The island of Taiwan (Figure 1) was formed by oblique collision of the passive Chinese margin and the Luzon Volcanic Arc in the Late Miocene–Early Pliocene (Suppe, 1981; Teng, 1990; Dadson et al., 2003). The West Taiwan Basin developed approximately 6.5 Ma ago by lithospheric flexure of the Chinese continental margin in front of migrating thrust and fold loads in the Taiwan orogenic belt (Yu and Chou, 2001; Lin and Watts, 2002). The basin is occupied by shallow sea and was subaerially exposed during the late Pleistocene, when the East China Sea was up to 140 m below its present level (Boggs et al., 1979). Small mountainous rivers draining Taiwan collectively can discharge ∼200 Mt/year sediment into the Taiwan Strait, mostly during typhoon-generated floods (Dadson et al., 2003). The Choshui River, the largest river in Taiwan, occupies only ∼2% of the watershed area of the Yangtze (Figure 1) (Xu et al., 2009). The mountain belt in central-southern Taiwan is still actively uplifting, while the northern Taiwan mountain belt is beginning to collapse (Teng, 1996; Lo and Hsu, 2005). As a result, the sea floor immediately off northern Taiwan is situated in a transition zone among the post-collision Taiwan mountain belt, the passive Chinese margin, and the N-S extension of the rifting SOT (Sibuet et al., 1998; Tsai et al., 2018).

Regionally, the continental margin off northeastern Taiwan consists mainly of the broad ECS shelf, the narrow East China Sea slope (ECS slope), and the deep SOT (Figure 1). The ECS shelf is relatively flat and wide, measuring about 400 km in width, and has a northeast trending edge about 140 m in water depth. The ECS slope near Taiwan is characterized by irregular sea floor features with an average slope angle of about 1.5° (Song et al., 2000; Tsai et al., 2018). Linear depressions such as sea valleys, canyons, channels, and gullies are prominent morphological features of the shelf-slope region off northeastern Taiwan. The Okinawa Trough (OT), lying between Japan and Taiwan, is a back arc basin formed by extension within the continental lithosphere behind the Ryukyu trench-arc system (Kimura, 1985; Sibuet et al., 1998; Chen et al., 2019). The SOT near northeastern Taiwan is bounded by the ECS slope to the north and by the Ryukyu Islands to the south. It extends from the southwest Kyushu Island to the Ilan Plain of Taiwan (Sibuet et al., 1998) (Figure 1). The floor of SOT is at a water depth of less than 2,300 m (Sibuet et al., 1998). The water depth is nearly 200 m close to the Taiwan coast. The SOT is actively rifting with the current N-S extension of the shelf-slope areas near the Huapinghsu Channel and the Mienhua Canyon. The areas surrounding the SOT are characterized not only by active tectonics but also by frequent typhoons and earthquakes (Hsu et al., 2004; Huh et al., 2004; Liang et al., 2019). Submarine slides and mass-wasting features are also observed in this region (Tsai et al., 2018; Chang et al., 2021).

