Lanthanide Atlantic Ocean

Neutron activation analyses of sixteen samples of sea xmter (eight in duplicate) taken at six widely spaced stations in the Central Atlantic Ocean between 16° N and Equator (depths below 1000 m,) showed that the lanthanide patterns are relatively conservative characteristics of water masses. The differences in lanthanide distribution and total abundance between different water masses are small but significant. The absolute mass abundances of the lanthanides can be illustrated by the following values for North Atlantic Deep Water ... 

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). 

There is a considerable variability in lanthanide patterns in sea water, both regionally and with depth (Elderfield and Greaves 1982, De Baar et al. 1983, 1985a). With the exception of a thin surface zone which may be dominated by aeolian input, there is a general increase of light lanthanides with depth. Total lanthanides are higher in surface waters of the Atlantic Ocean, but lower in deep waters, compared with those of the Pacific (De Baar et al. 1985a). 

Lanthanide complexation with (western North Atlantic Ocean) 543... 

A comparison of profiles from one station in the western North Pacific (Piepgras and Jacobsen 1992) and one station from the South Atlantic Ocean (German et al. 1995) are used below to introduce the major features (fig. 14) of oceanic lanthanide distributions. The following characteristics summarize the major features of lanthanides in the oceans ... 

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).

In 1982, the first oceanographically consistent vertical profiles were reported for nine out of ten lanthanides that could be measured by the ID-TIMS method in the North Atlantic. Since then, a significant amount of data on the distribution of REEs have accumulated from various oceanic regions. For example, Figure 2 shows the station locations where the REEs were measured in seawater, together with the... 

Copeland et al. (1971) describe clays from the mid-Atlantic ridge that are derived from local igneous material and whose lanthanide distribution therefore matches that of the ocean floor basalts. The lanthanides must have been released from the minerals of the parent igneous rock into solution, but they did not equilibrate in any general way with ocean water before being captured by the clays. 

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. 

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). 

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).

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. 

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