Deep-Sea Research I 51 (2004) 979–997
Observed sediment fluxes in the southwesternmost Okinawa
Trough enhanced by episodic events: flood runoff from Taiwan
rivers and large earthquakes
Shih-Chieh Hsu
a,*, Fei-Jan Lin
b, Woei-Lih Jeng
b, Yu-chia Chung
c,
Lih-Ming Shaw
b, Kuo-Wei Hung
caInstitute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC bInstitute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC
cInstitute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan, ROC
Received 18 December 2002; received in revised form 10 October 2003; accepted 20 January 2004
Abstract
The sediment fluxes to the southwesternmost Okinawa Trough (SOT) off northeast Taiwan have been measured with four sets of sediment traps. The sediment fluxes observed were much higher than those in many marginal seas and showed great variations in time and space. The fluxes exhibited a drastic seaward decline from northeastern Taiwan to the central trough and an increase with depth. Temporal variability of fluxes was in a wide range from three times to one order of magnitude for certain traps. The observed sediment fluxes had a positive correlation with river runoff from the Lanyang Hsi, a river in northeastern Taiwan. Usually, the highest fluxes could be attributed to episodic events, for example, typhoon passages, which always cause ample rainfall leading to high runoff and sediment discharge, and sediment fluxes that were observed during typhoon passages accounted for more than 50% of the long-term flux. Large earthquakes are suggested to be responsible for some sediment flux anomalies observed on the nearby steep slope because of a good match in time between sediment collection and earthquake occurrence. A conceptual model is proposed to illustrate the sources and transport of the SOT sediment. The study highlights the greater contribution of Taiwan fluvial sediments to the SOT compared to that of the Changjiang, and it provides evidence demonstrating the combined effects of river runoff and large earthquakes on the sediment fluxes observed in such a deep marginal sea. r2004 Elsevier Ltd. All rights reserved.
Keywords: Sediment flux; River runoff; Earthquake; Episodic event; Okinawa Trough; Taiwan
1. Introduction
Apart from source strengths, a variety of factors have been considered to be regulating the sediment fluxes in the seas, such as primary production (Nair et al., 1989), scales and directions of oceanic
*Corresponding author. Tel.: +886-2-27839910x252; fax: +886-2-27833584.
E-mail address:schsu@earth.sinica.edu.tw (S.-C. Hsu).
0967-0637/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2004.01.009
flows (Honjo et al., 1980), monsoons inducing changes of oceanic circulation patterns (Nair et al., 1989), and river discharge directly from land (Ittekkot et al., 1991;Wheatcroft, 2000). Recently, a comprehensive investigation, the STRATA-FORM program, was undertaken to evaluate the supplies of storm driven river sediment to the northern California shelf (Nittrouer, 1999; Wheat-croft, 2000). Researchers have documented that such episodic small-river floods could account for much of the annual sediment flux to the con-tinental shelf and slope. Large earthquakes are able to induce instabilities and then sliding/ slumping of slope sediments in the tectonically active continental margins (Hasiotis et al., 2002;
Lykousis et al., 2002). Owing to the natural forces mentioned above, medium- to large-scale submar-ine mass movements can occur and thus play a very important role in transport of terrigenous sediment to deep basins (Mulder and Cochonat, 1996).
Data on sediment fluxes obtained from four sets of sediment traps deployed in the southwestern-most Okinawa Trough (SOT), as well as data on particulate Al (PAl) in total suspended matter (TSM) along four transects (one each along shelf, slope and trough and one across shelf trough) were acquired in this study. Al was used as an indicator of the lithogenic (aluminosilicate) component of TSM and settling sediments (trap samples) (Hsu et al., 1998). During the collection period, there were several typhoons sweeping over northern Taiwan and large earthquakes (MX5:0 R) occur-ring with epicenters located beneath the study area. The study aimed to (1) determine the temporal and spatial variations in the sediment flux, (2) examine the source and transport mechan-isms of the sediments, and (3) evaluate the
significance of episodic events (i.e. flood runoff and large earthquakes) as they might be reflected in observed flux extremes.
2. Geological setting and circulation patterns Tectonically, the Okinawa Trough, situated between the East China Sea (ECS) shelf and the Ryukyu Islands (inset toFig. 1), is an active back-arc basin. The SOT borders northeastern Taiwan and is the deepest and narrowest part of the Okinawa Trough. Large earthquakes frequently occur in the region, where the Philippine Sea plate collides with and then subducts beneath the Eurasian plate. Usually, earthquake epicenters are located beneath the land area of eastern Taiwan and the seafloor off eastern Taiwan. In the SOT, sediments are approximately 1–3 km thick (Lee et al., 1980; Letouzey and Kimura, 1986;Sibuet et al., 1987). Sedimentation rates are from 0.18 to 0.87 cm/yr in a small quadrangle (ca. 2000 km2) in the SOT (Fig. 1) off northeastern Taiwan, and dramatically decrease to less than 0.1 cm/yr outside this quadrangle (Chen, 1995;
Chung and Chang, 1995). Also, the preliminary results from site 1202 (2448.240N, 12230.000E, 1275 m) of ODP Leg 195 in the SOT showed a very high-sedimentation rate of at least 325 cm/kyr (0.325 cm/yr) ( http://www-odp.tamu.edu/publica-tions/pubs.htm), comparable to the above values. This rate may be the largest ever found in seas with depths of the SOT or deeper.
