Formation processes of eolian loess in the northern subtropics of China: Evidence from multiple micromorphological metrics

Abstract

Loess-like sediments are widely distributed in the northern subtropics of China, stretching beyond the traditional boundary of typical loess. Compared with well-researched typical loess, loess-like sediments are much less studied and their origin (eolian or fluvial) and movement pathways under evolving environments remain unclear. Micromorphology offers novel insights into explaining sediment-forming mechanisms through reconstructing the sedimentary history. To examine the debates comprehensively, multiple micromorphological metrics were systematically analyzed, and three main results were derived from the analysis: (a) Hanjiang loess-like sediment can be directly diagnosed as eolian loess based on its differences from fluvial sediment with regard to particle composition and quartz roundness; (b) combined analyses of quartz configurations with the distribution patterns of modern winter winds/dust storms indicated that a portion of the small particles were transported by upper flow in the East Asian winter monsoon from the northwestern Gobi Desert, whereas coarse grains were mainly derived from the adjacent floodplain by creep and saltation; and (c) vertical heterogeneity of configurations and compositions among layers was mainly caused by different degrees of chemical/biological/physical weathering since the latest glaciation. All measurements showed the weathering intensity of sediments decreased in the order of S0/Ts, Lt, L0, L1, meaning that the East Asian summer monsoon weakened in this order during different formation periods. In summary, these metrics reconstructed the formation process and evolutionary history of monsoons in subtropical China excellently and would rival traditional indexes, but are easier to observe.