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Cesium-137, a unique geochemical tracer, behaves as a particle-reactive species in freshwater that is useful as a geochronometer, and soluble in seawater that is used as a water mass tracer, as evidenced by higher solid-solution partition coefficient, K d , in freshwater and much lower K d values in seawater. A suite of sediment cores were collected from the Southern Yellow Sea and coastal regions of Croatia in 2014 and analyzed for vertical profiles of excess 210Pb (210Pb ex), 239+240Pu, and 137Cs. A set of laboratory experiments were also conducted to investigate the partitioning of 137Cs between a suite of solid material comprised of commonly occurring minerals in the environment and solution under different contact times, pH, and salinity. The partition coefficient showed that for illite K d sharply increased during the first two hours and then slowly increased after 4 h. Overall, the salinity significantly affects the 137Cs behavior in aqueous environments. The K d values of 137Cs were higher (6.5 × 103–1.1 × 104 mL g−1) when the salinity was below 0.5 psu but drastically decreased to 9.0 × 102 mL g−1 when salinity was 3.6 psu and then slowly decreased to 2.7 × 102 mL g−1 with the salinity of 27.5 psu. Combining with the previously published results, it is observed that higher suspended particle (SPM) concentration could enhance the K d values of 137Cs when SPM was below 12 mg L−1. Our results show that post-depositional mobility of 137Cs in the marine environment, except for river-dominated coastal areas, result in the 137Cs chronology is not suitable to validate the 210Pb ex- based chronology. The penetration depths of 137Cs were found to be much deeper than it was expected based on the 137Cs and 239+240Pu peak and 210Pb chronology. Chitin showed a higher affinity for 137Cs than clay particles (e.g., smectite and illite). The desorption experiment showed that the NH 4 + could leach 5.6% of sorbed 137Cs, accounting for 79% of the total exchangeable fraction while 2.8% of the total 137Cs was bound to organic matter. These observations support the field observation degradation of organic matter in anoxic marine sediment could enhance the mobility of 137Cs. [Display omitted] • 137Cs particle affinity varied different particles, pH and particle component. • Higher SPM concentration enhance the K d values of 137Cs when SPM was <12 mg L−1. • Mobility of 137Cs cause it invalidate the 210Pb ex chronology in seawater systems. • NH 4 + can leach 5.6% of sorbed 137Cs and 2.8% of 137Cs was bound to organic matter. • The organic matter degradation could enhance 137Cs mobility in seawater systems. [ABSTRACT FROM AUTHOR]