The Trace Metal Geochemistry of Suspended Oceanic Particulate Matter

Vertical profiles of suspended particulate trace metals were measured in the Sargasso Sea near Bermuda and in the California Current, northwest Pacific. Using a new in situ pump, sufficiently large samples of particulate matter (order 10mg) were collected to allow measurement of a suite of trace metals as well as major component elements, using several different leaching techniques on separate subsamples. Concentrations of particulate Cu, Zn, Co, Cd, Ni, and Pb near Bermuda were determined to be substantially lower than estimates based on previously published work. Concentrations of the more abundant elements Al, Fe, and Mn were similar to previous estimates. Vertical profiles of Mn, Co, Pb, Zn, Cu, and Ni (mol/1) at Bermuda dispayed similar features: a relative depletion in surface waters, a relative maximum in the upper thermocline, and relatively constant deep water concentrations. The similarity in the vertical variations of dissolved/particulate fractionation for these metals may be caused by interaction of dissolved metals with authigenic Mn phases; the fractionation is anti-correlated with other major particle components (organic carbon, calcium carbonate, opal, and aluminosilicate). The vertical profile of particulate Cd displayed a different form: enrichment at the surface and decreasing concentrations through the thermocline to constant deep water values. These measurements were incorporated into a steady-state two-box flux model for the Bermuda station. Estimates of the residence time of small suspended particles suggest that metals associated with these particles contribute less than half of the total flux observed in deep ocean sediment traps. This result held for metals displaying a wide variety of behaviors in their dissolved profiles, and suggests that processes occurring in the upper ocean are more important determinants of metal removal from the open ocean environment than dissolved/particulate interactions in the deep ocean. The interactions between suspended particulate and dissolved Pb were investigated further by an analysis of the isotopic composition of Pb in each pool. Profiles of particulate Pb/ 2 10 Pb and 0Pb/20 Pb were virtually identical to contemporaneous seawater profiles for the same tracer ratios in the upper 2000m at Bermuda, indicating isotopic equilibration on a time scale which is rapid relative to the residence time of suspended particles in the water column (a few years), and to the time rate of change of the non-steady state dissolved profiles (years-decades). Analysis of the relative contribution of deep suspended particles to the total deep ocean downward flux suggests that the Pb isotopic composition of material delivered to the sediments will increasingly resemble that of deep ocean suspended matter as the anthropogenic input of Pb to the surface decreases over the next few years-decades. These results support a relatively rapid and reversible exchange of Pb between solution and particles. The particulate metals distributions in the Sargasso Sea were compared with those observed at a station in the California Current, characterized by higher productivity, closer proximity to continental and hydrothermal particle sources, and the distinct dissolved metal distributions of the northwest Pacific. This provided an opportunity to observe the effect of bulk particle composition and dissolved metal concentration on dissolved/particulate metal fractionation. Although concentrations of non-aluminosilicate particulate metals were of the same order (generally within a factor of two) at the two stations, the differences are significant and are not generally proportional to differences in the dissolved distributions. Particulate contents of Mn, Co, and Cu are lower by a factor of 2-3 in mid-waters at the Pacific site, despite similar dissolved concentrations. In contrast, particulate concentrations of the nutrient-type elements Zn, Ni, and Cd were less than 50% greater at the Pacific station, although dissolved metal concentrations are several times higher. Of the metals investigated, only Pb showed similar dissolved/particulate fractionation at both stations. The evidence indicates that metal partitioning in the open ocean water column varies in response to factors which are outside the predictive capability of simple chemical exchange models.