Locally, a relatively small portion of sea floor immediately off the northern coast of Taiwan, known as the Chilung Shelf, shows marked differences from the ESC shelf in terms of tectonics and topography (Song et al., 2000). The Chilung Shelf looks like a platform with rugged surface approximately bounded by isobath 200 m west of the Huapinghsu Channel and the Mienhua Canyon (Figure 1). The noticeable Chilung Sea Valley occurs along the northern coast of Taiwan and is characterized by a relatively small and short linear trough with low reliefs about 20 m along its valley axis (Figure 1). It is about 65 km long and ends near the shelf edge (Song and Chang, 1993; Song et al., 2000). East of the Chilung Sea Valley, two canyons were named the Mienhua Canyon and the North Mienhua Canyon, respectively, by Song and Chang (1993), after an islet near the head of the canyon on the shelf. These two canyons barely indent the shelf, before crossing the slope and extending southeastward into the SOT (Figure 1). The North Mienhua Canyon is a slope-confined canyon with four distinct heads immediately below the shelf edge, coalescing to form a single canyon down-cutting the sea floor with its end in the lower slope about 900 m in water depth (Yu and Lee, 1998). The stem of the Mienhua Canyon mainly developed on the continental slope. The head of this canyon barely indents the shelf edge northward at about 200 m in water depth near the islet of Mienhua (Figure 1). This canyon displays a relatively straight southeastward course but with two noticeable branches at the head and ends in the lower slope at approximately 1,000 m isobath. Yu and Hong (1993) noticed that a broad trough-shaped channel occurs northwest of the Mienhua Canyon with its head close to the islet of Huapinghsu where the water depth is around 120 m. This channel extends from its head near the Huapinghsu Island seaward in a slightly southeasterly direction and merges into the western side of the Mienhua Canyon where a canyon branch occurs at a water depth of about 300 m. The apparent connection of the Huapinghsu Channel to the Mienhua Canyon led Yu and Hong (1993) to name this linear depression the Huapinghsu Channel/Mienhua Canyon System, although the channel/canyon transition has not been clearly determined by the limited bathymetric data (Figure 1).

The Kuroshio current is an oceanic warm current flowing toward north or northeast off the eastern coast of Taiwan (Figure 1). The current shows the maximum velocity with over 100 cm/s near the surface of its core and gradually reduces it with depth (Tang et al., 2000). Seasonal migration of the Kuroshio current around the shelf edge off north of Taiwan interacts with the northward flowing Taiwan Warm Current (TWC) (Chuang et al., 1993; Tang and Yang, 1993; Tang et al., 1999; Tang et al., 2000; Tseng and Shen, 2003; Yin et al., 2020; Zhao et al., 2021), which yields a counterclockwise circulation of about 100 km in diameter centered around the Mienhua Canyon.

This circulation leads intrusion of the subsurface Kuroshio water onto the northern shelf of Taiwan, a deep southwestward countercurrent along the northern wall of the Mienhua Canyon, and a seaward outflow of continental shelf water around the northern coast of Taiwan. The mean velocity of the southwestward countercurrent showed a nearly constant value of 0.23 m/s over the upper 380 m water volume (Yin et al., 2020). The southwestward countercurrent can be significantly enhanced by a stronger offshore Kuroshio Current northeast of Taiwan. The northward flow of TWC in the Taiwan Strait has been formed about 7.3 Ka BP and has maintained its pathway to the present (Hsiung and Saito, 2017).

Wave condition around the study area is usually moderate. Based on the wave statistics of the Central Weather Bureau, Taiwan, 0.6–2.0 m of the mean wave height and 4.7–6.1 s of the rush period are reported in December, 2015–2019, near the Huapinghsu Channel head (northeast Taiwan; 25.304°N, 121.534°E; 34 m water depth). However, strong typhoons could generate giant waves offshore with the wave height (H _s) > 10 m and a period (T _p) of >10 s (Ou et al., 2002; Shih et al., 2018).

The SOT has been considered to be an ideal place to study regional source-to-sink process. As a main sediment sink, the SOT receives abundant terrigenous sediment from multiple sources (Figures 1 and 2), including major rivers on the Chinese mainland and small mountain rivers in Taiwan (Kao et al., 2003; Wei et al., 2006; Bentahila et al., 2008; Diekmann et al., 2008; Liu et al., 2008; Dou et al., 2016; Li et al., 2016; Chen et al., 2017; Hsiung and Saito, 2017; Xu et al., 2019; Hu et al., 2020).

Diekmann et al. (2008) have suggested that sediment supply increased from the northwestern Taiwan between 28 and 19.5 ka BP, rather than predominantly coming from East China. Other proxies, for instance, Pb and Sr isotopes (Bentahila et al., 2008), have indicated a mixture, with about 60% from Taiwan, 30% from Chinese loess, and 10% from Changjiang River (Yangtze River). The SOT has a very high sedimentation rate of 0.10–0.95 cm/year (Chung and Chang, 1995; Wei et al., 2005; Wei et al., 2006), providing a good archive for examining sedimentation history in the Late Quaternary. For instance, Dou et al. (2016) found changes in detrital sediment provenance in the SOT from a dominance of the paleo-Changjiang and/or continental shelf sediment during the late deglaciation and from western Taiwan rivers since 9.5 ka BP (Figure 2).

