James T. Liu, National Sun Yat-sen University, Taiwan
Yu-Huai Wang, National Sun Yat-sen University, Taiwan
Chih-An Huh, Institute of Earth Sciences, Academia Sinica Nangang, Taiwan
Ray T. Hsu, National Sun Yat-sen University, Taiwan
Rick J. Yang, National Sun Yat-sen University, Taiwan
Rebecca H. Rendle-Bühring, University of Bremen, Germany
Tidal versus hyperpycnal regimes in the dynamics and sedimentation in the Gaoping Submarine Canyon off SW Taiwan
The Gaoping Submarine Canyon (GPSC) is a river-fed canyon, whose head is located 1 km off the mouth of the Gaoping River, a small mountainous river in SW Taiwan, and meanders SW and SE that eventually merges into the northern end of the Manila Trench over a course of 260 km. The sediment dynamics and sedimentation in the canyon are dominated by the episodic hyperpycnal process triggered by typhoon floods in the river; and earthquakes and by normal tidal oscillations. These processes also create sediment records in the substrate of the canyon.
Sediment trap mooring studies in the GPSC have shown that under normal tide-dominated conditions, the net sediment transport direction near the canyon floor is up-canyon. The deployments of T6KP and T7KP moorings for two months each provide valuable contrasts in the net sediment transport in the near-bottom part of the GPSC under normal and hyperpycnal conditions. During the deployment of T6KP between Jan. 8-March 8, 2008, the net sediment transport was up-canyon, superimposed by tidal oscillations, both of which followed the orientation of the canyon axis at the mooring site. This net transport pattern is mainly controlled by the tidal regime.
The T7KP mooring (July 18-Sept. 18, 2008) reveals a complicated net sediment transport pattern, which shows that the normal up-canyon directed tidal regime was interrupted by a typhoon-triggered down-canyon hyperpycnal regime (July 18-25); that was followed by a transitional period of recovery (July 25-Aug. 24); after which the normal up-canyon tidal regime resumed.
The steady tidal oscillations and episodic gravity-driven hyperpycnal and gravity flows leave two distinct types of sedimentary signatures in the seafloor deposits of the canyon conduit. Trends similar to those observed in the sediment trap moorings were also observed in piston cores taken at three locations along the GPSC. They were taken in the 1) proximal, upper reaches (OR1-811 K08P), 2) at the transition between the upper and middle reaches (OR1-820 35), and more distally, 3) at the transition between the middle and lower reaches (OR1-811 K31A) of the GPSC. They are used to illustrate gradual down-canyon changes in the seafloor lithology in response to the relative dominance of these two sediment transport processes.
The sedimentary lithology observed in these three cores shows that the grain size decreases down-canyon in both the fine-grained hemipelagic sediments and in the coarse-grained gravity flow deposits. This demonstrates the down-canyon decrease in the strength of the fluvial-fed sediment load and/or the episodic gravity-driven turbidity currents. Due to the reduced dilution effect by these gravity flow deposits farther away from the head of the canyon, the fine-grained hemipelagic mud becomes the dominant lithology in the more distal, down-canyon sediments which is comparable to the hemipelagic sediments of the basin that ultimately form the deposits on the abyssal plain and in the Manila Trench in the SCS (the ultimate sink for sediments).
Theme 1: Canyon processes in the space-time continuum (formation, evolution, circulation)
hyperpycnal flows, sediment dynamics, typhoon floods, internal tides, Gaoping Canyon