Field:
Earth Science/Environment
Session Topic:
Primitive Earth and Atmosphere
Speaker:
Yuichiro Ueno/Tokyo Institute of Technology
Stable Isotope Geochemistry of 3.5-Billion-Years-Old Seafloor Hydrothermal Deposits
How to detect biological activities from ancient geological record is a difficult issue
for investigation of the earliest evolution of Earth’s life. The record of prokaryotic
microfossils dates back to ~3.5 billion years old (Shopf, 1993), however, biological
origins of these “fossils” have been still debated (Buick 1990; Brasier et al., 2002).
Moreover, morphology-based study of prokaryotic fossil provides only limited
information for physiology of the ancient life.
On the other hand, the ratios of stable carbon (12C/13C) and sulfur (32S/33S/34S/36S)
isotopes between reactant and product are useful tracers for specific metabolic
activities in the past. In principle, significant depletions of heavier (thus rarer)
isotopes in reaction products could be expected under low temperature condition, in
which chemical reactions are usually prohibited due to kinetic barrier. Hence, the
large isotopic fractionations could be a good indicator for “catalysis” accelerating the
low temperature reactions. This is a reason why stable isotope ratio could be a tracer
for biological activities, though we have to ask then whether the inferred reactions
were catalyzed by inorganic catalysts or by biological enzymes.
In order to detect the ancient metabolic activities, we have conducted stable carbon
and sulfur isotope geochemistry of the ~3.5-billion-years-old hydrothermal deposit
occurring in Western Australia together with detailed field geology. Recent geological
studies revealed that this deposit consists of hydrothermal sediments deposited in
caldera of submarine basaltic volcano (Isozaki et al., 1997; Ueno et al., 2004; Van
Kranendonk, 2006). Unique character of this deposit is the occurrence of prominent
swarm of hydrothermal dykes, which are rocks precipitated in the past conduits of the
hydrothermal fluid beneath the seafloor.
Significant isotope depletions have been observed for several reduced forms of
carbon and sulfur in the deposits: 1) 13C-depleted organic matter in seafloor sediments
and sub-seafloor dykes (Ueno et al., 2001; 2004), 2) 13C-depleted methane preserved in
the dyke (Ueno et al., 2006), and 3) 34S-depleted sulfide minerals in the seafloor
sediment (Shen et al., 2001; Ueno et al., 2003). The observed large isotopic
fractionations relative to co-existing oxidizing compounds could be expected from
enzymatically catalyzed biological reactions: 1) autotrophic carbon fixation, 2)
methanogenesis, and 3) sulfate reduction, respectively. Alternatively, these
isotopically depleted compounds may have possibly been produced by non-biological
reactions, for example, Fischer-Tropsch-type (FTT) reactions to produce CH4 and
hydrocarbons, and thermochemical sulfate reduction to produce sulfide minerals
(McCollom & Seewald, 2006; Runnegar et al., 2001). However, the lack of native metal
(i.e., effective catalyst of FTT reactions) in the deposit and isotopic relationships
among CH4, CO2, organic matter, and carbonate are inconsistent with the FTT origin
of the methane and the organic matter. On the other hand, it is still ambiguous
whether the 34S-depletion of the sulfide mineral was resulted from thermochemical or
biological reactions.
These results indicate that microbial ecosystem would have existed in the
3.5-billion-years-old hydrothermal system including sub-seafloor environment. The
ecosystem included methanogen (methane-producing microbe), which would have
gained energy through the conversion of CO2 and H2 into CH4. The methanogen may
have acted as a primary producer of organic compounds, which may possibly be
utilized by other heterotrophs possibly including sulfate-reducing microbes.
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