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

Oxygen and silicon isotope compositions of cherts and studies of protein evolution 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 temperature. 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 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. [ABSTRACT FROM AUTHOR]