Syn- and post-depositional modification of loess deposits and their paleoenvironmental significance

Yin, Qiuzhen [UCL] Xiao, G.Q. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Guo, Z.T. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Ge, J.Y. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Hao, Q.Z. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Yuan, B.Y. [Institute of Geology and Geophysics, Chinese Academy of Sciences]

Loess-paleosol sequences are widely used for paleoenvironmental reconstructions. As a sub-aerial deposit, any level of a loess-soil section was exposed to land surface, and is subject to syn-depositional affections by surface processes. Also, because loess is sensible to erosion, post-depositional modifications are highly common in space. Here, some most commonly found syn-depositional and post-depositional features of loess deposits are studied mainly based on field and micromorphological investigations on the eolian deposits in northern China. Since most of these features are formed under specific environmental conditions, they may be used as indicators for paleoenvironmental reconstruction. Meanwhile, a number of modifications may affect the significance of some climate proxies that are frequently used in loess studies. Consequently, their influence should be taken into considerations in the climate interpretations. Firstly, we show that the bottom parts for most of the loess-soil sections usually contain some water-reworking features. This is because the substratum of loess deposits was never flat enough so that eolian dust deposited in the earliest stage was more or less affected by surface water. These processes may lead to a progressive flatting of the land surface, resulting in the formation of typical loess-soil sequences. We show also that syn-depositional affection of groundwater oscillations may result in ambiguous distinctions of loess and soil layers because of the disturbances on zonal pedogenic processes, while loess deposits without these affections have usually clear cyclical occurrences of loess and soil layers that are mostly regulated by Milankovitch cycles. Vegetation degradations under extremely dry conditions may cause eolian and sheet-water erosions of loess deposits. These features are highly common in the loess deposits in northern China. Wind and sheet-water erosion features are most frequently observed across the boundaries between loess and soil layers. These appear to suggest that climate transitions at orbital scales are usually marked by a severe dry event, leading to vegetation degradation. Freeze-thaw features are much less common in the loess deposits in China than in the loess of Europe and Americas, probably due to the much drier conditions. However, some weak frost features are observed in the northern and northwestern Loess Plateau while they appear to be absent in the southern Loess Plateau. These are consistent with the present-day climate pattern in the region, as is marked by warmer conditions in the southeastern part and cooler conditions in the northwestern part. A number of post-depositional modifications are also described in this report as they may influence the significance of some climate proxies. Also, because some features of the loess-soil sequences are easy to be confused with the modifications described above. Their basic morphological properties, formation processes and their significance are also discussed.