Quantifying tidal signatures of the benthic nepheloid layer in Gaoping Submarine Canyon in Southern Taiwan

Abstract

The benthic nepheloid layer (BNL) has been observed in the head region of the Gaoping/Kaoping Submarine Canyon (KPSC) throughout a year. The top of the BNL could be as high as 100 m above the canyon floor in which the suspended sediment concentration (SSC) could be as high as 30 mg/l. In the BNL, sand-sized particles comprise the largest size-class in the suspended sediment population. Based on three one-month time series observations near the canyon floor of along-canyon velocity, water temperature, and the volume concentration (VC) of clay, very-fine-to-medium silt, coarse silt and sand size-classes in 2000, 2002, and 2004, the BNL is strongly modulated by the tides at semidiurnal, diurnal, quarter, and sixth diurnal and spring-neap frequencies. In the course of a semidiurnal tidal cycle, the flood (up-canyon) current brings colder water from the seaward part of the canyon causing the SSC and the thickness of the BNL to increase. The SSC immediately near the canyon floor also increases in response to the maximum flood and ebb currents of the M 2 tide. The tidal-to-total energy ratio (ER) of the along-canyon flow is between 70–80%, and between 50–80% among the suspended sediment of clay, very-fine-to-medium silt, coarse silt and sand size-classes. The M 2 is the most important tidal constituent in the temporal variations of along-canyon flow, water temperature, and the VC of the four size-classes. The local phase differences between the forcing (velocity), and the responses (temperature and VC) at the M 2 frequency show distinct phase-lock that suggests patterns of standing and progressive internal tides. Suspended sediments in the BNL also respond to the M 2 forcing in a coherent fashion. Suspended sediment movements are strongly affected by nonlinear processes as indicated by the elevated values of the amplitude ratio of M 4/M 2 of the suspended sediment size-classes comparing to that of the flow. The cross-canyon geometry and the slope of the submarine canyon floor affect the propagation of the barotropic and internal tides, which in turn, affect the nonlinearity generation in the BNL.