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

Alexandra Kunert,Brian Kendall,Johnathan Clarke

doi:10.3390/min10121075

Abstract

Vanadium is an important redox-sensitive trace metal for paleoenvironmental reconstructions. Modern organic-rich sediments persistently contain sediment V enrichments <500 μg/g, but many ancient marine organic-rich mudrocks record enrichments >500 μg/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 (δ98Mo) 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 μM) closely record global seawater δ98Mo 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 μM), 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 δ98Mo for a single time interval, including values at or below oceanic input δ98Mo (0.3–0.7‰). This observation suggests significant isotopic offset from reasonable seawater values (typically ≥1.0‰). 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 μM 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.

Highlights

Redox-sensitive trace metals in organic-rich mudrocks (ORM) are commonly used to infer ancient marine physicochemical conditions
Formation shales in the Williston Basin and the absence of V hyper-enrichments in modern sediments from euxinic basins whose water columns have hydrogen sulfide (H2 S) < 10 mM [9]
We do not rule out occurrence of Vhyper-enrichment under hyper-sulfidic conditions, instead we argue that this is not the primary mechanism for V hyper-enrichment in ORM from most environments

Summary

Introduction

Redox-sensitive trace metals in organic-rich mudrocks (ORM) are commonly used to infer ancient marine physicochemical conditions. Semi-quantitative trace metal proxies for oxygen (O2 ) and hydrogen sulfide (H2 S) levels in ancient bottom waters are well-defined based on modern basin analogs. Such proxies include redox-sensitive metal concentrations (e.g., Mo, Re; [1,2]), sedimentary. The average V concentration in modern oxygenated seawater is ~35–40 nM [12,13], which is regulated by dissolved fluxes from rivers and oxic marine sediments (~62 × 109 g/yr; [13]) into hydrothermal deposits and anoxic organic-rich sediments (~53 × 109 g/yr; [13]).

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