Sargasso suspended particles

Water for the analysis of suspended particles may be collected with any clean water sampler used for oceanographic work, e.g., Niskin bottles, Go-Flo bottles, Nansen bottles. Hydrobios water samplers (see also Chapter 1). The sample volume will depend on the expected concentration of POM. In nearshore and/or biologically productive water, 0.5-2 L usually is an adequate sample volume. Ten litres may be required in particle-poor open ocean waters such as the Sargasso Sea. For sampling large volumes of water, 30 L Niskin bottles are recommended. Even larger volumes may be sampled with in situ pumps. (See Chapters 1,2 and 13). 

Fig. 16. (a, b) Shale-normalized ratios of filtered seawater and suspended particles from a sample collected at 255 m in the Sargasso Sea in April 1989. Results from three sequential digestions of the suspended particles (acetic acid, strong acid and bomb) are compared, (c) Shale-normalized ratio of dust collected at Bermuda in August 1989. Dust data from Sholkovitz et al. (1993) include the results of a mild digestion with dilute HCl... 

Fig. 17. The lanthanide composition of acetic acid digestions of suspended particles normalized to the composition of filtered Sargasso Sea seawater, (a) for samples in the upper 340 m and (b) for samples between 340 m and 2000 m. Note log scale. From Sholkovitz et al. (1994).

The extensive fractionation of trivalent lanthanides between Sargasso Sea water and suspended particles can be qualitatively explained by a solution/surface competition model (fig. 15). The progressive decrease from La to Lu in the extent to which dissolved lanthanides are removed by suspended particles results from an increase in the solution complexation constants as predicted by chemical models (e.g., sect. 4 of this chapter Erel and Morgan 1991, Erel and Stolper 1993, Byrne and Li 1995). 

Fig. 20. Vertical profiles of La, Ce and Nd concentrations from the acetic acid digestion of suspended particles from the Sargasso Sea. The Ce anomaly profile is also shown. From Sholkovitz et al. (1994).

There have been only a few studies of the lanthanide composition of particles in the oceans. We will distinguish suspended particles collected in bottles from settling particles collected in sediment traps. Sediment trap data have been reported by Murphy and Dy-mond (1984), Masuzawa and Koyama (1989) and Fowler et al. (1992). Suspended-particle compositions have been reported for anoxic basins by De Baar et al. (1988) and German and Elderfield (1989, 1990). The only data for oceanic suspended particles are those of Bertram and Elderfield (1993) for the Indian Ocean and Sholkovitz et al. (1994) and Jeandel et al. (1995) for the Sargasso Sea (N. Atlantic). The latter study only measured Nd concentrations on suspended and trapped particles. A fuller discussion of the results and interpretations of particle studies is by Sholkovitz et al. (1994). A brief overview follows. 

Atlantic Ocean and the western North Pacific Ocean (as described in fig. 14). In both cases it is clear that Ce in upper water column is being removed from the dissolved phase. The study of particle surface coatings by Sholkovitz et al. (1994), (discussed in sect. 6.2), demonstrates in-situ oxidation of dissolved Ce to particulate Ce. As shown in fig. 20 with a Sargasso Sea profile, there is a large continuous increase in Ce associated with the surface coatings of suspended particles between surface and 500 m water. The in-growth of Ce in the upper 500 m of the water column corresponds with the subsurface maximum in particulate Mn oxyhydroxide (fig. 19), suggesting that the oxidation of Ce(lll) is directly coupled to the redox chemistry of Mn. 

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