“C” and “D” means a triangle node on the coastline corresponding to Fig. S1.

For stations C1-C18, the radium activities and salinity at each triangle node on the coastline were approximated by those at the nearest seawater station and the water depths at these nodes are set to be zero.

For stations D1-D4, ¹⁸O value, ²²⁸Ra activity, salinity and depth of these triangle nodes were calculated based on the distance-weighted method. The distance is between the triangle node and corresponding two or three near seawater stations.