『(Abstract)
Oxygen and silicon isotope compositions of cherts and studies
of protein evolution have been interpreted to reflect ocean temperature
s 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 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℃ to 35℃. 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.』
(Introduction)
(Result)
(Discussion)
Methods summary
(References)
Methods