Vestiges of a Beginning: Clues to the Emergent
Biosphere Recorded in the Oldest Known Sedimentary Rocks
Mojzsis, T. Mark
Harrison, Department of Earth and Space Sciences and IGPP
Center for Astrobiology, University of California, Los Angeles, CA
Recent explorations of the oldest known rocks of marine
sedimentary origin from the southwestern coast of Greenland suggest
that they preserve a biogeochemical record of early life. On the
basis of the age of these rocks, the emergence of the biosphere
appears to overlap with a period of intense global bombardment. This
finding could also be consistent with evidence from molecular
biology that places the ancestry of primitive bacteria living in
extreme thermal environments near the last common ancestor of all
known life. To make new advances in understanding the physical,
chemical, and biological states of early environments for life
through this unique Greenland record, we must fully exploit the
spectrum of biosignatures available; these efforts must also be
coupled with an understanding of the complex geologic history of the
rocks hosting these signatures. The new methods employed here will
eventually become applicable to other worlds when samples become
available for study early in the 21st century.
If the presence of a stable liquid water veneer on Earth is
necessary for the origin, evolution, and propagation of life (Fig. 1), then
the best sources of information to understand this phenomenon are
water-laid sediments preserved in the geologic record. Ancient
(>3800 Ma) water-sculpted terrains have been recognized on Mars,
and liquid water appears to exist beneath the icy crust of Jupiter's
moon Europa. As long as liquid water, energy sources, and organic
building blocks were present, life could have emerged on these
worlds as well. The discovery of terrestrial marine sediments
approaching 4 b.y. in age (Nutman et al., 1996) provides a
remarkable opportunity to investigate possible environments from
which life arose. Complicating this opportunity is their protracted
metamorphic history; the oldest known sediments have been
recrystallized under high-grade metamorphism (Griffin et al., 1980)
and do not contain recognizable microfossils (Bridgwater et al.,
1981). However, stable isotope fractionations of the bioessential
elements (e.g., C, N, and S) produced by different metabolic styles
can be preserved in ancient sediments (Schidlowski et al., 1983).
This permits understanding of the development of early life inferred
from chemical and isotopic information, rather than solely on
interpretation of microfossil-like shapes (e.g., Schopf, 1993; McKay
et al., 1996) (Fig. 2).
EARLY EARTH–SURFACE STATE
Early Earth was likely dominated by markedly different
hydrospheric (e.g., lower pH, much higher
[Fe2+]aq) and atmospheric (e.g., much lower
pO2, much higher pCO2) conditions (Holland,
1984) and a tectonic style reflecting higher heat flow through the
crust than at present. Several factors were unique to the early
Archean surface, including a higher ultraviolet flux from a Sun 30%
less luminous at 3800 Ma than today (Kuhn et al., 1989) and impact
rates from asteroids and comets many orders of magnitude greater.
Together, these conditions would presumably have restricted the
number of suitable environments for life to emerge. The minimum ages
of some of the oldest Greenland rocks (Nutman et al., 1996, 1997)
appear to overlap in time with a period of intense impacts peaking
at 3850 ± 100 Ma as recorded on the Moon (Ryder, 1990). Thermal and
shock effects associated with the Late Heavy Bombardment era (Tera
et al., 1974) are presumed to have rendered early Earth unsuitable
for the emergence of life until after the massive bombardments
ceased (e.g., Maher and Stevenson, 1988). During this bombardment
era, conditions may have favored the survival of certain bacteria
that survive (and even thrive) in environmental extremes of
temperature, pressure, and pH before diversifying into wider
ecological niches throughout the planet. Phylogenetic studies using
highly conservative ribosomal RNA sequences reveal that the deepest
branches of life derive from "heat-loving," or thermophilic,
bacteria (Pace, 1997). Such organisms could have survived thermal
assaults from giant impacts, especially if sequestered deep in the
oceans or in rocks away from a destructive surface zone bathed both
in the intense ultraviolet radiation of the early Sun and a rain of
extraterrestrial debris ~4 b.y. ago. To better understand this early
era, we have to unravel the timing of events from a heavily modified
early Archean rock record.
