Mean residence time of the shelf water in the East China and the Yellow Seas determined by ²²⁸Ra/²²⁶Ra measurements

Abstract

Increasing attention of oceanographers has recently been paied on East Asian marginal seas regarding their role on the global environment, yet geochemical investigations have been few to date. We here report new data on the distribution of 228 Ra and 226 Ra in the surface water of the East China and the Yellow seas in an effort to constrain the time necessary for the coastal and shelf waters to exchange with offshore waters. Such information is needed in evaluating the exchange of heat and water across the air‐sea interface that affects the local climate and the fate of pollutants, nutrients and weathering products supplied from the continent. Based on the Ra isotope signals, we have estimated that the shelf water component contributes ∼ 20 % of the Tsushima Current water passing through the Tsushima Strait and the mean residence time for the shelf water to mix with the Kuroshio surface water is ∼ 2.3 years. As many of materials derived from the continent such as heavy metals and the nutrients have their mean residence times less than a few months in the nearshore and shelf waters, they must largely deposit on the shelf sediments prior to the transport from the shelf to the open sea by mixing.