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Earth and Planetary Science L
Preservation of ~3.4–3.5 Ga microbial biomarkers in pillow lavas
and hyaloclastites from the Barberton Greenstone Belt, South Africa
Neil R. Banerjee a,b,*, Harald Furnes a, Karlis Muehlenbachs b,
Hubert Staudigel c, Maarten de Wit d
a Department of Earth Science, University of Bergen, Allegt. 41, 5007 Bergen, Norwayb Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3
c Scripps Institution of Oceanography, University of California, La Jolla, CA 92093-0225, USAd AEON and Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
Received 11 April 2005; received in revised form 3 November 2005; accepted 4 November 2005
Available online 19 December 2005
Editor: H. Elderfield
Abstract
Exceptionally well-preserved pillow lavas and inter-pillow hyaloclastites from the Barberton Greenstone Belt in South Africa
contain textural, geochemical, and isotopic biomarkers indicative of microbially mediated alteration of basaltic glass in the
Archean. The textures are micrometer-scale tubular structures interpreted to have originally formed during microbial etching of
glass along fractures. Textures of similar size, morphology, and distribution have been attributed to microbial activity and are
commonly observed in the glassy margins of pillow lavas from in situ oceanic crust and young ophiolites. The tubes from the
Barberton Greenstone Belt were preserved by precipitation of fine-grained titanite during greenschist facies metamorphism
associated with seafloor hydrothermal alteration. The presence of organic carbon along the margins of the tubes and low d13C
values of bulk-rock carbonate in formerly glassy samples support a biogenic origin for the tubes. Overprinting relationships of
secondary minerals observed in thin section indicate the tubular structures are pre-metamorphic. Overlapping metamorphic and
igneous crystallization ages thus imply the microbes colonized these rocks 3.4–3.5 Ga. Although, the search for traces of early life
on Earth has recently intensified, research has largely been confined to sedimentary rocks. Subaqueous volcanic rocks represent a
new geological setting in the search for early life that may preserve a largely unexplored Archean biomass.
D 2005 Elsevier B.V. All rights reserved.
Keywords: early life; biomarker; volcanic glass; pillow lava; greenstone belt; Archean
1. Introduction
During the last decade several studies have shown
that the upper volcanic part of the modern oceanic crust
0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2005.11.011
* Corresponding author. Present address: Department of Earth
Sciences, University of Western Ontario, London, Ontario, Canada
N6A 5B7.
E-mail address: neil.banerjee@gmail.com (N.R. Banerjee).
is a habitat for microorganisms. In this environment
microbes colonize fractures in the glassy selvages of
pillow lavas, extracting energy and/or nutrients from
the glass by dissolving it, leaving behind biomarkers
that reveal their former presence [1–12]. The biomar-
kers consist of (1) corrosion structures (commonly
filled by secondary minerals) that have textural criteria
indicative of a biogenic origin (size, morphology, dis-
tribution as populations), (2) enrichment of C, N, P, and
etters 241 (2006) 707–722
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722708
S associated with the corrosion structures, (3) charac-
teristically low d13C values of disseminated carbonate
within the altered glass rims of pillows compared to
their crystalline interiors, and (4) presence of DNA
associated with corrosion structures.
The methods developed for tracing biomarkers in
modern oceanic crust have been successfully applied to
pillow lava sections of ophiolites. The ophiolites inves-
tigated so far range in age from Cretaceous to Middle
Proterozoic, range in metamorphic grade from near
unmetamorphosed to lower amphibolite facies, and con-
tain all the principal components of a Penrose-type
ophiolite (summarized in [13]). Further, in a recent
study of pillow lavas of the ~3.2–3.5 Ga Barberton
Greenstone Belt (BGB) in South Africa, Furnes et al.
[14] reported biomarkers related to the initial alteration
of glassy pillow lava rims. Most products of biological
activity are too delicate to survive geological processes
like weathering, erosion, and dynamothermal-metamor-
phism. As such destructive processes compound through
geological time, it has proven to be increasingly difficult
to find preserved evidence for life as the age of a rock
approaches the age of the oldest rocks on Earth. Studies
of the earliest history of life are plagued also by pro-
blems of fossil preservation and poor and ambiguous
evidence for fossil material. In this paper we build upon
our previous work, present a new dataset of the biomar-
kers found in the volcanic rocks of the BGB, and stress
the importance of how the study of basaltic volcanic
rocks in Archean greenstone belts may contribute to the
discussion of the early life on Earth.
2. Basaltic glass as a geological setting for microbial
life
Biologically mediated corrosion of synthetic glass
is a well-known phenomenon [15] that has also been
proposed for the pitting of natural volcanic glass [16].
Thorseth et al. [17] first suggested that bio-corrosion
was produced by colonizing microbes that cause local
variations in pH which allows them to actively dis-
solve the natural basaltic glass substrates thereby pro-
ducing tubular structures. This process was later
verified in experiments [18–20]. The microbial disso-
lution experiments by Thorseth et al. [18] showed that
etch marks on the basaltic glass surface were produced
after a relatively short time (46 days). The etch marks
produced were of uniform size (0.3–0.5 Am in diam-
eter) and they had a chain or bcolonyQ shape, similar
to the size and arrangement of the live bacteria that
were removed from the glass surface. Although we are
unaware of any experiment that has produced long
(several tens of micrometers) tubular structures, the
experiment by Thorseth et al. [18] demonstrates the
onset of a dissolution process. We suggest that given
enough time the etching process, ultimately might
produce the long tubular structures. Over the past
decade numerous studies have shown that microbe-
sized corrosion structures are commonly produced by
biological activity in natural basaltic glasses through-
out the upper few hundreds of meters of the oceanic
crust of any age, including the oldest oceanic crust in
the western Pacific Ocean [18,2,21,3,4,22,5–7,9,11,
10,23]. These structures are very distinct and cannot
be explained by abiotic processes, as supported by
evidence from petrography, geochemistry and molec-
ular biology.
