Watershed and climate controls on rare earth element mobility

Stephen Norton1, Marie-Claire Pierret2, Jiri Kopacek3
1 Earth and Climate Sciences, University of Maine, USA
2 University of Strasbourg, France
3 Biology Centre ASCR, Czech Republic

O 1.10 in Long term trends in the functioning of ecosystems

15.07.2014, 10:20-10:40, H18

Sediment cores from Sargent Mountain Pond, Maine USA and Plešné Lake, southwestern Czech Republic, record the entire post-glacial palynological, algal (esp. diatoms), and chemical histories of the watershed and lake behavior (16,600 and 14,500 cal y BP to the present, respectively). This enables strong inferences about the behavior of the two watersheds and lakes since deglaciation. Both watersheds are underlain by granite, and overlain by till. Total rare earth element (REE) concentrations and fluxes declined from deglaciation until afforestation commenced (slightly prior to the Younger Dryas (YD) (ca. 12,600 to 11,600 cal Y BP) at Sargent Mountain Pond, and immediately after the YD at Plešné Lake), peaked in value just after the YD, and then declined to the present. Prior to the YD, REE concentrations were dominated by the abundance of detrital, acid-soluble apatite, which also controlled total NaOH-extractable phosphorus (P) in the lake sediment. After the YD, REE concentrations and fluxes behave coherently with NaOH-extractable aluminum (Al) and P after afforestation in each catchment.

We hypothesize that as climate ameliorated after glaciation and afforestation occurred, organic-rich forest soils developed. These soils produced higher concentrations of dissolved organic carbon (DOC) in soil water and runoff. The DOC complexed with, and mobilized, Al and REEs that were (and continue to be) 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 PO4 and REEs by the Al(OH)3, and irreversible sedimentation of the adsorbed REEs. REE concentrations and fluxes decline from the Younger Dryas to the present, because the most important primary source of them, apatite, was progressively depleted in the soil by weathering, and increasingly retained by secondary Al(OH)3 (and Fe(OH)3 to a lesser degree) in the soil. The adsorption ability of soil Al(OH)3 concurrently increased as the watershed soils, soil water and runoff naturally (and finally also atmospherically) acidified, as indicated by changes in diatom assemblages

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last modified 2014-06-19