Redai dili (Jun 2023)
Grain Size and Magnetic Characteristics of Floodplain Sediments in a Small Watershed at Northern Guangdong: Implications for Paleoflood Identification
Abstract
With the intensification of global climate change, floods caused by typhoons and heavy rainfall occur frequently. It is important to effectively identify flood events within sedimentary records to study the flood frequency patterns. Currently, grain size analysis is the commonly applied method for flood event identification. The hydrodynamic conditions and sediment sources identified from the grain size parameters are used to distinguish flood events. However, identification results from a single index can easily lead to inaccuracies due to complex river sedimentation processes. Therefore, it is necessary to select the most suitable proxy and, in combination with the implications of different types of indicators, identify flood events in study areas. In addition to grain size analysis, the technology of environmental magnetism has been widely applied in the sedimentology research of flood sediments owing to its advantages of sensitivity, rapidity, economy, and non-destructiveness. In this study, a longitudinal floodplain sediment profile collected from a small watershed in northern Guangdong was selected as the research object. Both grain size and magnetic characteristics of the entire profile were determined in detail. Based on the flood event records identified by sediment grain size characteristics, an in-depth analysis of the magnetic characteristic differences between the flood sedimentary layers and normal sedimentary layers, was performed. We aimed to establish a set of suitable magnetic parameters for identifying flood events in this region. Consequently, a new approach can be used to identify paleofloods within sedimentary records. The results indicate the following. 1) The sediment particle size composition is primarily composed of silt, which accounts for more than 70% of the total. Moreover, the sediment particle size tended to become finer from the flood sedimentary layer to the normal sedimentary layer. Three sublayers characterized by relatively high peak values of sand content and mean grain diameter were identified as flood event sediments within the flood sedimentary layer. 2) The composition and sources of magnetic minerals within the sediments from different sediment layers are similar. The predominant magnetite and some hematite within the sediments are from the natural weathering of parent materials within the watershed. Exogenous input from the tailings pond of the Dabaoshan mine is another source of magnetic particles. 3) As deposition is dominated by suspended sedimentation in the study area, sediment sorting is sufficient under the influence of a continuous long-term high-water level. As a result, the magnetic particles within the flood sedimentary layer are relatively fine. With the enhancement of hydrodynamic conditions, the grain size of the magnetic particles and the concentration of fine-grained magnetite and hematite became coarser and increased in the three identified flood event sublayers within the flood sedimentary layer (from the middle to the top of the sedimentary profile). Therefore, a combination of environmental magnetic parameters and magnetic mineral composition analysis can be used for the accurate identification of flood events recorded by sediments. In the study area, the magnetic characteristics of the relatively high contributions of fine-grained magnetite and hematite and the relatively low contribution of coarser magnetite to saturation isothermal remanent magnetization can be regarded as proxies for identifying flood events and strong hydrodynamic conditions.
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