Excess 222Rn profiles in the bottom layer of the Japan Sea and their implication for bottom water dynamics

• We measured excess 222Rn profiles for the first time in the Japan Sea bottom layer.
• All profiles showed mid-depth maximum, suggesting strong affects of horizontal advection.
• Lateral bottom currents or eddies with short timescales are suggested in bottom water.

Vertical profiles of excess radon-222 (222Rn; half life: 3.82 days) in bottom seawaters between 8 and 780 m above the seafloor were measured in the Japan Sea, a semi-closed deep marginal sea in the northwest Pacific Ocean. All of the profiles observed at three locations (two profiles in the Japan Basin and one in the Yamato Basin) show curious zigzag patterns with 222Rn maxima and minima, in contrast to a generally known steady-state profile of 222Rn exponentially decreasing with the distance above the bottom. The observed 222Rn profiles demonstrate the existence of horizontal supplies of 222Rn-enriched waters in the bottom layer of the Japan Sea with timescales as short as the half-life of 222Rn, probably in relation with the interaction between bottom currents and topographic features.