Distribution and budget of ¹³⁷Cs in the China Seas

Cesium-137 is one of the most abundant anthropogenic radionuclides released by atmospheric nuclear testing and nuclear accidents, and accordingly it may significantly impact the health of humans and marine environmental eco-systems. Documenting the distribution and inventory of ¹³⁷Cs is thus a crucial task. In this study, we collected a large number of datasets with field observations of ¹³⁷Cs in the China Seas, in order to provide an in-depth understanding of ¹³⁷Cs budgets and distributions. The activity and inventory of ¹³⁷Cs in China Seas' sediments showed large spatial variations, related to the ¹³⁷Cs source, sedimentation rates and the mineral composition of sediments. The ¹³⁷Cs concentration in sediments decreased with distance from the shore, generally tracing the distribution of sedimentation rates. High ¹³⁷Cs inventories in the water column indicated a high solubility and long mean residence times. The mean residence times of ¹³⁷Cs in the China Seas were determined to be 45.6 ± 3.8 years for the South China Sea (SCS), 36.8 ± 3.1 years for the East China Sea (ECS), and 12.0 ± 1.0 years for the Yellow Sea (YS). A ¹³⁷Cs mass balance suggests that oceanic input from the north Pacific is the dominant ¹³⁷Cs source to the China Seas, contributing about 96.9% of this substance. Furthermore, the bulk of ¹³⁷Cs remains dissolved in the SCS water column, while ¹³⁷Cs is mostly deposited to the sediments of the ECS and the YS. This new compilation of the activity level and inventory of ¹³⁷Cs help to establish background levels for future ¹³⁷Cs studies in the China Seas.