Key petrographic arguments for a biogenic origin for
the corrosion structures include their size similarity to
microbes, their biotic morphology, and distribution as
populations. In particular, these structures commonly
occur as irregular tubes that consistently originate from
fractures. The structures are also observed to bifurcate
and never occur with a symmetric counterpart on the
other side of the fracture. Geochemical evidence
includes the common enrichment of biologically im-
portant elements such as C, N, P, K, and S associated
with the microbial alteration structures (e.g. [4,6,7,11])
and characteristically low d13C values of disseminated
carbonate within microbially altered basaltic glass
[4,8,11]. Molecular arguments include the presence of
DNA associated with biological corrosion textures (e.g.
[21,4,11]). As to the timing of formation of microbial
alteration structures and subsequent filling of the struc-
tures, it is important to mention that we have found
filled tubules in the glassy rinds of Quaternary pillow
lavas (e.g. Fig. 3A of [8]). This shows that microbial
etching and subsequent filling of the empty structures
can be a penecontemporaneous process. In the absence
of abiotic explanations for these phenomena, microbial
etching is the most likely explanation for these petro-
graphic, geochemical, and biomolecular biomarkers in
the glassy margins of submarine lavas. The breadth of
these arguments and the abundance of these features
make it unlikely that microbial processes do not play an
important role during alteration of basaltic glass on the
present-day seafloor. Recent work by Lysnes et al. [24]
on the microbial community diversity in young (V1Ma) seafloor basalts has revealed eight main phyloge-
netic groups of Bacteria and one group of Archaea that
differ from those of the surrounding seawater including
autolitotrophic methanogens and iron reducing bacteria.
It should be stressed, however, that it has not yet been
possible to identify specific microbes or specific meta-
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722 709
bolic processes that cause the tubular corrosion struc-
tures described here.
3. Evidence for early life
The evidence for earliest life on Earth fall in three
main categories: chemical evidence (e.g., carbon isoto-
pic evidence), micro-morphological evidence (e.g., mi-
croscopic observation of microfossils), and macroscopic
interpretation of sedimentary structures preserved in the
rock record that are commonly associated with modern
microbial mats (e.g., stromatolites). The oldest proposed
evidence for life in the geological record traces back to
3.5–3.8 Ga and is based on chemical signatures in high-
grade schists and paragneisses of the Isua Supracrustal
Belt (ISB), Southwestern Greenland. Graphite from
these rocks and within apatite crystals has unusually
low d13C values indicative of biological fractionation
of carbon [25–30]. However, recent studies have pointed
out that low d13C values in at least some of the ISB
graphite occur in secondary carbonate veins and may
thus be also explained by abiotic processes, which brings
into question much of the evidence from Isua [31–33].
In addition, reports of low d13C signatures from
graphite inclusions in apatite crystals from ~3.85 Ga
granulite-facies rocks on Akilia island have also been
questioned [34]. However, the occurrence of low d13C
signatures in a sequence of graded metasediments inter-
preted as turbidites from the ISB [30], remains widely
accepted as biogenic and are thus possibly the oldest
chemical evidence of life on Earth.
The next-oldest evidence for life in the geological
record is based on micro-textural observations sup-
ported by laser-Raman imaging of features interpreted
as filamentous microfossils in ~3465 Ma metasedi-
ments (Apex chert) from the Pilbara Craton in South-
western Australia (e.g., [35,36]). However, Garcia-Ruiz
et al. [37] have recently shown that morphologically
similar filamentous microstructures can be generated
from abiotic processes. This specifically calls into ques-
tion the uniqueness of the biogenic interpretation for
the filaments in the Apex cherts. In addition, Brasier
et al. [38] interpreted the filamentous structures in the
Apex cherts as secondary artifacts consisting of amor-
phous graphite produced from inorganic synthesis or
organic compounds in hydrothermal veins. In contrast,
the 3472 and 3447 Ma low-grade metasediments (now
mostly cherts) from the middle and uppermost Onver-
wacht Groups of the BGB contain microstructures
and carbon isotope evidence for the presence of fos-
sil bacteria and biofilm [39–41]. Nevertheless, there is
widespread skepticism for the biogenic nature of these
micromorphs (F. Westall, personal communication
2004).
These searches for early life in Greenland, Australia,
and South Africa show very clearly that geochemical or
morphological evidence for life is controversial and
underscores the need for more and better evidence for
Archean life in the oldest rock sequences. In this paper,
we describe textures and associated geochemical data in
formerly glassy pillow lavas, a suite of rocks that has
not been previously considered in the search for Arche-
an life. This new morphological and geochemical evi-
dence provide a consistent set of criteria for biogenicity
because it is firmly based on observations of a modern
analogue in oceanic basalts that has not been credibly
explained to form through abiotic processes. This ap-
proach offers an integrated data set that is substantially
more powerful than evidence based on a single data
type.
