Late Holocene sediment provenance change in the Red River Delta: A magnetic study

Holocene climate change and human activities can lead to sediment composition variations. In this study, two drilled cores (VN and GA) from the Red River Delta, Vietnam, were subjected to magnetic measurement, as well as particle size, diffuse reflectance spectroscopy (DRS), and geochemical analyses, to infer Holocene sediment provenance changes. Both cores VN and GA cover estuarine, shallow marine, delta front slope, delta front platform, and delta plain facies in ascending order, with the former covering the whole Holocene while the latter covering the late Holocene. In the younger core GA, the clay content has a significant positive correlation with magnetic grain-size indicators (χ_ARM/SIRM and χ_fd%) and a negative correlation with demagnetization parameters (S_{-100 mT} and S_{-300 mT}), reflecting the enrichment of fine-grained ferrimagnetic minerals and antiferromagnetic minerals in finer sediments. On the contrary, the relationship between the above-mentioned magnetic properties and clay content is not significant in core VN, reflecting the impact of stronger post-depositional diagenesis. In addition, diagenesis in the shallow marine and delta front slope facies of two cores is more pronounced, which is due to the fact of higher input of marine-sourced organic carbon. A magnetic comparison of the upper parts of two cores indicates that there are sediment source changes within the last 2000 years, which is supported by DRS analysis. Sediments in delta front slope and delta front platform of two cores have similar goethite content, but core GA contains higher hematite content indicating a drier condition. Such a sediment source change can be interpreted by variations of monsoon climate and reworking of soil due to human disturbance. Surface soil erosion due to human activities over the last 0.5 ka is supported by the presence of ultra-fine grained magnetite. Our study demonstrates that magnetic properties together with DRS analysis can provide insights into paleoclimate and paleoenvironmental change in deltaic environments.