Molybdenum Isotope Constraints on the Origin of Vanadium Hyper-Enrichments in Ediacaran-Phanerozoic Marine Mudrocks

Vanadium is an important redox-sensitive trace metal for paleoenvironmental reconstructions. Modern organic-rich sediments persistently contain sediment V enrichments <500 mu g/g, but many ancient marine organic-rich mudrocks record enrichments >500 mu g/g. Previous studies propose that ancient V enrichments of these magnitudes (V hyper-enrichments) were deposited from hyper-sulfidic bottom-waters with higher H2S levels (>= 10 mM) than observed in modern euxinic basins. To test the importance of hyper-sulfidic conditions for generating V hyper-enrichments, we compare V concentrations with Mo isotope (delta Mo-98) compositions from mudrock samples ranging in age from Ediacaran to Pleistocene. In the modern ocean, sediments deposited from strongly euxinic bottom waters ([H2S](aq) > 11 mu M) closely record global seawater delta Mo-98 because conversion of molybdate to tri- and tetra-thiomolybdate is quantitative. By contrast, large Mo isotope fractionations occur during Mo adsorption to Fe-Mn particulates or because of incomplete formation of the most sulfidic thiomolybdates in weakly euxinic settings ([H2S](aq) < 11 mu M), which both favor removal of lighter-mass Mo isotopes to sediments. We find multiple examples when mudrocks with V hyper-enrichments are associated with a wide range of delta Mo-98 for a single time interval, including values at or below oceanic input delta Mo-98 (0.3-0.7 parts per thousand). This observation suggests significant isotopic offset from reasonable seawater values (typically >= 1.0 parts per thousand). Thus, we conclude that hyper-sulfidic conditions were not responsible for many V hyper-enrichments in Ediacaran-Phanerozoic mudrocks. Instead, sediment V hyper-enrichments can be explained by high Fe-Mn particulate fluxes to weakly euxinic sediments or by moderately restricted euxinic settings with strongly euxinic ([H2S](aq) > 11 mu M but not necessarily > 10 mM) or weakly euxinic (with slow clastic sedimentation rates and high organic carbon fluxes) bottom waters where vigorous water exchange provides a continuous V supply from the open ocean.