EARLY ARCHEAN (>3500 Ma) HISTORY OF WEST GREENLAND
The diverse rock types present in the Isua district of West
Greenland are all contained within extensive early Archean
(3600–3900 Ma) gneisses dominantly of tonalitic-granodioritic
composition (Black et al., 1971; Nutman et al., 1996) (Fig. 3). This
multiply metamorphosed terrane, termed the Itsaq Gneiss Complex
(Nutman et al., 1996), contains <10% volcano-sedimentary enclaves
in the gneisses. The identification of water-laid rocks, such as
banded iron-formation (BIF), metamorphosed chert, metapelites, rare
graywackes, and pillow lava basalts, attests to
a hydrosphere with an already mature cycling of sediments by
~3900 Ma. The paleogeography of this sedimentary system remains
poorly understood (e.g., oceanic arc-back-arc basin or deep abyssal
basins), but the apparent lack of continentally derived sediment
(Nutman et al., 1984) mitigates against the widespread presence of
nearby exposed continental crust at the time of deposition.
Banded Iron-Formation >3850 Ma on Akilia Island
oldest known sediment (Nutman et al., 1997), and the oldest known
rock with evidence of biological processes active during time of
deposition (Mojzsis et al., 1996), is a layer ~3 m thick of BIF
within a body of amphibolite on the southern tip of Akilia island,
West Greenland (Fig. 4). This
BIF on Akilia was chosen as the type locality for the Akilia
association, a term used for volcano-sedimentary enclaves found
throughout the Itsaq Gneiss Complex that are not part of the larger
and better preserved Isua supracrustal belt (McGregor and Mason,
1977). More detailed field studies through the 1980s and 1990s,
combined with geochronological work on the intruding gneisses in the
coastal regions of southern West Greenland, demonstrated that the
rocks around Godthåbsfjord (in the southwestern coastal area),
including those of the Akilia association, contain older components
(Kinny, 1986) than most rocks in the Isua district, 150 km distant.
On Akilia island itself, gneissic sheets crosscut the amphibolite
enclave containing BIF and yield U-Pb zircon ages as old as ca. 3850
Ma, interpreted as the age of crystallization of the magmatic
protolith by Nutman et al. (1996, 1997). If this interpretation is
correct, then the BIF is at least as old as 3850 Ma. Carbon isotopic
evidence of bio-organic activity during deposition of the BIF
sediments (Mojzsis et al., 1996) would then suggest that the
emergence of life on this planet occurred much earlier than
previously thought (Hayes, 1996; Holland, 1997). This age for the
Akilia island sediments is also significant because it would place
their deposition simultaneous with the Late Heavy Bombardment of the
Moon at 3800–3900 Ma, and coeval with the presence of abundant
liquid water on the surface of Mars.
Several of the Akilia gneisses that cut the BIF generally contain
three zircon age populations: ca. 3850, 3650, and 2700 Ma (Fig. 5)
(Mojzsis and Harrison, 1999; Whitehouse et al., 1999). One possible
explanation for this zircon age distribution is that the ~2700 Ma
grains are metamorphic in origin, the ~3650 Ma grains are igneous,
and the oldest grains are xenocrysts inherited from an older rock.