The Changjiang annually delivers 478 Mt of sediment onto the ECS shelf (Milliman and Meade, 1983), of which a substantial fraction (ca. 60%) is transported southward along the mainland China coast by the Changjiang Coastal
Fig. 1. (a) Regional map of circulation patterns over the ECS and the Okinawa Trough. There are three main currents occurring in this large-scale region: the Kuroshio, the TWC and the CCC. The Kuroshio collides with the shoaling ECS shelf off northeastern Taiwan; as a consequence, a portion of the Kuroshio intrudes upon the ECS shelf and thus forms a branch called the Kuroshio Branch (KB, shown inFig. 1b). At the same time, a cyclonic eddy (CE) develops around the shelf break, shown inFig. 1b. In addition, another current that may be involved in the flow systems and influence the transport and sedimentation of terrigenous sediments is the Yellow Sea Coastal Current (YSCC). (b) Sites of four sets of sediment trap mooring array deployments in the SOT and four transects (22 sites) along shelf, slope and trough and crossing the shelf–slope–trough for TSM sampling. The plaided quadrangle indicates a high-sedimentation area (>0.2 cm/yr). Earthquake data were recorded at the Toucheng observation station of the Central Weather Bureau, Taiwan, ROC.
Current (CCC) (Fig. 1a) or tidal currents ( Milli-man et al., 1985). The wide ECS outer shelf, however, is covered by abundant relict sediments on the seafloor; they are composed mainly of biogenic debris, shell fragments and light brown sand remaining from the last glaciation (Boggs et al., 1979); no apparent sedimentation of contemporary sediments was observed on the outer shelf (Chen et al., 1992). Additionally, because of the high denudation rate (Li, 1976) and ample rainfall, Taiwan’s rivers annually deliver an enormous 263 Mt of sediment to the sea (191 Mt from western Taiwan and 72 Mt from eastern Taiwan) (Milliman, 1991;Water Resources Planning Commission, 1996; Water Resources Bureau, 1997a, b), a portion of which could be carried northward by currents (i.e. the Kuroshio off eastern Taiwan and the Taiwan Warm Current (TWC) in Taiwan Strait, see Fig. 1a) toward the study area. The large supply of terrestrial sedi-ments from Taiwan and China thus appears to be sufficient to account for the accumulation in the high-sedimentation-rate part of the SOT (Fig. 1b), which is less than 10 Mt/yr (Hsu, 1998). Never-theless, the source of the SOT sediment has been debated for over two decades. Earlier efforts to identify the sources (e.g. the Changjiang or Taiwan’s rivers) and transport pathways of the SOT sediment were based on diverse tools, e.g. chemical characteristics, mineralogy and physical properties of the sediments (Lin and Chen, 1983;
Chen et al., 1992; Li, 1994; Chung and Chang, 1995, 1996). On the basis of the spatial distribution of PAl concentrations, hydrographic properties (such as temperature and salinity) as well as circulation patterns, Hsuet al. (1998)
concluded that terrigenous sediment from eastern Taiwan rivers is probably the main source.
This study further explored the positive relation-ship between the observed sediment fluxes in the SOT and runoff of the Lanyang Hsi, a river in northeastern Taiwan. The annual sediment dis-charge and sediment yield of the Lanyang Hsi are 8.0 Mt and 8154 Mt/km2, respectively. The annual runoff is 2773 106m3and transient runoff always rises sharply from a baseline level of several tens of m3/s (mean rate 62 m3/s) to an abnormal level of
thousands of m3/s after heavy rainfall (average annual rainfall 3256 mm). The historical maxima of the daily runoff and of suspension concentra-tion are 4580 m3/s and 118 g/l, respectively, since 1949 (Water Resources Bureau, 1997a, b). The sediment discharges are dependent on river runoff, and a function relating the two has been estab-lished (Kao, 1995).
The main flow affecting this study area is the Kuroshio, which flows northward along the east-ern coast of Taiwan (Fig. 1). As it approaches northeastern Taiwan, it impinges on the shoaling ECS shelf; thereafter the Kuroshio mainstream turns northeastward almost along the shelf break. Part of the Kuroshio water intrudes onto the shelf forming a branch current, called the Kuroshio Branch Current. On the western side of the branch current, a cyclonic eddy about 70 km in diameter is generated over the shelf–slope as a result of the westward deflection of part of the branch current. According toHsuet al. (1998), the Kuroshio may bring abundant fluvial sediments exported from eastern Taiwan rivers, in particular the Lanyang Hsi, to the SOT. The TSM in the upper water can be entrained into the cyclonic eddy and accumu-late around the eddy’s center. Hence, for the shallower water the cyclonic eddy may play an important role in transporting surface suspensions seaward.
There are two other major current systems involved in the circulation of the ECS (Fig. 1a). One, the previously mentioned CCC flows south-ward along the mainland China coastline, and the other, the TWC flows northward along Taiwan Strait and is expected to bring fluvial sediments exported from western Taiwan rivers. It has been demonstrated that the TWC prevents the south-ward-transported Changjiang sediments from spreading to the outer shelf and thus confines these sediments to the inner shelf (Milliman, 1991;
Su, 2000).
3. Materials and methods
Sediment fluxes were obtained at four moorings (T5, T6, T12 and T13) with sediment traps at two or three depths per mooring in the SOT (Table 1,
Fig. 1b). French-made time-series PPS3 sediment traps (Piege a Particules Sequentiel, Model 3) were used (Heussner et al., 1990), which comprise 12 receiving cups and have a receiving area of 0.125 m2. Current meters were attached between traps. Sample treatment procedures are described elsewhere (Chung and Hung, 2000). The Al abundance in samples from T5 and T6 was measured; samples from T12 and T13 arrays were not analyzed because they were expected to be similar to those from T5 and T6, which were dominated by lithogenic sediments, as described below.
