25th International Meeting on Organic Geochemistry IMOG 2011

P-195
Molecular fossils and the late rise of eukaryotes and oxygenic
photosynthesis
Jochen J. Brocks
Research School of Earth Sciences, The Australian National University, Canberra, Australia (corresponding
author:Jochen.Brocks@anu.edu.au)
In 1999, a study reported the discovery of molecular
fossils in 2.7 to 2.5 Ga old, mildly-metamorphosed
shales from the Pilbara Craton in Western Australia 1 .
Based on apparently typical Precambrian composition
and high thermal maturity, the biomarkers were
characterized as ‗probably syngenetic‘ with the
Archean host rocks 2 . The findings led to far reaching
conclusions about the Archean biosphere. 2�methylhopanes
were interpreted as evidence for the
existence of cyanobacteria more than 300 million
years before the atmosphere became widely
oxygenated 1,3 , and the presence of steranes
suggested that ancestral eukaryotes existed 2.7 Ga
ago, 800 million years before the first eukaryotic body
fossils appear in the geological record 1 . However,
here we present evidence that the biomarkers entered
the Archean rocks at a much later point in time.
In a recent study, Rasmussen et al. 4 detected
microscopic spherules of thermally solidified bitumen
in the Archean shales and compared their carbon
isotopic composition with liquid bitumen and kerogen
in the same rocks. The indigenous pyrobitumens had
approximately the same carbon isotopic composition
as co-occurring kerogens, but were depleted in 13 C by
10 to 20‰ relative to extractable alkanes. The
isotopic discrepancy between solidified and liquid
bitumens indicates that the extractable phase entered
the rock after peak-metamorphism 2.2 Ga ago.
If the hydrocarbons were indeed later additions, then
they should have left a distinct spatial distribution in
the Archean rocks. In the simplest case, recent
contaminants should be entirely surficial, while
syngenetic hydrocarbons should be homogeneously
distributed throughout the rock. However, liquid
contaminants might also diffuse into fissures and pore
space creating distinct concentration gradients 5 .
Similarly, an indigenous oil is not necessarily
homogeneously distributed in its host rock.
Hydrocarbons might be driven to rock surfaces by
pressure release when the core is recovered from
great depths after drilling, leading to an accumulation
on rock surfaces. To study these phenomena in the
Archean rocks, we determined the spatial distribution
of biomarkers in drill core material. Rock samples
were cut into millimeter thick slices, and the molecular
content of each slice was quantified. In the drill core,
saturated and aromatic hydrocarbons of low
molecular weight had gradually increasing
concentrations from the surfaces to the center of the
rock while the abundance of higher molecular weight
hydrocarbons, including steranes and hopanes,
steeply decreased with distance from the outer
surfaces.
We propose two mechanisms that may have caused
the inhomogeneous distribution: diffusion of
petroleum-based contaminants into the rock
(‗contamination model‘) and leaching of indigenous
hydrocarbons out of the host shales driven by
pressure release after drilling (‗condensate-escape
model‘). To test these models, we compared the
hydrocarbon distribution in the Archean shales with
artificially contaminated rocks, and with younger
mudstones where leaching of ‗live-oil‘ had been
observed after drilling. Although the condensateescape
model is consistent with some of the observed
patterns in the Archean shales, the observed relative
migration depth of different hydrocarbon classes is
only consistent with chromatographic effects caused
by diffusion of contaminants from the surfaces of the
rock towards the centre. The results confirm that the
biomarkers are not of Archean age and entered the
samples during and after drilling.
[1] Brocks, J. J., Logan, G. A., Buick, R. &
Summons, R. E. (1999) Science 285, 1033-1036.
[2] Brocks, J. J., Buick, R., Logan, G. A. &
Summons, R. E. (2003) Geochim. Cosmochim. Acta
67, 4289-4319.
[3] Summons, R. E., Jahnke, L. L., Hope, J. M.
& Logan, G. A. (1999) Nature 400, 554-557 (1999).
[4] Rasmussen, B., Fletcher, I. R., Brocks, J. J.
& Kilburn, M. R. (2008) Nature 455, 1101 - 1104.
[5] Brocks, J. J., Grosjean, E. & Logan, G. A.
(2008) Geochim. Cosmochim. Acta 72, 871-888.
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