4. Geological background and sampling locations
The Mesoarchean BGB of South Africa contains
some of the world’s oldest and best-preserved pillow
lavas [42,43]. The magmatic sequence, consisting of
the Theespruit, Komati, Hooggenoeg, and Kromberg
Formations (the Onverwacht Group) comprises 5–6 km
of predominantly basaltic and komatiitic extrusive (pil-
low lavas, minor hyaloclastite breccias and sheet
flows) and intrusive rocks. This sequence is inter-
layered with cherts and is overlain by cherts, banded
iron formations (BIF) and shales of the Fig Tree and
Moodies Groups (Fig. 1). The Onverwacht Group has
been interpreted to represent fragments of Archean
oceanic crust, termed the Jamestown Ophiolite Com-
plex [42,44], that developed in association with sub-
duction and island arc activity approximately 3550 to
3220 Ma [45–47]. The magmatic sequence of the
Onverwacht Group is exceptionally well-preserved,
relatively undeformed away from its margins with
the surrounding granitoids, and upwards from the mid-
dle to the upper part of the sequence the metamorphic
grade decreases from greenschist to prehnite–pumpel-
lyite facies. Tectono-stratigraphically downward into
the Theespruit Formation and across a major shear
zone (the Komatii Fault), there is a rapid increase in
the metamorphic grade to higher pressure–lower tem-
perature amphibolite facies concomitant with the de-
velopment of tectonic fabrics related to structural
emplacement (~3.4 Ga) and subsequent exhumation
(~3.2 Ga) of the BGB [46,48,49].
Well away from the margin of the greenstone belt,
about midway into the Komati Formation, 40Ar / 39Ar
Fig. 1. (A) Location of the BGB and adjacent lithologies of South Africa. (B) Map of BGB showing location of study area. (C) Schematic map of
study area within the BGB with location of sampling sites. Samples listed in Table 1 come from sites 3, 4, 6, 7, 10 and 11 (filled circles). (D)
Reconstructed profile of the BGB showing the relative stratigraphic position of sampling sites. Samples listed in Table 1 are shown in bold.
Modified from [42].
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722710
step-heating analyses on amphiboles from serpenti-
nized komatiitic basalts give a metamorphic age of
3486F8 Ma [50]. This 40Ar / 39Ar age overlaps with
an ~3482 Ma U/Pb date of magmatic zircon from an
interbedded airfall tuff in the same outcrop [51,52].
The overlapping metamorphic and igneous crystalliza-
tion ages, perfect preservation of fine igneous micro-
structures such as spinifex tectures pseudomorphed by
metamorphic minerals, abundant pillow lavas, and
oxygen isotope stratigraphy through the sequence,
are taken as evidence that the metamorphism repre-
sents ocean-floor type hydrothermal alteration that
occurred penecontemporaneously with igneous activity
[53,42,54,44].
Fig. 2. Examples of well preserved pillow lavas and interpillow hyaloclastite from the BGB. (A) Pillow lavas surrounded by hyaloclastite breccia
from the upper part of the Hooggenoeg formation at location 6. Note the dark chilled margins (up to 2 cm thick). Field of view is ~1 m. (B)
Vesicular pillow lavas from the lower part of the Kromberg Formation at location 7. Note the well developed dark chilled margins and excellent
preservation of spherical vesicles and interpillow hyalocastite. Field of view is ~50 cm. (C) Pillow lavas and hyaloclastite breccias from the middle
part of the Hooggenoeg Formation at location 5. Field of view is ~40 cm. (D) Well preserved pillow lavas and interpillow hyaloclastite from the
upper part of the Hooggenoeg Formation at location 6. Field of view is ~80 cm.
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722 711
We collected samples of pillow lava and interpillow
hyaloclastite from the best-exposed parts of the
Komati-, Hooggenoeg-, and Kromberg Formations
(Fig. 1). Pillow lavas were sampled from locations 3,
4, 6, 7, 10 and 11 (Fig. 1). Individual pillows are highly
variable with respect to size and vesicle density (Fig. 2).
The pillow lavas invariably display a well-developed
chilled margin (commonly 10 mm thick) grading in-
wards into a variolitic zone (5–10 mm thick), consisting
of a mixture of altered glass and microcrystalline ma-
terial. Interpillow hyaloclastites were sampled from
locations 6 and 7 (Fig. 1). Hyaloclastite breccias are
confined to minor inter-pillow occurrences (Fig. 2).
5. Analytical methods
Samples were first carefully trimmed with a saw and
the sawn surfaces ground to remove any trace of surface
contamination. Samples containing open fractures or
pore spaces were avoided completely. No open pore
spaces were observed in the sample collection as con-
firmed by SEM and petrographic analysis.
Scanning electron microscopy (SEM) observations
were performed on a JEOL JSM-6301FXV instrument
at the University of Alberta connected to a Princeton
Gamma Tech IMIX energy-dispersive spectrometer
system. The analyses were performed at an accelerating
voltage of 20 kV and a working distance of 15 mm.
Thin sections and grain mounts were sputter coated
with a thin film of iridium, approximately 40 A thick
[11].
X-ray mapping on the same iridium-coated thin
sections was carried out with a JEOL JXA-8900R
electron microprobe at the University of Alberta,
using an accelerating voltage of 15 kV, and a probe
current of 3.0�10�8 A. Carbon and nitrogen peak
positions were determined using synthetic silicon car-
bide and boron nitride standards, respectively. Instru-
ment calibration for all other elements was performed
on natural standards. Carbon was routinely measured
on two different spectrometers to monitor the reproduc-
ibility of observed signals.
Stable C-isotope analyses of carbonates were per-
formed by pouring 100% phosphoric acid on whole-
rock powders under vacuum [55] and analyzing the
exsolved CO2 on a Finnegan MAT 252 mass spec-
trometer at the University of Alberta. Yields of CO2 in
the samples varied from 0.011% to 19% by weight as
carbonate. The error in calculated carbonate yields
range from ~F1% to ~F15% for samples rich and
Fig. 3. Healed fractures within pillow rims displaying irregular patches consisting of extremely fine-grained titanite (brown mineral in A–C).
