Exobiology in the Solar System & The Search for Life on Mars - ESA

SP-1231
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<strong>The</strong> isotopic difference thus established between organic (biogenic) carbon and <strong>the</strong>
surficial bicarbonate-carbonate pool is largely reta<strong>in</strong>ed when organic and carbonate
carbon enter newly-<strong>for</strong>med sediments. As is obvious from Fig. I.4.3.2.2/1, both <strong>the</strong>
carbon isotope spreads of extant primary producers and those of mar<strong>in</strong>e carbonate and
bicarbonate are basically transcribed <strong>in</strong>to <strong>the</strong> sedimentary record back to 3.5 Gyr or
even 3.8 Gyr ago. This would imply that organic carbon and carbonate carbon had
always been transferred from <strong>the</strong> surficial environment to <strong>the</strong> crust with relatively
little change <strong>in</strong> <strong>the</strong>ir isotopic compositions. For example, <strong>the</strong> δ 13 C org spread <strong>in</strong> recent
mar<strong>in</strong>e sediments (cf. Fig. I.4.3.2.2/1) faithfully <strong>in</strong>tegrates over <strong>the</strong> spread of <strong>the</strong>
contemporary liv<strong>in</strong>g biomass with just <strong>the</strong> extremes elim<strong>in</strong>ated, <strong>in</strong>dicat<strong>in</strong>g that <strong>the</strong>
effect of a later diagenetic overpr<strong>in</strong>t on <strong>the</strong> primary isotope values is ra<strong>the</strong>r limited
(usually below 3‰) and, <strong>for</strong> <strong>the</strong> most part, gets lost with<strong>in</strong> <strong>the</strong> broad scatter of <strong>the</strong>
orig<strong>in</strong>al values. Consequently, <strong>the</strong> k<strong>in</strong>etic isotope effect <strong>in</strong>herent <strong>in</strong> photosyn<strong>the</strong>tic
carbon fixation is propagated from <strong>the</strong> biosphere <strong>in</strong>to <strong>the</strong> rock section of <strong>the</strong> carbon
cycle almost unaltered, which opens up <strong>the</strong> possibility of trac<strong>in</strong>g <strong>the</strong> isotopic
signature of this process back <strong>in</strong>to <strong>the</strong> geologic past.
With <strong>the</strong>se relationships established, decod<strong>in</strong>g of <strong>the</strong> vast body of isotopic
<strong>in</strong><strong>for</strong>mation stored <strong>in</strong> <strong>the</strong> sedimentary record is fairly straight<strong>for</strong>ward. <strong>The</strong>re is little
doubt that <strong>the</strong> conspicuous 12 C-enrichment displayed by <strong>the</strong> data envelope <strong>for</strong> fossil
organic carbon (Fig. I.4.3.2.2/1) constitutes a coherent signal of autotrophic carbon
fixation over almost 4 Gyr of recorded Earth history as it ultimately rests with <strong>the</strong>
process that gave rise to <strong>the</strong> biological precursor materials. Moreover, <strong>the</strong> long-term
uni<strong>for</strong>mity of <strong>the</strong> signal attests to an extreme degree of conservatism of <strong>the</strong> basic
biochemical mechanisms of carbon fixation. In fact, <strong>the</strong> ma<strong>in</strong>stream of <strong>the</strong> envelope
<strong>for</strong> δ 13 C org depicted <strong>in</strong> Fig. I.4.3.2.2/1 can be most readily expla<strong>in</strong>ed as <strong>the</strong>
geochemical manifestation of <strong>the</strong> isotope-discrim<strong>in</strong>at<strong>in</strong>g properties of one s<strong>in</strong>gle
enzyme, namely, ribulose-1,5-bisphosphate (RuBP) carboxylase, <strong>the</strong> key enzyme of
<strong>the</strong> Calv<strong>in</strong> Cycle.
It is well known today that <strong>the</strong> carbon transfer from <strong>the</strong> <strong>in</strong>organic to <strong>the</strong> organic
world largely proceeds via <strong>the</strong> RuBP carboxylase reaction that feeds CO 2 directly <strong>in</strong>to
<strong>the</strong> Calv<strong>in</strong> Cycle as a 3-carbon compound (phosphoglycerate). Most autotrophic
microorganisms and all green plants operate along this pathway of carbon
assimilation; higher plants rely<strong>in</strong>g on it entirely are termed C3 plants. As a result, <strong>the</strong>
bulk of <strong>the</strong> Earth’s biomass (both extant and fossil) bears <strong>the</strong> isotopic signature of C3
(or Calv<strong>in</strong> Cycle) photosyn<strong>the</strong>sis characterised by <strong>the</strong> sizeable fractionations of <strong>the</strong>
RuBP carboxylase reaction that assigns a mean δ 13 C org range of -26±7‰ to most
biogenic matter.
Occasional negative offshoots from this long-term average (Fig. I.4.3.2.2/1) are
commonly restricted to <strong>the</strong> Precambrian (Hayes et al., 1983; Schidlowski et al., 1983)
and suggest <strong>the</strong> <strong>in</strong>volvement of methanotrophic pathways <strong>in</strong> <strong>the</strong> <strong>for</strong>mation of <strong>the</strong>
respective kerogen precursors. Basically, <strong>the</strong>se excursions appear to be oddities
conf<strong>in</strong>ed to side stages of <strong>the</strong> carbon cycle that have never affected <strong>the</strong> economy of
<strong>the</strong> global cycle as a whole.
<strong>The</strong> only apparent discont<strong>in</strong>uity <strong>in</strong> <strong>the</strong> δ 13 C org record depicted <strong>in</strong> Fig. I.4.3.2.2/1 is
<strong>the</strong> break between <strong>the</strong> ~3.8 Gyr-old Isua metasediments from West Greenland and <strong>the</strong>
whole of <strong>the</strong> post-Isua record. <strong>The</strong> observed isotope shift is, however, fully consistent
with <strong>the</strong> predictable effects of an isotopic re-equilibration between coexist<strong>in</strong>g organic
carbon and carbonate <strong>in</strong> response to <strong>the</strong> amphibolite-grade metamorphism
experienced by <strong>the</strong> Isua suite. Both currently available <strong>the</strong>rmodynamic data on 13 C/ 12 C
exchange between C org and carbonate carbon (C carb) as a function of <strong>in</strong>creas<strong>in</strong>g
metamorphic temperatures and observational evidence from a host of geologically
younger metamorphic terranes make it virtually certa<strong>in</strong> that <strong>the</strong> ‘normal’ sedimentary
δ 13 C org and δ 13 C carb records had orig<strong>in</strong>ally extended back to 3.8 Gyr ago, and that <strong>the</strong>
Isua anomaly is clearly due to a metamorphic overpr<strong>in</strong>t (Schidlowski et al., 1979;
1983). Apparently prist<strong>in</strong>e δ 13 C org values <strong>in</strong> <strong>the</strong> range -21‰ to -49‰ (average -7±3‰)
have been recently reported <strong>for</strong> m<strong>in</strong>or carbon <strong>in</strong>clusions <strong>in</strong> apatite gra<strong>in</strong>s from
3.85 Gyr-old Isua banded iron-<strong>for</strong>mation (Mojzsis et al., 1996). Here, carbonaceous