Oxygen and silicon isotope compositions of cherts1,2,3 and studies of protein evolution4 have been interpreted to reflect ocean temperatures of 55–85 °C during the early Palaeoarchaean era (∼3.5 billion years ago). A recent study combining oxygen and hydrogen isotope compositions of cherts, however, makes a case for Archaean ocean temperatures being no greater than 40 °C (ref. 5). Ocean temperature can also be assessed using the oxygen isotope composition of phosphate. Recent studies show that 18O:16O ratios of dissolved inorganic phosphate (δ18OP) reflect ambient seawater temperature as well as biological processing that dominates marine phosphorus cycling at low temperature6,7. All forms of life require and concentrate phosphorus, and as a result of biological processing, modern marine phosphates have δ18OP values typically between 19–26‰ (VSMOW)7,8, highly evolved from presumed source values of ∼6–8‰ that are characteristic of apatite in igneous rocks9,10 and meteorites11. Here we report oxygen isotope compositions of phosphates in sediments from the 3.2–3.5-billion-year-old Barberton Greenstone Belt in South Africa. We find that δ18OP values range from 9.3‰ to 19.9‰ and include the highest values reported for Archaean rocks. The temperatures calculated from our highest δ18OP values and assuming equilibrium with sea water with δ18O = 0‰ (ref. 12) range from 26 °C to 35 °C. The higher δ18OP values are similar to those of modern marine phosphate and suggest a well-developed phosphorus cycle and evolved biologic activity on the Archaean Earth.
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We thank M. Kastner and J. R. O’Neil for editorial suggestions and G. Olack and K. Fornash for technical assistance with isotopic analyses.
Author Contributions R.B. and A.L. conceived the study; A.L. performed field work, sample collection, petrographic and chemical analyses; S.J.C. and R.B. developed methods of sequential PO4 extraction, purification, micro-precipitation and PO4 oxygen isotope analysis. All authors contributed to the interpretation of results and the writing and editing of the manuscript.
The authors declare no competing financial interests.
This file contains Supplementary Tables S1-S3, Supplementary Sample Descriptions 2.1-2.3, Supplementary Figures S1-S2, a Supplementary Discussion about the oxygen isotopic composition of seawater and Supplementary References. (PDF 1305 kb)
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Blake, R., Chang, S. & Lepland, A. Phosphate oxygen isotopic evidence for a temperate and biologically active Archaean ocean. Nature 464, 1029–1032 (2010). https://doi.org/10.1038/nature08952
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