The TSM samples for analyzing Al were collected on cruises OR331B (10/4–9/1992) and OR397 (8/10–17/1994) of R./V. Ocean Researcher I of National Taiwan University (Fig. 1b). Cruise OR397 comprised three NE–SW parallel transects, along the shelf (transect A), slope (transect B) and trough (transect C), and each transect contained five stations. Cruise OR331B sampled seven stations in one NW–SE transect (transect D), crossing the ECS shelf to the trough and perpen-dicular to transects A, B and C. Cruise OR397 was taken 3 days after the landfall of typhoon Doug on northern Taiwan, and was in the collection period of trap T5. TSM samples were obtained through a filtration of 20–60 l seawater. Seawater samples were collected from varying depths with a CTD rosette equipped with twelve 20-l precleaned GO-FLO bottles.
Al and Fe determinations were made with a Hitachi 8100 flameless atomic absorption spectro-photometer (graphite furnace) after total digestion. Compositional data quality was validated by determinations of standard reference material BCSS-1 (marine sediment). The details of sampling and analysis are given inHsuet al. (1998, 2003).
4. Results and discussion
4.1. Spatial and temporal variations of the observed sediment fluxes
Temporal variations of the observed sediment fluxes for four sets of trap arrays are displayed against cup sequences in the lower panels of
Figs. 2–5. Ranges and means of the sediment flux (Table 2) were 1.7–43 and 8.8 g/m2/d for array T5 (n ¼ 36), 1.3–7.5 and 4.0 g/m2/d for array T6 (n ¼ 24), 4–95 and 46 g/m2/d for array T12 (n ¼ 43) and 6.1–36 and 17 g/m2/d for array T13 (n ¼ 48), respectively. They show large spatial and temporal variability, and are much larger than those of the Middle Okinawa Trough (Narita et al., 1990) and many marginal seas (Gardner, 1989;
Monaco et al., 1990;Biscaye and Anderson, 1994;
Heussner et al., 1999). The time-series patterns of sediment flux showed: (1) a strong time-depen-dence, for example, the T5 fluxes varying over one
Table 1
Basic parameters for sediment traps deployed off northeastern Taiwan Station ID Latitude (N) Longitude (E) Bottom depth (m) Cup depth (m) Current meter depth (m) Deployment period Collection duration (days/cup) T5 2506.450 12230.060 1060 560 (12)a, 760 (12), 960 (12) 365, 565, 765 5/5/1994– 9/1/1994 10 T6 2459.630 12239.260 1440 740 (12), 1340 (12) 896, 1196 11/14/1994– 5/12/1995 15 T12 2453.370 12212.380 1017 569 (15), 768 (16), 967 (12) 581, 780, 972 3/23/1996– 8/19/1996, 10/7/1996– 1/4/1997 15 T13 2455.360 12219.570 1392 946 (16), 1144 (16), 1343 (16) 951, 1244 3/23/1996– 8/19/1996, 10/7/1996– 1/4/1997 15 a( )=sample numbers.
Fig. 2. Time-series of the Lanyang Hsi mean daily runoff (m3/s) (upper panel) and the observed fluxes (g/m2/d) at three depths from the T5 array (lower three panels). In the former, time was shifted back 5 days (from 4/30 to 8/27/1994) relative to the observation period of the T5 fluxes (from 5/5 to 9/1/1994). Three typhoons struck Taiwan during the T5 collection period; the runoff peaks caused by them may correspond to high fluxes, indicated by gray bars. Two flux maxima (cups 11 and 12) were evidently responsive to runoff peaks occurring during passage of typhoons Doug (8/6–8/8/1994) and Fred (8/18–8/21/1994). On the other hand, the extraordinary high runoff resulting from typhoon Tim’s passage (7/10/1994) did not enhance the flux recorded in cup 8. Note that a logarithmic scale is used for the mean daily runoff.
Fig. 3. As inFig. 2, but for the T6 fluxes. Time-series of the Lanyang Hsi mean daily runoff was shifted back 5 days (from 11/9/1994 to 5/7/1995) relative to the observation period of the T6 fluxes (from 11/14/1994 to 5/12/1995). No typhoon took place during the T6 collection period. One runoff peak may correspond to the flux maximum observed in cup 3 of the 1340 m trap, but not to that of the 740 m trap, as indicated by the gray bar.
Fig. 4. As inFig. 2, but for the T12 fluxes. Time-series of the Lanyang Hsi mean daily runoff was shifted back 3 days (from 3/20 to 8/16/1996) relative to the observation period of the T12 fluxes (from 3/23 to 8/19/1996). Typhoons Gloria (7/25–7/27/1996) and Herb (7/29–8/1/1996) occurred during the T12 collection period.
Fig. 5. As inFig. 4, but for the T13 fluxes. Time-series of the Lanyang Hsi mean daily runoff was shifted back 4 days (from 3/19 to 8/15/1996) relative to the observation period of the T13 fluxes (from 3/23 to 8/19/1996).
order of magnitude, (2) synchronized variations between various depths for each set of trap arrays, and (3) constant (less variable) but low fluxes for T6 relative to others. These three features reveal three facts. First, many flux extremes (e.g. cups 11 and 12 of T5) could probably be attributed to certain episodic events (e.g. heavy rain during typhoon passages) that enhance quantities and rates of sediment supply, which will be discussed below. Second, the sources of settling sediments were almost similar at least for individual trap arrays, but the source strength was temporally changeable. Third, constant but low fluxes for T6 indicate that there were persistent and stable sources of sediment laterally contributing to the central SOT sediments.
