Journal of Earth Science & Climatic Change
Open Access

Abstract

A global cooling of the oceans since 3.5 Gyr is inferred from the continuous record of thecoupled 18O/16O and 30Si/28Si isotope ratios (expressed in δ18O and δ30Siunits) in siliceous seawater sediments (cherts). 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 preserve disotope 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 were used 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.) exhibit low δ18O compatible only with a silica precipitation from hot (80ºC) hydrothermal fluids, a warm temperature (�50ºC) for the Archean oceans remains the most plausible conjecture to account for the secular isotope variations in cherts.

Biography

François Robert is an isotope geochemist specialized in Cosmochemistry, Precambrian sediments and organic geochemistry.