Redox lanthanide fractionation

The oceanic chemistry of the lanthanides distributions, biogeochemical cycles, mass fractionation, Ce redox reactions and Nd isotopes... 

This section describes and discusses lanthanide distributions in the oceans and the biogeochemical and physical processes responsible for these distributions. Three interrelated subjects will be developed. These are (a) profiles of lanthanide concentrations in the water column and their variations within and between ocean basins, (b) the redox geochemistry of Ce as revealed by vertical and horizontal variations in the Ce anomaly and (c) the inter-oceanic mass fractionation of the strictly trivalent lanthanides. 

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. 

The increase in concentration between oxic and anoxic waters is accompanied by large scale fractionation of the trivalent-only lanthanides. Although these elements have no redox chemistry of their own, their absolute and relative abundance are altered by variations in redox conditions, in particular the cycling of iron and manganese. 

A time-series study of Chesapeake Bay demonstrated that the dissolved lanthanides have a large seasonal cycle in both the water column and pore water in response to the development of anoxia in the spring and reoxygenation in the fall (Sholkovitz et al. 1992). Short-term variations in fractionation accompany the release and removal of lanthanides from the water column under seasonally-varying redox conditions. 

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... 

In summary, the lanthanides undergo large scale diagenetic reactions under sub-oxic and anoxic conditions. Pore water concentrations increase greatly over those of oxic seawater. Large cerium anomalies develop and large scale fractionation occurs as the strictly-trivalent lanthanides are added to and removed from pore waters. These features develop in vertical profiles within sediments and in surface sediments exposed to seasonally varying redox conditions. 

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