For work on abiotic synthesis of organics, graduate student Karen Whelly initiated new experiments and spent the summer at NASA Goddard with Jason Dworkin analyzing the products. We also published several papers outlining advances in the application of secondary ion mass spectrometry on microorganisms (House et al., 2009; Orphan et al., 2009; Orphan and House, 2009). The PSARC group working on developing DNA as a biosignature had two lines of inquire. Co-PI Beth Shapiro, her graduate student, and their collaborators headed to the Yukon Territory to scout out new sites for sampling new session (Figure 1). Recently, Dr. Shapiro published two important papers on ancient DNA preservation (Drummond et al., 2009; Collins et al., 2009), and was also awarded a 2009 Macarthur Fellowship. PSARC graduate student Mandi Martino continued work using Equatorial Pacific marine sediments on a new method for amplifying DNA from very low quantities in a way that is suitable for metagenomics.

Biosignatures in relevant microbial ecosystems

PSARC is investigating microbial life in some of Earth’s most mission-relevant ecosystems. These environments include the Dead Sea, the Chesapeake impact structure, the methane seeps of the Eel River Basin, and Greenland glacier ice. PSARC is targeting environments that, when studied, provide fundamental information that can serve as the basis for future solar system exploration. Combining our expertise in molecular biology, geochemistry, microbiology, and metagenomics, we propose to decipher the microbiology, fossilization processes, and recoverable biosignatures from these mission-relevant environments.

Graduate student Moshe Rhodes continues to work on samples from the Dead Sea. He has also travel there for field work again this year. He has an initial paper submitted. The paper deals with amino acid bias of hypersaline genomes. It also uses sequence analysis coupled with amino acid bias to investigate which microbial groups are indigenous to the Dead Sea and what microorganisms have transferred genetic material to Dead Sea microbiota. For our work on subsurface life in impact basins, we have a new student initiating work on the project this year. Our work on marine methane seeps has gone very well this year. We published a paper in Science showing methane oxidation linked to manganese and iron reduction (Beal et al., 2009). We have also made good progress studying the possibility of preserving cells in authigenic carbonates. The Brenchley laboratory and colleagues have characterized two novel ultramicrobacterial species isolated from Greenland ice (Figure 2; Lovelad-Curtze et al., 2009a; Lovelad-Curtze et al., 2009b). They also published a comparison of microbial diversity at different depths of a Greenland ice core in relation to climate at the time of deposition (Miteva et al., 2009). These results suggested that microbiological records in ice cores may provide exciting new information about past climates and underscored the importance of further searching for spatial and temporal diversity patterns that could lead to microbial markers as new tracers of specific past climates. We anticipate improving our understanding on how local and global climate processes may have shaped the composition of the microbial populations transported and deposited over Greenland. As a part of our work on biosignatures in modern environments, we have also investigated biosignatures in both pelagosite and anoxic biofilms.

Biosignatures in ancient rocks

The Earth’s Archean and Proterozoic eons offer the best opportunity for investigating a microbial world, such as might be found elsewhere in the cosmos. The ancient record on Earth provides an opportunity to see what geochemical signatures are produced by microbial life and how these signatures are preserved for geological time. Researchers have recognized a variety of mineralogical and geochemical characteristics in ancient rocks (sedimentary and igneous rocks; paleosols) that may be used as indicators of: (i) specific types of organisms that lived in the oceans, lakes and on land; and (ii) their environmental conditions (e.g., climate; atmospheric and oceanic chemistry). Our project addresses the following questions: Are some or all of these characteristics true or false signatures of organisms and/or indicators of specific environmental conditions? Do a “biosignature” in a specific geologic formation represent a local or global phenomenon? How are the biosignatures on Mars and other planets expected to be similar to (or different from) those in ancient terrestrial rocks?

