Reactive vanadium and iron fluxes in different modern sedimentary environments

Abstract

This study evaluated the removal of the metals (Me) vanadium and iron associated with the operationally defined HCl-extractable (Me_HCl), pyrite (Me_pyr), and reactive (Me_reac = Me_HCl + Me_pyr) fractions in the sediments of five contrasting sedimentary environments: (1) the oxygen minimum zone (OMZ; San Lázaro and San Blas basins); (2) the anoxic/anoxic-sulfidic sediments of the California Continental Borderland basins and Todos Santos Bay Canyon; (3) the oxic sediments of the Patton Escarpment, Baja California continental shelf, and deep sediments of the Gulf of Mexico (GoM); (4) the Guerrero Negro Hypersaline System (GNHS) in Baja California Sur, Mexico; and (5) the turbiditic deposits of the GoM. The average V_HCl concentration ranged from 8 ± 15 nmol g⁻¹ in turbidites to (7.9 ± 4.3) × 10² nmol g⁻¹ in the OMZ, respectively. The V_pyr concentrations were within the narrow range of 6.6 ± 4.2 nmol g⁻¹ (oxic sediments) to 8.6 ± 5.4 nmol g⁻¹ (turbidites), which indicates that the pyrite fraction is not an important reservoir of reactive V. The relative consistency of the V_pyr concentrations (7.2 ± 5.2 nmol g⁻¹; n = 1098), regardless of the sedimentary environment and redox state of the system, allowed us to calculate a global burial value for oceanic sediments of 4.2 ± 3.0 Gg y^–1. The V_HCl enrichment observed in OMZ and anoxic/anoxic-sulfidic sediments may be due to its incorporation into acid volatile sulfide. The lowest average values of the degree of V pyritization corresponded to the OMZ (0.98 ± 0.68%) while the highest average values corresponded to turbidites (69 ± 32%), with the latter being due to their extremely low V_HCl values. The mass accumulation rate (MAR) calculations for reactive Fe and V show that the sediments of all OMZ regions could annually incorporate (12.1 ± 8.6) × 10⁴ Gg and (6.3 ± 3.8) × 10² Gg of reactive Fe and V, respectively. Although the GNHS exhibited the greatest MAR value of reactive Fe and V [(32 ± 28) × 10² g m⁻² ky⁻¹ and 35 ± 35 g m⁻² ky⁻¹, respectively], this environment currently does not contribute an important percentage of the MAR at the global level due to its small geographical area. However, in the geological past when its geographical area was much greater, the MAR values of reactive Fe and V may have been more important. Although deep sediments (>1000 m water depth) cover a substantial area, they contribute a relatively small, yet significant proportion of the total vanadium (V) deposited in ocean sediments, ranging from 9.7% to 24.8%.