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

Abstract

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 + 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 ²¹⁰Pb, ¹³⁷Cs, ^{239 + 240}Pu geochronology, microplastics content, and optically stimulated luminescence (OSL) dating. The radionuclide profiles show an irregular profile of ²¹⁰Pb, 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 ²¹⁰Pb, ¹³⁷Cs, ^{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 ²¹⁰Pb, ¹³⁷Cs 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.