From the highest to the deepest: The Gaoping River-Gaoping Submarine Canyon dispersal system

Abstract

There are many different source-to-sink dispersal systems around the world, and the Gaoping River (GPR)-Gaoping Submarine Canyon (GPSC) provides an example especially as a canyon-captured system. The GPR, a small mountainous river having an average gradient of 1:150, and the GPSC, which links the river catchment to the deep-sea basin, represent two major topographic features around SW Taiwan. Together, they constitute a terrestrial-to-marine dispersal system that has an overriding impact on the source-to-sink transport of sediment in this region. The GPSC extents from the mouth of the GPR through the shelf and slope and into the northeastern Manila Trench, a distance of about 260 km. It is a major conduit for the transport of terrestrial sediment and carbon to the South China Sea and the landward transport of particles of marine and biological origin.
In the GPSC the dominant mode of suspended-sediment transport is tidal oscillations and the net direction is up-canyon. In contrast, sediment transport associated with episodic gravity-driven events is down-canyon. The steady sedimentation of the tidal regime results in hemipelagic mud across the canyon floor, whereas the gravity-driven (hyperpycnal) regime causes turbidite erosion and deposition along the canyon thalweg.
Typhoon-induced river floods often lead to hyperpycnal plumes at the river mouth, which directly and indirectly ignite hyperpycnal turbidity currents in the canyon forming an effective agent for transporting large amounts of terrestrial organic material (modern and fossil carbon) to the South China Sea basin. Therefore, the GPR-GPSC represents a source-to-sink system in which terrestrial sediment in a mountainous catchment is promptly removed and transported to the deep sea by episodic gravity flows. This is also a pathway by which modern terrestrial organic carbon is quickly and effectively delivered to the deep sea.

Full text

From the highest to the deepest: The Gaoping River–Gaoping Submarine
Canyon dispersal system
James T. Liu
a,
⁎, Ray T. Hsu
a
, Jia-Jang Hung
a
,Yuan-PinChang
a
,Yu-HuaiWang
a
, Rebecca H. Rendle-Bühring
b
,
Chon-Lin Lee
c
, Chih-An Huh
d
,RickJ.Yang
a
a
Department of Oceanography, National Sun Yat-sen University, Kaohsiung 80424, Taiwan ROC
b
Department of Geosciences, University of Bremen, PO Box 330440, D-28334 Bremen, Germany
c
Department of Marine Environment, National Sun Yat-sen University, Kaohsiung 80424, Taiwan ROC
d
Institute of Earth Sciences, Academia Sinica, Taipei, 11529, Taiwan ROC
abstractarticle info
Article history:
Received 2 September 2014
Received in revised form 22 September 2015
Accepted 27 October 2015
Available online 30 October 2015
Keywords:
Small mountainous river
Submarine canyon
Typhoon
River flood
Hyperpycnal flow
Turbidite
Terrestrial sediment
Hemipelagic sediment
There are many different source-to-sink dispersal systems around the world, and the Gaoping River (GPR)–
Gaoping Submarine Canyon (GPSC) provides an example especially as a canyon-captured system. The GPR, a
small mountainous river having an average gradient of 1:150, and the GPSC, which links the river catchment
to the deep-sea basin, represent two major topographic features around SW Taiwan. Together, they constitute
a terrestrial-to-marine dispersal system that has an overriding impact on the source-to-sink transport of
sediment in this region. The GPSC extents from the mouth of the GPR through the shelf and slope and into the
northeastern Manila Trench, a distance of about 260 km. It is a major conduit for the transport of terrestrial
sediment and carbon to the South China Sea and the landward transport of particles of marine and biological
origin.
In the GPSC thedominant mode of suspended-sediment transport is tidal oscillations and the net direction isup-
canyon. In contrast, sediment transport associated with episodic gravity-driven events is down-canyon. The
steady sedimentation of the tidal regime results in hemipelagic mud across the canyon floor, whereas the
gravity-driven (hyperpycnal) regime causes turbidite erosion and deposition along the canyon thalweg.
Typhoon-induced river floods often leadto hyperpycnal plumes at the river mouth, whichdirectly and indirectly
ignite hyperpycnal turbidity currents in the canyon forming an effective agent for transporting large amounts of
terrestrial organic material (modern and fossil carbon) to the South China Sea basin. Therefore, the GPR–GPSC
represents a source-to-sink system in which terrestrial sediment in a mountainous catchment is promptly
removed and transported to the deep sea by episodic gravity flows. This is also a pathway by which modern
terrestrial organic carbon is quickly and effectively delivered to the deep sea.
© 2015 Elsevier B.V. All rights reserved.
Contents
1. Introduction.............................................................. 275
2. Background............................................................... 275
2.1. Systemmorphology ....................................................... 275
2.2. Strongforcing:typhoonsandearthquakes ............................................. 275
2.3. AhighlyperturbedGPRcatchment:complexsignalgeneration.................................... 276
2.4. Aneffectiveconduitforland-seaexchanges:theGaopingSubmarineCanyon.............................. 278
3. Approachandmethods......................................................... 278
4. The environmental units in the Gaoping River–GaopingSubmarineCanyonsystem.............................. 278
4.1. The fluvialsystem ........................................................ 278
4.1.1. Hydrologicalcyclesandepisodicevents .......................................... 278
4.1.2. Sedimentgenerationanddelivery............................................. 278
4.2. Theestuary........................................................... 281
4.2.1. The estuarine filterrelatedtothesalt-wedge........................................ 281
Earth-Science Reviews 153 (2016) 274–300
⁎Corresponding author.
E-mail address: james@mail.n sysu.edu.tw (J.T. Liu).
http://dx.doi.org/10.1016/j.earscirev.2015.10.012
0012-8252/© 2015 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Earth-Science Reviews
journal homepage: www.elsevier.com/locate/earscirev

