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

Abstract

“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 234Thex/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 210Pbex in MM.

Particle Dynamics in a Managed Navigation Channel Under Different Tidal Conditions as Determined Using Multiple Radionuclide Tracers

“…At S3, the 7 Be/ 210 Pb ex AR during the low tide of neap tide (0.40) was higher than that during the high tide of neap tide (0.22) and the 234 Th ex / 210 Pb ex AR was high during neap tide (0.11-0.13), indicating that particle sources contained terrigenous input as well as suspended particles or oceanic input. Overall, the 137 Cs/ 210 Pb ex AR at all four stations (0.058-0.12, mean 0.11) was higher than that of XLJ (0.058-0.12, mean 0.046) during 2007 (Du et al, 2010) but was in the same range as that beyond the estuary (0.040-0.18, mean 0.12) during 2011 (Wang et al, 2016). This further suggested that ocean-derived particle input played a significant role in particle sourcing in channels of the Changjiang Estuary such as the North Passage.…”

“…Mean 7 Be/ 210 Pb ex and 137 Cs/ 210 Pb ex ARs in XLJ could reflect the Changjiang input signal (1.0 and 0.046, respectively). Mean 7 Be/ 210 Pb ex and 137 Cs/ 210 Pb ex ARs beyond the estuary were reported to be 0.14 and 0.12, respectively (Wang et al, 2016(Wang et al, , 2018. Observed mean 7 Be/ 210 Pb ex and 137 Cs/ 210 Pb ex ARs in the North Passage were 0.16 and 0.11, respectively.…”

“…As 234 Th is the decay product of soluble 238 U in seawater, the 234 Th ex / 210 Pb ex AR can be used to indicate the resuspension process of particles in the estuary (Rutgers van der Loeff & Boudreau, 1997). The 137 Cs/ 210 Pb ex AR is also useful for reflecting the transport and suspension of riverine particles in the estuary (Wang et al, 2016). The foundation of using these three pairs as tracers in particle transport is these radionuclides tightly adsorbed onto particles and transport together with particles as well as the different ratios for different sources (i.e., riverine input, oceanic input, "old"/"new" resuspended particles, etc.)…”

“…Wang et al, 2016), and the specific activities of particulate 210 Pb ex and 234 Th ex were lower Th ex Activity in the North Passage than those in surface sediments of the East China Sea (5.3-171 Bq kg −1 for 210 Pb ex , 0-167 Bq kg −1 for 234 Th ex )(Wang et al, 2016).…”

Plutonium Isotopes Research in the Marine Environment: A synthesis

“…18,87 Combined with other isotopes-indicated results, it can be concluded that these sediments are from the Changjiang inputs. 81,114,121 3) Implications for sediment transport, mixing and erosion:…”