THE TRACE-METAL COMPOSITION OF SUSPENDED PARTICLES IN THE OCEANIC WATER COLUMN NEAR BERMUDA

Trace metal partitioning between seawater and particles, both suspended and sinking, must be determined to understand and model oceanic trace metal scavenging processes. In order to improve the oceanic particulate trace metal data base, a new in-situ pump was developed and deployed to collect suspended particles from 0-4000 m in the Sargasso Sea near Bermuda. These particles were analyzed for Al, Fe, Mn, Co, Zn, Cu, Ni, Cd and Pb; major particulate carrier phases were estimated from Ca, opaline Si, and P. Cd is the only trace element of this group with a particulate maximum in near-surface waters. The other metals have low particulate concentrations (mol/l) in near-surface waters, and increase with depth into the upper thermocline. Particulate Al and Fe are then uniform with depth below about 1000 m until increasing in the bottom nepheloid layer. The remaining elements (Mn, Co, Zn, Cu, Ni and Pb) decrease from the mid-thermocline values to lower and relatively uniform concentrations in deep waters. The similarities among the vertical profiles for these metals suggest that authigenic Mn oxides influence their uptake from the dissolved poll, although an important or dominant role for other host phases, specifically particulate organic matter, is not ruled out by the data. Mean intemediate and deep water concentrations are: Al, 3600; Fe, 1200; Mn, 146; Co, 1.5; Zn, 7.4; Cu, 15; Ni, 6.0; Cd, 0.06; and Pb, 2.1 pmol/l. These particulate Al and Mn values agree with previous measurements in this region, but the other elements are lower by a factor of 3 (Fe, Co) to 30 (Zn), suggesting that earlier data were compromised by contamination. The suspended metal data are used in a simple steady-state two-box flux model to evaluate the role of suspended particles in determining the vertical flux of metals in the Sargasso Sea. The model calculations indicate that removal of suspended particles in the deep (> 500 m) water column contributes less than half of the total deep flux for all of the elements investigated except for Cd. This result implies that uptake of these metals onto particles in the deep central ocean is a minor factor in their oceanic mass balance. If this finding is generally true for oligotrophic ocean environments, whole-ocean residence times for these elements must be governed largely by processes occurring either in near-surface waters or at ocean margins.