Comparing mineral magnetic properties of sediments in two reservoirs in "strongly" and "mildly" eroded regions on the Guizhou Plateau, southwest China: A tool for inferring differences in sediment sources and soil erosion

• Shibanqiao Reservoir (0.24 km(2)) and Xiaohe Reservoir (0.28 km(2)) have catchments of 6.00 km(2) and 22.69 km(2), respectively. They essentially represent "strongly eroded" (Shibanqiao) and "mildly eroded" (Xiaohe) regions in Guizhou Plateau, southwestern China. Sediment cores were retrieved from the two reservoirs and soils were also sampled from their catchments. Mineral magnetic measurements were performed on the sediments and soils, and the particle-size analysis of selected sediment cores and soil profiles were also made. SIRM versus chi if was used to compare sediment and soil samples. In addition, correlations between percentages of different particle-size fractions and magnetic parameters/ratios of sediment samples were also checked. These results revealed differences in sediment sources and soil erosion in these two representative catchments. The sediments in Shibanqiao Reservoir are derived from both the topsoil and subsoil. When erosion is relatively weak, it is limited to the topsoil and preferentially transports the relatively finer magnetic grains into the reservoir, which may imply rain splash and/or sheet erosion. In this relatively steep catchment, stronger erosional forces are capable of not only moving relatively coarser ferrimagnetic grains from the topsoil, but also incising the subsoil and moving anti-ferromagnetic minerals into the reservoir, which seems to hint at not only sheet-wash but also rill and gully erosion. As there are no stable streams nor channel networks developed in this mountainous catchment, contributions from river-bank erosion to sediment yield are almost negligible. The sediments in Xiaohe Reservoir, which has a lower relief, are predominately derived from topsoil erosion. Here, erosion is concentrated to the topsoil and preferentially transports the relatively fine magnetic grains into the reservoir when erosional forces are weak. However, even when erosional forces increase in this relatively flat catchment, erosion is still restricted to the topsoil. Thus, relatively coarser ferrimagnetic grains from the topsoil are transported to the reservoir, but the subsoil remains undisturbed and there are no significant changes in the proportion of anti-ferromagnetic minerals in the sediments. Consequently, only sheet-washing occurs even when erosion is intensifying in this hilly plain catchment. In general, the more effective erosion in the Shibanqiao Catchment is probably due to its steeper topography than Xiaohe Catchment, although higher and more seasonal rainfalls and lower percentages of woodlands may also contribute to the differences in susceptibility to erosional forces. (C) 2011 Elsevier B.V. All rights reserved.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

Nyckelord

Mineral magnetism

Sediment source

Soil erosion

Southwest China

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