Hsiung and Saito (2017) linked the abrupt provenance shift in SOT sediments from mainland China to Taiwan with the birth of the TWC since 7.3 ka BP, which transports sediments derived from western Taiwan rivers to the ECS shelf. From Sr-Nd isotopes and geochemical measurements, Hu et al. (2020) proposed that terrigenous sediment supply has increased from western Taiwan to the SOT over the last 3 ka BP (Figure 2).

Under the present sea-level highstand, sediments on the sea floor in the Taiwan Strait are composed mainly of silt, whose coarse fractions (fine sand to medium silt) have mostly been derived from large hyperpycnal discharges from rivers in the western Taiwan (Xu et al., 2009; Hsiung and Saito, 2017; Jin et al., 2021). Much of the sediments were carried northward by the TWC through Taiwan Strait and formed a thick (>20 m) sediment body on the shelf (Liu et al., 2008; Hsiung and Saito, 2017). Furthermore, Hsiung and Saito (2017) suggested that western Taiwanese river sediments are transported northward by the TWC to the ECS shelf and finally to the SOT with the evidence from provenance studies of SOT. Liu et al. (2008) conducted a comprehensive study of post-glacial northwestern Taiwanese sediment distribution and transport in the Taiwan Strait. Geochemical evidence has led Chen et al. (2017) to suggest that some Changjiang River-derived sediment is carried by the China Coastal Current southward and subsequently joins the northward flowing Kuroshio along the western Taiwan coast to be transported to the SOT.

This section describes major seismic characteristics and morpho-sedimentary features from three seismic reflection profiles crossing the segment of the Mienhua Canyon at a water depth of more than 600 m. Seismic profiles penetrate downwards into the slope strata below the present canyon floor, allowing for examinations of characteristic features of the infilled sediments accumulated during the preceding canyon developing stages and interpretations of sedimentary processes related to the present Mienhua Canyon. Seismic profile ACT-04 (Figure 4A) shows distinct morpho-sedimentary features of a major erosional trough with steep flanks and flat-bottom cut by an axial incision. The main body of the major erosional trough consists of layered parallel to sub-parallel reflectors with high to low amplitudes. These seismic reflectors show variations in lateral continuity. Tilting and chaotic seismic facies are also present. The surface of the southwestern flank of the Mienhua Canyon is an inclined seismic reflector that extends down-slope and is intercepted by the canyon floor, which is a relatively smooth and flat seismic reflector. The shallow successions below the canyon floor are characterized by stratified, continuous, and parallel reflectors, which form layers that terminate at the inclined reflector below the canyon floor, forming an onlap seismic configuration (Figure 4A). Moreover, the northeastern flank of the Mienhua Canyon is an inclined seismic reflector with an irregular surface associated with several steps extending down-slope to the flat canyon bottom. It is noted that sub-parallel reflections below the northeast canyon flank terminate against the inclined reflection, implying that the northwestern canyon flank is an erosional surface. The axial incision is a sharp V-shaped depression without sediment accumulation at the thalweg. Apparently, layers of reflections below the canyon floor are truncated by both flanks of the axial incision, indicating down-cutting of the canyon bottom. Below the axial incision, chaotic seismic reflections occur, overlain by high-amplitude bedded facies that are probably incised axially. Ancient axial incisions likely formed during the early stages and were infilled by sediment later, resulting in axial incision-like features. Farther down-canyon, seismic profile ACT-03 (Figure 4B), parallel to seismic profile ACT-04, shows two distinct morpho-seismic features of a major erosional trough, flat bottom, and entrenched axial thalweg, closely resembling those on profile ACT-04. However, there are noticeable differences in these two features. Both major erosional trough tops are well recognized on seismic section, showing a symmetrical V-shaped depression with relatively narrow canyon walls and broad flat canyon floors. In the middle of the canyon floor is an entrenched axial incision with similar dimensions to those on profile ACT-04. However, the axial incision is characterized by a narrow U-shaped depression rather than a V-shaped depression. Note that the axial incision on profile ACT-03 is partially infilled by sediment, resulting in a relatively flat reflector at the incision bottom with a U-shaped depression.

