Rare earth elements in a stalagmite from southwestern Japan: A potential proxy for chemical weathering

抄録

This study examines the distribution of rare earth elements (REEs) in an 18–4.5 ka stalagmite in southwestern Japan, with the aim of assessing the utility of such data in paleoenvironmental reconstructions. Shale-normalized REEs (REE_N) of this stalagmite generally contain two features: (1) a positive Eu anomaly and (2) heavy REE (HREE) enrichments. The magnitude of the positive Eu anomaly is most likely controlled by the relative contribution of REEs from weathered wall-rock andesites, which is estimated to be 19–96% of the total REEs in the stalagmite. The HREE enrichments in the stalagmite are likely to have formed by ionic radii-dependent fractionation between mineral and aqueous phases, where parental waters for the stalagmite preferentially incorporated the HREEs with relatively low partition coefficients (D_REE) during bedrock dissolution. In contrast, calcite precipitation causes quantitative incorporation of dissolved REEs by the stalagmite, all of which are highly compatible with calcite (D_REE > 100). This indicates that stalagmite REE patterns should be identical to dripwater REE patterns. The REE characteristics of the stalagmite also changed temporally with the period of deglaciation (16.6–7.1 ka) that was associated with slow stalagmite growth rates, and this coincided with high La_N/Nd_N ratios (>1). La (and Ce) have effective ionic radii that are smaller than that of Ca²⁺ (100 pm), and these were enriched in aqueous phases during slow dissolution of the bedrock. The data presented here indicate that the stalagmite REE patterns may be useful as potential indices for local weathering intensity.