We have found that primary hematite formed in an oxygenated sea 3.46 billion years ago. During this reporting period, we (Hoashi et al., 2009) published a paper in Nature Geoscience, reporting a discovery of primary hematite (Fe2O3) crystals in the 3.46 Ga old Marble Bar Chert/Jasper Formation in ABDP #1 drill hole. Also, analyses of redox sensitive elements (e.g., U, Cr, V, Mo, Mn, Fe, Ce) in 3.43 Ga altered basalts in ABDP #1 drill hole and in other submarine basalts of ~3.5-3.2 Ga in age indicate that the behavior of the elements in these rocks were essentially the same as those in altered submarine basalts in modern oceans. Work by PSARC graduate student Lev Horodyskyj has found evidence for the early life on land. He has investigated middle Cambrian rocks for an active role for fungi and bryophytes in soil weathering, using cores from Iowa and South Dakota. Kump and Arthur and their students have progressed on work involving the Great Oxidation Event and Proterozoic “Euxinic” Oceans. They have focused their initial foray into Proterozoic environmental evolution on the FAR-DEEP drill cores collected from Fennoscandia in 2008 with funding from NAI, NSF, the International Continental Drilling Program, and the Norwegian and German governments. We have established coarse-resolution C and N isotope stratigraphies for the critical “Shunga” interval of exceedingly high organic carbon deposition. We have preliminary evidence that the sediments were deposited in an anoxic, sulfidic ocean basin, perhaps the first hint of the “Canfield Ocean”, the putative sustained euxinic Proterozoic oceanic state.
PSARC researchers have also made great progress on the mass-independent fractionation of sulfur. The presence of MIF-S signatures in many (but not all) sedimentary rocks older than ~2.4 Ga and the virtual absence of MIF-S in younger rocks have been considered by many investigators as unequivocal evidence for a dramatic change from an anoxic to oxic atmosphere. This is because, until recently, the only known mechanism to cause MIF-S signatures in natural samples has been the UV photolysis of volcanic SO2 in an O2-poor atmosphere. However, we (Watanabe, Farquhar and Ohmoto) reported in a March issue of Science that reactions between amino acid powder and sulfate powder (+ H2O) at 150-200°C generated reduced S compounds (H2S and polysulfides) with significant MIF-S signatures. In separate work, Co-PI Lyons has worked to determine whether SO2 photolysis in the atmosphere can explain the S-MIF record observed in Archean and Paleoproterozoic rocks. We have determined and published the range of mass-dependent isotope fractionation that could be expected, and demonstrated that recent low-resolution SO2 isotopologue spectra yield incorrect S-MIF results (Lyons et al., 2009). We are working to measure isotopic SO2 spectra that can be used for accurate calculation of S-MIF effects during SO2 photolysis. This is necessary for Archean atmospheric models of sulfur isotope processes.
Finally, PSARC has also worked on the genomic history of biosignatures. In January, Hedges and coworkers published the results of a large study showing that two-thirds of prokaryotes had a common ancestor that lived on land about 3 billion years ago. In March we published a book, Timetree of Life (Hedges and Kumar, Eds.; Oxford) with 81 chapters, 10 of which were authored or coauthored by Hedges, and most of those involved new analyses.

Biosignatures in extraterrestrial settings

Our work on atmospheres and climate of young terrestrial planets is proceeding well. We have at least two papers on a NO2 warming mechanism for terrestrial planets being prepared for submission. Recently, we have been concentrating on atmospheric SO2. The work includes deriving thermal-IR absorption coefficients for SO2 and modeling so that we can calculate surface temperature as a function of CO2 partial pressure and SO2 outgassing rate. Our work on the magmatism on young terrestrial planets is also going well. Ohmoto presented a paper at the Goldschmidt Conference, suggesting that the volcanic and hydrothermal inputs from the mantle to the atmosphere on early Earth and Mars were dominated by CH4, and that the early life utilized CH4, rather than CO2. A collaborative assessment of CO self-shielding in the solar nebula and the presolar molecular cloud showed that 3 environments are viable locations for producing oxygen isotope fractionation consistent with the CAI mixing line. N2 self-shielding can be a source of large 15N enrichment.

