Phosphate oxygen isotopic evidence for a temperate and biologically active Archaean ocean

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 O-18:O-16 ratios of dissolved inorganic phosphate (delta O-18(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 O-18(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 O-18(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 O-18(P) values and assuming equilibrium with sea water with delta O-18=0% (ref. 12) range from 26 degrees C to 35 degrees C. The higher delta O-18(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.