Composition and temperature of Archean Oceans

Like the existing carbonate sediments, Archean sedimentary formations (BIFs and cherts) provide access to several physico-chemical parameters of the Archean ocean (such as pH, T, and fO2). Our work in recent years has shown that studies on a micrometric scale of the isotopic compositions of O and Si in Precambrian cherts provide markers of their origin (diagenetic-sedimentary, hydrothermal, or by silicification), and in the case of a diagenetic-sedimentary origin, they also serve as markers of the compositions and temperatures of the diagenetic fluids and of seawater. Our goal is to continue in this direction by establishing new criteria for recognizing the origin and preservation of Precambrian cherts by combining the isotopic and elemental geochemistry of Ge (which is the main trace element that replaces Si in quartz) with those of Si and O. We will also move forward in the use of the isotopic compositions of Si by a more precise determination of isotopic fractionation during the formation of amorphous silica and opal through a study of current analogues of siliceous deposits in African alkaline lakes. Finally, BIFs of different ages will be studied in detail with high spatial resolution. The laminations of the BIFs constitute temporal records of sedimentation and thus, of the composition of seawater. We will measure the Ge/Si ratio with a laser coupled ICP-MS (in collaboration with Georessources-Nancy) at the same scale as the isotopic compositions of Si, O, and Fe. Coupled with the study of the isotopic compositions of Fe and Ge, these measurements will help to clarify the detrital, diagenetic, and hydrothermal sources of these formations. These tracers will also provide information on the possible genetic links between these sedimentary formations and will help in understanding the changes in the physico-chemical parameters that led to the almost total disappearance of BIFs during the oxygenation of the atmosphere at around 2.3 Gy.