Farther down-canyon, seismic profile ACT-02 (Figure 4C) crossing the Mienhua Canyon shows a small V-shaped depression about 2 km wide and 190 m deep, cutting downward into the slope strata. Apparently, the canyon shape changes drastically from a well-defined major erosional trough with a distinct axial incision to a hardly recognizable major erosional trough with only a thalweg incision down-cutting the present canyon floor.

In general, channels and canyons are formed by downslope erosion of the sea floor on both active and passive margins. The major difference is that tectonics control the location and orientation, while sedimentary processes contribute to the excavation and enlargement of channels and canyons (Hagen et al., 1994; Yu and Hong, 2006; Bernhardt et al., 2015; Chiang et al., 2020). This section focuses on the sedimentary processes related to the role of the Huapinghsu Channel as a modern sediment conduit. Moreover, the orientation and location of the Huapinghsu Channel are related to tectonic influence on the development of this channel.

V-shaped channel bottom indicates that the channel is not filled by sediments, implying gravity flows sweep unconsolidated sediments in the channel or even actively incises the channel floor. Here, we discuss the possibility of the occurrence of such density currents around the channel head.

Large waves during typhoons can often resuspend seafloor sediments on shelves and generate density current (e.g., Fan et al., 2004; Puig et al., 2004; Palanques et al., 2008). Because the study area is a place where intensive typhoons repeatedly attack every year, density flows may be formed by resuspended sediments from the seafloor during and after the events. Recent typhoons Soudelor (August, 2015) and Megi (September, 2016) are examples that caused heavy damages and numbers of fatalities in Taiwan. Both typhoons recorded large wave heights and long wave periods, whose maximum significant waves and the periods were 14.2 m and 15.1 s for the Soudelor and 8.2 m and 11.9 s for the Megi, respectively, at the Fugui Cape Buoy (Central Weather Bureau, Taiwan).

Based on the Airy wave theory (e.g., Komar, 1998), the maximum orbital velocities U _w for the maximum significant wave heights reach 139 cm/s for the Soudelor and 44 cm/s for the Megi at the water depth of 80 m around the head of the Mienhua Canyon (Figure 5A). Here, the threshold velocities for the motion of sand by the bottom orbital velocity U _w are obtained from the equations of Komar and Miller (1975), U w 2 ( s - 1 ) g D = 0.21 ( d o D ) 1 / 2   when   D <0 . 5 mm U w 2 ( s - 1 ) g D = 0.463 π ( d o D ) 1 / 4   when   D >0 . 5 mm where s is the ratio of densities of grain and sea water (1,650 kg/m³), g is the gravitational acceleration (9.81 m/s²), D is the grain size, and d _o is the orbital diameter by wave at the seafloor. Combining the depth-dependent orbital velocities (Figure 5A) with the threshold grain size under the waves (Figure 5B) yields a diagram showing the relationship between water depth and the threshold grain size (Figure 5C). This suggests that the typhoon waves are large enough to entrain coarse sand (1 mm) shallower than 80 m water depth and fine sand (0.1 mm) at ∼120 m water depth, although the normal wave base for coarse sand and fine sand are 16 and 27 m, respectively (Figure 5C). Because the wave-induced bottom shear stress is commonly enhanced by tides and surges (e.g., Shih et al., 2018), it is likely that sediment resuspension and generation of sediment gravity flows around the channel head occur during typhoon events.

