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Phosphate oxygen isotopic evidence for a temperate and biologically active Archaean ocean
Blake, R.E.; Chang, S.J.; Lepland, A. (2010). Phosphate oxygen isotopic evidence for a temperate and biologically active Archaean ocean. Nature (Lond.) 464(7291): 1029-1032.
In: Nature: International Weekly Journal of Science. Nature Publishing Group: London. ISSN 0028-0836; e-ISSN 1476-4687, meer
Peer reviewed article  

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  • Blake, R.E.
  • Chang, S.J.
  • Lepland, A.

    Oxygen and silicon isotope compositions of cherts(1-3) and studies of protein evolution(4) have been interpreted to reflect ocean temperatures of 55-85 degrees C during the early Palaeoarchaean era (similar to 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 degrees C (ref. 5). Ocean temperature can also be assessed using the oxygen isotope composition of phosphate. Recent studies show that (18)O:(16)O ratios of dissolved inorganic phosphate (delta(18)O(P)) reflect ambient seawater temperature as well as biological processing that dominates marine phosphorus cycling at low temperature(6,7). All forms of life require and concentrate phosphorus, and as a result of biological processing, modern marine phosphates have delta(18)O(P) values typically between 19-26 parts per thousand (VSMOW)(7,8), highly evolved from presumed source values of similar to 6-8 parts per thousand that are characteristic of apatite in igneous rocks(9,10) and meteorites(11). 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 delta(18)O(P) values range from 9.3 parts per thousand to 19.9 parts per thousand and include the highest values reported for Archaean rocks. The temperatures calculated from our highest delta(18)O(P) values and assuming equilibrium with sea water with delta(18)O=0% (ref. 12) range from 26 degrees C to 35 degrees C. The higher delta(18)O(P) 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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