REE geochemistry of fine-grained sediments from major rivers around the Yellow Sea

To investigate the distribution pattern and controlling factors of rare earth elements (REEs) in riverine sediments, river mouth sediments were collected at five geographically different rivers around the Yellow Sea. Two- (1 M HCl leached and residual fractions) and five-step sequential extraction schemes (the SEDEX method) were applied to size-separated sediments < 20 µm. For the total REE composition, patterns normalized relative to the upper continental crust (UCC) showed light REE (LREE) and middle (MREE) enrichments in the Korean river sediments and MREE enrichment in the Chinese river sediments. LREE and MREE enrichments in 1 M HCl leached fractions played a major role in the distribution patterns of the total compositions in all the river sediments. About half of LREE enrichment in Korean river sediments was explained by the residual fraction. Comparison of the REE composition with major elements (Al, Fe, Ca, P) in each fraction of the SEDEX scheme revealed that MREE enrichment could be explained by reactive iron minerals, including goethite and hematite, although REE/Fe ratios showed different trends among the rivers due to different major REE-host iron minerals. After extracting reactive irons, authigenic phosphate, and carbonate, the sequential 1 M HCl fraction indicated that LREE enrichment in Korean river sediments may have originated from clay minerals, such as chlorite. These observations suggest that LREE enrichment may be a good tracer, while MREE enrichment should be used cautiously considering diagenetic modification, when using the REE composition to identify the sources of terrestrial materials.