Lanthanide Pacific 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). 

The tholeiitic basalts of Japan that have light-lanthanide depleted distributions were collected on the eastern (Pacific Ocean) side. These islands, like the Caucasus geosyncline region discussed earlier (Ronov et al., 1974) show how lavas with ocean floor affinities and lavas with continental or oceanic island affinities both appear in island arcs. The basalts of the Crescent formation (northwestern Washington) are a good example of welding of materials partly of ocean floor affinity onto a continent (Glassley, 1974). The lower basalts of the... 

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).

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).

Masuzawa and Koyama (1989) attributed the positive Ce anomalies of their sediment trap samples (Japan Sea) to a biologically-mediated oxidation process associated with the presence of Mn oxide particles. This process is discussed in detail in the subsection on Ce redox chemistry. The shale-normalized patterns of Masuzawa and Koyama (1989) do not show any consistent form from which to draw conclusions about lanthanide(lll) fractionation. Only their 2750 m sample is slightly light-element enriched the other four samples have flat or heavy-enriched patterns. Sediment trap particles from the eastern equatorial Pacific Ocean (Murphy and Dymond 1984) are strikingly different in that their shale-normalized patterns are like those of seawater heavy-lanthanide enrichment and negative Ce anomalies. 

When REE fractionation is discussed, it is common to normalize the data to the values in shale which are thought to be representative of the REEs in the upper continental crust. The shale-normalization not only helps to eliminate the well-known distinctive even-odd variation in natural abundance (the Oddo-Har-kins effect) of REEs but also visualizes, to a first approximation, fractionation relative to the continental source. It should be noted, however, that different shale values in the literature have been employed for normalization, together with the ones of the Post-Archean Australian Sedimentary rocks (PAAS) adopted here (Table 1). Thus, caution must be paid on the choice of the shale values if one ought to interpret small anomalies at the strictly trivalent lanthanides such as Gd and Tb. Alternatively, for detailed arguments concerning fractionation between different water masses in the ocean, it has been recommended that the data are normalized relative to the REE values of a distinctive reference water mass, for example, the North Pacific Deep Water (NPDW, Table 1). The NPDW-normalization eliminates the common features of seawater that appeared in the shale-normalized REE pattern and can single out fractionation relative to the REEs in the dissolved end product in the route of the global ocean circulation. 

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). 

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. 

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 ... 

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). 

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