Lanthanide Indian Ocean

The total concentrations of the lanthanides in the Indian Ocean (J) are about 100 times greater than in the deep Central Atlantic Ocean, but comparable with that of surface waters of the Pacific Ocean near California (4). 

Five papers form the core of this section s discussion Elderfield (1988), Piepgras and Jacobsen (1992), Bertram and Elderfield (1993), Sholkovitz et al. (1994) and German et al. (1995). While other papers make important contributions and will be mentioned to different extents, these five papers provide the most detailed coverage of this subject. Elderfield (1988) reviewed the field using data from papers prior to 1988. The two detailed profiles from Piepgras and Jacobsen (1992) best illustrate vertical distributions in the North Pacific Ocean. Bertram and Elderfield (1993) used all existing data including their detailed study of the Indian Ocean to provide a broad discussion of the distribution and fractionation of lanthanides in the world s oceans. The study by Sholkovitz et al. (1994)... 

Figures 23 and 24 (from Bertram and Elderfield 1993) illustrate interelement fractionations. There is a clear regional variation between oceans [fig. 23]. Surface water concentrations of both Er and Nd increase in the following sequence Atlantic > Indian > Pacific. .. This is the reverse of the situation in the deep waters of the oceans.. .. deep waters concentration of the REEs increase in the following sequence Atlantic < Indian < Pacific. The oceanwide trend in surface waters REE concentrations suggests that the primary control is that of REE input from the continents rather than scavenging efficiency for REE removal. Studies of Nd isotopes confirm this key point -continents as the dominant source of lanthanides to oceans (sect. 5.1).

Bertram and Elderfield (1993) point out the major features with respect to fractionation. As expected from their close positions in the lanthanide series, the oceanic distributions of Nd and Sm are tightly coupled. Sm and Nd molar ratios range between 0.181 and 0.203, with very minor variability and a mean value equal to 0.189. Sm/Nd ratios decrease in the following sequence Atlantic (0.203 0.009, mean and standard deviation) > Indian Ocean (0.196 0.013) > Pacific (0.181 0.024). Thus, the inter-basin differences are not significant relative to the range for each ocean. Since the Sm/Nd ratio for average shale is identical to the oceanic mean but is smaller than Sm/Nd for dissolved lanthanides (Sm/Nd w 0.25) in many rivers (Elderfield et al. 1990), oceans are fractionated relative to the dissolved river input with a small enrichment in Nd relative to Sm (Bertram and Elderfield 1993). 

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. 

Redox potential (oxidation-reduction) is considered a master variable with respect to controls on the concentration and speciation of many trace elements in natural waters (Stumm and Morgan 1981). Shifts between oxic, suboxic and anoxic conditions represent one of nature s most dramatic chemical variations. The response of lanthanides to variations in redox conditions has been studied in many of the world s classic anoxic and suboxic basins. These include (1) the Black Sea (German et al. 1991, Schijf et al. 1991, 1994, Schijf and De Baar 1995), (2) Saanich Inlet (Canada) (German and Elderfield 1989), (3) Chesapeake Bay (Sholkovitz and Elderfield 1988, Sholkovitz et al. 1992), (4) the Cariaco Trench (De Baar et al. 1988), (5) the Mediterranean Sea (Schijf et al. 1995) and (6) the northwest Indian Ocean (German and Elderfield 1990). The latter two regions are located on ocean shelves while the first three basins are estuarine and coastal. Data from the papers cited above are compiled in table A12. 

Since the first reliable measurements of seawater by Elderfield and Greaves (1982), approximately 25 publications have been reported on the distribution of lanthanide concentrations in the oceans and inland seas. Table A4 provides a bibliography of these papers sorted by region (Atlantic, Indian, Pacific and Arctic Oceans and the Mediterranean, Black and Baltic Seas). There are also approximately ten papers which have focused on the Nd isotopic composition of seawater they usually report the concentration of Nd and Sm. These papers also listed in table A5. 

Fig. 24. Lanthanide interelement ratios (a) Sm vs. Nd, (b) Er vs. Nd, and (c) Ce vs. Ce for the Atlantic, Indian, and Pacific oceans. Ce = [2(La/La tate) + (Nd/Nd,teie)]/3, where the subscripts refer to concentrations in shale. Data as in fig. 23. From Bertram and Elderfield (1993).

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