Extending from these titanite patches are mineralized tubular structures 1–10 Am in width and up to 200 Am in length (A and B=Sample 27C-BG-
03; C=Sample 29-BG-03). Detail of tubular structures within the white boxes in A and B are shown in the insets. Some of these tubular structures
exhibit well-defined segmentation where they have been overprinted by the chlorite, indicating that they predate the alteration process (C). Modern
microbial tubular structures in basaltic glass (from ODP sample 148-896A, 11R-01, 73–75) are shown for comparison (D). Note the similarity in
size, shape, and distribution between the modern and ancient tubular textures.
Fig. 4. Tubular structures mineralized by titanite observed in samples of interpillow hyaloclastite (brownish-black mineral in A–C; Sample 27C-BG-
03). The structures in the interpillow hyaloclastite also have the same size, shape, and distribution as those shown in the pillow rims (Fig. 3).
Modern microbial tubular structures (from DSDP sample 46-396B-20R-4, 112–122) are shown for comparison (D). Again note the similarity in
size, shape, and distribution between the modern and ancient tubular textures.
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722712
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722 713
poor in carbonate, respectively. The errors on isotopic
analyses for carbon are better than F0.1x. The data
are reported in the usual delta-notation with respect to
VPDB for carbon [56,57].
6. Results
6.1. Transmitted light petrography
The outermost 10–20 mm of most pillows is defined
by a dark zone that represents the chilled, originally
glassy rim (Fig. 2). In many cases part of the glassy
margin spalled off during pillow growth to form inter-
pillow hyaloclastite (Fig. 2; see also [52]). Due to the
pervasive greenschist facies metamorphic overprint,
these rims now consist of very fine-grained chlorite
with scattered grains of quartz, epidote, and amphibole.
Within this originally glassy zone, there are healed
fractures along which occur dense, irregular patches
Fig. 5. Photomicrographs of well-preserved interpillow hyaloclastites.
(A) Original glass shards are completely replaced by chlorite, quartz,
and epidote and the interstitial spaces are filled with quartz and calcite
(Sample 119-BG-04). There is very little evidence of deformation and
preservation of original jigsaw breccia textures between individual
glass shards is clearly visible in thin section. (B) Tubular structures
mineralized by titanite are present both within the glass shards and
along the margins of the shards in the surrounding interstitial quartz
(Sample 27C-BG-03). Inset shows interpretation of original margin of
an individual glass along which numerous titanite tubules are now
located in quartz.
Fig. 6. Individual glass shards in interpillow hyaloclastites also pre
serve fractures along which incipient alteration is observed (A). Areas
of quartz along these fractures contain irregular patches of individua
and/or coalesced spherical bodies mineralized by titanite that protrude
away from the filled fracture (Sample 119-BG-04; inset A; B). The
individual spherical bodies or patches are commonly 1–4 Am in
diameter. These textures are similar to granular microbial alteration
patterns observed at the interface between fresh glass and microbia
alteration fronts in pillow basalts (from ODP sample 148-896A, 9R-1
17–21) from in situ oceanic crust (C).
-
l
l
,
consisting of very fine-grained titanite (Fig. 3). The
development of these alteration features along fractures
is irregular and non-symmetric. Extending from these
titanite patches are mineralized tubular structures 1–10
Am in width (average 4 Am) and up to 200 Am in length
(most commonly about 50 Am; Fig. 3). Some of these
tubular structures exhibit well-defined segmentation.
The segmentation results from the partial replacement
of the titanite tubule by chlorite. Since the tubule pre-
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722714
dated the chlorite formation they must have formed
early in the alteration process (Fig. 3C). These struc-
tures have largely similar shape and size as tubular
textures found in glassy pillow rims of young pillow
lavas of in situ ocean crust (Fig. 3D).
The mineralized tubular structures are most com-
mon and best developed in the interpillow hyaloclastite
(Fig. 4) in the upper part of the Hooggenoeg Forma-
tion (dated between 3472 and 3456 Ma; [45,46]; and
de Wit, unpublished data). The interpillow hyaloclas-
tite samples show no evidence of deformation and
preserve original jigsaw breccia textures with individ-
ual glass shards clearly visible in thin section (Fig. 5)
Fig. 7. Series of X-ray element maps (C, Ca, N, P, and Ti) and backscattered
Sample 29-BG-03 from location 6. Carbon 1 and Carbon 3 refer to carbon m
the patterns observed in the two carbon maps are an artifact produced in sam
of the X-rays and the different physical orientation of the spectrometers on
spatial distribution of P and N are due to the same artifact. In the maps label
the BSE image showing the association of carbon with the margins of the t
green–yellow–red–pink. Scale bar is 20 Am.
and also in hand specimen. Original glass shards are
completely replaced by chlorite, quartz, and epidote
and the interstitial spaces are filled with quartz and
calcite. Tubular structures mineralized by titanite are
present both within the glass shards and along the
margins of the shards in the surrounding interstitial
quartz (Fig. 5). The mineralized tubular structures in
the interpillow hyaloclastite also have the same size,
shape, and distribution as those observed in the pillow
rims (Fig. 3).