Publications

  • Battistuzzi, F. U., & Hedges, S. B. (2008). A Major Clade of Prokaryotes with Ancient Adaptations to Life on Land. Molecular Biology and Evolution, 26(2), 335–343. doi:10.1093/molbev/msn247

  • Beal, E. J., House, C. H., & Orphan, V. J. (2009). Manganese- and Iron-Dependent Marine Methane Oxidation. Science, 325(5937), 184–187. doi:10.1126/science.1169984

  • Cameron, V., Vance, D., Archer, C., & House, C. H. (2009). A biomarker based on the stable isotopes of nickel. Proceedings of the National Academy of Sciences, 106(27), 10944–10948. doi:10.1073/pnas.0900726106

  • Collins, M. J., Penkman, K. E. H., Rohland, N., Shapiro, B., Dobberstein, R. C., Ritz-Timme, S., & Hofreiter, M. (2009). Is amino acid racemization a useful tool for screening for ancient DNA in bone?. Proceedings of the Royal Society B: Biological Sciences, 276(1669), 2971–2977. doi:10.1098/rspb.2009.0563

  • Czaja, A. D., Kudryavtsev, A. B., Cody, G. D., & William Schopf, J. (2009). Characterization of permineralized kerogen from an Eocene fossil fern. Organic Geochemistry, 40(3), 353–364. doi:10.1016/j.orggeochem.2008.12.002

  • Hausrath, E. M., Neaman, A., & Brantley, S. L. (2009). Elemental release rates from dissolving basalt and granite with and without organic ligands. American Journal of Science, 309(8), 633–660. doi:10.2475/08.2009.01

  • Hausrath, E. M., Treiman, A. H., Vicenzi, E., Bish, D. L., Blake, D., Sarrazin, P., … Brantley, S. L. (2008). Short- and Long-Term Olivine Weathering in Svalbard: Implications for Mars. Astrobiology, 8(6), 1079–1092. doi:10.1089/ast.2007.0195

  • Hedges, S. B., Marin, J., Suleski, M., Paymer, M., & Kumar, S. (2015). Tree of Life Reveals Clock-Like Speciation and Diversification. Molecular Biology and Evolution, 32(4), 835–845. doi:10.1093/molbev/msv037

  • Hedges, S. B., Marin, J., Suleski, M., Paymer, M., & Kumar, S. (2015). Tree of Life Reveals Clock-Like Speciation and Diversification. Molecular Biology and Evolution, 32(4), 835–845. doi:10.1093/molbev/msv037

  • Hoashi, M., Bevacqua, D. C., Otake, T., Watanabe, Y., Hickman, A. H., Utsunomiya, S., & Ohmoto, H. (2009). Primary haematite formation in an oxygenated sea 3.46 billion years ago. Nature Geosci, 2(4), 301–306. doi:10.1038/ngeo465

  • House, C. H. (2009). The Tree of Life Viewed Through the Contents of Genomes. Methods in Molecular Biology, None, 141–161. doi:10.1007/978-1-60327-853-9_8

  • House, C. H., Orphan, V. J., Turk, K. A., Thomas, B., Pernthaler, A., Vrentas, J. M., & Joye, S. B. (2009). Extensive carbon isotopic heterogeneity among methane seep microbiota. Environmental Microbiology, 11(9), 2207–2215. doi:10.1111/j.1462-2920.2009.01934.x

  • Jones, D. S., Tobler, D. J., Schaperdoth, I., Mainiero, M., & MacAlady, J. L. (2010). Community Structure of Subsurface Biofilms in the Thermal Sulfidic Caves of Acquasanta Terme, Italy. Applied and Environmental Microbiology, 76(17), 5902–5910. doi:10.1128/aem.00647-10

  • Loveland-Curtze, J., Miteva, V. I., & Brenchley, J. E. (2009). Herminiimonas glaciei sp. nov., a novel ultramicrobacterium from 3042 m deep Greenland glacial ice. INTERNATIONAL JOURNAL OF SYSTEMATIC AND EVOLUTIONARY MICROBIOLOGY, 59(6), 1272–1277. doi:10.1099/ijs.0.001685-0

  • Loveland-Curtze, J., Miteva, V., & Brenchley, J. (2009). Novel ultramicrobacterial isolates from a deep Greenland ice core represent a proposed new species, Chryseobacterium greenlandense sp. nov.. Extremophiles, 14(1), 61–69. doi:10.1007/s00792-009-0287-6

