Towards a holistic sulfate-water-O2 triple oxygen isotope systematics

Triple oxygen isotope (Delta O-17 with delta O-18) signals of H2O and O-2 found in sulfate of oxidative weathering origin offer promising constraints on modern and ancient weathering, hydrology, atmospheric gas concentrations, and bioproductivity. However, interpretations of the sulfate-water-O-2 system rely on assuming fixed oxygen-isotope fractionations between sulfate and water, which, contrastingly, are shown to vary widely in sign and amplitude. Instead, here we anchor sulfate-water-O-2 triple oxygen isotope systematics on the homogeneous composition of atmospheric O(2 )with empirical constraints and modeling. Our resulting framework does not require a priori assumptions of the O-2- versus H2O-oxygen ratio in sulfate and accounts for the signals of mass-dependent and mass-independent fractionation in the Delta O-17 and delta O-18 of sulfate's O-2-oxygen source. Within this framework, new Delta O-17 measurements of sulfate constrain similar to 2.3 Ga Paleoproterozoic gross primary productivity to between 6 and 160 times present-day levels, with important implications for the biological carbon cycle response to high CO2 concentrations prevalent on the early Earth.

Automated summary

Triple oxygen isotope signals are useful for studying modern and ancient weathering, hydrology, atmospheric gas concentrations, and bioproductivity, but interpretations of the sulfate-water-O-2 system must consider varying oxygen-isotope fractionations. By anchoring the systematics on the homogeneous composition of atmospheric O(2) through empirical constraints and modeling, new insights without a priori assumptions are gained for the response of the biological carbon cycle to high CO2 concentrations on the early Earth.