Biological control of Cl/Br and low sulfate concentration in a 3.5-Gyr-old seawater from North Pole, Western Australia

The concentration of halogens (Cl, Br) and sulfate in seawater during the Archaean eon have important implications for the evolution of Earthʹs hydrosphere and atmosphere and the development of early life. Insights into the composition of Archaean seawater and hydrothermal fluids can be obtained by direct analysis of fluid inclusions preserved in Archaean sediments and hydrothermal systems. Here, we investigated a suite of well-preserved intrapillow quartz–carbonate pods that formed during oceanic hydrothermal alteration of the 3.49 Ga Dresser Formation, North Pole Dome, Western Australia. Texturally, the pods seems to contain a unique population of primary fluid inclusions which were analyzed individually using microthermometry and synchrotron radiation X-ray microfluorescence (μ-SR-XRF) techniques. Bulk chemical analyses were also performed using crush-leach method. hermometric data combined with crush-leach and μ-SR-XRF analyses yielded a model composition of 1100 mM Na, 2250 mM Cl., and 375 mM Ca, which corresponds to a bulk fluid salinity of 12 wt.% salt equivalent. This high Cl concentration (ca. four-times present-day value) reflects a typical modern-day seawater evaporation trend in a shallow marine, closed basin environment. Individual fluid inclusion analysis using μ-SR-XRF revealed the presence of three main fluid populations: a metal-depleted fluid, a Ba-rich and S-depleted fluid, and a Fe–S-rich end-member. The Cl/Br ratio of metal-depleted fluid inclusions (630) is similar to the modern seawater value (649). By contrast, Ba- and Fe-rich brines have Cl/Br ratios (350 and 390) close to bulk Earth value (420), hence arguing for a mantle buffering and a hydrothermal origin of these fluids. The metal-depleted fluid displays low sulfate concentration (0–8 mM compared to 28 mM in present-day ocean). Sulfur content of the Fe-rich fluids ranges between 41 and 82 mM. rock interaction processes occurring in pillow basalts located on the seafloor are unlikely to be the cause of Cl/Br fractionation between hydrothermal fluids and the “North Pole seawater” end-member. We hypothesize that high biological activities associated with the specific geological settings of North Pole (small isolated basin exposed to relatively intense terrigenous sedimentation) are responsible for the buffering of North Pole seawater Cl/Br value. Although the exact effect of water–rock interaction, barite precipitation, and biological activity on the sulfate concentration of analyzed fluids is difficult to asses, the low sulfate concentration recorded for North Pole seawater argues in favor of a poorly oxygenated Archaean environment.