Carbon isotope evidence for early life

1: Division of Geological and Planetary Sciences, California Institute of Technology,
    Mail code 170-25, Pasadena, California 91125 USA eiler@mail.gps.caltech.edu

2:  Scripps Institution of Oceanography, University of California San Diego,
    La Jolla, California 92093-0220 USA smojzsis@ucsd.edu; arrhenius@ucsd.edu

Sir — The recent Letter by Mojzsis et al.1 on evidence for life on Earth before 3,800 million years ago was accompanied by a discussion of the effects of prograde thermal metamorphism on carbon isotope ratios.  This discussion included an analysis of the effect a Rayleigh distillation process might have in reducing the d¹³C of organic carbon that is residual to oxidation during diagenesis and metamorphism.  The calculations presented in that discussion (including Fig. 3 and its caption, and text on page 58 of ref. 1) contained errors that contributed to the conclusion that it would have been physically impossible for such a process to produce the observed d¹³C values of approximately -35‰ (PDB) from initial abiotic values of -10‰.  The purpose of this correspondence is to correct the errors in this calculation and to discuss their significance for the interpretation of the data reported.
        Two related errors are present in the Rayleigh distillation calculation as published in ref. 1:  1) the Rayleigh equation in the caption of Fig. 3 uses d¹³C values where 13C/12C ratios should be used, and 2) the direction of the equilibrium isotopic fractionation between CO2 and graphite is opposite that necessary for use of the Rayleigh equation as it is commonly derived for stable isotopes, and its absolute magnitude differs substantially from that estimated by experimental studies.  The correct form of the Rayleigh equation for the distillation of CO2 from residual graphite is:
   R_f = R_i^.F(a-1)       eqn. 1
Where R_i is the isotope ratio ¹³C/¹²C in the graphite prior to oxidation and distillation of CO₂, R_f is the same ratio in the residual graphite after this reaction, F is the mole fraction of the residual phase (i.e. graphite) remaining after this reaction and a is the equilibrium isotope fractionation factor between the evolved CO₂ and residual graphite at any given step in the Rayleigh distillation process, defined as a = R_CO2/R_graphite².  Translation of eqn. 1 into standard d notation yields:
  d_f = 1000^.(F(a-1) - 1) + d_i^.F(a-1)    eqn. 2

Where d values are defined by the equation:
   d = (R/R_std - 1)^.1000      eqn. 3
with R_std being the ¹³C/¹²C ratio of a reference standard (e.g. PDB).
        The correct value for a(CO₂-graphite) at 400 C, the temperature appropriate for modeling prograde metamorphism of the samples in question, is 1.0111 (ref. 3), corresponding to an 11‰ difference in d¹³C between CO₂ and graphite.  Using this value for a, an initial d¹³C value of -10‰PDB (the lower limit of ‘abiotic’ carbon from ref. 1), and eqn. 2, a d¹³C value of -35‰ in residual graphite will be attained when F = 0.1 (10% of the original carbon remains in the rock as graphite), substantially different from the value of F = 2.5^.10^-11 estimated by Mojzsis et al.¹  It was argued that the extreme value of F they calculated disproved the possibility that low d¹³C was the result of oxidation during metamorphism because virtually no C would remain in the rock if it had experienced such extreme extents of reaction.  The correct value of F = 0.1 (90% reaction) is not sufficiently extreme to offer such disproof, and instead the calculation permits that under the assumed conditions the oxidation of carbonaceous matter during metamorphism could produce residual graphite with d¹³C values in the range that is often regarded as diagnostic of biogenic carbon (i.e. less than -20‰).
        The debate over the C-isotope shifts that are expected to accompany diagenesis and metamorphism of carbonaceous matter is old (e.g., refs. 4,5), and controversy as to the origin and initial isotopic composition of carbon in Archean rocks remains today (e.g., ref. 6).  Resolving this issue for a given sample requires that one prove or disprove the action of oxidation reactions that proceed by Rayleigh distillation during hydrocarbon maturation and metamorphism.  The requirements for such a process to lead to low d¹³C residual carbon are: 1) fluids in equilibrium with residual hydrocarbons and/or graphite must have a high ratio of CO₂/CH₄, and 2) isotopic exchange between residual hydrocarbons and/or graphite and fluids must be rapid.  Under common fO₂ conditions, mid-crustal pressures, and at temperatures less than 500 C, C-O-H fluids are generally CH₄ dominated⁷.  Thus, these requirements are not expected to be met unless metamorphic conditions are unusually oxidizing or if temperatures are higher⁷.  A number of previous studies have demonstrated that metamorphism of carbonaceous matter in sedimentary rocks commonly leads to increases, not decreases, in d¹³C due to the distillation of CH⁴ and exchange with high d¹³C carbonate minerals (e.g., 8,9).  Armoring of carbonaceous matter by crystals of a C-poor phase, such as is the case for the graphite analyzed by Mojzsis et al.,¹ would tend to further reduce the opportunities for isotopic fractionation during metamorphism (e.g., 10).  For these reasons, the authors of the original paper (ref.1) support the initial interpretation that measured values of d¹³C in graphite define maximum limits on the initial d¹³C values of precursor carbonaceous matter.
 

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