The link is here: Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada (Tsuyoshi Komiya et al, Nature 549, 516–518 (28 September 2017)).
From the introduction:
The presence of life on early Earth is still controversial owing to the scarcity and poor preservation of the Eoarchean records. Isotopic compositions of graphite in the Eoarchean sedimentary rocks in the Isua supracrustal belt (ISB) suggest that the graphite grains have biogenic origins because of the enrichment of light carbon isotope4–7. However, biogenic graphite has not been discovered in the 3.83 Ga Akilia association and 3.75 Ga Nuvvuagittuq supracrustal belt1–3. Recent reassessment of uranium–lead (U–Pb) dating and cathodoluminescence observation of zircons from the Uivak Gneiss in Saglek Block (Fig. 1), northern Labrador, Canada indicated the presence of the oldest supracrustal rock in the world, intruded by the more than 3.95 Ga Uivak- Iqaluk Gneiss8,9. Here we report for the first time, to our knowledge, on the occurrence and geochemical characteristics of the oldest graphite
The authors are evaluating the changes in the ratio between the carbon 13 and carbon 12 isotopes:
Carbonate rocks are also found in St. John’s Harbour East, and have some chert nodules (Extended Data Fig. 2c). Graphite grains form elongated shapes or aggregates of globules. The carbonate rocks have distinctive positive La, Eu, and Y anomalies on the shale-normalized rare earth element patterns, diagnostic of chemical sediments, precipitated from seawater mixed with hydrothermal fluid (Extended Data Fig. 3b). The TOC contents of carbonate rocks range between 0.09 and 0.16 wt%, and the ? 13Corg and ? 13Ccarb values are between ? 6.9 and ? 9.9‰ and between ? 3.8 and ? 2.6‰, respectively (Extended Data Table 1, Fig. 2). It is well known that the ? 13Ccarb value decreases with later alteration7 so that a ? 13C value of marine inorganic carbon in the Eoarchean is estimated to be higher than the maximum value (? 2.6‰).
The authors evaluate some possible confounding issues such as a geochemical (abiotic) Fischer-Tropsch type reaction, or possible intrusion of grains from younger rocks; they date the oldest rocks, via a U/Pb dating technique as being 3.95 billion years old, in some cases the grains of older rocks are inclusions in carbonate rocks, which owing to anomalies in the distribution of lanthanum, europium and yttrium are inferred to have abiotic marine origins.
They conclude their paper thusly:
..On the other hand, the siderite decomposition pathway cannot be excluded for the carbonate rocks because they contain magnetite, and the isotopic differences, 4 to 6‰, between graphite and carbonate (? 13Ccarb–? 13Corg) in the carbonate rocks are consistent with equilibrium isotopic fractionation during graphite formation from siderite at around 700 °C20. The Fischer–Tropsch-type synthesis requires an appropriate catalyst such as Ni-Fe metal and magnetite, and a source of H2 and CO, and the reaction is operated between 200 and 350 °C21,22. Although the H2-rich reducing conditions can be produced by hydrothermal alternation of ultramafic rocks21,22, the highly 13C-depleted graphite grains are present only in the clastic sedimentary rocks without ultramafic rock-derived chromite, hence the contribution of the Fischer–Tropsch-type synthesis is insignificant. We conclude that the graphite from clastic sedimentary rocks in the Saglek Block has a biogenic origin and the primary ? 13Corg and ? 13Ccarb values were estimated to be less than ? 28.2 and greater than ? 2.6‰, respectively. As a result, the isotopic fractionation between graphite and carbonate (? 13Ccarb–? 13Corg) reached ? 25.6‰ more than those in turbidite-derived sedimentary rocks of the Isua supracrustal belt6. The large fractionation provides the oldest evidence for autotrophs, using the reductive acetyl-CoA pathway or the Calvin cycle, over 3.95 Ga (Extended Data Fig. 5).
It does seem to me - and I'm in no way an expert in this field - that it is something of a leap, possibly of faith - to attribute this isotopic shift to something as detailed as an acetyl-CoA Calvin cycle type metabolism. The calvin cycle is orchestrated enzymatically, and there is no
chemical evidence of life here, merely a physical apparent fractionation of isotopes that
may be consistent with life.
I note as well, that the distribution of carbon isotopes can clearly be inconsistent in different regions of space, depending on the origins of the elements in supernovae (or similar) cataclysmic stellar events, or the simple boiling off of the elements in AGB type stars which tend to dominate the universe.
These isotopic variations can be seen, for example, in many types of meteorites with many elements, calcium for example. Nearly pure calcium-44 has been found in some meteorites which clearly originated from the decay of titanium-44 during a rather rapid fractionation event that probably derived from supernovae, however it does seem that most of the carbon on earth, reflected by the carbon isotope distribution ratios we see locally, probably originated in fluxes from AGB stars.
ISOTOPIC ANALYSIS OF SUPERNOVA SiC AND Si3N4 GRAINS FROM THE QINGZHEN (EH3) CHONDRITE (Cf, Zinner
et al, The Astrophysical Journal, 709:1157–1173, 2010 February 1)
Isotopic anomalies, from my less than comprehensive understanding, might well originate from localized distribution of isotopes that may have prevailed on earth very early in its history before homogenization of distribution of isotopes took place by geochemical and aqueous interactions.
Just saying...
Have a nice day tomorrow.
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