Oxygen and hydrogen isotope evidence for a temperate climate 3.42 billion years ago

A cooler Archaean ocean The widely held view that the Archaean climate around 3.5 billion years ago was remarkably warm — with ocean temperatures perhaps as high as 80 °C — has been questioned on the grounds that the established method of estimating ancient ocean temperature (from the oxygen isotope ratios of sedimentary deposits) is subject to significant uncertainty. Michael Hren et al . have adopted a different approach to estimating ocean temperature, based on an analysis of both oxygen and hydrogen isotopes of 3.4 billion-year-old Buck Reef Chert sediments from South Africa. The isotopes sampled are consistent with formation in waters no warmer than about 40 °C, suggesting that Earth's early oceans may have been far cooler than previously thought. The study of stable oxygen isotope ratios (δ 18 O) of Precambrian cherts suggests that ocean temperatures during the Archaean era (about 3.5 billion years ago) were between 55 °C and 85 °C, but uncertainty about the δ 18 O of the primitive ocean has led to considerable debate regarding this conclusion. Here, a combined analysis of oxygen and hydrogen istopes sampled from 3.42-billion-year-old Buck Reef Chert rocks in South Africa indicates that the ancient ocean was much cooler than previously thought. Stable oxygen isotope ratios (δ 18 O) of Precambrian cherts have been used to establish much of our understanding of the early climate history of Earth 1 , 2 , 3 and suggest that ocean temperatures during the Archaean era (∼3.5 billion years ago) were between 55 °C and 85 °C (ref. 2 ). But, because of uncertainty in the δ 18 O of the primitive ocean, there is considerable debate regarding this conclusion. Examination of modern and ancient cherts indicates that another approach, using a combined analysis of δ 18 O and hydrogen isotopes (δD) rather than δ 18 O alone, can provide a firmer constraint on formational temperatures without independent knowledge of the isotopic composition of ambient waters 4 , 5 . Here we show that δ 18 O and δD sampled from 3.42-billion-year-old Buck Reef Chert rocks in South Africa are consistent with formation from waters at varied low temperatures. The most 18 O-enriched Buck Reef Chert rocks record the lowest diagenetic temperatures and were formed in equilibrium with waters below ∼40 °C. Geochemical and sedimentary evidence suggests that the Buck Reef Chert was formed in shallow to deep marine conditions, so our results indicate that the Palaeoarchaean ocean was isotopically dep