Individual glass shards in interpillow hyaloclastites
also preserve fractures along which incipient alteration
is observed (Fig. 6A). These fractures contain patches
electron (BSE) images within the formerly glassy chilled pillow rim of
aps collected on spectrometers 1 and 3, respectively. The differences in
ples that are not perfectly flat by the combination of the take off angle
the microprobe (approximately 1508 apart). Slight differences in the
ed C1+BSE and C2+BSE the carbon map has been superimposed on
ubular features. Increasing order of elemental abundance black–blue–
Fig. 8. SEM–BSE images and transmitted light photomicrograph o
area mapped in Fig. 7 from Sample 29-BG-03. (A) BSE image clearly
shows a healed fracture trending from upper left to bottom right of the
image cutting the formerly glassy margin now replaced by predomi
nantly quartz and chlorite. Two titanite patches (light gray material
with mineralized tubules extend away from the healed fracture. Area
in box is shown in B. Scale bar is 100 Am. (B) Detailed BSE image o
titanite tubules mapped in Fig. 7 (area in box). The healed fracture is
clearly visible at the center of the titanite patch. Scale bar is 50 Am(C) Transmitted light photomicrograph of titanite tubules mapped in
Fig. 7 (area in box). This image clearly shows the titanite tubules are
connected to the main titanite patch below the surface and are no
isolated grains. The apparently isolated nature of some tubules in the
BSE images is an artifact of the thin section making process. Scale ba
is 50 Am.
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722 715
of quartz along their length that host irregular patches
of individual and/or coalesced spherical bodies miner-
alized by titanite that protrude away from the filled
fracture (Fig. 6B). The individual spheres or patches
are commonly 1–4 Am in diameter and resemble mi-
crobial alteration patterns observed at the interface
between fresh glass and microbial alteration fronts
observed in basaltic glass from in situ oceanic crust
(Fig. 6C) (e.g., [5,8,11]).
6.2. Element mapping
X-ray element maps collected by electron micro-
probe on iridium-coated thin sections show elevated
levels of carbon, and possibly nitrogen and phospho-
rus, associated with the mineralized tubular features
(Fig. 7). SEM images of the area mapped in Fig. 7
clearly show the tubules extending away from a healed
fracture (Fig. 8A and B). A transmitted light photo-
micrograph of the tubules mapped in Fig. 7 is shown
in Fig. 8C. The observed enrichments are highly re-
stricted to the margins of the mineralized tubes and
diminish sharply away from these areas. Although the
intensity of the carbon signal observed in Fig. 7 repre-
sents a qualitative indication of the amount of carbon
present and does vary, elevated levels of carbon are
observed in several samples where mineralized tubes
occur. Element maps for calcium, magnesium, iron,
aluminum, sodium, potassium, silicon, sulfur, chlorine,
and titanium were also routinely produced. Most of
these elements do not show enrichments and argue
against the possibility that carbon highs are due to
inorganic carbonate material (e.g., Ca, Mg, Fe) or
epoxy (e.g., Cl).
6.3. Carbon isotopes
A series of sub-samples from the outermost glassy
rims and crystalline interiors of individual pillow lavas
were carefully prepared. The bulk-rock carbonate from
the bglassyQ and bcrystallineQ sub-samples was ana-
lyzed for carbon isotope ratios in order to investigate
if there was any indication of biological fractionation.
The results are given in Table 1 and Fig. 9. No trend
with depth within the stratigraphic sequence or corre-
lation between isotope ratio and carbonate abundance
is observed.
Carbon-isotope analyses of disseminated carbonate
in formerly glassy pillow rims and interpillow hyalo-
clastites, in which there are textural and geochemical
evidence for microbial activity, display a significantly
greater range in the d13C values (+3.9x to �16.4x)
f
-
)
f
.
t
r
than the crystalline interiors of pillow lavas (+0.7xto �6.9x). Secondary carbonate in vesicles has d13C
values that cluster near zero. The d13C values from
the crystalline pillow lava interiors are bracketed
between primary mantle CO2 (�5x to �7x;
[58,59]) and values close to marine carbonates (0x;
[59,60]). This distribution is very similar to that
observed in studies of ophiolites (Fig. 9B) and in
Table 1
Carbon isotope analyses of disseminated carbonates
Sample wt.% carb d13C
Glassy samples
P1A-02 1.253 �0.3
P1A-02 1.261 1.8
P1A-02 0.016 �5.7
P1A-02 1.322 0.1
P1A-02 0.02 �3.5
P1A-02 3.453 0.4
P1A-02 0.765 0.3
P1B-02 0.054 �2.6
P1B-02 2.4 3.9
P1B-02 0.057 �1.9
P2B-02 0.025 �12.7
P2B-02 0.035 �3.4
P2B-02 0.037 �4
P2B-02 0.019 �9.7
P2B-02 0.02 �7.1
