Earth Science & Climate Change

Biography

Biography: Francois Robert

Abstract

A global cooling of the oceans since 3.5 Gyr is inferred from the continuous record of the coupled ¹⁸O/¹⁶O and ³⁰Si/²⁸Si isotope ratios (expressed in δ¹⁸O and δ³⁰Siunits) in siliceous seawater sediments (cherts) [1-2]. During the past decade, marked advances in this interpretation were made from in-situ analysis at a micrometric spatial resolution. Among the most remarkable findings of this type of analysis is the discovery of a large internal distribution of δ¹⁸O (up to 8‰) in the microquartz. This mineral stands for the first recrystallization step of amorphous opal CT precipitated from seawater and, in this respect, should exhibit the best preservedisotope compositions.Such internal δ¹⁸O distribution is irreconcilable with a thermal isotope equilibrium between seawater and precipitated silica: large isotope fractionation must have taken place in closed micrometric systems, likely through dissolution-reprecipitation of opal CT during diagenesis.Several petrographical and geochemical criteria wereused to reconstruct the original δ¹⁸O of the precipitated silica.These criteria should be regarded as guides to better constrain seawater paleo-temperature reconstructions. Although some Archean microfossil-rich cherts from the Farrel Quartzite (Pilbara Cration /Australia - 3.0 Gyr.) [4] exhibit low δ¹⁸O compatible only with a silica precipitation from hot (80°C) hydrothermal fluids,a warm temperature (≥50°C) for the Archean oceans remains the most plausible conjecture to account for the secular isotope variations in cherts.