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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 6^th 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
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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
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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)
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