Another possibility for shelf-to-channel flows at the edge of the shelf northeast of Taiwan may be also caused by the seasonal migration of the Kuroshio current (Chuang et al., 1993; Tang et al., 1999; Tang et al., 2000; Tseng and Shen, 2003). More than 30 cm/s of the flow velocity were reported along the shelf margin of 100–300 m water depth (Tang et al., 1999; Tang et al., 2000; Yin et al., 2020). For the case of unidirectional steady flows, threshold grain size can be estimated by using the following equations (p. 100 in Soulsby, 1997), U cr = 7 ( h D ) 1 / 7 [ g ( s − 1 ) D f ( D ∗ ) ] 1 / 2   for   D ∗ > 0.1   with f ( D ∗ ) = 0.3 1 + 1.2 D ∗ + 0.055 ( 1 − exp [ − 0.020 D ∗ ] ) D ∗ = ( g ( s − 1 ) v 2 ) 1 / 3 D where h is the height from the seafloor and ν is the kinematic viscosity of water (10⁻⁶ m²/s). This equation indicates that the threshold current velocities at 1 m above the seafloor are 0.29 m/s for fine sand (0.1 mm) and 0.41 m/s for coarse sand (1 mm), respectively. The observed flow velocities of the currents satisfy the critical values to threshold of motion of fine sand. Therefore, such southwestward countercurrent in this region may also cause density currents from the outer shelf where unconsolidated sediments are supplied.

Based on bathymetry, seismic profiles, and previous studies, we focused on the Mienhua Canyon as a main sediment conduit for receiving shelf sediments from the Huapinghsu Channel and transporting them down slope towards the SOT. The stem of the Mienhua Canyon is a slope-confined canyon characterized by a head terminating at the upper slope immediately below the shelf-break (Figure 1). Therefore, in the present sea-level highstand, the stem of the Mienhua Canyon has no apparent sediment input from fluvial sources to the canyon head. Hsu et al. (2004) clearly illustrates developments of intermediate and bottom nepheloid layers that were responsible for suspended sediment transportation from the shelf to the trough. Hung et al. (2003) also suggests the suspended particles might be sourced from resuspended bottom sediments. Submarine canyons can act as preferential pathways for sediment escaping from the continental shelf to the deep sea (Puig et al., 2014; Wilson et al., 2015; Haalboom et al., 2021), In other words, the present Mienhua Canyon acts as a sediment conduit, delivering mainly in situ sediments generated within proximal reach with minor amounts of ECS shelf sediments farther down canyon.

Instead, the western canyon branch and distal part function as a main sediment conduit delivering sediments sourced from Taiwan rivers to the SOT. Our results showed that the distal reach of the Mienhua Canyon is characterized by distinct morpho-seismic features of a major erosional trough with steep flanks and flat bottom cut by an axial incision (Figures 4A,B). The major erosional trough and axial incision observed in the Mienhua Canyon closely resemble the morpho-seismic features found in other submarine canyons (Popescu et al., 2004; Baztan et al., 2005). For the Mienhua Canyon, the major erosional trough and the filling of the canyon below the flat canyon bottom have resulted from multiple episodes of erosion and deposition throughout its evolution, in agreement with the findings of previous studies on canyon development (Shepard, 1981; Pratson and Coakley, 1996; Popescu et al., 2004; Baztan et al., 2005). For example, the stratified sediments of flat canyon floor overlie the steep canyon flanks to form an onlap seismic configuration (Figure 4A), indicating that the major erosional trough is subsequently and partially filled with sediment. In general, the occurrences and formation of axial incision in submarine canyons are mainly formed by erosive sediment flows generated by high sediment input into the canyon head areas. High sediment supply to the canyon is usually related to paleo-fluvial drainage during sea-level lowstand. Specifically, present-day axial incisions in submarine canyons (Black Sea and Gulf of Lion) have resulted from erosive sediment flows down-cut into the flat canyon bottom during the last sea-level lowstand. By analogy, the axial incisions in the Mienhua Canyon were formed by erosive sediment flows during the last sea-level lowstand 18 ka BP. Subsequent transgression took place some 9 to 9.5 ka BP (Figure 2). The ECS shelf north of the Mienhua Canyon head became a sediment-starved shelf, and the sediment supply to the canyon head was cut off with reduced canyon erosion and sediment transport. The Mienhua Canyon remained active, transporting sediment down-canyon even without high sediment supply from the head areas during the sea-level highstand. Due to the high sediment supply from the western canyon branch, the stem of the canyon head kept the canyon segment south of the head areas active during the subsequent transgression and sea-level highstand. In addition, intensities or magnitudes of erosion and transportation of sediment flows within the Mienhua Canyon may have been enhanced by episodic floods and earthquakes (Hsu et al., 2004). As long as shelf sediments are adequately and continuously supplied to the western canyon branch via the Huapinghsu Channel, the Mienhua Canyon maintains an active sediment conduit, delivering river-borne sediments from western Taiwan to the SOT even during sea-level highstand.