In the latter interpretation, the actual intrusive age of the
tonalitic protolith of the gneiss would be only 3650 Ma. For
example, Kamber and Moorbath (1998) and Whitehouse et al. (1999)
argued that the ca. 3850 Ma ages were assimilated at about 750 °C
from adjacent, zircon-bearing rocks. However, there is no identified
candidate rock for the assimilant in the whole of the Itsaq Gneiss
Complex. The granitoid protolith of the orthogneisses is
characterized by relatively low Zr contents (~120 ppm) and high
crystallization temperatures (>900 °C). The likelihood of
strongly zircon undersaturated tonalitic-granodioritic melts
(Harrison and Watson, 1983) intruding into zircon-poor rocks and
preserving widespread inherited zircon is low (Watson, 1996). Kamber
and Moorbath (1998) argued that the lack of 3850 Ma Pb-Pb ages in
feldspars from gneisses collected throughout southern West Greenland
specifically preclude a 3850 Ma protolith age for any rocks in West
Greenland. However, their conclusion overstates the clarity of
interpretations that can be drawn from Pb isotopes in feldspars.
Peak metamorphism experienced by these rocks at 3650 Ma occurred
under conditions (Griffin et al., 1980) that could permit exchange
of primitive Pb isotopes in feldspar (Cherniak, 1992) with
radiogenic Pb released from U-rich phases. McGregor (2000) pointed
out that the petrologically unrelated samples analyzed by Kamber and
Moorbath (1998) are dominated by relatively late phases of the
gneiss complex that underwent partial melting under granulite facies
metamorphism, and thus their data "do not preclude the existence of
very old (greater than or equal to 3800 Ma) rocks" on Akilia island.
Existing evidence points to a 3850 Ma age for granitoids
intruding BIF on Akilia island, but the great significance of this
age for early terrestrial evolution leads us to continue our
geochronological investigations in more detail. We are testing more
directly the hypothesis that the 3850 Ma zircon ages are the result
of xenocrystic contamination by determining in situ U-Pb ages of
zircons included in preserved, early-crystallizing phases of the
tonalite protolith. Also, the three-dimensional distribution of Pb/U
revealed by isotopic depth profiling (e.g., Grove and Harrison,
1999) and applied to the oldest zircons helps us address whether
zircon overgrowths reflect magmatic or metamorphic processes. If
these rocks indeed provide a glimpse into pre–3800 Ma Hadean Earth,
then the time of the emergence of the biosphere appears to have
coincided with a period of intense bombardment in the inner solar
system, as recorded on the Moon.
LATE HEAVY BOMBARDMENT OF THE MOON AND EARTH AT CA.
The surface of the Moon displays evidence of an intense
bombardment that took place at some time between original crust
formation and the outpourings of lava that form the dark mare
plains. The oldest of the lunar mare are dated at ~3800 Ma (see
review in McDougall and Harrison, 1999). Breccias from the lunar
highlands generally yield radiometric ages between 3800 and 3900 Ma
that are interpreted as reflecting impact resetting (Dalrymple and
Ryder, 1996). The narrow distribution of ages and rapid transition
from older, impact-dominated landforms to volcanic plains has
generally been attributed to a cataclysmic spike in impacts at
3800–3900 Ma (Tera et al., 1974; Ryder, 1990). The present evidence
is consistent with an abrupt termination of this intense activity
coinciding with nearly simultaneous creation of the large Imbrium,
6), and Schrödinger basins at 3850 Ma, or even as early as 3870
Ma (G. Ryder, personal communication). These timing details are
important for at least two reasons: An age of 3850 Ma closely
corresponds to that of the oldest sediments in West Greenland cited
above, and that age has been taken as a temporal marker horizon for
estimating the age of heavily cratered crust on Mars and Mercury.
