Using Multi-tracers Technique To Understand The Transport Of Sediment In The East China Sea

The river-loaded sedimentary particles are not only the important source materials for morphotectonics, but also play a major role in the transport-transformation-burial of particle-positive materials in coastal environment as a carrier of many pollutants and nutrients. Some portion of newly deposited sediment could be resuspended and transported under certain hydrodynamic condition. This part of sediment ?defined as "Mobile mud" ?MM??, characterized by high porosity and relatively strong mobility, plays a major role in better understanding the sedimentological and biogeochemical processes in the estuarine and coastal areas. Rdionuclides (7Be,234Th,²¹⁰Pb, ¹³⁷Cs, 239Pu and ²⁴⁰Pu), with different half time, are particle-reactive and serve as natural tracer for the source and transport of sediment. Magnetic properties could reflect the mineral component and are sensitive to the variation of particle size, which is often used to indicate the sorting process of minerals. Therefore, a suite of MM samples was collected in the East China Sea ?ECS? to observe their spatial and temporal distribution, grain size, magnetic parameters and the concentration and inventories of radionuclides. The record of multi-source tracers in MM were evaluated to elucidate the source and transport of MM in different temporal and spatial scales. The radio-chronology based on ²¹⁰Pb_ex,¹³⁷Cs and Pu isotopes were also conducted in the ECS to evaluate the application of these methods in marine environment. Meanwhile, mobility of ¹³⁷Cs was studied to evaluate its application of sediment transport and chronology.?1? The granulometric composition of MM was mainly fine grain size ?>95% clay +silt, and<5% sand? and high porosity ?> 0.50?, except for several offshore locations, where the MM was sandy. The MM thickness in May and August of 2011 changed with magnitude variation, from 0.5 to 7.0 cm ?average:2.2 cm? and 0.5 to 11 cm ?average:3.9 cm?, respectively. In general, the thickness of MM was thicker in south of the ECS during spring but in north during summer. In summer of 2013, there were two regions with thicker MM, one of which is in the Zhe-Min coast where the thickness of MM was decreased toward offshore and the other region is in the north offshore where the thickness of MM was decreased southeastward. The MM along the Zhe-Min coast during summer of 2013 was thicker than that during spring and summer of 2011. In the north offshore, the MM during spring of 2011 was extended much close to south compared to that during summer of 2013. The estimated mass of MM in the Zhe-Min coast of ECS was 7.8×108 ton in May and 8.8×108 ton in August, and these values are approximately 4 times higher than the annual sediment discharge from the Yangtze River. Using a simple mass balance calculation and inventory of ²¹⁰Pb_ex, the residence time of MM was estimated to be 3-6 years. This residence time indicates that the MM undergoes multiple cycles of deposition-resuspension-transport-deposition over a period of 3-6 years before eventual burial and/or transport off the shelf.?2? In 2011, the inventories of 7Be of MM were higher along the Zhe-Min coast during spring and in the north of the Yangtze Estuary during summer. Moreover, the magnetic parameter ?ARM/SIRM increased from estuary to southeast during spring and increased from estuary to northeast during summer. These variations were corresponding to the seasonal change of the Changjiang Diluted Water ?CDW?, indicating that the seasonal offshore transport of MM near the Yangtze Estuary was influenced by CDW. In the Zhe-Min coast, the inventory of 234Th_ex in MM was 2.5 times higher than the water column supported 234Th, suggesting that there was notable sediment focusing in this region. From ? vs. SIRM distribution, MM of this region was mainly from the Yangtze River input. The inventory of 7Be and ¹³⁷Cs decreased from the coast to offshore and this was corresponding to the increase of ?ARM/SIRM, which indicated that the offshore transport of MM. This offshore transport was observed in both two seasons, suggesting that the transport process was not very likely dominated by the seasonally varied coastal current. In the north of the Yangtze Estuary, the bimodal distribution of grain size frequency was observed, indicating that the MM materials in this region were derived from the mixture of multi-sources materials. From ? vs. SIRM distribution, MM of this region was mainly from the mixture of the finer materials in the North Jiangsu coast and the relict sands in the north offshore.?3? 