Variability and Export Timescales of Upper Ocean Particulate Trace Metals in the North Pacific Subtropical Gyre

Abstract

The transport of trace metals (TMs) in and out of the upper ocean is largely controlled by particulate phases, but the seasonal variability and residence time of these particles are not well known. Over three years and 21 cruises, we measured upper ocean particulate trace metal concentrations and export fluxes at 150 m at Station ALOHA in the North Pacific Subtropical Gyre. Vertical profiles for most metals were highest near the surface, but Al, Fe, Ni, and Cu also showed evidence of scavenging below the mixed layer. In contrast, labile particulate Mn and Cd had unique subsurface maxima driven by the photoinhibition of Mn-oxides in the euphotic zone and uptake of Cd near the base of the euphotic zone. Our sampling period captured pulses of lithogenic dust input, evident in high export fluxes of particulate Al, Ti, and Fe, which were exported on a timescale of ∼10 days. The mean export-based residence time for labile particulate Fe was 36 days, three times longer than that for recalcitrant Fe, indicating that labile particulate Fe is recycled several times before export. Particulate Cu and Co had mean residence times of 6 months, similar to particulate C, N, and P, suggesting that their export is controlled by the export of biomass. Labile particulate Mn, Ni, and Cd appear to be exported more efficiently with mean residence times of 1.8–3 months. The range of TM residence times underlines the differences in the recycling, biotic utilization, and scavenging of these metals in the upper ocean.