In this section, we intend to integrate the shelf sediments sourced from western Taiwan rivers, Huapinghsu Channel, Mienhua Canyon, and the SOT into a unified source-to-sink scheme. We propose a simplified model for the Huapinghsu Channel/Mienhua Canyon System as a modern sediment conduit with the main controls of changes in sea level, sediment supply to the heads of channel and canyon, sedimentary processes, and tectonics. This simplified model is described as follows:

At about 18 ka BP, a series of subaerial rivers and submarine channels and canyons developed in northeastern Taiwan (Boggs et al., 1979). No names were given to these linear troughs on the exposed shelf and submerged channels and canyons, which were assumed to be the seaward extensions of subaerial rivers. These linear troughs are generally oriented in a NW-SE direction and normal to the paleo-shelf edge of the ECS shelf (Figure 6A). Some of these linear troughs are considered the precursors of the Chilung Sea Valley, Huapinghsu Channel, and Mienhua Canyon. During the last sea-level lowstand, most of the present-day Huapinghsu Channel was exposed subaerially as river channel. However, the distal part at a water depth of >140 m might have remained a submarine channel. The stem of the Mienhua Canyon and the Huapinghsu Channel, close to the paleo-shoreline, possessed the capacity for receiving nearby fluvial and shelf sediments. Fluvial sediment input and shelf sediments, together with in situ sediments, resulted from slope failures of both canyon walls in the canyon head areas with transport down-canyon and final delivery to the SOT. Dou et al. (2016) suggested that sediments sourced from paleo-Changjiang and mainland Chinese rivers transported to the SOT were prevalent before 9.5 ka BP (Figure 2). These results indicated that the paleo-Changjiang and mainland Chinese rivers dominated the early Holocene deposition in the OT before 9.5 ka BP, despite the paleo-Changjiang shoal and mainland Chinese rivers being submerged and estuaries retreating landward to the Chinese coasts due to sea-level rise (Li et al., 2002). Judging from the canyon setting, we suggest that the Mienhua Canyon and the Huapinghsu Channel can be considered viable sediment conduits for delivery of shelf sediments to the sediment sink of the SOT before 9.5 ka BP. At the end of the Younger Dryaswas (11.55–12.7 ka BP), there was a sudden sea-level rise from about −60 to −45 m below the present level, followed by another rapid 15-m rise between 9.5 and 9.2 ka BP. The Yangtze mouth shifted close to its present position in response to these sea-level rises (Liu et al., 2004). In addition, the paleo ESC shelf-break might have rapidly retreated landward, resulting in less sediment input to these canyons and sediment sourced from Chinese rivers deposited in the SOT (Xu et al., 2019). It is noted that local depocenter of post-glacial fluvial sediments is located near the head of the Huapinghsu Channel (Figure 4D). These deposits could be formed in the past 10 ka BP (Liu et al., 2008). Assumed continuous sediment input to the head of the Huapinghsu Channel from these deposits might have kept this channel/canyon system actively transporting sediments to the SOT via the distal Mienhua Canyon since the 9.5 ka BP. Shen et al. (2021) noted that these sediments derived by Taiwanese rivers are carried northward by TWC and deposited near the head of the Huapinghsu Channel. The deposits were formed after the 7 ka BP (Jin et al., 2021). However, continuous sediment input to the head of the Huapinghsu Channel from the deposits has kept this channel/canyon system actively transporting sediments to the SOT via the distal Mienhua Canyon since the 7 ka BP. The Mienhua Canyon might have become less active 9.5–7 ka BP due to the lack of connection between the head and fluvial sediments.

