Phosphate oxygen isotopic evidence for a temperate and biologically active Archaean ocean. 2010. R. E. Blake, et al. Nature 464, 1029-1032.
Abstract : 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), highly evolved from presumed source values of ~6–8‰ that are characteristic of apatite in igneous rocks and meteorites.
Here we report oxygen isotope compositions of phosphates in sediments from the 3.2–3.5-billion-year-old (Palaeoarchaean) 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.