  • Luo, G., Kump, L. R., Wang, Y., Tong, J., Arthur, M. A., Yang, H., … Xie, S. (2010). Isotopic evidence for an anomalously low oceanic sulfate concentration following end-Permian mass extinction. Earth and Planetary Science Letters, 300(1-2), 101–111. doi:10.1016/j.epsl.2010.09.041

  • Lyons, J. R. (2009). Atmospherically-derived mass-independent sulfur isotope signatures, and incorporation into sediments. Chemical Geology, 267(3-4), 164–174. doi:10.1016/j.chemgeo.2009.03.027

  • Lyons, J. R., Bergin, E. A., Ciesla, F. J., Davis, A. M., Desch, S. J., Hashizume, K., & Lee, J-E. (2009). Timescales for the evolution of oxygen isotope compositions in the solar nebula. Geochimica et Cosmochimica Acta, 73(17), 4998–5017. doi:10.1016/j.gca.2009.01.041

  • Lyons, J. R., Lewis, R. S., & Clayton, R. N. (2009). Comment on “Experimental Test of Self-Shielding in Vacuum Ultraviolet Photodissociation of CO”. Science, 324(5934), 1516–1516. doi:10.1126/science.1167717

  • Miteva, V., Teacher, C., Sowers, T., & Brenchley, J. (2009). Comparison of the microbial diversity at different depths of the GISP2 Greenland ice core in relationship to deposition climates. Environmental Microbiology, 11(3), 640–656. doi:10.1111/j.1462-2920.2008.01835.x

  • Morrow, A. L., Luhman, K. L., Espaillat, C., d’Alessio, P., Adame, L., Calvet, N., … Bohac, C. J. (2008). Observations of Disks around Brown Dwarfs in the TW Hydra Association with the Spitzer Infrared Spectrograph. The Astrophysical Journal, 676(2), L143–L146. doi:10.1086/587462

  • Orphan, V. J., & House, C. H. (2009). Geobiological investigations using secondary ion mass spectrometry: microanalysis of extant and paleo-microbial processes. Geobiology, 7(3), 360–372. doi:10.1111/j.1472-4669.2009.00201.x

  • Orphan, V. J., Turk, K. A., Green, A. M., & House, C. H. (2009). Patterns of 15 N assimilation and growth of methanotrophic ANME-2 archaea and sulfate-reducing bacteria within structured syntrophic consortia revealed by FISH-SIMS. Environmental Microbiology, 11(7), 1777–1791. doi:10.1111/j.1462-2920.2009.01903.x

  • Rambaut, A., Ho, S. Y. W., Drummond, A. J., & Shapiro, B. (2008). Accommodating the Effect of Ancient DNA Damage on Inferences of Demographic Histories. Molecular Biology and Evolution, 26(2), 245–248. doi:10.1093/molbev/msn256

  • Rhodes, M. E., Fitz-Gibbon, S. T., Oren, A., & House, C. H. (2010). Amino acid signatures of salinity on an environmental scale with a focus on the Dead Sea. Environmental Microbiology, 12(9), 2613–2623. doi:10.1111/j.1462-2920.2010.02232.x

  • Schopf, J. W. (2009). The hunt for Precambrian fossils: An abbreviated genealogy of the science. Precambrian Research, 173(1-4), 4–9. doi:10.1016/j.precamres.2009.01.003

  • Schopf, J. W., & Bottjer, D. J. (2009). World summit on ancient microscopic fossils. Precambrian Research, 173(1-4), 1–3. doi:10.1016/j.precamres.2009.06.001

  • Schopf, J. W., & Kudryavtsev, A. B. (2009). Confocal laser scanning microscopy and Raman imagery of ancient microscopic fossils. Precambrian Research, 173(1-4), 39–49. doi:10.1016/j.precamres.2009.02.007

  • Tian, F., Kasting, J. F., & Solomon, S. C. (2009). Thermal escape of carbon from the early Martian atmosphere. Geophysical Research Letters, 36(2), n/a–n/a. doi:10.1029/2008gl036513

  • Wander, M. C. F., & Schoonen, M. A. A. (2008). Reduction of N2 by Fe2+ via Homogeneous and Heterogeneous Reactions. Orig Life Evol Biosph, 38(2), 127–137. doi:10.1007/s11084-008-9122-9