The vertical profiles of mean fluxes at certain depths for each set of trap arrays are shown inFig. 6a; horizontal trends of mean fluxes for each set of trap arrays are shown against relative distances to
station T12 (Fig. 6b). The spatial patterns of observed fluxes were characterized by two major features. First, the fluxes increase with increasing depth (except for T13); this was usually found in marginal seas (Honjo et al., 1980, 1982;Saito et al., 1992; Biscaye and Anderson, 1994), illustrating that the transport of SOT sediments depended upon lateral advection in mid-depth water and bottom transport in deep water. Second, the fluxes largely decrease offshore from the northern slope of the Ilan Shelf and Ridge (T12 and T13) to the outlet of the Mien-hwa Canyon (T5) and the central trough (T6). Generally, the decreasing trends of fluxes are consistent with those of the PAl distribution (Hsuet al., 1998), as will be discussed in Section 4.3. Furthermore, the abnor-mally high fluxes observed on the lower slope north to the Ilan Shelf and Ridge demonstrated that the depositing environment is very dynamic, which will always cause sediment redistribution
Table 2
Sediment fluxes (g/m2/d) measured for four sets of sediment traps deployed in the southwestern Okinawa Trough off northeastern
Taiwan Cup no. T5 T6 T12 T13 560 m 760 m 960 m 740 m 1340 m 569 m 768 m 967 m 946 m 1144 m 1343 m 1 2.0 3.0 4.7 3.9 6.9 16.9 67.9 94.8 9.4 31.6 21.8 2 2.1 2.7 4.1 3.4 7.1 12.2 36.4 59.5 14.0 21.3 14.4 3 9.8 8.2 14.7 3.2 7.5 6.9 26.4 44.1 6.1 14.2 9.2 4 3.0 4.6 5.7 5.3 6.5 9.7 23.4 41.4 10.2 20.0 18.1 5 5.4 7.9 9.8 2.6 2.2 13.7 38.2 69.2 6.1 10.2 9.9 6 3.5 7.3 8.0 2.6 2.3 9.9 71.9 84.6 8.5 16.6 14.7 7 1.7 2.8 4.0 3.3 7.4 4.0 37.2 78.8 6.9 14.3 8.9 8 6.1 8.8 9.7 3.3 5.5 __ 32.5 26.3 6.7 15.8 11.6 9 3.1 5.1 9.5 1.9 2.9 33.4 44.0 __ 11.4 26.2 17.1 10 6.6 6.1 7.2 2.7 3.3 27.6 72.6 __ 16.4 32.6 22.8 11 14.6 20.7 42.9 3.0 3.3 12.1 64.4 72.2 13.5 24.4 20.3 12 5.1 15.6 39.5 1.3 3.4 21.8 71.0 80.1 13.5 22.8 21.0 13 30.4 78.1 78.1 16.7 35.7 23.9 14 39.2 89.4 36.7 15.0 30.8 27.4 15 27.8 87.4 __ 12.7 32.1 23.6 16 31.6 79.5 __ 9.4 20.5 17.3 Meana 5.3 7.7 13.3 3.0 4.9 19.8 57.7 63.8 11.0 23.1 17.6 Meanb 8.8 4.0 46.2 17.2 __ Sampling failures.
aMean values of various depth trap fluxes. bMean values of each set of trap fluxes.
and then contribute sufficient sediments to the SOT (Chen et al., 1992; Hsuet al., 1998; Lee, 2001).
4.2. Components and chemical compositions of settling sediments
Al abundance of the settling sediments from both T5 and T6 trap arrays averages 7.8% and 8.8%, respectively; Fe abundance, 3.9% for T5 samples and 4.1% for T6 samples (Table 3). This is in agreement with those determined for the SOT surface sediments, Al and Fe abundances being 8.271.3% and 4.470.7%, respectively (Hsuet al., 2003). Given that the Al abundance of lithogenic particles is similar to that of the average crust composition (8%) (Taylor, 1964), it allowed us to estimate the fraction of lithogenic components accounting for the bulk sediment (at least 80% or more). Such an estimate is reasonable because the total organic carbon (TOC) and carbonate of T6 trap samples account for a minor fraction, less than 2% and less than l0%, respectively (Sheu et al., 1999). This also agrees with the result of grain size analysis which shows that both T5 and T6 samples are dominated by silt and clay (more than 90% for most samples) (Chung and Hung, 2000). No obvious trends were found to illustrate the seasonal variations in TOC (or total organic matter, TOM) related to the primary production in the euphotic zone and those in grain size related to hydrodynamics (Sheuet al., 1999; Chung and Hung, 2000). In addition, based on the study of
Chen et al. (1992), the mean grain size of the SOT surface sediment is quite fine and uniform, i.e. 7– 8f; the clay fraction accounts for B50% of the
Fig. 6. (a) Vertical distribution of mean fluxes (g/m2/d) at
certain depths for four sets of sediment trap mooring arrays. (b) Horizontal distribution of mean fluxes for each trap array relative to station T12.
Table 3
Ranges and means (71 s.d.) of Al and Fe abundances (wt%) for T5 and T6 trap samples
Depth (m) (sample no.) Al (%) Fe (%)
Range Mean71 s.d. Range Mean71 s.d.
