Holocene loess accumulation and soil development at the western edge of the Chinese Loess Plateau: implications for magnetic proxies of palaeorainfall

Abstract

A high-resolution Holocene sequence of loess, palaeosols and incipient soils at the western edge of the Chinese Loess Plateau, dated using optically stimulated luminescence techniques, is used to identify the respective influence of dust accumulation rate, time and climate on soil magnetic properties. Dust deposition and soil formation have both been quasi-continuous through the Holocene at this site; the soils are thus accretionary in nature. The degree of soil development (as indicated both by geochemical and magnetic properties, which correlate strongly) varied through the Holocene. Compared with the less-weathered loess units, each palaeosol is enriched in nitrogen and organic carbon, depleted of carbonate due to leaching, and displays higher values of pedogenic magnetic susceptibility, frequency-dependent susceptibility (%), remanences and magnetisation. Magnetic grain size indicators show that the pedogenic ferrimagnets are ultrafine, of single domain and superparamagnetic dimensions. Sediment accumulation rates were lowest from ∼12 to 2.5 ka, providing ample time (100 s of years) for weathering and soil formation to proceed, yet pedogenesis through this interval was relatively weak. Conversely, soil development was stronger during later intervals—when loess accumulation rates were higher, and soil-forming intervals were correspondingly shorter. In terms of Jenny's (1941) soil-forming equation, accumulation rate (and thus time), sediment source (parent material) and topography show no systematic variation between the loess and the palaeosols. Therefore, climate seems to be the key soil-forming factor which has controlled the geochemical and magnetic properties of these Holocene palaeosols.