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Multiple dating approaches applied to the recent sediments in the Yangtze River (Changjiang) subaqueous delta
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This study applied multiple radioactive dating techniques to recent sediment deposits in the Yangtze River delta to determine accumulation rates. While focused on geological processes, accurate sediment dating is also valuable for tracking the history of contaminant deposition, including microplastics, in river systems.
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. 210 Pb, 137 Cs and 239 + 240 Pu), 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 210 Pb, 137 Cs, 239 + 240 Pu geochronology, microplastics content, and optically stimulated luminescence (OSL) dating. The radionuclide profiles show an irregular profile of 210 Pb, while 239 + 240 Pu 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 210 Pb, 137 Cs, 239 + 240 Pu, 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 210 Pb, 137 Cs and 239 + 240 Pu 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.
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