Search

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 ( 210 Pb, 137 Cs, 234 Th, 7 Be, 235 U, 228 Th, 228 Ra, 226 Ra and 40 K) 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 210 Pb profiles in marine sediments, and specified which ones could be used as possible evidence of reworked muds. The types of 210 Pb 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 150 cm deep in the Changjiang River LDE, and up to 50 cm 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 7 Be and 234 Th, the top layer (ca. 2–4 cm 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., 137 Cs, 228 Th ex , and 228 Ra/ 226 Ra 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. [ABSTRACT FROM AUTHOR]

Beibu Gulf is a highly dynamic and complex coastal environment that is currently experiencing one of the largest rates of development and urbanization in west China. Little is known about the effects of this increased human activity on coastal sedimentation processes and on the rates of sediment accumulation and the variation of organic materials to the coast. In this study, four sediment cores were collected and applied the 210 Pb dating method to reconstruct sedimentation rates and historical changes of materials to the northern Beibu Gulf over the past century. Depth profiles of excess 210 Pb ( 210 Pb ex ) showed highest activity values at the surface (28.4-104.0 Bq kg -1 ) followed by a linear or exponential decay with depth for all but one study site. 137 Cs activity ranged between 0.236 and 2.034 Bq kg -1 , and a distinct peak activity - representing the 1963 fallout maximum - was observed at all but one site. Sediment chronologies were determined using the Constant Rate of Supply (CRS) model. Calculated accumulation rates in the studied sites were the lowest in the late 1920s and early 1930s (mass accumulation rate (MAR): 0.06 ± 0.01 g cm -2  y -1 ; sediment accumulation rate (SAR): 0.08 ± 0.01 cm y -1 ) and increased gradually until reaching maximum values in the 2010s (MAR: 0.22 ± 0.09 g cm -2  y -1 ; SAR: 0.46 ± 0.32 cm y -1 ). Current accumulation rates are up to 800% higher than rates observed in the 1920s, with most of the increase happening after 1970, coinciding with the increasing rate of urbanization and development in the region. The highest increase in SAR over the last century (+877%) was observed in Sanniang Bay, with the lowest rate of increase (+283%) observed in Lianzhou Bay. TOC content in these sediments has also increased over the last 100 years. Current values (0.98-1.28%) are about 170% higher than historical concentrations (before 1970). The positive correlations between TOC and population density and GDP growth in major cities surrounding the gulf, provide further indication that human activities have significantly altered the sedimentary environment in recent decades along the northern Beibu Gulf coast., (Copyright © 2019. Published by Elsevier Ltd.)