With the subsequent transgression some 5–7 ka BP, when the sea level rose to close to the present position (Figure 2), the entire Huapinghsu Channel was a submerged submarine channel, while the stem of the Mienhua Canyon was a slope-confined canyon far from the present shoreline (Figure 1). As shown in Figure 1, there exists bathymetric continuity of the deepest part of the sea floor along the courses of the Huapinghsu Channel and the distal part of the Mienhua Canyon, allowing inter-connections along their axes to form a continuous bathymetric trough across the shelf and slope region. It is worth noting that the Mienhua Canyon preferentially developed along the western canyon branch, which connects to the distal part of the Huapinghsu Channel to form a channel/canyon transition pathway, allowing shelf sediments to bypass the Huapinghsu Channel and be transported laterally to the Mienhua Canyon. Finally, the modern sediment conduit developed in response to the progressive changes in sea level from lowstand to highstand with shifting sediment input from the head of the Mienhua Canyon (18–9.5 ka BP) to the head of the Huapinghsu Channel from 9.5 ka BP to the present. The shifting of high-sediment input to the Huapinghsu Channel head led to generation of gravity-driven sediment flows, with transport of river-borne sediments from western Taiwan to the ultimate sink of the SOT via the Huapinghsu Channel/Mienhua Canyon System. High sediment input to the head of Huapinghsu Channel during the present sea-level highstand has been supported by a provenance study (Hu et al., 2020). Hu et al. (2020) showed that terrigenous sediment supply from western Taiwan to the SOT progressively increased from 3 ka BP to the present (Figure 2).

Wide continental shelves are commonly considered as an important sediment sink for terrestrial sediments due to very low slope gradient (Nittrouer and Demaster, 1986; Thomas et al., 2004; Shen et al., 2021). In the modern day, the East China Sea shelf with a width more than 500 km is then considered as a sediment sink as much of Yangtze River-derived fine-grained sediments are transported southward by the China Coast Current and deposited along the Zhejiang–Fujian coast. Less sediments from the Yangtze River can be transported across shelf east of the coast of about 120–150 km where the TWC is present. The TWC hinders most cross-shelf coastal sediment transport (Milliman et al., 1989; Bian et al., 2013; Chen et al., 2021). Then, the modern sediments from the Yangtze River are unlikely transported to the outer shelf of East China Sea. On the other hand, the northern Taiwan Strait has high energy with strong currents and high-sediment-yield environment, where sediments derived from Taiwanese rivers, mainly the Choushui River, are carried northward by the north-flowing Taiwan Warm Current and tidal currents and deposited near northern Taiwan island in outer East China Shelf (Liao et al., 2008; Liu et al., 2008; Shen et al., 2021). This study recognized the head of the Huapinghsu Channel being immediately in front of the local depocenter of shelf sediments about 20 m thick sourced from western Taiwan rivers (Figure 1). It is noted that the Huapinghsu Channel may play an important role as a sediment conduit transporting sediment derived from Taiwan rivers along the channel course, with merging into the Mienhua Canyon and eventually into the Southern Okinawa Trough. Nepheloid layers are significant contributors to the cross-shelf transport of particle from the ECS shelf through the Mienhua Canyon (Hung et al., 2003). The linkages between the north-flowing currents in the northern Taiwan Strait, the deposition of sediments derived from western Taiwan rivers, and the sediment conduits of the Huapinghsu Channel and Mienhua Canyon may explain the increase of terrestrial sediment supply from western Taiwan rivers to the final sediment sink of Southern Okinawa Trough over the last 3 ka BP.