Chenyin Dong

Magnetic mineral diagenesis in the river‐dominated inner shelf of the East China Sea, China

The inner shelf of the East China Sea is a river-dominated margin characterized by fine-grained mud deposits and a rapid sedimentation rate. Three short sediment cores (~2.7 m in length) were examined to characterize spatial variations in magnetic mineral diagenesis. The sediment cores were analyzed for sedimentation rates, magnetic properties, particle size distribution, organic carbon, and total sulfur content. The two more proximal cores with higher sedimentation rates (~2.2 cm/yr and ~0.96 cm/yr) do not exhibit obvious effects of reductive dissolution of magnetite with increasing depth, which is consistent with their lower total sulfur content. The offshore core, A12-4, which has a lower sedimentation rate, contains clear evidence of magnetite dissolution and increasing total sulfur content with depth. The three cores have a similar sediment source and organic matter input; therefore, we suggest that a higher sedimentation rate will lead to less reductive diagenesis of magnetite, assuming that other factors are constant. The iron- to sulfate-reduction boundary, i.e., revealed by the onset of a rapid decline of magnetic susceptibility, is located 1.0 m below seafloor in core A12-4. This is much deeper than is reported in many other coastal marine environments and can be explained by the higher sedimentation rate, the presence of refractory terrestrial organic matter, and an abundant input of detrital iron oxides. This study demonstrates that analyses of the magnetic mineral zonation provide a straightforward approach to assess diagenetic organic carbon decomposition pathways in marine environments.

Multiple dating approaches applied to the recent sediments in the Yangtze River (Changjiang) subaqueous delta

The accumulation rate of recent deposits in a delta environment is critical to the study of delta dynamics and their sustainable management. The most commonly used dating approach for recent (<100 years) deposits is based on radionuclide analyses (e.g. 210Pb, 137Cs and 239 + 240Pu), while alternative techniques, such as microplastics dating, are emerging. In this study, a 180-cm sediment core from the Yangtze River (Changjiang) subaqueous delta was dated using multiple techniques, including 210Pb, 137Cs, 239 + 240Pu geochronology, microplastics content, and optically stimulated luminescence (OSL) dating. The radionuclide profiles show an irregular profile of 210Pb, while 239 + 240Pu exhibit a clear peak of activity at 74 ± 2 cm, which is linked to the maximum global fallout in 1963. Microplastics were not detected below a depth of 90 cm with maximum counts occurring in the top 16 cm. OSL analysis was conducted on the dominant grain size of the quartz (around 4–11 µm) and the ages were ~60 years older than those derived from 210Pb, 137Cs, 239 + 240Pu, and microplastics analyses. We infer that the relatively old quartz OSL ages are most likely caused by residual OSL signals arising from poorly bleached grains at the time of deposition. The profiles of 210Pb, 137Cs and 239 + 240Pu activities, microplastics content, and OSL ages indicate a variable sedimentation rate over the last ~200 years reflecting the dynamic nature of delta deposits. This study shows that both OSL and microplastics particles are promising dating tools for recent young deltaic sediments, and their combined use, alongside radionuclide methods, increases the reliability of age determination.

Recent Applications of Mineral Magnetic Methods in Sediment Pollution Studies: a Review

This paper reviews recent progress in applying mineral magnetic methods in sediment pollution studies. Such applications include its use as a dating marker, as a proxy for heavy metal concentrations and to trace metal pollutant dispersal. The mineral magnetic method has been found to be a promising tool in a wide range of sediment metal pollution studies. However, its use as a proxy of heavy metal concentrations is not always straightforward. This reflects the potentially mixed origins of magnetic minerals in sediments which may have an anthropogenic, natural or mixed source. Furthermore, anthropogenic magnetic particles may not have a common source with heavy metals. The possible linkage between magnetic minerals and heavy metals is discussed. The role of sorting, sorption/desorption and post-depositional diagenesis on the magnetic mineral-heavy metal linkage is highlighted as still requiring careful consideration. It is suggested that detailed characterisation of magnetic mineralogy using combined magnetic, geochemical and mineralogical methods is critical to the optimization of sediment pollution studies using a mineral magnetic approach.