Lanthanide shale-normalized

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. 

Fig. 1. (a) The concentration of dissolved lanthanides in the surface waters of the Sargasso Sea. A composite of data measured by TIMS (Sholkovitz and Schneider 1991) and INAA (De Baar et al. 1983). Note classic sawtooth abundance pattern. Pm does not exist in nature, (b) Shale-normalized pattern of the composite seawater shown in (a) using shale concentrations of table 1. Tb, being inconsistent, probably reflects an incorrect concentration of the seawater. 

Fractionation of the lanthanides is a major theme of the literature and hence this article. Fractionation in natural waters is often quantified by computing shale-normalized ratios... 

Lanthanide distribution models (Elderfield 1988, Byrne and Kim 1993, Erel and Morgan 1991, Erel and Stolper 1993) provide a basis for the comparative shale-normalized lanthanide concentrations (lanthanide fractionations) which are observed in the oceans. Lanthanide fractionation models are formulated in terms of competitive complexation equilibria involving, on one hand, solution complexation, and on the other, surface complexation on marine particles. Following the developments of Elderfield (1988) and Byrne and Kim (1993), shale-normalized lanthanide concentrations (Mj)sn in seawater can be expressed as... 

Fractionation of the lanthanides is often quantified by shale-normalized patterns. Normalization to shale represents an abundance relative to that of the upper crust of the continents. A flat shale pattern for river suspended particles would indicate a composition similar to that of averaged continental crust. To study fractionation in rivers, it is also instructive to normalize the dissolved composition to that of suspended particles on the assumption that the particles better represent the solids being weathered in the watershed. 

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).

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... 

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. 

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... 

Fig. 37. Temporal variation of the Nd/Yb ratio in the bottom water and upper porewater for the CHEER time-series study of Chesapeake Bay. Four symbols refer to estimated values, as the heavy-lanthanide (Yb) fractions from these samples were lost (see sect. 3). The Yb concentrations were estimated by using the shale-normalized patterns generated from the light lanthanide and middle lanthanide fiactions and extrapolating to Yb. This leads to good ( 10%) estimates as the lanthanide patterns are smooth (see fig. 7). From Sholkovitz...

Fig. 38. Shale-normalized lanthanide patterns for CHEER 6 (26 July 1988) filtered samples comparison of oxic Surface waters, anoxic bottom waters, and anoxic upper porewater. Also shown is the pattern resulting from the normalization of the lanthanide concentrations of the anoxic bottom waters relative to the lanthanide concentrations of the oxic surface waters. From Sholkovitz et al. (1992).

Fig. 38. Lanthanide abundance patterns for selected iron formations and iron-rich sedimentary rocks. Data are normalized to average shale values of the same period, Archean iron formation being normalized to average Archean shale (McLennan and Taylor 1984) and the others normalized to PAAS. In detail, iron formations exhibit considerable variability in lanthanide patterns. These samples illustrate the general feature of Eu enrichment, relative to contemporaneous upper continental crust, for Archean and early Proterozoic iron formations. The younger examples display no such Eu enrichment. This feature has been used to suggest that early Precambrian seawater was dominated by a hydrothermal signature, enriched in Eu (see fig. 30). (Data are from table 22.)...

Fig. 41. Lanthanide patterns for various grain size fractions of an unconsolidated sediment. Data normalized to the <2 pm fraction, the closest approximation to a shale derived from the same source. (Data are from table 24.)...

Fig. 6a. Lanthanide concentrations in seawater normalized to North American Shale Composite (Piepgras and Jacobsen 1992) are shown for average deep water and average surface water.

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