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 <5% sand) and high porosity (≥0.50), plays an important role in the biogeochemical cycles in the estuarine areas and the inshore shelf. A suite of MM samples from the coastal area of the East China Sea (ECS) was collected in spring and summer of 2011 to observe their spatial and temporal distribution, grain size, and radionuclides concentrations. The MM thickness ranged from 0.5 to 11 cm (average: 2.2 cm (May) and 3.9 cm (August)). The thick mud layer is mainly distributed along the coast, with an area of 2.2 × 10⁴ km² in May and 1.5 × 10⁴ km² in August, with corresponding masses of 8.8 × 10⁸ and 7.8 × 10⁸ t, respectively. The estimated masses of MM are considerably larger than the annual sediment discharge mass of the Changjiang River. The distribution of ¹³⁷Cs inventories in MM indicates that ¹³⁷Cs can be effectively utilized as a transport tracer of MM in the river-dominated estuaries and coastal areas. The higher inventories of ⁷Be in MM in the river mouth in spring are attributed to higher depositional flux and higher sediment discharge. The ratio of the MM inventory of ²³⁴Th_ex/production in the overlying water column of >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 ²¹⁰Pb_ex in MM.