  • Wander, M. C. F., Kubicki, J. D., & Schoonen, M. A. A. (2008). Reduction of N2 by Fe2+ via Homogeneous and Heterogeneous Reactions Part 2: The Role of Metal Binding in Activating N2 for Reduction; a Requirement for Both Pre-biotic and Biological Mechanisms. Orig Life Evol Biosph, 38(3), 195–209. doi:10.1007/s11084-008-9133-6

  • Young, S. A., Saltzman, M. R., Foland, K. A., Linder, J. S., & Kump, L. R. (2009). A major drop in seawater 87Sr/86Sr during the Middle Ordovician (Darriwilian): Links to volcanism and climate?. Geology, 37(10), 951–954. doi:10.1130/g30152a.1

  • Battistuzzi, F.U. & Hedges, S.B.  (2009).  Archaebacteria.  In: Hedges, S.B. & Kumar, S. (Eds.).  The Timetree of Life.  New York: Oxford University Press.
  • Battistuzzi, F.U. & Hedges, S.B.  (2009).  Eubacteria.  In: Hedges, S.B. & Kumar, S. (Eds.).  The Timetree of Life.  New York: Oxford University press.
  • Beal, E.J.  (2009).  Geochemical Requirements of the Anaerobic Oxidation of Methane in the Eel River Basin.  Geosciences.  The Pennsylvannia State University.
  • Bhattacharya, D., Yoon, H-S., Hedges, S.B. & Hackett, J.D.  (2009).  Eukaryotes (Eukaryota).  In: Hedges, S.B. & Kumar, S. (Eds.).  The Timetree of Life.  New York: Oxford University Press.
  • Hedges, S.B. & Kumar, S.  (2009).  Timetree (www.timetree.org) [Online].  Website: www.timetree.org.
  • Hedges, S.B. & Vidal, N.  (2009).  Lizards, snakes, and amphisbaenians (Squamata).  In: Hedges, S.B. & Kumar, S. (Eds.).  The Timetree of Life.  New York: Oxford University Press.
  • Hedges, S.B.  (2009).  Life.  In: Hedges, S.B. & Kumar, S. (Eds.).  The Timetree of Life.  New York: Oxford University Press.
  • Hedges, S.B.  (2009).  Vertebrates (Vertebrata).  In: Hedges, S.B. & Kumar, S. (Eds.).  The Timetree of Life.  New York: Oxford University Press.
  • Heinicke, M.P.  (2009).  A molecular phylogenetic perspective on the evolutionary history of terraranan frogs, a vertebrate mega-radiation.  Department of Biology.  University Park: The Pennsylvania State University.
  • Heinicke, M.P., Naylor, J.P. & Hedges, S.B.  (2009).  Cartilaginous fishes (Chondrichthyes).  In: Hedges, S.B. & Kumar, S. (Eds.).  The Timetree of Life.  New York: Oxford University Press.
  • Heinicke, M.P., Sander, J.M. & Hedges, S.B.  (2009).  Lungfishes (Dipnoi).  In: Hedges, S.B. & Kumar, S. (Eds.).  The Timetree of Life.  New York: Oxford University Press.
  • Hoashi, M., Watanabe, Y. & Ohmoto, H.  (2009).  Primary hematite formation in an oxygenated deep sea 3.46 billion years ago.  Goldschmidt conference.  Davos, Switzerland.
  • Horodyskyj, L.  (2009).  Soil formation and terrestrial biosignatures in the Middle Cambrian [electronic resource].  Geosciences.  University Park: The Pennsylvania State University.
  • Johnson, I.  (2009).  The Earth’s oldest (~3.4 GA) paleosol at Trendall Ridge in the North Pole Dome region of the Eastern Pilbara Craton, Western Australia.  Geosciences.  University Park: The Pennsylvania State University.
  • Johnson, I., Watanabe, Y., Stewart, B. & Ohmoto, H.  (2009).  Earth’s oldest (~3.4 Ga) lateritic paleosol in he Pilbara Craton, Western Australia.  Goldschmidt conference.  Davos, Switzerland.
  • Jones, D.S., Schaperdoth, I. & MacAlady, J.L.  (2010).  Microbial communities from extremely acidic sulfidic cave biofilms.  Environmental Microbiology, submitted.
