Extractable manganese in the southeastern East China Sea shelf and the Okinawa Trough

Abstract

The distribution of extractable manganese, which is likely to be similar to dissolved manganese, in a transect from the mid-shelf of the southeastern East China Sea Shelf across the Kuroshio to the Okinawa Trough was determined in May 1991. In the surface layer above 200 m, the higher concentrations of extractable manganese (5 to 6 nM) are found at the most shoreward station. Mixing between the shelf water and the Kuroshio results in a tongue of manganese-rich water (> 3 nM) extending from the shelf into the Okinawa Trough. In contrast, the Kuroshio surface water and the upwelling Kuroshio subsurface water, both with concentrations of < 3 nM, are manganese-poor. Thus, during upwelling and frontal exchanges, manganese-poor water is imported into the East China Sea Shelf while manganese-rich water is exported to the Kuroshio. A box-model calculation for the cycling of extractable manganese in the East China Sea Shelf–Huang Hai-Bo Hai system indicates that the manganese cycle in this shelf-sea system is dominated by the exchanges between the water column and the sediments. The benthic flux of manganese from the shelf sediments to the water column (190 × 10 7 mole·year –1) and the scavenging removal of manganese from the water column to the shelf sediments (180 × 10 7 mole·year –1) are about equal to each other so that they may constitute an approximately closed cycle. While the shelf is a net source of manganese to its adjoining waters, the export of manganese from the shelf (20 × 10 7 mole·year –1) is about an order of magnitude smaller than the benthic flux. The inputs of manganese to the system from the rivers (3 × 10 7 mole·year –1), the intrusion of Kuroshio surface water (4 × 10 7 mole·year –1) and the upwelling of Kuroshio subsurface water (2 × 10 7 mole·year –1) are of similar magnitude but are yet another order of magnitude smaller. Thus, the export of manganese from the shelf cannot be sustained by these inputs of manganese to the shelf. Furthemore, the sum of the inputs from the Kuroshio is larger than the riverine input. In the deep water (> 600 m) of the Okinawa Trough, as the concentration of oxygen decreased steadily with depth, higher concentrations of extractable manganese (3 to 15 nM) were found. Superimposed on these generally elevated concentrations of extractable manganese, there were two distinct layers of manganese-rich water at about 700 to 1 000 m and 1 300 to 1 500 m. The former coincided approximately with the depth of the bottom of the pycnocline and with the depth of the band of organic-rich fine-grain sediment which lies along the lower shelf-upper slope at the northern flank of the Okinawa Trough. The latter coincided approximately with the depth of some known hydrothermal vent fields in the Okinawa Trough. However, whether these layers of manganese-rich water are linked to these oceanographic phenomenon cannot yet be established definitively.