T5 560 (n ¼ 9) 6.8–9.1 7.770.7 3.4–4.3 3.870.3
760 (n ¼ 12) 7.1–8.8 7.970.6 3.7–4.2 3.970.2
960 (n ¼ 9) 7.2–8.4 7.870.5 3.6–4.2 4.070.2
T6 740 (n ¼ 12) 7.9–9.1 8.870.4 3.9–4.1 4.070.1
bulk sediment; the carbonate consists ofB10% of the bulk sediment. They concluded that the terrigenous sediment is the most predominant component of the SOT sediment, consistent with results of the trapped sediment in this work and of the surface sediment (Hsuet al., 2003). Accord-ingly, high terrigenous fractions may mask the signatures of low biogenic fractions (Sheuet al., 1999), thus resulting in insignificant seasonal patterns of the biogenic fluxes observed.
4.3. Nepheloid layers elucidated by the PAl spatial distribution
For cruise OR331B, PAl concentrations range from 0.4 to 36.7 mg/l with a mean of 4.477.1mg/l; for cruise OR397, PAl concentrations range from 0.2 to 135 mg/l with a mean of 7.0720.3 mg/l. The PAl depth profiles along four transects (A, B, C and D) investigated are displayed in Fig. 7, and have provided more insights into the spatial distribution of aluminosilicate (i.e. terrigenous) particles in the water column for tracing possible sources and depicting transport routes of the SOT sediment.
As shown in Fig. 7a, the PAl concentrations along transect A decrease seaward from the Taiwan coast (station 1) to offshore regions (station 13). They are low but uniform (approxi-matelyp10 mg/l) throughout the water column at offshore stations (station 6 and farther stations). It reveals that the PAl in the shelf water has undergone thorough mixing, possibly driven by tidal currents or internal waves. It further indicates that terrigenous particles transported offshore were quickly deposited (Windom and Gross, 1989;Naudin et al., 1997) or that the particle-rich coastal water was diluted by the particle-poor Kuroshio water.
As shown in Fig. 7b, high PAl concentrations along transect B were found in the upper water levels, extending from the coast (e.g. station 2) to the center of a cyclonic eddy (station 8) located around the shelf break (Fig. 1b) (Hsuet al., 1998;
Tang et al., 1999). This suggests that terrigenous particles were entrained into the eddy and then accumulated around its center (Hsuet al., 1998).
Hsuet al. (1998) observed low temperature and
low salinity in water occupying the coastal region (e.g. stations 2, 3, 4 and 5) and suggested large volumes of river discharge exported from eastern Taiwan (e.g. Langyang Hsi), particularly during or after typhoon passages. Meanwhile, rivers can be expected to export huge volumes of suspended sediments. PAl concentrations in the offshore water (stations 11 and 14) drastically decline throughout the water column, with a low PAl zone appearing at a depth of 200–500 m around the eddy center (station 8). Apparently, the PAl-poor water observed on transects A and B reflects the intruded ‘‘clean’’ Kuroshio water. By contrast, high PAl was found at B600 m around the northern slope of the Ilan Shelf (e.g. stations 2 and 5) where the environment may be unstable primarily owing to steep topography (>3) and tectonic settings as well as strong flows. Such a high PAl plume likely forms nepheloid layers (represented by shaded area inFig. 7b), dispersing suspended sediments into the SOT.
As shown inFig. 7c, the general patterns of the PAl profile along transect C were similar to those along transect B. There were elevated PAl concentrations near the coast and low PAl concentrations (less than 10 mg/l) off the coast. Two slightly higher PAl concentrations extending northeastward from the coastal zone (station 3) were found in the middle water layers. Addition-ally, a pronounced PAl concentration (135 mg/l) was found in near-bottom water layers on the slope (920 m at station 3) that most likely originated from the same source (or mechanism) as that observed at station 2, and it declined sharply toward the trough. The Al plume probably forms nepheloid layers. Note that the isopleth of 20 mg/l seems not to extend as far as station 15 (Fig. 7c) because sampling intervals were not close enough to verify the dispersing distance for the strong PAl plume. Nevertheless, it is believed that the plume can influence the PAl concentration to a large extent, at least up to the trough (e.g. station 10).
As shown in Fig. 7d, the PAl distribution was characterized by two main features along transect D: (1) PAl concentrations were generally lower in the shelf water than those in the slope and trough water, and (2) two PAl maximum zones were
found in the subsurface water (about 100 m) and at mid-depth (500–700 m) at slope–trough stations (stations D, E, F and G). Apparently, this high PAl subsurface zone did not result from cross-shelf
transport of suspended particles from the ECS shelf (e.g. stations A, B and C) to the slope and trough. It revealed that high terrigenous particles in subsurface slope water would come
Fig. 7. Vertical distributions of the PAl concentrations (mg/l) on four transects (a) A (along the shelf), (b) B (along the slope), (c) C (along the trough) and (d) D (crossing the shelf–slope–trough). Several PAl plumes are represented by shaded areas, which may be nepheloid layers. Note that different contour intervals were used for individual plots.
predominantly from eastern Taiwan (or the north slope of the Ilan Shelf and Ridge); this is consistent with results from stations 2 and 3 of cruise OR397. Moreover, the PAl mid-depth maximum could be caused by lateral transport of resuspended slope sediments (on the ECS slope or the north slope of Ilan Shelf and Ridge). However, it did not allow us to conclude if particle contributions from station D (ECS slope) were adequate for station E because gradients of PAl concentrations between 600 m at station D (15.5 mg/l) and 500–700 m at station E (11.0–15 mg/l) were insignificant. It is therefore suggested that there was another source from a different direction accounting for the high PAl layer. Downslope transport of redistributed sedi-ments on the north slope of Ilan Shelf and Ridge, which were caused by disturbances such as resuspension, slumping or sliding and had a dispersal direction perpendicular to transect D, appeared to be the major source of sediments. Apparently, this source is identical to the middle and deep PAl (or suspended matter) plumes observed on transects B and C (Figs. 7b and c).
