Sediment-trap material

The Zn isotope composition of sediment trap material collected over more than one year at the site of the 1991-1992 EUMELI experiment (18° 28 N, 21° 03 W, z = 3851 m)(0° 11.59 N, 110° 31.18 W, z = 3100 m) near the upwelling off the coast of Mauritania, central Atlantic (Marechal et al. 2000), is on average similar to that of clay minerals (0.24 0.14%o), which is consistent with the composition of the settling material. A subtle increase of 8 Zn values of 0.20%o over Spring and Summer in the fractions collected at depths of 1000 and 2500 m suggests, however, that surface productivity preferentially removes the light isotopes from surface water. 

Marechal (1998) reports a relatively large seasonal fluctuation of 0.2%o in sediment trap material at the EUMELI site (see above) at depths of 1000 and 2500 m with maximum depletion of Cu during Spring and Summer. In contrast to Zn, Archer and Vance (2002) found that Cu (and Fe) in Belingwe black shales is signiflcantly lighter with 8 Cu values down to -1.0%o. 

Zooplankton population samples for isotope analysis were composites of 50-200 individuals. Population samples are less variable in isotope composition than are samples of individuals. Replicate isotope analyses of composite samples of zooplankton or POM collected at different locations within the lake varied by no more than 0.5%. Larger organisms such as molluscs, insects, and fish were analyzed individually. Molluscs were soaked in dilute HCl to remove carbonates and then rinsed copiously with distilled water. Fish muscle was analyzed. Sediment trap material was collected in replicate cylinders (11.4-cm diameter, 76.2-cm length) suspended at 4.5-m depth. All isotope samples were dried at 60 °C before analysis. 

Total-P Flux. Two approaches were combined to calculate removal by deposition. During the stratified period deposition was determined from the mass flux, measured by using sediment traps suspended at eight levels in the water column, and the P concentration in the sediment trap material. Positioning of traps at key depths allowed primary and resuspended flux components to be deconvoluted. 

Deposition during the mixed period (up to day 165) was calculated from a mass balance on water-column Si and the Si P ratio in sediment trap material, because sediment traps overestimate the net particle deposition flux in a mixing water column (19). Our calculations assumed that losses of dissolved reactive Si resulting from diatom uptake that are not accounted for by increases in particulate biogenic Si are caused by Si deposition. The estimate of mixed-period P deposition was conservative because we assumed that nondiatom particulate P was removed at a rate similar to diatom P. We also assumed that loss of P in traps resulting from diagenesis-dissolution was negligible. The use of short collection periods (2-3 weeks) and a poison should minimize loss. 

Resuspension of bottom sediments presents a potential problem for flux estimates. However, our results suggest minimal resuspension during stratification. As a part of a separate study, Hurley (unpublished data) measured pigment fluxes to the sediment surface. Sediment trap material was dominated by chlorophyll a and pheophorbide a (a grazing indicator). Surface sediments, however, were dominated by pheophytin a, a relatively stable chlorophyll degradation product. The lack of any substantial amounts of pheophytin in trap material suggested that if resuspension of particulates from the surface sediment was important, it was probably minimal. 

Particle-bound Hg concentrations of sediment trap material exhibited strong seasonal response and accounted for the differences between the Hg flux and mass and carbon fluxes late in the summer. Particle-bound HgT content in spring and early summer was below 200 ng/g, but during late summer stratification it reached levels between 200 and 400 ng/g. Levels were highest following breakdown of thermal stratification and remained high throughout the fall (>350 ng/g). The elevated HgT levels after overturn most likely represented a shift from dissolved to particle-bound Hg. 

Fig. 4 Phaeocystis pouchetiildiatom (P/D) cell carbon ratio is compared suspended (integrated mg C m-2, 0-40 and 0-100 m) and in the corresponding sediment trap material (mg C m-2 d-1) at (a) 40 m and (b) 100 m depth during spring bloom events. The 40 m data includes measurements from North Norwegian fjords (Balsfjord in 1996 and Balsfjord, Ullsfjord and Malangen 1997, n = 26), and the 100 m data includes measurements from...

During an extended station in a Trichodesmium bloom in the Indian Ocean, sediment trap material from that station clearly reflected diazotrophic input, compared to stations away from the bloom (Capone et al., 1998). Another example was observed during the massive bloom of R. intracellularis within H. hauckii off the NE coast of South America (Carpenter et al., 1999). Suspended particles and zooplankton collected within the bloom were depleted in reflecting the dominant contribution of N2 fixation to the planktonic N budget. 

Sediment trap material provides a valuable view of the U37 and quantity of alkenones transiting to the seafloor. Few time-series experiments have been reported to date, although data from the Gulf of California (Goni et al., 2001) and off the coast of Angola (Muller and Fischer, 2001) offer reasonable resolution for 1.5 yr and 4 yr... 

Liebezeit G. (1991) Analytical phosphorus fractionation of sediment trap material. Mar. Chem. 33, 61—69. 

Sediment cores were taken with a gravity corer. They were sliced with 0.5-cm resolution and processed the same way as sediment-trap material. One of the cores (code SE-8802) was dated by Wieland et al. (41), who used the 137Cs record from bomb fallout and the Chernobyl accident. The authors determined the sediment accumulation rate at this site to be 1.84 g/m2 per day. This value agrees with the average sedimentation rate of 2.17 g/m2 per day gleaned from the sediment-trap record of the years 1984-1991 at a depth of 81 m. 

The coordinative environment of manganese in sediment-trap material was evaluated with EXAFS spectra recorded at the Mn K-edge. Figure 7 shows the radial distribution function (RDF) around Mn atoms of a sediment-trap sample. The RDFs of pyrochroite, manganite, Na-bimessite, todorokite, and vemadite are also shown. A comparison of EXAFS data with results from X-ray diffraction (XRD) is shown in Table III for all three oxidation states of manganese. 

Figure 7. Radial distribution functions (RDF), not corrected for phase shift from EXAFS spectra, of sediment-trap material from Lake Sempach and from reference oxides. Pyrochroite, Mn(OH)-, and bimessite [a Mn(IV) oxide] have the same layered structure with edge-sharing Mn octahedra. Todorokite is a Mn(IV) oxide with a 3 X 3 tunnel structure. A shift to longer distances occurs in going from the Mn(IV) oxide bimessite to the Mn(II) hydroxide pyrochroite. Contributions from double-comer Mn-Mn linkages are clearly seen in sediment-trap material and in todorokite and vemadite but not in the layered minerals bimessite and pyrochroite.

The RDF of the sediment-trap material clearly shows two Mn(IV)-Mn(IV) contributions at distances characteristic of edge- and comer-sharing linkages. Therefore, we can exclude the mineral bimessite. This result is consistent with the XRD pattern obtained for sediment-trap material it displays... 

In summary, the EXAFS data of the sediment trap material are compatible with both todorokite and vemadite (8-Mn02). However, the fact that this Mn oxide is X-ray-amorphous suggests that vemadite is the prevalent Mn mineral that forms in the deep waters of Lake Sempach. The formation of... 

Analysis of Sediment-Trap Material. The freeze-dried material from the sediment traps was digested with HCl-HN03 in a microwave digestion device (MLS-1200). Teflon beakers, cleaned with NH03, were used. Fe, Mn, Ca, Zn, Cu, and Cr were determined by inductively coupled plasma emission spectrometry, P by the molybdate spectrophotometric method (25) and organic C and N on a C, H, N analyzer (Heraeus). 

TABLE 13.6 Comparison of Enrichment Factors in the Sediment Trap Material and... 

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