P2B-02 0.03 �6.6
PB3-02 0.027 �16.7
PB3-02 0.967 0.3
PB3-02 0.011 �1.2
PB3-02 0.016 �15.6
PB3-02 0.99 0.4
PB3-02 0.81 0.5
PB3-02 1.08 0.3
8A-BG-03 0.756 �1.1
9A-BG-03 0.03 �4.4
12A-BG-03 2.336 3
14A-BG-03 4.372 0.3
17A-BG-03 0.04 �4.2
17B-BG-03 0.042 �5.7
24-BG-03 2.219 �0.1
26-BG-03 0.018 �5.1
29-BG-03 5.327 �4.8
30B-BG-03 0.027 �6.1
33-BG-03 0.033 �3.3
37A-BG-03 0.045 �6.9
37B-BG-03 0.035 �5.2
38-BG-03 0.014 �7.1
39A-BG-03 0.039 �5.2
39B-BG-03 0.035 �9.4
39C-BG-03 0.035 �6
40A-BG-03 0.031 �7.9
40B-BG-03 0.038 �1.6
40C-BG-03 0.053 �3.7
40D-BG-03 0.015 �5.8
41A-BG-03 0.027 �5.9
41B-BG-03 0.03 �7
41C-BG-03 0.029 �9.1
41D-BG-03 0.025 �4.4
42A-BG-03 0.025 �5.8
42B-BG-03 0.018 �11.3
43A-BG-03 0.019 �8.1
43B-BG-03 0.011 �8.4
43C-BG-03 0.054 �9
56-BG-03 0.05 �8.8
57-BG-03 0.065 �6.5
58A-BG-03 0.071 �1.5
58B-BG-03 0.106 �1.6
Table 1 (continued)
Sample wt.% carb d13C
Glassy samples
59A-BG-03 0.019 �2
59B-BG-03 0.025 �6.9
60A-BG-03 0.015 �5.4
60B-BG-03 0.031 �4.3
Crystalline samples
P1A-02 0.944 �0.1
P1A-02 1.989 0.7
P1A-02 3.538 0.6
P1B-02 0.7 �0.1
P1B-02 5.501 0.1
P1B-02 2.555 0.5
P1B-02 18.287 0.7
P1B-02 4.211 0.6
P1B-02 4.306 0.6
P2B-02 0.42 0
P2B-02 0.03 �6.1
PB3-02 0.332 �1.1
PB3-02 0.287 0.4
PB3-02 0.45 0
8B-BG-03 0.377 �1.8
10B-BG-03 0.152 �1.6
13B-BG-03 0.664 �0.4
15B-BG-03 0.053 �1.9
25-BG-03 5.264 �1.4
31-BG-03 0.289 �1.6
34-BG-03 3.529 �0.5
37D-BG-03 1.037 �1.1
37E-BG-03 5.903 �0.5
39D-BG-03 0.018 �2.4
39E-BG-03 3.232 �0.9
39F-BG-03 0.997 �0.9
39G-BG-03 1.208 �6.5
40E-BG-03 0.022 �2.6
40F-BG-03 0.013 �6.6
43E-BG-03 0.779 �6.4
58C-BG-03 0.049 �5
59C-BG-03 0.037 �5.9
59D-BG-03 0.052 �5.4
60C-BG-03 0.039 �6.5
Vesicles
P1A-02 15.414 0.7
P1B-02 19.094 0.5
PB3-02 2.878 0.5
wt.% carb=weight percentage carbonate. Samples listed come from
the following sites: Site 3=06 to 08-BG-03; Site 4=09 to 20-BG-03;
Site 6=24 to 29-BG-03; Site 7=30 to 46-BG-03; Site 10=56 to 58-
BG-03; and Site 11=59 to 60-BG-03. All samples listed as Pxx-02
were collected at Site 7.
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722716
situ oceanic crust (Fig. 9C). The observed shift to
lower d13C values of disseminated carbonates in the
outer glassy rim of pillow lavas is a pattern that is
interpreted to result from microbial fractionation
[7,61,62,13,11,14,63].
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722 717
7. Discussion
It is perhaps surprising to find that evidence for early
life has come from igneous rather than sedimentary rocks,
which to date have been the only Archean rocks subjected
to close scrutiny for signs of early life. The granular and
tubular structures reported here from the pillow rims and
interpillow hyaloclastite of the Hooggenoeg and Krom-
berg Formations are interpreted as the mineralized
remains of microbial borings in previously glassy rocks.
Those familiar with early accounts of microscopic tubular
quartz or iron-carbonate pseudofossil trails extending
fromminute pyrite grains observed in Precambrian organ-
ic rich cherts (i.e., [64]) may initially attach some simi-
larity to the tubular structures in the present study. These
Fig. 9. Relationship between weight percent carbonate versus d13C for
the originally glassy rims (filled circles) and crystalline interiors (open
squares) of pillow lavas. (A) Analyses from pillow lavas of the Komati,
Hooggenoeg, and Kromberg Formations of the Onverwacht Group,
BGB. (B) Compilation of analyses from ophiolites worldwide [13,63].
(C) Compilation of analyses from modern oceanic crust [7,8,11].
Fig. 10. Comparison of tubular structure diameters in the BGB
samples with tubular microbial alteration textures in modern oceanic
crust. The mineralized structures in BGB samples are similar in size
but generally slightly larger than microbial alteration textures in fresh
oceanic basaltic glass.
structures, termed ambient inclusion trails, are inter-
preted to form by pressure solution initiated by gas
evolution from organic material that drives minute min-
eral grains (commonly pyrite) through the chert matrix
[65,66]. These trails commonly display spectacular
morphologies with straight, curved, coiled, and pseudo-
branching patterns having been observed [65,66]. These
complex tubular morphologies were originally misinter-
preted by Gruner [64] as bmicrofossilsQ but were later
shown to form through abiotic pressure solution [65,66].
Although ambient inclusion trail can also be caused by
minerals such as titanite or magnetite, the BGB tubules
are mineralized by titanite, not associated with mineral
grains at their tips, and do not occur in organic-rich
cherts, which are necessary requirements for this process
to occur [65,66]. For these reasons it is unlikely that the
tubular structures in the formerly glassy BGB rocks
formed by a similar pressure-solution process.
