Geoscience Reference
In-Depth Information
tions of sulfate in the water. The sulfate was likely sourced, ultimately,
from sulfur gases discharging from the local volcanoes. Sulfate miner-
als, at least some of which were originally precipitated as gypsum (they
are now all barite, BaSO
4
), are associated with ine-grained pyrites and
form an important component of the sediments. My then postdoc Yanan
Shen, together with Roger Buick and I looked at the sulfur isotope
composition of the sulfate and the pyrites in these rocks. I will say more
about what these isotopes tell us in
chapters 7
and
9,
but our results
suggested that the sulfide in the pyrites formed from microbial sulfate
reduction. We were quite proud of this result because it documented
both the early evolution of sulfate-reducing bacteria and the earliest spe-
cific microbial metabolism in the geologic record.
As one might expect, this finding came under intense scrutiny, with
the discussion centering on whether the sulfide was instead formed from
a thermochemical process not involving organisms at all. This can hap-
pen if you heat sulfate up with organic matter to a sufficiently high tem-
perature. Yanan Shen has since revisited these rocks with a more sophis-
ticated sulfur isotope approach and has generated new results that are
consistent with our earlier finding.
There's still more. The basaltic rocks lying just below the sediments
explored by Yanan, Roger, and me are crisscrossed with a number of
silica-rich dikes, and these contain tiny inclusions of fluid and gases.
Since the dikes formed at about the same time as the sediments, these
inclusions could, in principle, hold further clues as to the microbes liv-
ing some 3.5 billion years ago. Indeed, Yuichiro Ueno from the Tokyo
Institute of Technology, along with his colleagues, looked at the gases
present in these inclusions and found that many contained a good deal
of methane. They divided the inclusions into those that looked primary,
dating back to the time that the dikes were formed, and those that looked
secondary, having formed after the original emplacement of the dikes.
They then measured the isotopic composition of the methane, as bio-
genic methane yields a distinct isotopic signal. They found that the pri-
mary inclusions contained a methane isotopic composition (don't worry
about the details) consistent with a biogenic origin through microbial
methanogenesis. In contrast, the secondary inclusions had methane
isotopic compositions more consistent with nonbiogenic sources. This
