Lanthanide composition

Lanthanide distribution models 521 7.2. The lanthanide composition of marine ... 

This handbook article combines an up-to-date tabulation of the lanthanide composition of the ocean with a description of lanthanide distributions in the context of physical, chemical and biogeochemical processes controlling these distributions. The focus of this chapter is water column biogeochemistry. While pore waters and hydrothermal waters will be considered in this article, the extensive literature on the lanthanide geochemistry of minerals and marine sediments will not be discussed. 

The objective of this section is to introduce some of the broad characteristics and properties of lanthanides in seawater along with the rationale for studying the lanthanide composition of natural waters. The themes developed below will be expanded upon in subsequent sections. 

Lanthanide composition and aquatic chemistry of river water... 

Fig. 8. Shale-normalized lanthanide compositions of 0.22 (tm filtrates of the Amazon, Fly (Papua New Guinea) and Mississippi River waters. Amazon and Mississippi data from Sholkovitz (1993, 1995) Fly River data from unpublished work of Sholkovitz. G/J Avg. refers to the averaged river water composition of Goldstein and Jacobsen (1988a).

The importance of colloids on the lanthanide composition of rivers waters was developed in Elderfield et al. (1990) who suggested that lanthanides are contained in three pools particles, colloids and solution. The combination of the latter two pools determined the lanthanide composition reported as dissolved concentrations (i.e., 0.22 and 0.45 [xm filtrates). In fig. 11 we have up-dated their plot of Nd versus the Er/Nd (mole) ratio of river waters using data collated in table Al. This... 

Shale-normalized data from a 255 m sample illustrate the major features observed by Sholkovitz et al. (1994) (fig. 16). Included in this comparison is the lanthanide composition of dust collected in Bermuda within a few months of the water sample collection (Sholkovitz et al. 1993). The major observation is that surface coatings have a lanthanide composition which is distinct from that of seawater, dust and the two mineral phases of the suspended particles. The mineral matrix of suspended particles and the atmospheric dust are similar in composition. Both mineral matrices have crustlike patterns indicating a detrital source from the atmosphere. About 40-70% of the lanthanides are contained in the acetic acid digest the strong acid digest carries 10-30% and the bomb digest carries 15-25%. At the heavy end of the series (Yb and Lu) the three fractions contribute equal proportions. These observations indicate that the surface... 

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).

Acetic acid digestions of suspended particles demonstrate that removal and fractionation of lanthanides in seawater are caused by processes which result in particle surface coatings. In contrast, the detrital phases have dust and crust-like lanthanide compositions indicative of a continental source. 

Fig. 30. A comparison of the lanthanide composition of the Pacific Deep Water (PDW) with the composition of North Atlantic Deep Water (NADW) and the composition of Antarctic Bottom Water (AABW). Plots of PDW/NADW ratios and PDW/AABW ratios. Data for water masses from German et al. (1995).

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. 

Shale-normalized patterns quantify the extent of fractionation during periods of anoxia as illustrated by the water column data collected from different depths of Chesapeake Bay on 26 July 1988 (fig. 38). The oxic surface water exhibits a heavy-enrichment and large negative Ce anomaly while anoxic bottom water has an almost flat pattern and a small negative Ce anomaly. Hence the lanthanide composition shifts away from the heavy-enriched pattern of oxic seawater toward one that is more crust-like. As redox conditions become more reducing, the relative order of trivalent lanthanide release to the... 

There is only one paper with data on the pore water composition of deep ocean sediments deposited under oxygenated water (Ridout and Pagett 1984). This paper mostly deals with extraction methods and provides poorly documented lanthanide data for a single sample. It is fair to state the lanthanide composition and chemistry of oxic marine sediments is unknown. 

Byrne and Sholkovitz review the lanthanide composition of the ocean and the processes that control their distribution. Particular attention is given to research on the lanthanide geochemistry of marine hydrothermal vent systems. They find that the lanthanides are good indicators of the reactions between hydrothermal fluids and basalt and between iron particles and sea water, but these reactions are not quantitatively significant with respect to river water fluxes and oceanic cycles and inventory. 

---