Holocene colluviation and its implications for tracing human-induced soil erosion and redeposition on the piedmont loess lands of the Qinling Mountains, northern China

Colluvial components incorporated in loess-soil sequences have preserved evidence of soil erosion and redeposition in the middle reaches of the Yellow River drainage basin. Holocene loess-soil sequences on the gentle sloped lands were studied by measuring: magnetic susceptibility, particle-size distribution, CaCO3 and extraction and identification of the non-dust materials. The analytical results indicate that in the early Holocene soil erosion and redeposition by overland flow occurred, even on the loess lands with small ground gradient, in response to climatic amelioration linked to global climatic change. No obvious soil erosion and redeposition events were found to correspond with the climatic fluctuations recorded in the loess-soil sequence during the Holocene Climatic Optimum. The primary Neolithic farming did not cause apparent erosion and redeposition because the loess landscape was mantled by well-developed Ustic Luvisols and probably by forest vegetation, both provided adequate resistance to sheet erosion by overland flow. Enhanced soil erosion and redeposition are traceable back to ca. 4000 a B.P. It is attributed to intensified land use of arable farming and human settlement from the early Bronze Age onward. A major event of sheet erosion and redeposition by overland flow occurred in the loess landscape during 4000-3600 a B.P. in the Xia dynasty. Soil degradation and reduced resistance caused by climatic aridity and intensified dust accumulation may have played a secondary role in promoting soil erosion since 3100 a B.P. These results provide new insights into the history of soil erosion in the semi-arid and semi-humid loess regions and into the evaluation of human-induced accelerated soil erosion against the background of the Holocene natural erosion forced by monsoonal climatic fluctuation.