There were several PAl plumes found in the upper (B100 m), middle (B600 m) and deep (B900 m) waters (Fig. 7); in general, they extended from the coastal zone to the offshore region (or the trough). On the basis of hydrographic measure-ments, river-borne sediments delivered from the Lanyang Hsi were the most probable source responsible for the upper PAl plume (Hsuet al., 1998). In addition, sediments deposited on the northern slope of the Ilan Shelf and Ridge off northeastern Taiwan can be easily disturbed by diverse dynamic flows (including currents and tides) and tectonic movement (e.g. submarine earthquakes). Turbidites have been commonly found in the lower slope sediments in the south-western Okinawa Trough (Chen et al., 1992), so they must frequently lead to mass movement of slope sediments. The reworked sediments thus were laterally advected toward the trough and enhanced the PAl content in the water. These plumes might become nepheloid layers that always serve as a critical transport mechanism (or source) for the basin sediment (Monaco et al., 1990; De Hass and Van Weering, 1997).
4.4. Sediment fluxes related to river runoff
A graph-matching method (comparing the time-series of sediment fluxes observed from each trap and of the mean daily runoff of the Lanyang Hsi undergoing shifts of 0–7 days) was used to examine the correlation of the above two para-meters; results are shown inFigs. 2–5. Some peak data of daily runoff were identified to correlate with high fluxes. For example, typhoons Doug (8/ 6–8/8/1994) and Fred (8/18–8/21/1994) caused such high runoffs (652 and 468 m3/s, respectively) that sediment fluxes peaked at cups 11 and 12 of array T5 (Fig. 2and Table 2). Typhoon Herb (7/ 29–1/8/1996) also caused relatively high runoff (1180 m3/s), which consequently caused a high sediment flux observed at cup 10 of both arrays T12 and T13 (Figs. 4 and 5). These facts revealed that once typhoons struck Taiwan, heavy rainfall always caused high runoff together with huge sediment export from the land, thus contributing to the source of settling sediments. Unfortunately, it appeared that not all flood events necessarily cause high fluxes. For example, typhoon Tim (7/9– 7/11/1994) created very high runoff (up to 1440 m3/s) and thus very high suspensions of about 10,000 mg/l (Kao, 1995), but it did not cause remarkable sediment flux as expected (Fig. 2). Even so, we cannot be sure if this resulted from sampling biases, because trapped sediments seemed to overstuff the receiving cup (T5-cup 8).
Furthermore, a relationship was examined between the sediment flux and the Lanyang Hsi river discharge. The total river discharge for certain periods, corresponding to the flux observed in a given cup, was the integration of the Lanyang Hsi mean daily runoff for the collection periods (shifted). For instance, the collection period of array T5-cup1 was from 5/5 to 5/14/1994, so the total river discharge corresponding to the T5-cup1 is the sum of the mean daily runoff for a period of 10 days, i.e. 4/30–5/9/1994, this period was shifted back 5 days (relative to 5/5–5/14/1994). As a consequence, we found a significant log–log positive correlation between the observed flux and the total runoff for almost all traps even though there are a few outliers from regression lines (Fig. 8). If there were any typhoon passing
through Taiwan during the collection period for certain cups, the data of these cups were labeled by a ‘‘t’’ in related plots. Moreover, the high fluxes due to flood events, particularly during typhoon passages, generally account for a major fraction of the long-term fluxes, particularly for the central trough; for example, cups 11 and 12 accounted for at least 52% of the total fluxes for half a year for array T5. In fact, the peak fluxes would have appeared in several days (very likely in less than 3 days) and would be much larger than observed values (i.e. averaging the whole collection period), so the significance of the flood events to the SOT
sediment is inferred to be much greater than the above estimate. In addition, the log–log relation-ship between the flux and river runoff reflects two facts: (1) there is an exponential relationship between the riverine particle concentration and river discharge, as discussed by Kao (1995), and (2) concentrations of the suspended particulate plumes that developed after tremendous fluvial sediments were exported from the Lanyang Hsi river mouth declined exponentially with dispersing distances (this is in agreement with sediment transport models) (Lyne et al., 1990;Zhang et al., 1999).
Fig. 8. Positive log–log relations between the observed sediment fluxes (g/m2/d) for all traps and total daily runoff, integrating
Lanyang Hsi mean daily runoff by selecting durations corresponding to the shifted periods for every cup. Dashed regression lines were drawn for all data points and their equations are shown in the lower part of plot; solid regression lines were constructed by excluding some anomalous data points (open circles) that cannot be simply interpreted by river sources, and their equations are listed in the upper part of the plot. Data points were marked with a ‘‘t’’ (typhoon) or an ‘‘e’’ (earthquake) if typhoons or large earthquakes (MX5:0 R) took place during the collection periods of each cup. Most high flux data can be interpreted by either flood events or large earthquakes.