7Be and ²¹⁰Pb of MM in the ECS mainly transport from the coastal area to offshore, but Pu isotopes of MM are mainly from open sea. The ²⁴⁰Pu/239Pu atom ratios in MM of the ECS, ranging from 0.16 to 0.30 ?mean:0.23±0.02?, which were mostly higher than the global fallout value of 0.18 while lower than the signature value ?0.36? of the Pacific Proving Grounds ?PPG?, indicating an influence of PPG Pu input to the ECS. The mean inventory of 239,²⁴⁰Pu in sediment of ECS was ?1.4×10² Bq m^-2 ?including this study and literature report results?. The Yangtze River input dominates as the source of Pu in the estuary area in the ECS. However, the annual Yangtze River input 239,²⁴⁰Pu ?2.4×1010 Bq? is small compared with the total Pu inventory of 4.5×1013 Bq in the ECS shelf. The global fallout 239,240pu was estimated to be 1.3×1013 Bq. The PPG input ?2.6×1013 Bq? is the main source of 239,²⁴⁰Pu in ECS sediment, accounting for 64% of the total 239,²⁴⁰Pu estimated in the entire ECS. Thus, ²⁴⁰Pu/239Pu atom ratio could be potentially used to estimate the nutrients that behave similarly to Pu transported by the Kushiro Current and Taiwan Warm Current into the ECS. Moreover, the sedimentation rates estimated from the 239,240pu were similar to the values based on ²¹⁰Pb_ex, which indicates that 239,240pu is a good time marker and can be used to calibrate the ²¹⁰Pb_ex chronology in marine environment.?4? In the estuarine and coastal areas, the global fallout curve of ¹³⁷Cs could not be observed in some sediment cores. Thus, the laboratorial work was conducted to certify that ¹³⁷Cs could move downward in the marine sediment and the Kd value of ¹³⁷Cs in oxic sediment was higher than that in anoxic sediment. Considering the mobility and relative short half time of ¹³⁷Cs ?30.2 y?, its application in marine radio-chronology are facing more and more challenge. The mobility and particle reactivity of ¹³⁷Cs is influenced by salinity, pH and particle component. The experimental results showed that the particle-reactivity of ¹³⁷Cs in 53 mg L"1 illite suspended solution was higher, with a distribution coefficient ?Kd? value of >6.5×10³ ml g-1 when the salinity was 0-0.5 psu, but the Kd value of ¹³⁷Cs was sharply dropped to lower than 10³ ml g-1 when the salinity increased from 3.6 to 28 psu. When the pH was low ?i.e.,5?, the Kd value of Cs was very low due to the competition of multi-cation ?e.g., H+?, whereas that was higher when pH was 7. The particle species would also influence the Kd value of ¹³⁷Cs in seawater system. The Kd value of ¹³⁷Cs was higher in illite, semcite and chitin, with the value of ?2.0-4.0?×10²ml g-1. In the freshwater,¹³⁷Cs could be tightly absorbed into particles, and transported to ocean environment together with particles. The experimental results showed that ¹³⁷Cs in the sediment could be desorbed by seawater for less than 10%. Meanwhile, the river-loaded particles could only absorb very few oceanic dissolved ¹³⁷Cs because of the solubility of ¹³⁷Cs in marine environment. The measured ¹³⁷Cs inventory in MM showed clearly linear correlation with the thickness of MM. Therefore,¹³⁷Cs could be potentially utilized as a transport tracer of terrigenous particles in river-dominated estuaries and coastal areas around the world.In summary, this study creatively utilized the multi-tracer, including radionuclides and magnetic properties, to investigate the source and seasonal transport of MM in the ECS and evaluate how the mobility and particle reactivity of ¹³⁷Cs affect its application for tracing sediment transport and marine radio-chronology. This study also evaluated the budge of Pu in sediment and its application in radio-chronology in the ECS. This study offers the complementary information for the estimation of MM residence time around the world, which is not only useful for better understanding of the transport-deposition-resuspension processes of sedimentary particles and biogeochemical processes in the mud area, but also for the research of shelf processes around the world. This study also gives some references for choosing better dating methods in the marine environment.