16-12 Mineralogical Analysis of Sediment Exported from a First Order Post-Glacial Watershed in New Hampshire

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

Presenting Author:

Authors:

Abstract:

The Hubbard Brook Experimental Forest (HBEF) in New Hampshire is a well-instrumented, post-glacial, temperate watershed where long-term hydrologic measurements allow evaluation of material fluxes at the catchment scale since 1955. Although river sediment loads are commonly used to infer watershed hillslope weathering and erosion rates, previous work at the HBEF indicates that physical denudation of upland soils is minimal, raising the possibility that sediment export is disconnected from hillslope soil erosion. Here, we investigate the provenance of particulate material exported from HBEF watersheds by using the degree of biotite weathering as a mineralogical tracer. In temperate climates, biotite weathers rapidly under soil-forming conditions and is therefore expected to be scarce or highly altered in upland Spodosols, but relatively unweathered if derived from in situ breakdown of streambed material. Clay-sized (<2 µm) sediments collected from weir stilling basins were analyzed using oriented X-ray diffraction (XRD) to identify mineral assemblages. Initial XRD results reveal abundant biotite and kaolinite, but an absence of vermiculite, interstratified mica-vermiculite, or chlorite phases characteristic of upland soils in the region. The presence of unaltered biotite in exported sediments, contrasted with its absence in soil clay mineral assemblages documented in previous studies, indicates that particulate material exported from the HBEF is derived predominantly from streambed weathering rather than erosion of upland soils. These findings demonstrate a disconnect between soil weathering and sediment export in this low-denudation, forested watershed and emphasize the importance of in-channel sediment production in controlling solid phase catchment losses.

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