7.1. Textural evidence for microbial alteration of the
BGB lavas
The tubular and granular structures from both the
pillow rims and interpillow hyaloclastites are compara-
ble in size, shape, and distribution to microbial alter-
ation features reported from glassy rims in pillow lavas
from the Troodos ophiolite ([61]; Fig. 3A and B), and
basaltic glass from in situ oceanic crust [18,2,21,3–
7,9,11,10] (Fig. 3C and D). Fig. 10 compares the
diameter of tubular structures in the BGB samples
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722718
with the diameter of tubular microbial alteration fea-
tures in modern oceanic crust. The observed distribu-
tion shows that on average the mineralized structures
found in the formerly glassy BGB samples are slightly
larger than predominantly unfilled microbial alteration
features in fresh oceanic basaltic glass. This can be
explained if the mineralized structures found in the
BGB samples were once empty corrosion structures
because it would be impossible for a mineralized tube
to be smaller than the original channel it subsequently
filled. Additionally, they may have become thicker by
metamorphic growth. The shape and distribution of the
tubular and granular structures along healed micro
fractures in the BGB lavas is identical to those found
in modern oceanic basalts (Figs. 3D, 4D, 6C).
7.2. Element distributions
The presence of carbon along the margins of the
titanite tubules in the BGB samples unrelated to carbo-
nates is interpreted to represent residual organic matter
[14]. The common association of carbon, and to a lesser
extent nitrogen and phosphorus, with suspected micro-
bial alteration textures and not elsewhere is a common
observation in basaltic glasses from modern oceanic
crust and ophiolites affected by microbial alteration
(e.g., [7,11]). The association of carbon and nitrogen
with the mineralized tubes argues for a biological origin
because abundances of these elements in igneous and
metamorphic rocks are commonly low. Our interpreta-
tion is that these elements were most likely concentrated
from seawater by microbes that colonized the originally
glassy surfaces. As the microbes dissolved the glass,
multiplied, and died, organic remains containing carbon
and nitrogen were left behind within the alteration
textures produced. These organic remnants were then
later trapped along the margins of the tubes as they
became mineralized by titanite, resulting in the elevated
signals observed. Conversely, phosphorous would have
been present in the glass matrix but likely in very low
concentrations relative to seawater. It is uncertain if
the microbes are able to extract P from the glass during
dissolution but element maps show elevated concentra-
tions of P in microbial alteration textures, likely due to
incorporation in cells.
The metabolic requirements of microbes responsible
for alteration of basaltic glass in modern samples have
not been determined. It is thought that elements in the
glass such as iron act as nutrients sought by the
microbes and are released during glass dissolution.
The possibility that the titanite tubules in the BGB
samples represent vestiges of the same microbial pro-
cess found in basaltic glass from the modern seafloor
suggests that endolithic microbes may have been an
early form of life on Earth.
7.3. Interpretation of carbon isotopes
Disseminated carbonate in the relic glassy rocks from
the BGB is lower in d13C on average (�4.6x) than in
crystalline rocks (�2.1x). The crystalline interior sam-
ples all display d13C values bracketed between Archean
marine seawater (~0F2x; [60]) and primary mantle
CO2 (�5x to �7x; [59,58]). The relic glassy samples
also extend to much lower d13C values (�16.7x) than
the crystalline rocks (�6.9x). The lowest d13C values
occur in relic glassy samples with very low carbonate
abundances (V0.054 wt.%). Such isotopic contrasts
have been documented in pillow lava rims from Phan-
erozoic and Proterozoic ophiolites (Fig. 9B) and recent
oceanic crust (Fig. 9C). The generally low d13C values of
disseminated carbonate are attributed to metabolic by-
products formed during microbial oxidation of dissolved
organic matter from pore waters [7,11,13,63]. Two relic
glassy samples from the BGB with relatively high car-
bonate contents (N2 wt.%), have d13C values (3.0x and
3.9x) above those expected for seawater carbonate
(Table 1; Fig. 9A). These high carbonate contents
argue against a seawater fingerprint and are likely the
result of an outside, possibly later, source of inorgani-
cally precipitated carbonate.
Prior to alteration and metamorphism all the basaltic
samples would have had initial magmatic d13C values
in the range of �5x to �7x [58,59]. Abiotic inter-
action with seawater would have introduced inorgani-
cally precipitated marine carbonate with d13C values
close to zero [58,60]. Based on these starting conditions
we can make some predictions regarding the observed
distribution of d13C values in the glassy and crystalline
samples. Forty-seven of the 63 relic glassy samples, all
of the crystalline samples, and all of the vesicle fillings
have d13C values between �7x and 2x. These values
are best explained as being derived from some combi-
nation of carbonate inorganically precipitated from sea-
water and magmatic CO2 values [7,11,13,63].
In contrast, the samples with d13C values lower than
�7x most likely contain an amagmatic carbon com-
ponent with low d13C. The low d13C values of carbon-
ate in the relic glassy BGB samples, particularly those
samples that contain low abundances of carbonate and
d13C valuesb�7x, are interpreted to be the result of
microbial fractionation. If the proportion of inorgani-
cally precipitated calcite (with d13C near 0x) exceeds
that resulting from precipitation of carbonate from
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722 719
microbially produced CO2 during oxidation of organic
carbon, the d13C value of a sample may be greater than
�7x. This is commonly seen for samples that are
relatively rich in carbonate (Fig. 9A; Table 1).
The carbon isotope signatures are unlikely to be the
result of some later carbonate formation, more recent
extraneous respired organic matter, or a Rayleigh frac-
tionation of 13C / 12C associated with decarbonation
reactions. If the observed d13C values were due to
deposition of carbonate (such as from more recent
fluid flow) unrelated to biological activity in the glassy
margin, there is no reason that the pillow margins
should be different from the pillow interiors. Instead,
one would expect a homogenization of the isotope data
which is clearly not observed (Fig. 9A). It is also
difficult to explain why extraneous respired organic
matter would preferentially affect the pillow rims
since these would have devitrified early in the alteration
process through replacement by secondary minerals.
This is confirmed by independent petrographic evi-
dence that demonstrates alteration occurred soon after
eruption of the pillow lavas (Fig. 3C; see Section 7.4).
