Rare earth elements: a potential proxy for identifying the lacustrine sediment source and soil erosion intensity in karst areas

Purpose

The geochemistry of rare earth elements (REEs) taken from the sediments of a karst lake was analyzed to track the provenance of the sediments and the history of soil erosion in the Jiuzhaigou Nature Reserve (JNR) in Sichuan Province in southwest China.

Materials and methods

The REE concentrations of 177 samples including 150 lake sediments, 13 soils, 8 loess, 3 carbonate rocks, and 3 water were determined using a quadruple inductively coupled plasma mass spectrometer (Q-ICP-MS). The grain size distributions of 139 sediment samples were measured using a Malvern Master Size 2000 laser particle size analyzer. The carbonate contents of 159 sediment samples were determined using a gasometric method. The dried remains after reaction with HCl were defined as acid-insoluble residues. The mineral composition of the sediments was measured using a Philips X'Pert Pro X-ray diffractometer.

Results and discussion

The lake sediments in the JNR exhibit features similar to those of the neighboring soils, i.e., enriched REE content, a greater ratio of light REEs to heavy REEs, greater δCe values, and lower δEu values relative to those in the carbonate rocks. The REE signatures in the silt and sand fractions of the sediments are similar to those in the soils and carbonate rocks, respectively. The REE content, LREE/HREE, and δCe values in the sediments strongly negatively correlate with the carbonate content, indicating an intense carbonate effect on the REE geochemistry. The content of acid-insoluble residues is obviously positively correlated with ∑REEs in the sediments, and a negative relationship is observed between ∑REEs and the ratio of calcite to quartz in the sediments.

Conclusions

The sediments in the JNR originate from eroded soils and authigenic travertine deposits in the lakes. It is proposed that the increased ∑REEs in the sediments resulted from the elevated proportion of soil particles in lacustrine deposits was due to increasing soil erosion intensity. This hypothesis is evidenced by the coupling of ∑REEs with the intensity of soil erosion and anthropogenic activities in the JNR. In karst drainage, the total REE concentration might be a potentially powerful indicator for reconstructing the history of soil erosion.