Three Gorges Dam controls sediment coarsening of the mud patch on the inner East China Sea shelf

Abstract

The well-known Three Gorges Dam (TGD) within the Yangtze catchment launched its operation in 2003. The effect of the TGD operation on the sediment size on the East China Sea shelf is rarely known. High resolution (0.5 cm sampling) grain size analysis and 137 Cs and 210 Pb dating of the DH8-1 core were conducted with core collected from the distal part of a main sink for the modern Yangtze sediment entering the sea, the Min-Zhe Coastal Mud Deposits (MZCMD) on the inner East China Sea shelf. The 137 Cs dating results show that the core DH8-1 formed during 1946-2012 with a mean deposition rate of 0.65 cm yr −1 , indicating that the 0.5 cm sampling for grain size analysis in this local area could reflect environmental changes generally on a one-year time scale. The mean grain size of DH8-1 core sediment that deposited after 2003 is significantly larger than that deposited during 1988-2002. After ruling out other possible factors, we infer that the sediment coarsening of DH8-1 core after 2003 is attributed to the TGD operation which causes the erosion of the Yangtze subaqueous delta. Specifically, the TGD operation significantly intensifies the declining trend of the Yangtze sediment loads to the sea despite no decreased water discharge, which results in extensive erosion of the Yangtze subaqueous delta. The relatively coarse sediment of the subaqueous delta is eroded and resuspended by ocean dynamics and then transported by coastal current, finally depositing on the MZCMD area. In addition, the general sediment fining of core DH8-1 that deposited during 1988-2002, comparing with 1946-1987, is mainly caused by dam construction and soil and water conservation within the Yangtze catchment. Our findings are helpful for better understanding the effects of such a huge dam as the TGD on a sediment sink like the MZCMD of such a large river as the Yangtze River.

Normalization of excess 210Pb with grain size in the sediment cores from the Yangtze River Estuary and adjacent areas: Implications for sedimentary processes

“…Although the SAR was lower than the previously reported values of 1–5.4 cm/a in the YEMZ (DeMaster et al, 1985; Huh and Su, 1999; McKee et al, 1983), this case may be closely associated with the context of the declines in sediment discharge from the Yangtze River, which were driven by human activities in the river basin, such as the construction of TGD in 2003 (this can be regarded as the most important factor). Also, SSC and SAR decreased dramatically in response to the reduction of sediment supply in YEMZ, which were confirmed by Gao et al (2011) and Zhu et al (2015), but if the sediment rate continues to decline, erosion of the Yangtze subaqueous delta could occur (Miao et al, 2016; Yang et al, 2006). After applying the correction, the low activity of 210 Pb remained at depths of 1 and 5 cm in this core, which indicates that this case was not because of the change of the clay content; instead, it may have been related to weak mixing of the re-suspended sediment caused by energetic physical processes during the winter or the episodic delivery of low activity sediment during flood/typhoon events.…”

Variations in the transport, distribution, and budget of 210Pb in sediment over the estuarine and inner shelf areas of the East China Sea due to Changjiang catchment changes

“…Because only the sediment cores with interpretable 210 Pb ex profiles were selected for publications in the literature, the data from many profiles associated with an eroding setting are not included in the present study. Since an accreting environment is beneficial to 210 Pb accumulation and an eroding environment tends to have a reduced 210 Pb ex activity [ Qian et al , ], the use of the 210 Pb ex activity data from accreting sites may result in an overestimate of the actual values [ Xu et al , ; Miao et al , ]. Nevertheless, the changes in the 210 Pb flux from the Changjiang into the sea and the 210 Pb ex budgeting of the study area (in section 5.3) demonstrate that since 2003, the 210 Pb ex activity in the surficial sediment of the CSD and ZFCMB regions has been indeed reduced.…”

Modern variations in clay minerals in mud deposits of the Yellow and East China Seas and their geological significance

“…The Pb 210 and Cs 137 dating of cores B44, YSZD01, and DH8-1 was conducted with a γ spectrometer, showing a deposition period and yearly deposition rates of 97 years and 0.49 cm/ yr, 110 years and 0.21 cm/yr, and 62 years and 0.69 cm/yr, respectively. Generally, the sedimentary record with sediment sampling at an interval of 0.5 cm has been proven to be able to show yearly changes in environment (Miao et al, 2016;Shen et al, 2015).…”

“…All samples were pretreated to remove carbonate with distilled water and 0.25 mol/L HCl and organic matters with 30% H 2 O 2 . Then, 5 mol/L (NaPO 3 ) 6 was used to disperse sediment samples, and the sediment sample stood for more than 24 h. The sample was then placed into an ultrasonic disperser and vibrated for 10–15 min (Miao et al, 2016). Finally, the particles smaller than 2 µm were extracted based on Stoke’s law, and the ethylene glycol–saturated oriented slide (EG) was made.…”