Rare earth element patterns in a Chinese stalagmite controlled by sources and scavenging from karst groundwater

Abstract

The rare earth elements and yttrium (REY) in a Chinese stalagmite (SJ3) have been studied to delineate their possible sources and controlling mechanisms. The following results are obtained: (1) The REY in karst groundwater at the study site have very low concentrations, e.g. ranging from 59×10−6 to 614×10−6μmol/L for La, and are transported predominantly in particle/colloidal phases.These elements show a high linear positive correlation with Fe, Mn and Al (r2=0.98, 0.95 and 0.91, respectively), which are also transported largely in particle/colloidal phases, suggesting a close association of REY with Fe, Mn and Al in colloidal/particle phases; (2) REY mobilization from the sources, i.e. the limestone host rock and the overlying soil layer, may not have exerted a significant influence on the REY patterns for SJ3 including anomalies of La, Ce and Y and fractionation between light and heavy REY; (3) The REY patterns of SJ3 display significant variations which are controlled in part by change in contributions from the limestone host rock and the overlying soil layer and can be associated with stadial–interstadial changes in past climate and environment. The REY contribution from the overlying soil layer is higher under a warm–humid climate phase than a cold–dry one and the REY contribution from the limestone host rock is reversed; (4) Some of the variations of the SJ3 REY patterns could not be explained with a simple mixing of different REY sources. Removal of REY from the groundwater in association with precipitation of colloidal and particle materials, an analogue to REY scavenging from seawater, is the most important mechanism influencing the SJ3 REY patterns. Strong REY “scavenging”, which may be caused by both less dynamic hydrology, longer travel-time and increased ionic strength of karst groundwater, is suggested to be responsible for the large positive Y anomalies and negative Ce anomalies and depletion of light REE relative to heavy REE during the cold–dry climate phases (e.g. the periods corresponding to the last glacial maximum and Heinrich event 1).