Exploring the sensitivity of the vanadium redox proxy to Fe/Mn-(oxyhydr)oxide cycling in a basinal oxic margin setting

Nicol D Udy, Sune Nielsen, Maureen Auro, Kasper Olesen, Don Eugene Canfield

Accurately interpreting the redox state of ancient seawater using the Vanadium (V) paleo-redox proxy requires us to have a comprehensive understanding of the sinks that might impact vanadium’s cycling in the ancient ocean. Ferromanganese (oxyhydr)oxides accumulating in sediments beneath oxic seawater probably constitute the largest modern sink of marine V. In many oxic sediment sinks, V enrichments are low relative to the crustal background, and authigenic δ51V values (where δ51V = 1000 ‰ * [(51V/50VSample – 51V/50VAlfa-aesar)/(51V/50VAlfa-aesar)]) are offset from seawater δ51V by -1.1 ‰. However, recent evidence supports the existence of an efficient V-Fe-Mn-(oxyhdyr)oxide sink on some portions of the oxic continental margin (OCM). Prior to the observation of V enrichments in the aforementioned OCM sediments, efficient V removal from oxic seawater had primarily been described near hydrothermal plumes and seeps. Moreover, most of the OCM is likely not enriched in V, implying that OCM V enrichment may be produced by an unconstrained, and potentially inhomogeneous, V sink. One such V enriched OCM basin can be found in the Skagerrak seaway, located between Norway and Denmark. In this study, we report δ51V values and V, Mn, and Fe concentrations from sediments collected from the shelf, slope, and deep basin of the Skagerrak. Elemental concentrations were measured from porewaters, and from sediment solids using a novel sequential extraction method that distinguishes between V bound to Fe- and Mn- (oxyhydr)oxides.
Vanadium sequestration rates into the deep basin of the Skagerrak are at least 10x higher than rates reported from typical OCM sediments. Authigenic V delivery into the Skagerrak basin is primarily driven by an allochthonous Fe-(oxyhydr)oxide sink that reproduces the typical V-(oxyhydr)oxide isotopic fractionation factor of -1.1 ‰. Additionally, we hypothesize that Mn-oxides precipitating from Mn2+ effluxing from shelf sediments enhance the V sink in the deep basin of the Skagerrak. Authigenic V concentrations are dominated by Mn-oxides in the top sediment of the Skagerrak basin, the only sediments in our data set where Mn concentrations are diagenetically enriched enough to approach Fe concentrations. These deep basin top sediments display a unique authigenic V isotope fractionation relative to seawater <0.9‰, suggesting that V-Mn-oxide precipitation may attenuate the V isotopic fractionations typically induced by adsorption onto (oxyhydr)oxide minerals. The allochthonous Fe-(oxyhydr)oxide V sink increases in importance from shelf → basin while the diagenetic Mn-oxide V sink is uniquely important in the basin, suggesting that a basin shaped hydrography may be one critical factor enhancing V delivery into OCM sediments. Much of the sedimentary archive heretofore analyzed for V was deposited in epicontinental basins. Caution should therefore be applied when using V to interpret the redox state of the ancient seawater that once over lied sediments depositing in basins. Moreover, our results imply that Mn-oxides will be an important V sink from seawater when they are diagenetically enriched, and that diagenetically precipitated (oxyhydr)oxides in general may induce distinct, and previously unrecognized, V isotopic fractionations from seawater.