Consequences of Bombardments to Early Life
energetic impacts would have been deleterious for near-surface
emergent ecosystems as conceptualized in the classic "Darwin Pond"
hypothesis of the origin of life. Impact "erosion" of planetary
atmospheres and wholesale destruction of hydrosphere and crust from
the most massive infalling bodies are some of the effects postulated
to have rendered the surface zone truly Hadean in character before
3800 Ma (Sleep et al., 1989). However, if life originated in deep
marine or crustal nurseries (Shock et al., 1995) and continued to
reside there until bombardment ceased, events at the surface might
have had little immediate consequence to survivability of the
To search for evidence of impacts in the geologic record, workers
have taken advantage of the geochemistry of iridium. Earth's crust
is highly depleted in Ir and other platinum-group metals relative to
its mantle, which is itself depleted relative to chondritic
meteorites by partitioning of Ir into the core. Iridium content of
sediments is therefore used as a sensitive indicator of the presence
of extraterrestrial debris (e.g., Alvarez et al., 1980). Sources of
Ir in ancient sediments might represent interplanetary dust
particles, micrometeorites, local- to world-spanning impacts,
airbursts, and products of asteroid or comet showers. If
depositional rates can be estimated, measurements of Ir in sediments
can provide information on the accretion of extraterrestrial
material during the time of deposition. On the basis of timing of
the lunar bombardment and age of the oldest terrestrial sediments,
the Akilia rocks might therefore be expected to preserve a signal of
markedly higher flux according to their precise age correlation with
the lunar bombardment record.
However, unless sedimentation rates were anomalously rapid (»1
mm/yr), thereby diluting a signal of intense impacts, the Ir
contents for all West Greenland samples studied thus far are lower
than that expected from simple models for the Late Heavy Bombardment
(Anbar et al., 1999). This observation may be explained by the fact
that the bombardment was not an era of continuous massive impacts.
It was most likely dominated by quiet conditions punctuated by
relatively infrequent episodes of extreme thermal shocks, none of
which could have erased a deep-seated biosphere already in place.
Furthermore, evidence for life from carbon isotopic distributions in
rocks from the early Archean of Greenland would appear to indicate
that life withstood the Late Heavy Bombardment, whatever its
intensity, relatively unscathed.
CHEMICAL AND ISOTOPIC FOSSILS OF EARLY LIFE
On a global scale, kinetic isotope fractionations during
enzymatic activity produce distinctive variations in stable isotope
ratios of bioessential elements (e.g., carbon, nitrogen, and sulfur)
in various reservoirs. A record of these fractionations can be
obtained by measuring the isotopic ratios of these elements in
bio-organic materials preserved in sediments. It has long been
recognized (e.g., Thode et al., 1949; Wickman, 1952; Craig, 1953)
that life systematically and significantly affects the isotopic
composition of the biologically important elements and that these
distinctive biosignatures can be preserved during diagenesis and
moderate-grade metamorphism. After more than 50 years of
investigations, geochemists now agree that postdepositional
processes tend to obscure the original carbon isotopic signal of
bio-organic material, but only toward values characteristic of
inorganic carbon (Schidlowski, 1988; Kitchen and Valley, 1995; Des
Marais, 1997). If this isotope record can survive very high grade
metamorphism, then it becomes possible to analyze the oldest
sediments for evidence of life (e.g., Mojzsis et al., 1996).
The primary metabolism of organisms on
Earth that manufacture their own food (autotrophs) imposes a kinetic
isotope fractionation that discriminates against 13C in the fixation
of atmospheric CO2 (d13CPDB = –7.9‰) and
dissolved marine HCO–3 (d13C = 0 ± 1‰). Because of the
intrinsic lower reactivity of 13C vs. 12C,
enzymatic processes discriminate against CO2 during
carbon fixation (by the enzyme ribulose bisphosphate carboxylase in
cyanobacteria and plants; Park and Epstein, 1960). These mechanisms
induce up to 5% differences in isotopic composition of bio-organic
carbon with respect to inorganic carbon sources in the atmosphere
and hydrosphere. The average isotopic composition of Phanerozoic
bio-organic matter (d13C = –27‰)
is far away from the inorganic field. While Horita and Berndt (1999)
postulated that the inorganic production of CH4 from
serpentinization could create a carbon isotope fractionation of
similar magnitude, subsequent work suggests that the evolved methane
in these experiments derives exclusively from bio-organic
contaminants in the reactant olivine (McCollom and Seewald, 1999).
No known abiotic process mimics the large carbon isotopic signal of
life in sediments.
