Long-term dynamics of watershed leaching and lake sediment sequestration of rare earth elements following deglaciation of two mountain watersheds

Abstract

Sediment cores from Sargent Mountain Pond, Maine, USA and Plesne Lake, southwestern Czech Republic, record the entire post-glacial terrestrial vegetation, lake algal communities, and chemical dynamics of the watershed and lake from 16,600 and 14,600 Cal Yr BP, respectively, to the present. Both watersheds are underlain by granite, and overlain by till. Total rare earth element (REE) concentrations and fluxes to the sediment declined from deglaciation until forest developed slightly prior to the Younger Dryas (YD) (12,600–11,600 Cal Yr BP) at Sargent Mountain Pond, and immediately after the YD at Plesne Lake, peaked in value just after the YD, and then declined to the present. Before the YD, REE concentrations were dominated by detrital, soluble apatite [Ca5(PO4)3(OH)]. Weathering of apatite contributed the phosphorus (P) sequestered as NaOH-extractable P in the lake sediment. After the YD, REE concentrations and fluxes behave coherently with NaOH-extractable aluminum (Al) and P after forestation in each catchment. Concentrations of extractable REEs, P, and Al peak after the YD and decline coherently to the present. We hypothesize that as climate ameliorated after glaciation, afforestation occurred and organic-rich forest soils developed. These soils produced higher concentrations of dissolved organic carbon (DOC) in soil water and runoff. Concurrently, the weathering of apatite released PO4 and Ca to runoff, raising pH to ~8. The soil DOC complexed with and mobilized Al and REEs, which were transported to the lakes in runoff. There, photo-oxidation of the Al–DOC and REE–DOC complexes caused release of ionic, inorganic Al and REEs, precipitation of Al(OH)3 in the higher-pH environment, adsorption of REEs and PO4 by the Al(OH)3, and irreversible sedimentation. REE concentrations and fluxes declined from the YD to the present because the apatite was progressively depleted in the soil by weathering, while REEs were increasingly retained by secondary Al(OH)3 [and Fe(OH)3 to a lesser degree] in the soil. The adsorption ability of soil and water-column Al(OH)3 for P increased, but for REEs likely declined, as the watershed soils developed and acidified. This natural process was further magnified by atmospheric acidification, as indicated by changes in diatom assemblages.