Plutonium in Southern Yellow Sea sediments and its implications for the quantification of oceanic-derived mercury and...

The spatial distributions of mercury (Hg) and zinc (Zn) concentration and the isotopic composition of plutonium (Pu) were investigated in surface sediments and sediment cores collected from the Southern Yellow Sea (SYS) during May 2014. The variation of the ²⁴⁰Pu/²³⁹Pu atom ratio (0.18-0.31) in the surface sediments of the SYS clearly indicated a signal of close-in fallout input from the Pacific Proving Ground (PPG). The buried ^239+240Pu in the sediment of the SYS was estimated to be (4.7 ± 0.5) × 10¹⁰ Bq y^-1 during the period from 2011 to 2014, of which ∼33% (1.5 × 10¹⁰ Bq y^-1) was derived from the PPG by long-range transport via ocean currents (e.g., the North Equatorial Current and Kuroshio Current). The concentrations of Hg and Zn varied from 0.003 to 0.067 mg kg^-1 and from 43.9 to 137 mg kg^-1, respectively, and exhibited positive correlations with the ^239+240Pu activity both in the surface sediments (0-1 cm) and upper layers (7 cm) of the sediment cores. Therefore, by using Pu as a tracer, we estimated that the oceanic input contributed 2.0 tons y^-1 of Hg and 1.0 × 10³ tons y^-1 of Zn to the SYS sediments between 2011 and 2014, which accounted for 33% and 3% of total buried Hg and Zn, respectively. These findings indicate that environmental pollution control should also consider the oceanic contribution of some pollutants. The results of the present work help to elucidate the biogeochemical cycling of trace metals in marginal seas, and are helpful for managing environmental pollution in marine environments.