Microbial methanogenesis is a widespread phenomenon whereby
organic matter decomposes in anaerobic sediments and in the
intestinal tracts of ruminants. The acetyl-CoA pathway — a metabolic
system utilized by methanogens — is responsible for carbon isotope
fractionations up to –40‰ from inorganic carbon (Preuß et al.,
1989). Methanogens are a group of chemosynthetic (rather than
photosynthetic) microbes that are among the most primitive life
forms (Woese, 1987). Chemoautotrophes can produce biomass with d13C values as low as –60‰ (Summons et
al., 1998). The recycling of carbon by these primitive microbes, and
the isotopically light (d13C =
–30‰ to –60‰) composition of their organic remains are considered
diagnostic of contemporary microbial habitats characterized by
extreme environments. These habitats (e.g., high salinity, high
water temperature, or variable pH) are considered analogous to some
of the earliest environments for life on Earth.
Analyses of bulk carbonaceous matter trapped within early
Archean, Proterozoic, and younger sediments has demonstrated the
ubiquity of isotopically light carbon (Schidlowski et al., 1979,
1983; Hayes et al., 1983; Des Marais, 1997; Rosing, 1999). These
results are consistent with a bio-organic origin for sedimentary
reduced carbon; yet, generalized concerns regarding later
contamination or open-system behavior during metamorphism have been
raised. While specific criticisms of this kind (e.g., Sano et al.,
1999) can be shown to be unwarranted on geologic grounds alone
(e.g., Mojzsis et al., 1999a), newly developed isotopic measurements
permit textural and mineralogical preservation of the analyzed
material and potentially transcend these two issues. Mojzsis et al.
(1996) used a high-resolution ion microprobe to analyze extremely
small (~10 mm) carbonaceous inclusions locked within mineralogic
domains that had remained stable since diagenesis (Mojzsis et al.,
1999a). Carbon isotopic values for carbonaceous inclusions in
apatite from the greater than or equal to 3770 Ma Isua sediments
(d13C = –30‰) and the greater
than or equal to 3850 Ma BIF from Akilia island (d13C = –37‰) closely match those for
similar rocks from a variety of younger, less metamorphosed rocks.
The simplest interpretation of the carbon isotopic data in
Mojzsis et al. (1996) is that the organisms responsible for the
light carbon signature in the oldest known terrestrial sediments
were metabolically complex, perhaps comprising populations of
phosphate-utilizing photoautotrophs and chemoautotrophs. These data
may point to the presence of diverse photosynthesizing,
methanogenic, and methylotrophic bacteria on Earth before 3850 Ma
(Mojzsis and Arrhenius, 1998; Mojzsis et al., 1999b). Not only had
life taken firm hold on Earth by the close of the Hadean era, but it
also appears to have evolved far enough away from its origin to
create an interpretable signature in carbon isotopes.
Our group at the University of California, Los Angeles, has
undertaken study of the stable isotope composition of individual
microfossils by the techniques described above; these studies
indicate the opening of new vistas in the isotopic paleontology of
ancient life forms where recognizable fossil microbes are preserved.
Furthermore, the utility of sulfur isotope
(32S,33S, 34S) measurements of
individual sulfide minerals in ancient rocks (Greenwood et al.,
1999) potentially provides information not only about primitive
metabolic cycling of sulfur in the biosphere, but also of unique
atmospheric fractionations and atmosphere-crust interactions early
in Earth history.
We thank Kevin McKeegan and Christopher Coath for ongoing
scientific advice, and David Des Marais, Christopher House, and an
anonymous reviewer for their constructive comments. We received
research support from the National Science Foundation's Life in
Extreme Environments (LexEn) and Earth Sciences Postdoctoral
Fellowship programs, from NASA's Astrobiology Institute, from the
Society of Economic Geologists Foundation (Hickok-Radford Award),
and from the GSA research grants program; facility support came from
the NSF Instrumentation and Facilities Program. This is a
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Manuscript received November
3, 1999; accepted February 9, 2000.
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