IRON ISOTOPE CONSTRAINTS ON FE CYCLING AND MASS BALANCE IN OXYGENATED EARTH OCEANS

Abstract

The Fe isotope composition of Proterozoic to modern clastic sedimentary rocks and aerosols defines a range in 56Fe values that is only slightly more variable than the range of Fe isotope compositions measured in terrestrial igneous rocks, indicating that chemical weathering, sedimentary transport, and diagenesis play only a minor role in producing Fe isotope variations in environments where Fe redox conditions have been controlled by current levels of atmospheric oxygen. In contrast, the Fe isotope compositions of hot fluids (>300 degrees C) from mid-ocean-ridge (MOR) spreading centers define a narrow range that is shifted to lower 56Fe values by 0.2%-0.5% as compared to igneous rocks. These new data allow a conceptual model for the Fe isotope composition of the oxic oceans that predicts large ranges in Fe isotope composition under conditions of changing aerosol and MOR Fe fluxes, such as during periods of major worldwide glaciation. [PUBLICATION ABSTRACT]