"What is a ‘sulfur-based’ organism? Did I step in a pile of science fiction? Please clean up my brain on this one."
University of California, Los Angeles
Executive Summary
In response to the new NASA budget we redistributed the resources of the UCLA lead team of the NASA Astrobiology Institute (NAI). The result is a more focused research program that emphasizes linking astronomical studies to the evolution of our own solar system, studies of early Earth and the Earth-Moon system, and isotope ratios as biosignatures. Our progress in these areas during the course of the past year is described below.
Evolving planetary systems: Our work on characterizing young stellar systems has progressed substantially this past year. UCLA NAI team members Edward Young and Mark Morris and graduate student Rachel Smith collaborated with Caltech Hubble Fellow Klaus Pontoppidan to obtain new measurements of the oxygen isotope ratios of young stellar objects that emphasize the distinction between oxygen comprising the solar system and normal oxygen in the galaxy (Smith et al., 2007).
UCLA and NAI lead team astronomers Ben Zuckerman, Michael Jura and Brad Hansen, together with UCLA graduate student Carl Melis and Kiel (Germany) graduate student Detlev Koester, report this year that a white dwarf known as GD 362 (in the constellation Hercules) was polluted by a large asteroid that had a significant impact on the chemistry of the star’s atmosphere (Zuckerman et al., 2007). The pollution resembles closely the compositions of rocky bodies in the inner part of our solar system. This work shows that Earth-like planets may have once existed around such an object. The study offers a preview of what may become of our own solar system five billion years from now.
Early Earth: Impacts may well have played an important role in the development of life on Earth billions of years ago. UCLA team members Don Lowe (Stanford) and Gary Byerly (Louisiana State) report their discovery of a new spherule bed in the Barberton Belt (Africa). They note that this is the 6th major impact unit discovered to date in this Precambrian rock belt. The bed is interesting because it is overlain by a sandstone unit containing detrital zircon crystals that are the oldest zircons reported from this cratonic region of Africa, representing crust that is 3.72 billion years old. Clearly the Earth was still subject to major continuing bombardment and excavation of deep crustal units not otherwise available for erosion as late as 3.2 billion years ago. This work is being written up for publication at present.
Establishing the roots of the tree of life serves as a means for synthesizing geological, climatological, and paleontological events with genomics. Results can elucidate the triggers for the development of life on Earth. UCLA team member Jim Lake and his group in UCLA's Department of Molecular, Cellular, and Developmental Biology report 2 evidence that the root of the tree of life is within the eubacteria (Skophammer et al., 2007). The importance of this finding, according to Lake, is that some eubacteria have been present on the Earth longer than the Archea. The paper is based on genomic analyses using indels (interstions and deletions) in genes to provide information on the location of the root. Studies such as these may one day determine the root more accurately.
The veracity of the oldest signature of life on Earth has remained controversial since the initial discovery by the group from UCLA in 1996. This year UCLA team members Kevin McKeegan, Anatoli Kudryavtsev and Bill Schopf reported discovery of graphite inclusions in apatite from 3830 million year old rocks from Akilia, Greenland that substantiate the original finding. The carbon isotopic composition of one such graphite inclusion was measured by secondary ion mass spectrometry and was found to have a δ13C value of –29‰ ± 4‰ (McKeegan et al., 2007). Such a low value is taken to be indicative of biological processing of carbon. These results are in agreement with earlier analyses obtained in 1996 and so confirm the original discovery of a biosignature in apatites from these rocks. The results are consistent with the hypothesis that graphitecontaining apatite grains from the 3830 Ma Akilia metasediments represent chemical fossils of early life.
This year saw completion of a project to obtain a high-resolution carbon isotope stratigraphy across a well-preserved section of the Precambrian-Cambrian boundary from the northwestern Siberian platform. UCLA team members Bruce Runnegar, Edward Young and Karen Ziegler, together with former team member Artem Kouchinsky and colleagues from the Institute of Physics of the Earth in Moscow, Russia, published the high-resolution carbon isotope section that shows secular oscillations in carbon isotope ratios (δ13C) with amplitudes of several per cent (Kouchinsky et al., 2007). These swings in carbon isotope ratios are being used to facilitate correlations between sections worldwide and to establish links between environmental changes and the rise of metazoan life.
