Jinlong Wang

Mobile mud dynamics in the East China Sea elucidated using 210Pb, 137Cs, 7Be, and 234Th as tracers

“Mobile mud” (MM), which has fine grain size distribution (>90% clay + silt, and 2.5 in south inshore indicates that the sediment focusing resulted in the increased mass flux. The residence time of MM is estimated as 3–6 years both by mass balance of MM and 210Pbex in MM.

Natural radioactivity assessment of surface sediments in the Yangtze Estuary.

The activities of the natural radionuclides (238U, 232Th, 226Ra and 40K) of the surface sediments in the Yangtze Estuary were determined and used to evaluate radiation hazards in the study area. The of activities of 238U, 232Th, 226Ra and 40K ranges from 14.1 to 62.3, 26.1 to 71.9, 13.7 to 52.3, and 392 to 898Bqkg-1, respectively, which were comparable to values of other regions in China. The activities of 232Th, 40K and 226Ra were clearly different from the global recommended values. The radium equivalent activity was less than the recommended limit of 370Bqkg-1; therefore, the sediment in this area can be safely used for reclamation. The external hazard index values were less than one. The average absorbed gamma dose rate and annual effective dose equivalent values were slightly greater than the world average value. 226Ra/238U and 232Th/238U ratios could potentially be applied for tracing sediment source.

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

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

Submarine Groundwater‐Borne Nutrients in a Tropical Bay (Maowei Sea, China) and Their Impacts on the Oyster Aquaculture

Submarine groundwater discharge (SGD) has been recognized as an important pathway for nutrients into estuaries, coasts and the adjacent seas. In this study, 222Rn was used to estimate the SGD-associated nutrient fluxes into an aquaculture area in a typical tropical bay (Maowei Sea, China). The SGD into the Maowei Sea during June of 2016 was estimated to be 0.36±0.33 m d−1 and was associated with SGD-derived dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved silicon (DSi) fluxes (mol d−1) of (4.5±5.5)×106, (5.3±9.1)×104 and (9.4±9.3)×106, respectively. The SGD-derived nutrients (i.e., DIN, DIP and DSi) were more than 1.9, 0.9 and 3.6 times the amounts in the local river input and served as dominant sources in the nutrient budgets in the Maowei Sea. Moreover, the N/P ratios in the SGD around the Maowei Sea were high (mean: 64), and these ratios likely exceeded the environmental self-purification capacity, thereby enhancing the biomass and changing the phytoplankton community structure. Therefore, SGD processes with derived nutrients may affect the biogeochemical cycles and marine ecological environment in the Maowei Sea. Furthermore, the N/P ratios (∼67) in oysters are very close to those in the SGD in the Maowei Sea; this coincidence suggests that the high N/P ratios in the SGD are likely to be one of the most important sources that support oyster aquaculture, which might weaken the burden of water eutrophication in the Maowei Sea.

Spatial and vertical distribution of 129I and 127I in the East China Sea: Inventory, source and transportation

Iodirne-129 is useful for tracking water mass movement in the ocean. In this study, the concentration of iodine isotopes in seawater of the East China Sea (ECS) in October 2013 were analyzed to investigate the spatial and vertical distribution of 129I and 127I to understand water mass exchange. Results showed that the 129I/127I atomic ratios varied with the water mass, with higher values of (10-20) × 10-11 in the coastal regions and lower values of <8 × 10-11 offshore. Inventories of 129I were estimated to be (0.23-1.7) × 1012 atoms m-2 (n = 18) in upper 100 m waters, which is comparable to those of other regions without being contaminated by the nuclear accidents or nuclear reprocessing facilities. The total amount of 129I in the ECS water column was estimated to be 88 g in which over 90% is attributed to the oceanic input (e.g., West Pacific) via the Kuroshio Current (KC). The contributions of 129I from Changjiang (Yangtze River) terrestrial watershed (<7.5%) and atmospheric fallout (<2.7%) were small. Those from the Fukushima accident were negligible during this investigation. The 129I/127I ratios versus salinity distribution showed the range and stratification of the Changjiang, Yellow Sea, and KC waters in the ECS. Our study shows that the Changjiang fresh water could be transported to the North Jiangsu coast in October; the Taiwan Warm Current water could intrude to Northern part of the Changjiang Estuary (32°N). Besides, our results suggest that the 129I/127I profile is useful to indicate the seawater mixing process in ocean marginal systems.

Spatial and vertical distribution of radiocesium in seawater of the East China Sea

The 137Cs activity in surface water of the East China Sea (ECS) was 0.66-1.36 Bq m-3 during May of 2011. The low activities were observed in the Changjiang Estuary and Zhejiang-Fujian coast and high activities were observed in the south offshore and Kuroshio Current pathway, suggesting that the influence from the current system in the ECS. The 134Cs were undetectable (<0.03 Bq m-3) and the contribution of the Fukushima accident to ECS is estimated to be below 3%; hence it is negligible during the investigation period. Using the vertical profiles of 137Cs in the ECS, the mass balance is obtained, which suggests that the oceanic input dominates the 137Cs source in the ECS. 137Cs is potentially useful to trace water mass movement in the ECS. Our study provides comprehensive baseline of 137Cs in the ECS for evaluation of the possible influence of the nuclear power plants in the future.

Fingerprinting Sediment Transport in River‐Dominated Margins Using Combined Mineral Magnetic and Radionuclide Methods

Both magnetic properties and radionuclides are widely used to trace sediment transport in aquatic environments; however, these methods have not been used in combination. In this study, the East China Sea (ECS), a typical river-dominated margin, was chosen to demonstrate the advantages of combining these two methods to track sediment movements on a seasonal to annual timescale. The ratios between saturation isothermal remnant magnetization and anhysteretic remnant magnetization (χARM/SIRM) and Be/Pbex activity ratios as well as mass balance of Be provide information on the seasonal transport of sediment from the Changjiang Estuary to the neighboring shelf. Both Pb budget and SIRM distribution in the inner shelf of the ECS show that a small fraction (at most 14% of annual Changjiang sediment discharge) of particles could be transported offshore. Most of Be activities in inner shelf sediments of the ECS were below detection limit due to relatively lower residence times and dilution by the older sediment. The observation that radionuclide activities exhibit a better correlation with χARM/SIRM ratios than with grain size suggests that iron oxides are the primary carriers of Be, Pb, and Th. The absorption of radionuclides onto magnetic minerals further reinforces the reliability of this combined approach in tracing sediment transport. Our study indicates that radionuclides, with different half-lives, can be utilized for quantifying sediment dynamics, whereas magnetic properties can yield more detailed information on sediment transport directions. The combined analysis of magnetic parameters and radionuclides offers a better understanding of sediment transport in river-dominated areas.