4.5. Enhancement of sediment fluxes by large earthquakes
In light of a few flux maxima (shown by open circles inFig. 8) that could not be explained by the flood runoff alone, especially those observed on the northern slope of the Ilan shelf (T12), it was inferred that earthquakes might be either the direct or indirect cause because this area is in a tectonically active region. Also, two other facts were considered. First, the measured dates of the flux anomalies were usually in accord with the striking dates of some earthquakes of MX5:0 R (or MX5:5 R) with foci in the 24.50–25.50N,
121–123E (or 24–25.50N, 121–124E) quadran-gle (Table 4) (http://www.iris.washington.edu). Second, because the slope north of the Ilan Shelf
is rather steep (>3), the surface sediment on the slope readily becomes unstable and is then subject to resuspension or slumping once large earth-quakes strike. For all individual flux data, the data points in Fig. 8 were labeled by an ‘‘e’’ if large earthquakes occurred in their measurement peri-ods. For instance, an earthquake of M ¼ 6:9 R striking on May 23, 1994 is presumed to be responsible for the third largest flux (cup 3, i.e. it would be the largest flux if the two fluxes of cups 11 and 12 driven by the typhoon flood are excluded) from the T5-960 m trap (Fig. 2). In particular, the two highest fluxes from the T12-960 m trap (95 g/m2/d for cup 1) and the T12-768 m trap (80 g/m2/d for cup 16) are quite abnormal because they corresponded to low run-off (Fig. 4). They apparently fall outside the
predicted relationships with the Lanyang Hsi runoff (Figs. 8g and h). Similarly, they seem to be caused by earthquakes of approximately MX5:0 R (Table 4). Even though many flux data points deviating farther from regression lines could be explained by postulating a cause of earth-quakes, not all earthquakes of MX5:0 R induce elevated sediment fluxes. For example, the earth-quake of M ¼ 6:9 R striking on June 5, 1994 did not cause a pronounced increase in sediment fluxes observed at cup 4 of array T5. A variety of complicated parameters must be taken into account in the mechanisms of earthquake-driven sediment mass movements, such as frequencies and foci of large earthquakes. The measured
impacts of earthquakes on sediment fluxes would also depend on deployment sites of trap arrays, i.e. the effect becomes weaker from the slope (i.e. T12 and T13) to the central trough (e.g. T6).
4.6. Uncertainties of the deep flows to sediment plumes
It is evident that the strength of the sediment plumes is controlled mainly by river runoff and occasionally by earthquakes. Besides, the deep flows should also be considered in driving the magnitudes of sediment fluxes. In this study, the hydrodynamic conditions were recorded by cur-rent meters attached to the trap mooring arrays. The hydrodynamic results recorded on both T5 and T6 trap arrays have been published elsewhere (Chung and Hung, 2000). Chung and Hung (2000), however, have not found any definite relations between the observed fluxes and the deep flows. The stick diagrams of low-frequency flow speeds and directions recorded at 951 and 1244 m of array T13 are displayed inFig. 9. The flow at 951 m has changeable speeds of 1 toB10 cm/s and a diverse N (including NE and NW) direction; whereas at 1244 m, it is characterized by an almost constant speed of less than 5 cm/s and a persistent direction of SE. The occasional high speeds and variable directions suggest that high-frequency flows (e.g. wave and tide) and the northward Kuroshio could sometimes appear at depths around 1000 m. Nevertheless, the influence of deep flows on the strength of suspended sediment plumes (or observed fluxes) seems not to be conclusive so far. On the other hand, it is similarly unclear whether the strong moving sediment plumes would drive deep flow, as found byOgston et al. (2000). The only exception was that the elevated fluxes observed at cup 1 of both arrays T12 and T13 (Table 2) could likely be postulated to correlate with recorded high-speed flows (Fig. 9), even though they could also respond to a large earthquake, as mentioned above. In summary, the effects of the deep flows on the observed fluxes appear to be less significant compared to the flood runoff and large earthquakes.
Table 4
Records of earthquakes larger than magnitude 5 (or 5.5) occurring in the 24.50–25.50N, 121–123E (or 24–25.50N,
121–124E) quadrangle during the collection periods of selected
traps reported Date (yymmdd)
Foci lat.(N), long. (E)
Magnitude Trap samples 940523 24.10, 122.48 6.9 T5-cup 3 (o)a 940605 24.46, 121.86 6.9 T5-cup 4 ( ) 940606 24.42, 121.19 5.1 T5-cup 4 ( ) 940608 24.47, 121.75 7.9 T5-cup 4 ( ) 940716 24.48, 121.78 6.8 T5-cup 4 ( ) 941126 24.67, 122.24 5.5 T6-cup 2 (o) 941204 25.01, 122.77 5.0 T6-cup 2 (o) 941212 24.69, 122.54 5.0 T6-cup 3 (o) 941213 25.14, 122.86 5.6 T6-cup 3 (o) 950223 24.17, 121.68 6.5 T6-cup 7 (o) 950324 24.60, 121.93 5.6 T6-cup 9 ( ) 950401 24.91, 122.11 5.3 T6-cup 10 ( ) 950403 24.06, 122.30 6.1 T6-cup 10 ( ) 950424 24.61, 121.73 5.3 T6-cup 11 ( ) 960329 24.08, 122.20 5.8 T12/T13-cup 1 (o) 960609 24.92, 123.40 5.7 T12/T13-cup 8 (o) 960727 24.53, 122.05 5.0 T12/T13-cup 9 (o) 960729 24.49, 122.23 6.1 T12/T13-cup 9 (o) 960810 24.04, 122.67 5.7 T12/T13-cup 10 (o) 961126 24.08, 121.76 5.5 T12/T13-cup 14 (o) 961221 24.80, 122.82 5.3 T12/T13-cup 16 (o) Note: Earthquake data were taken from http://www.iris.wa-shington.edu.
a
If the observed fluxes of the related trap samples are judged to be influenced by large earthquakes, they are labeled by circles; if not, by crosses. The time lag between earthquake and collection of affected sediment flux was taken into considera-tion when the judgments were made.