The timing of differentiation (i.e., crust formation) of early Earth and Moon can be constrained by high-precision measurements of isotope ratios of Hf in ancient zircons. This past year UCLA team members Mark Harrison, Kevin McKeegan and Edward Young established the capability to use our laser ablation multiple collection plasma source mass spectrometer system to measure Hf isotopes in ancient zircons. We have succeeded in showing that the technique works well at UCLA and analysis of samples will begin within a month of this writing.
Isotope biosignatures: This past year the UCLA team continued to engage in studies of iron isotope fractionation in systems devoid of biological activity. The goal is to establish the degree to which Fe isotope fractionation can be used as a biosignature. Team members Edwin Schauble, Pam Hill, Anat Shahar and Edward Young are in the final stages of completion (manuscript in preparation) of an experimental study of equilibrium Fe isotope fractionation in ferric aquo-chloro complexes. In this work the Fe-Cl-H2O system is used as a simple analogue in which to explore the large variety of iron-ligand compounds that exist in nature and the isotopic fractionations that may exist between them. The Fe-ligand compounds in this study are surrogates for not only chlorides and sulfides, but also simple organic acids. Results of the study show that 3 substantial equilibrium Fe isotope fractionation among aqueous ferric chloride complexes (Hill et al., 2006a; Hill et al., 2006b). These fractionations have nothing to do with changes in oxidation state or with biological activity. On this basis we conclude that Fe isotope fractionation on the per mil level is not a biosignature in all circumstances, nor is it a sure sign of changes in oxidation state of iron.
With recognition of the importance of sulfate formation on Mars, there is a need to understand the roles that water, atmospheric gases, and biota play in producing sulfates. One convenient marker for tracing the chemical pathways is the isotopic composition of oxygen. On Earth, the oxygen isotopic composition of tropospheric oxygen is distinct from that of waters in having a low Δ17O (Δ17O refers to the departure from normal massdependent fractionation). UCLA PI Young and his stable isotope group at UCLA have been collaborating with Max Coleman (JPL) to use Δ17O as a tracer of the sources of oxygen during sulfate formation both in the presence of microbes (Acidithiobacillus ferroxidans) and in the absence of microbes. Our results thus far show that conventional thinking about the relative importance of atmospheric and aqueous sources of oxygen are not always correct. In particular, we find that oxygen from water is more dominant than predicted and sulfite is an important mediator for isotope fractionation. We also find that there is a source of +Δ17O oxygen that is as yet unaccounted for in both experiments and in natural samples. We will be searching for this source in the coming months.
References cited
Hill PS, Schauble EA, Shahar A, Tonui E, Young ED (2006a) Ab initio and experimental studies of equilibrium isotopic fractionation in aqueous ferric chloride complexes. In prep.
Hill PS, Schauble EA, Shahar A, Tonui E, Young ED (2006b) Ab initio and experimental studies of equilibrium isotopic fractionation in aqueous ferric chloride complexes. Geochimica et Cosmochimica Acta 70, (18): A251
Kouchinsky A, Bengtson S, Pavlov V, Runnegar B, Torssander P, Young E, Ziegler K (2007) Carbon isotope stratigraphy of the Precambrian-Cambrian Sukharikha River section, northwestern Siberian platform. Geological Magazine 144, (4): 609-618
McKeegan KD, Kudryavtsev AB, Schopf JW (2007) Raman and ion microscopic imagery of graphitic inclusions in apatite from older than 3830 Ma Akilia supracrustal rocks, west Greenland. Geology 35: 591-594
Skophammer RG, Servin JA, Herbold CW, Lake JA (2007) Evidence for a grampositive, eubacterial root of the tree of life. Molecular Biology and Evolution 24: 1761-1768
Smith R, Pontoppidan KM, Young ED, Morris MR, van Dishoeck EF (2007) Detection of rare CO isotopologues in protostellar disks: an infrared investigation of molecular self shielding. The Chronology of Meteorites and the Early Solar System, 158-159
Zuckerman B, Koester D, Melis C, Hansen B, Jura M (2007) The chemical composition of an extrasolar minor planet. Astrophysical Journal(in press) 4
