The Distribution of Dissolved and Particulate Mo and V along the U.S. GEOTRACES East Pacific Zonal Transect (GP16): The Roles of Oxides and Biogenic Particles In Their Distributions In the Oxygen Deficient Zone and the Hydrothermal Plume
Molybdenum (Mo) and (V) vanadium are under-studied elements in the ocean due to their largely conservative natures. Thus, no detailed ocean sections of these elements exist although previous reports do suggest possible anomalies in their distributions due to biological, redox, or sorptive processes. Here we present the first detailed ocean sections of dissolved and particulate Mo and V, obtained as part of the 2013 U.S. GP16 GEOTRACES East Pacific Zonal Transect (EPZT) from Peru to Tahiti. Similar to previous work, the distribution of dissolved Mo was largely conservative, while dissolved V showed a ~ 5% depletion in the upper waters. For dissolved Mo, a small number of samples showed significantly depleted concentrations which, in most cases, gradually increased with time after samples were acidified. This implies the original sample had experienced a partial change in speciation of dissolved Mo from the predominant molybdate to another, as yet, unknown form. In the oxygen deficient zone (ODZ) off the Peru margin, depleted dissolved Mo and V in a few samples corresponded with the nitrite maximum, suggesting the possible involvement of both elements in the nitrogen cycle. Particulate Mo and V enrichments in the ODZ are likely indicative of scavenging by Fe oxyhydroxides and/or biogenic particles. In near surface waters close to the Peru margin, dissolved Mo and V concentrations slightly decreased with increasing total chlorophyll a, suggesting the removal of both elements by biological uptake and/or adsorption onto biogenic particles. In contrast to previous reports that removal to reducing coastal/estuarine sediments resulted in surface water depletion of dissolved V, there is no evidence from the EPZT section that this process plays a strong role in the development of the ~ 5% dissolved V depletion in the surface ocean. Additionally, dissolved V and Mo depletions were seen in some hydrothermal plume waters above the ridge crest, likely due to adsorption onto Fe/Mn oxides and suggesting that these plume waters are a net sink for these two elements. Associations of ridge crest particulate Mo and V with the particulate Mn and Fe carrier phases suggests V was largely scavenged by Fe oxyhydroxides while Mo was likely scavenged by both Fe oxyhydroxides and Mn oxides. Away from the ridge crest, depletions of dissolved V and Mo are seen along the boundaries of the far field hydrothermal plume, though the reasons for this remain obscure. Future studies of these elements could benefit from determination of speciation as well as increased focus on margin areas.