4.7. A conceptual model illustrating sources and transport of the SOT sediments
A conceptual model is presented in Fig. 10 to briefly illustrate sources and transport of the SOT sediment. Also, it can explain what results of sediment fluxes were observed in the study and what forces drove these results. Terrigenous sediments from eastern Taiwan rivers (e.g. La-nyang Hsi) are delivered into the seas in rainy seasons; they may be transported quickly into the SOT during high runoff periods or remain temporarily on the Ilan Shelf and Ridge during low runoff periods. On the northern slope of the Ilan Shelf and Ridge, massive resuspension and slumping/sliding of the slope sediment, induced mainly by flood runoff (particularly during typhoon passages) and sometimes by large earth-quakes, drive large-scale mass movements. These sediments would subsequently be transported downslope. The majority of reworked sediments are dispersed toward the central SOT in a matter of several days and the remainder of reworked sediments might be deposited on the lower slope.
Together with various dynamical forces (e.g. steep topography, small submarine earthquakes, hydro-dynamic disturbance, etc.), local resuspension could frequently take place along the slope–trough routes; this is based on the PAl spatial distribu-tion. A scenario describing sediment transport of marginal sediments to deep basins in two steps, i.e. transport–deposition and resuspension–transport, has also been proposed byHeussner et al. (1999). The fluvial and reworked sediments that were originally deposited on the slope during flood events would be transported within discrete water layers and advected toward the trough; the upper, mid-depth and near-bottom nepheloid zones are perhaps developed in this way (Monaco et al., 1990; De Hass and Van Weering, 1997) and become the predominant supplier of the SOT sediments. The dense downslope sediment plumes would gradually go deeper with dispersal distance since they were generated on the northern slope of the Ilan Shelf and Ridge. With lateral transport the sediments would gradually fractionate their particle sizes and mineral assemblages and thus become diluted because of weakening of sediment
Fig. 9. Stick diagrams of flow speeds and directions recorded at 951 m (upper panel) and 1244 m (lower panel) from the first phase of the T13 deployment. Note that the two plots have different scales in both flow speed and time period. Results recorded at 1244 m in August were incomplete because of mechanical failure of the recorder.
strength and addition of biogenic matter. This can explain why at station T12 the observed fluxes peaked at the deepest trap (967 m) and at station T13 at the intermediate trap (1144 m), and why the observed sediment fluxes decreased from traps T12 to T13, and from T13 to both T5 and T6.
5. Conclusions
A number of conclusions may be drawn from results of this study.
(1) The observed sediment fluxes in the SOT are much higher than in many other marginal seas. They exhibited synchronized variations at discrete depths for certain trap arrays. Spatial variability showed increasing trends with increasing depths and a decreasing
tendency from the slope to the trough. Compositional results suggested the lithogenic (aluminosilicate) particles being the predomi-nant components, accounting for at least 80%.
(2) The PAl spatial distribution showed a similar pattern to that of the observed fluxes, decreas-ing from the coastal to the offshore regions. Elevated PAl concentrations were found in the near-bottom water of the slope north to the Ilan Shelf and Ridge, which is a very unstable sedimentary environment. Three PAl plumes occurred at distinct depths and prob-ably formed nepheloid layers that are an important source for the SOT terrigenous sediments.
(3) A positive relationship was found to exist between the observed fluxes and river runoff from eastern Taiwan (i.e. the Lanyang Hsi),
Fig. 10. A conceptual model describing a synoptic picture of the sources and transport of suspended and settling sediments along the vertical section from the Ilan Shelf/Ridge to the SOT. The SOT sediments may be supplied from the Lanyang Hsi (or eastern Taiwan rivers) primarily during typhoon passages or from the reworked slope sediments with a riverine origin caused by resuspension and slumping/sliding driven by earthquakes and topographic effects as well as hydrodynamic disturbances. The downslope and bottom transports are suggested to be the most important processes driving large-scale mass movements. Suspended particle plumes can develop at certain depths. The tongue-like, dotted areas indicate the suspended particle plumes that can form nepheloid layers; dot sizes and density represent particle size and the density of the plumes, respectively, illustrating that the particle size becomes smaller with transport distance and that the plume density becomes less with dispersal. The mid-depth nepheloid zone might explain why the observed sediment fluxes in the intermediate trap are higher than those in the upper and lower traps of array T13. Additionally, a small amount of bottom resuspension occurs locally on the transport route, particularly on the lower slope and trough seabed.
indicating that the magnitudes of particle fluxes are dependent on the contributions of river-borne particles, particularly during flood periods. It is elucidated how the flood sediments can be transported to the central trough so quickly. The contribution of episo-dic floods can account for at least 50% of the long-term (half a year) observed sediment fluxes in the SOT.
(4) A few flux extremes that cannot be explained by river runoff alone may be caused by large-scale mass movements induced by large earth-quakes (MX5:0 R). Our results demonstrated the observed fluxes dominated by episodic events.
(5) Coupling the data on sediment fluxes and PAl concentrations in the water column revealed that sediments from Taiwan rivers may be the most important source of the SOT sediments, as opposed to the sediments transported across the shelf, which are presumably from the Changjiang. Our conceptual model em-phasizes that the down-slope transport of reworked sediments, driven by resuspension and slumping/sliding, was the major process dispersing the slope sediments to the trough.
Acknowledgements
We are grateful to three anonymous reviewers for their constructive comments. We thank the technicians and crew of R./V. Ocean Researcher I for their help with sampling. Thanks are also extended to Mr. K. Huang for pretreatment of samples and to Drs. S.J. Kao and W.T. Liang for their helpful comments. This research was financed by the National Science Council, ROC. SCH was supported through NSC grants (NSC 84-2611-M002A-007K2 and NSC 85-2611-M002A-026K2) to FJL.
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