Particulate fluxes and transports on the slope between the southern East China Sea and the South Okinawa Trough

摘要

This paper presents time-series variations of the particulate fluxes in resolutions of 3-15 d from 10 mooring sites on the continental slope between the southern East China Sea and the South Okinawa Trough, northeast of Taiwan. Three of these sites were located in a canyon cutting across the slope. Temporal and spatial variations in particulate flux in the study area were strongly affected by the bottom topography, the topographically induced eddies, the intensity of tidal currents, the depth and height above bottom where the traps were deployed, and episodic events. The apparent mass fluxes measured at depths from about 300 to 1350 m at different locations varied with time and showed synchronous variations among the traps deployed at the same site for some sites. The time-averaged mass flux for each individual trap deployed at different depth and in different length of mooring time ranged from 0.77 to 53.7 gm(-2) d(-1). In the canyon the average fluxes were generally greater than 30 g m-(2) d(-1) with large standard deviations; at other slope sites the average fluxes were generally less than 10 g m(-2) d(-1), excluding one or two unusually high values which were observed in some individual traps probably reflecting episodic events. The particulate fluxes always increased toward the ocean bottom at each mooring site but the increment was much greater in the canyon. This suggests that the particulates were transported out of the shelf or upper slope area essentially through the canyon where large mass fluxes were observed. The current meter data processed with low-pass filtration and expressed in stick diagrams and progressive vector diagrams showed large temporal and spatial variations which were generally not correlated with those of particulate fluxes. Although strong tidal velocity oscillations in the canyon were found to be associated with high mass fluxes in some instances (Hung and Chung, 1998, Continental Shelf Research 18, 1475-1491), episodic events with very high mass fluxes were generally not related to the current systems. The deduced prevailing flow was consistently along the isobaths of the lower slope toward the southwest even though the current meters were deployed in different seasons for variable length of time. The mean current speed varied from 30 cm s(-1) within the Kuroshio at 305 m depth to 3 cm s(-1) at about 250 m above the bottom of the deepest site near the South Okinawa Trough. Most values were about 5-10 cm s(-1). In contrast to the canyon where the trapped particulates were mostly silt and sand (Hung and Chung, 1998), the particulates collected from the general slope area were mainly silt. High mass fluxes were found to be associated with high sand and silt fluxes in the canyon (Kung and Chung, 1998) probably due to strong tidal currents and episodic events that had occurred in this high-energy environment. Lower mass fluxes with less sand contents observed at general slope sites suggest that no significant amounts of particulates may be transported via the intracanyon areas. (C) 2000 Elsevier Science Ltd. All rights reserved. [References: 25]