  • Kimball, B.  (2009).  Biogeochemical Cycling of Copper in Acid Mine Drainage.  Geosciences.  University Park: The Pennsylvania State University.
  • Kump, L.R., Condie, K.C. & Arthur, M.A.  (2009).  Co-evolutionary implications of alternative models of plate tectonics in Earth history.  Geol. Soc. America Ann. Meeting.  Portland, OR.
  • Kump, L.R., Meyer, K.M., Ridgwell, A. & Payne, J.L.  (2009).  Biosphere response to volcanic CO 2 release: Siberian Traps and End-Permian Mass Extinction.  Geol. Soc. America Ann. Meeting.  Portland, OR.
  • Lal, A.K.  (2008).  Origin of life.  Astrophysics and Space Sceince, 317: 267-278.
  • Meyer, K.M., Kump, L.R., MacAlady, J., Schaperdoth, I. & Freeman, K.  (in review).  Benthic production of a putative planktonic biomarker.  Geobiology.
  • Miteva, V.  (2009).  Microorganisms associated with glaciers.  In: V. P. Singh, P.S.a.U.K.H. (Eds.).  Encyclopedia of Snow, Ice and Glaciers.  Springer (submitted).
  • Moody, K.  (2009).  Three Body Dynamics in Dense Gravitational Systems.  Department of Astronomy.  University Park: The Pennsylvania State University.
  • Ohmoto, H.  (2009).  Redox evolution of volcanic gas through geologic time.  Goldschmidt conference.  Davos, Switzerland.
  • Ohmoto, H., Bevacqua, D.C. & Watanabe, Y.  (2009).  Alteration of submarine volcanic rocks in oxygenated Archean oceans.  American Geophysical Union Annual Meeting.  San Francisco, CA.
  • Saltzman, M.R., Young, S.A., Kump, L.R., Foland, K.A. & Leslie, S.  (2009).  The Late Ordovician glaciation and mass extenction: Relation to basaltic weathering and volcanic degassing.  Geol. Soc. America Ann. Meeting.  Portland, OR.
  • Schopf, J.W. & Bottjer, D.J.  (2009).  World Summit on Ancient Microscopic Fossils, Special Issue.  Vol. 173.  Precambrian Research.
  • Schopf, J.W.  (2009).  Emergence of Precambrian paleobiology: a new field of science.  In: Sepkoski, D. & Ruse, M. (Eds.).  The Paleobiological Revolution, Essays on the Growth of Modern Paleontology.  Chicago: University of Chicago Press.
  • Schopf, J.W.  (2009).  Organismal evolution and radiation before the Cambrian.  In: Ruse, M., Travis, J. & Zinser, J. (Eds.).  Evolution: The First 4 Billion Years.  Cambridge: Harvard University Press.
  • Schopf, J.W.  (2009).  Paleontology, Microbial.  In: Lederberg, J. & Schaechter, M. (Eds.).  Encyclopedia of Microbiology, 3rd Edition.  Amsterdam: Elsevier.
  • Urban, N., Bralower, T., Keller, K. & Kump, L.R.  (2009).  Statistical interpretation of the rate of carbon isotope changes at the onset of the Paleocene-Eocene Thermal Maximum.  American Geophysical Union Fall Meeting.  San Francisco, CA.
  • Vidal, N., Rage, J-C., Coulous, A. & Hedges, S.B.  (2009).  Snakes (Serpentes).  In: Hedges, S.B. & Kumar, S. (Eds.).  The Timetree of Life.  New York: Oxford University Press.
  • Yamaguchi, K.E., Kato, Y., Nakamura, K., Suzuki, K., Watanabe, Y., Nedachi, M. & Ohmoto, H.  (2009).  REE+Y geochemistry of the 3.46 Ga Marble Bar Chert recovered by the Archean Biosphere Drilling Project.  Goldschmidt Conference.  Davos, Switzerland.
  • Zerkle, A.L., Junium, C.K., Canfield, D.E. & House, C.H.  (2008).  Production of 15 N-depleted biomass during cyanobacterial N 2 -fixation at high Fe concentrations.  J. Geophys. Res, 113(G03024).