Origin and depositional background of the Holocene black soil in Northeast China: Evidence from grain-size analysis and optically stimulated luminescence dating

Abstract

The fertile Holocene black soil of Northeast China make it a major grain producing region, however, this soil is increasingly vulnerable to erosion. Knowledge of the origin and depositional background of the black soil in Northeast China can provide a reference for the conservation of the soil resources of this region. To this end, we examined the grain-size characteristics of the Holocene black soil across the plains in Northeast China, and we conducted optically stimulated luminescence (OSL) dating of the underlying stratum. The western Holocene black soils have a bimodal grain-size distribution similar to that of the sandy loess deposits on the Chinese Loess Plateau. In contrast, the eastern Holocene black soils mostly have a trimodal grain-size distribution similar to that of reworked loess in flood plains. From west to east, the median grain size and the sand content gradually decrease from >120 μm to <20 μm and from >80 % to <20 %, respectively. This eastward fining trend is consistent with the prevailing westerly winds during the dust storm season, indicating a wind-blown origin for the black soil material which was transported primarily from the adjacent sandy lands. Furthermore, our OSL ages reveal that the underlying stratum of the Holocene black soil was primarily deposited during the last glacial period, with the relatively wide depositional age range of 2–65 ka. This suggests unstable geomorphological conditions in Northeast China prior to the formation of the black soil, which may have resulted from the enhanced erosion and decreased vegetation cover.