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

摘要

Oxygen and silicon isotope compositions of cherts and studies of protein evolution have been interpreted to reflect ocean temperatures of 55-85 ℃ 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 ℃ (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 (δ~(18)O_p) 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 δ~(18)O_P 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 δ~(18)O_P values range from 9.3‰ to 19.9‰ and include the highest values reported for Archaean rocks. The temperatures calculated from our highest δ~(18)O_p values and assuming equilibrium with sea water with δ~(18)O = 0‰ (ref. 12) range from 26 ℃ to 35 ℃. The higher δ~(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.