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Sulfur-isotope measurements in 1.18-billion-year-old rocks from Scotland point to a terrestrial environment with enough oxygen to support sulfur-cycling microbes. They do not measure the oxygen content of the ancient atmosphere directly: the result is a geochemical clue, and the biological and environmental conclusions are interpretations of that clue.
What the 2010 study found
In a study published in Nature in November 2010, John Parnell and colleagues reported sulfur-isotope fractionation values (Δ34S) exceeding 50‰ in a terrestrial rock succession dated to 1.18 billion years ago. The rocks include red beds and lake deposits known as lacustrine black shales.
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The paper’s authors interpreted the large fractionation as evidence of sulfur-cycle disproportionation. They concluded that the terrestrial environment at the study site was sufficiently oxygenated to support an adapted biota, including activity extending into sediment below the surface. This is evidence about conditions at the Scottish site, not a direct reading of global atmospheric oxygen.
How sulfur isotopes support the interpretation
From sulfur-bearing rocks to isotope ratios
Chemistry World’s 10 November 2010 report describes sulfur-bearing samples, including pyrite, collected in the Lochinver area of northwest Scotland. It says researchers extracted sulfur chemically or with a laser, then used mass spectrometry to measure isotope ratios. These method details are reported by Chemistry World; the primary paper is the authority for the reported isotope results and their interpretation.
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From fractionation to a possible microbial process
Sulfur has different stable isotopes, and biological and chemical reactions can leave sulfur compounds with different isotope proportions. The study’s large Δ34S values were interpreted as evidence for disproportionation: sulfur compounds are transformed through reactions that can include both oxidation and reduction. The authors proposed that sulfide-oxidizing bacteria probably played a role. “Probably” matters: the isotope pattern supports a process-based inference, not a direct observation of ancient bacteria.
The authors argued that the evidence in both red beds and lacustrine black shales indicated enough oxygen in the terrestrial environment for an adapted microbial community. It does not establish a precise oxygen concentration in the air or show that all land environments shared the same conditions.
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How the terrestrial result compares with the marine record
The paper set its terrestrial measurements against a marine sulfur-isotope record. In the authors’ 2010 summary, marine Δ34S values were below 25‰ before 1 billion years ago and at least 50‰ after 0.64 billion years ago. The Scottish terrestrial succession, by contrast, had values above 50‰ at 1.18 billion years ago.
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What it does—and does not—say about early life
The findings support the possibility that oxygen-adapted microbes lived in a terrestrial environment about 1.2 billion years ago. They do not report fossils of those microbes, directly date the origin of complex animals, or demonstrate that complex life was present at the Scottish site.
Chemistry World framed the result as suggesting complex life could have existed 400 million years earlier than previously thought. That is the report’s broader implication, not a direct measurement or fossil-based conclusion of the study. The primary evidence concerns sulfur isotopes and microbial sulfur cycling.
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Paper and reporting
The primary study is John Parnell, Adrian J. Boyce, Darren Mark, Stephen Bowden, and Sam Spinks, “Early oxygenation of the terrestrial environment during the Mesoproterozoic,” published online on 10 November 2010 and in the 11 November issue of Nature 468, 290–293. Read the paper via its DOI. The article is indexed by PubMed as 2010 Nov 11, Nature 468(7321):290–293, PMID 21068840.
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