Using multi-radiotracer techniques to better understand sedimentary dynamics of reworked muds in the Changjiang River estuary and inner shelf of East China Sea

Abstract

Reworked muds, including mobile muds, resuspended sediments, and bio-mixed muds are ephemeral layers in the benthic boundary of large-river delta-front estuaries (LDEs). In this study, multiple radiotracers (210Pb, 137Cs, 234Th, 7Be, 235U, 228Th, 228Ra, 226Ra and 40K) were combined to evaluate the sedimentary dynamics of the mud belt on the inner shelf of the East China Sea (ECS) adjacent to the Changjiang River Estuary. We first categorized eight types of typical 210Pb profiles in marine sediments, and specified which ones could be used as possible evidence of reworked muds. The types of 210Pb profiles observed in previous studies and this study were then used to examine the depth distribution of reworked mud layers of the benthic boundary of ECS. We found that reworked mud depth in the along-shore mud belt of the inner shelf of ECS was deeper than those in the outer shelf and slope area, reaching up to 150cm deep in the Changjiang River LDE, and up to 50cm deep in the Zhejiang–Fujian coast mud area. Sediment core tracer profiles also showed that the reworked mud layer could be further divided into several sub-layers with distinct redox conditions and microbe (sulfate-reducing bacterial) abundance. SRB abundance intensively fluctuated with changing redox environment, and larger abundance values were usually observed in anoxic conditions. Based on 7Be and 234Th, the top layer (ca. 2–4cm thick) was the most modern and active zone, showing evidence of reworking on seasonal timescales. The deeper sections of the reworked mud layer were generally modified over longer timescale (years) which were examined using the more long-lived tracers, e.g., 137Cs, 228Thex, and 228Ra/226Ra ratio. We found for the first time in this region that age of the reworked layers was usually much younger than underlying accumulating (burial) layers. We posit that this multi-radionuclide approach is critical for evaluating the complex biogeochemical sediment record on delta-front estuaries of large rivers subject to changing climate conditions and human impacts over the last several centuries.