Because the metamorphism occurred early in the histo-
ry of these rocks there would have been no mineralogic
(i.e., different assemblage) or textural (i.e., glass versus
mineral) advantages for respired organic matter to be
deposited in the pillow margins preferentially over the
crystalline interiors since the time of initial alteration. In
addition, if the isotopic signatures observed were a
result of decarbonation reactions one would expect to
see a correlation between weight percent carbonate and
d13C value that fits a Rayleigh fractionation curve.
Instead, we observe no correlation over the d13C
range from �17x to 0x at low carbonate abundances
(b0.1 wt.%; Fig. 9A). At relatively high carbonate
contents (above ~0.1 wt.%) there is again no trend
observed and the d13C values typically fall within the
range of magmatic (�7x to �5x) and Archean ma-
rine carbonate values (~0x). Hence we consider it
unlikely that Rayleigh fractionation, a process that
may fractionate carbon isotope ratios during basalt
degassing and subsequent alteration [59], is responsible
for the pronounced spread in the d13C values of the
glassy components. Instead, the d13C pattern observed
in the Barberton pillow lavas (Fig. 9A) is best inter-
preted as resulting from similar processes observed in
recent oceanic crust and ophiolites [7,11,13,63].
7.4. Timing of microbial alteration
The question remains as to when the microbial
activity occurred. To address this question we investi-
gated the overprinting relationships between the tubular
structures and the metamorphic mineral growth. Chlo-
rite is the predominant mineral in the originally glassy
margin of the pillows, and is observed to overgrow the
titaniferous tubular structures and thus obliterate them
in some samples. In other samples fine chlorite has
caused the tubular structures to take on a segmented
character by partly overgrowing them (Fig. 3C). These
petrographic observations indicate that the tubular
structures are pre-metamorphic. Previous 40Ar / 39Ar
step-heating analyses of komatiitic basalts from the
Komati Formation give metamorphic ages of
3486F8 Ma [50]. The 40Ar / 39Ar ages overlap with
the igneous U/Pb ages of the Onverwacht Group [50–
52]. This is taken as evidence that the metamorphism
represents ocean-floor type hydrothermal alteration that
occurred soon after the crust was formed [42,54,44].
7.5. Preservation of microbial alteration textures in the
rock record
The mineralization of tubular microbial alteration
textures in the BGB samples by titanite is not a unique
occurrence. It is well known from studies of recent
oceanic basaltic glass that titanium can be passively
accumulated during etching of the glass by microbes
(e.g., [11]). This process preferentially concentrates
titanium in the channels produced by the microbes.
Titanium enrichments in tubular microbial textures are
also observed in the glassy pillow margins of ophiolitic
rocks. In samples from the Jurassic Mirdita ophiolite
(Albania) and the Stonyford Volcanics (California),
zeolite facies alteration has begun to replace the glass
(Banerjee unpublished data). In these samples open or
clay-filled tubular structures within the glass are min-
eralized by titanite as they pass into the zone of zeolite
alteration. This direct link between open or clay-filled
tubes with titanite-filled tubes suggests that the miner-
alization process follows a step-wise sequence during
progressive alteration conditions (Banerjee unpublished
data). The formation of titanite is thus an early process
that occurs at relatively low temperatures and this
mineralization process is essential if the microbially
produced structures are to be preserved for extended
periods of geological time.
8. Concluding remarks
Evidence for microbial alteration of relic glassy
basaltic rocks in the Archean BGB is widespead in
the former glassy margins of pillows and interstitial
hyaloclastites. The integrated observations suggest that
N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722720
these features are the result of microbial activity and
that this microbial colonization of the glassy basaltic
rocks took place soon after eruption of the ~3.4–3.5
Ga pillow lavas. During the last decade there has been
an intense search for traces of early life on Earth. This
search has largely been confined to sedimentary rocks,
or rocks interpreted to represent sediments. The search
for traces of early life is difficult and purported bio-
markers found in Archean metasediments, either in the
form of microfossils or as chemical and isotopic tra-
cers, have been questioned (e.g., [38,31,37,34]). Our
claim of ~3.5 Ga traces of life in the originally glassy
selvages of pillow lavas and inter-pillow hyalocastites
is a new geological setting in the search for early life.
Indeed, the birthplace of life may have been connected
to the volcanic environment of the oceanic crust, such
as deep-sea hydrothermal vents (e.g., [67]). Pillow
lavas are the most common rock sequences in Archean
greenstone belts [68]. The early Earth would have
been very different if these did not represent to a
large degree submarine eruptions and some form of
oceanic crust. It may therefore be profitable for the
study of early life on Earth to examine other green-
stone belts and specifically relic glassy lavas for signs
of microbial activity. This new niche has the potential
to represent a largely unexplored Archean biomass.
Acknowledgements
This work benefited from discussions with I. H.
Thorseth and B. Robins. T. Chacko and two anony-
mous reviewers improved an early version of the
manuscript. We thank D. Resultay and M. Labbe for
help preparing thin sections, N.R. Sandsta and G.B.
Carbno for help in the field, O. Levner for help with
carbon isotope analyses, G. Braybrook for help with
the SEM, and S. Matveev for help with the electron
microprobe. Thank you to Fred Daniel of Nkomazi
Wilderness for hospitality and the Mpumalanga Parks
Board for access during field work. Financial support
to carry out this study was provided by the Norwegian
Research Council, the National Sciences and Engi-
neering Research Council of Canada, the US National
Science Foundation, the Agouron Institute, and the
National Research Foundation of South Africa. This
is AEON contribution 007.
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