Phosphate oxygen isotopic evidence for a temperate and biologically active Archaean ocean
Nature 464, 1029 (2010). doi:10.1038/nature08952
Authors: Ruth E. Blake, Sae Jung Chang & Aivo Lepland
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.