Extensive accumulation of rare earth elements in estuarine sediments affected by leaching of acid sulfate soils

The concentrations, loads and speciation of rare earth elements (REEs) were studied in a 3.5 m thick mud depositional succession from an estuary in the Gulf of Bothnia. The uppermost 182.5 cm of the mud, estimated to have deposited from the early 1970s to 2011 (sampling year), had very high REE concentrations (596−1456 ppm) and accumulation rates (5.2−28 g m−2 year−1). This was explained by large REE export from acid sulfate soils after they became efficiently drained with modern drainage techniques. Geochemical and synchrotron-based spectroscopic (XANES) analyses showed that the REEs in the mud are relatively firmly bound in non-clastic phases, likely adsorbed by clay minerals and also to some extent by iron oxyhydroxides. Below 182.5 cm, the REE concentrations successively decreased down to background values at the base at 3.5 m, reflecting less efficient drainage and leaching of the acid sulfate soils in previous decades and centuries.