The isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans

Abstract

Despite important biological and biogeochemical consequences of extensive ocean anoxic events, their identification is controversial. The marine isotope geochemistry of molybdenum can help quantify the past oxygenation state of the ocean if the riverine input of Mo isotopes is known. Analysis of a set of rivers that account for 28% of global river runoff suggests more variable Mo isotopic ratios in rivers that are also isotopically enriched in the heavy isotopes, suggesting near-total anoxia in the Proterozoic ocean and during Mesozoic ocean anoxic events. Despite its important biological and biogeochemical consequences1,2,3, the identification of extensive oceanic anoxia in the geological record is controversial. In particular, global anoxia is difficult to distinguish from spatially restricted anoxia in the deep ocean, or in tectonically isolated basins such as the modern Black Sea. The marine isotope geochemistry of molybdenum (Mo) can help quantify the past oxygenation state of the ocean4,5,6,7,8,9,10,11,12,13,14, because to first approximation under oxic conditions lighter isotopes of Mo are preferentially removed to sediments, whereas in euxinic conditions quantitative removal leads to no fractionation. However, the isotopic composition of the Mo input from rivers, the main contributor of Mo to the oceans, is poorly constrained and had been assumed to be isotopically comparable to the narrow range found in a small dataset of continental rocks4,5,6,7,8,9,10,11,12,13,14. Here we present an isotopic analysis of Mo in a set of rivers that together account for 22% of the global riverine water discharge. We find a broad range of variability in the Mo isotopic composition of these rivers, with almost all samples enriched in the heavy isotopes compared with continental rocks. Our data remove key uncertainties associated with the marine Mo isotope budget14 and strongly suggest near-total anoxia in the mid-Proterozoic ocean8 as well as during Mesozoic ocean anoxic events13.