4.2.2. Flocculation....................................................... 281
4.3. TheGaopingSubmarineCanyon.................................................. 281
4.3.1. Physicaloceanographyofthecanyon ........................................... 282
4.3.2. Benthicnepheloidlayer ................................................. 283
4.3.3. Dynamicsandsourcesofthesettlingparticles....................................... 285
4.3.4. Sedimentation: tide-dominated versus gravity-flowdominatedregimes............................ 285
4.3.5. Tracersforterrestrial-andmarine-sourcedparticles..................................... 286
5. The source-to-sink sediment processes in the GPR–GPSCsystem ...................................... 287
5.1. Fine-grainedsedimentsascarriersofgeochemicalsignalsthroughthesystem............................. 288
5.2. The river–canyontransportpathwayundernormalconditions.................................... 289
5.3. The river–canyontransportpathwayunderhyperpycnalconditions.................................. 290
5.4. Internalsourcesandsinks:thenewisold ............................................. 294
5.5. Sedimentrecordsignaturesofthedualsedimentationpatternalongthecanyonconduit........................ 296
6. Summaryandfuturework........................................................ 297
Acknowledgments.............................................................. 297
AppendixA. Supplementarydata...................................................... 297
References................................................................. 298
1. Introduction
In a sediment-routing system the erosion of mountainous regions
and the final deposition are linked when the sediment is moved from
its source to its sink (Allen, 2008). Globally the amount of sediment
delivered to the oceans by rivers is influenced by tectonic processes,
which control the topography along the sediment-routing system, and
by the climate, which controls the weathering and erosion in river
catchments (Allen, 2008).
There is a great diversity of sediment-routing systems in the world,
which have a range of source and sink characteristics on continental
margins (Walsh and Nittrouer, 2009). There are numerous examples
including systems on passive continental margins such as the Po River
(Fox et al., 2004; Traykovski et al., 2007), the Amazon River (Nittrouer
and DeMaster, 1996), the Mississippi River (Bianchi and Allison,
2009), the Yangtze River(Changjiang), andthe Yellow River (Huanghe).
Some of these systems form active deltas (Bianchi and Allison, 2009;
Saito et al., 2001; Wu et al., 2007). Examples on active continental
margins include the Ganges-Brahmaputra River (Goodbred, 2003),
and the Eel River (Warrick, 2014). Along the Pacific Rim there are
small mountainousrivers on high standing islands, suchas the Lanyang,
Zhuoshui, and Gaoping in Taiwan (Liu et al., 2009b, 2013) and the
Waipaoa River in New Zealand (Kuehl et al., in this volume; Marsaglia
et al., 2010; Parra et al., 2012). A great majority of these rivers flow
into a bay or a gulf and wide or narrow shelves. There is another kind
of sediment-routing system that includes river-associated submarine
canyons (Baker and Hickey, 1986; Choi and Wilkin, 2007; Hsu et al.,
2014; Lopez-Fernandez et al., 2013; Palanques et al., 2005; Walsh and
Nittrouer, 2009), which comprise only 2.62% of all the 4025 submarine
canyons in the world (Harris and Wihteway, 2011). Most of these
submarine canyons are separated from their associated river mouths
by a portion of the continental shelf (Liu and Lin, 2004). Only in rare
cases does the submarine canyon head reach into or very near the
mouth of the river, such as the Sepik River (Kineke et al., 2000), the
Biobio River (Sobarzo et al., 2001), and the Gaoping River (GPR) (Liu
et al., 2002).
Among all the river dispersal systems, the Gaoping is particularly in-
teresting because there is a strong coupling between the dynamics that
move the sediment and thesediment record in the system that links the
catchment of a mountainous river and a submarine canyon on an active
tectonic setting. The characteristics of this sediment-routing system
makes it an ideal natural laboratory to study source-to-sink processes
and responses across the boundaries of different environmental units
within short time and space scales.
Based on topographic features, the Gaoping sediment dispersal sys-
tem in southern Taiwan is geographically divided into three segments
(Fig. 1A), each withits own source-to-sink implications:1) the southern
part of the Taiwan orogen (Fig. 1B); 2) the tectonically active drainage
basin of the GPR which includes the Gaoping coastal plain; 3) the
Gaoping Submarine Canyon (GPSC) that extends from the mouth of
the GPR into the NE corner of the South China Sea basin, which is
fronted by the steep slope of Taiwan Strait to the north (Yu et al., 2009).
This paper reviews the three key segments of the GPR–GPSC system
and discuss their influence on the form and function of this dispersal
system. The emphasis is on the process–response linkages between
sediment processes and the associated geochemical signals carried by
fine-grained sediment through the system.
2. Background
2.1. System morphology
The GPR is the largest river in Taiwan in terms of drainage area
(3257 km
2
) and the second largest in terms of suspended-sediment
load (49 MT per year) (Liu et al., 2009a)(Fig. 2A). The headwater of
the GPR is located in the southern part of the Central Range near Mt.
Jade (Yu-Shan) whose elevation is 3952 m above sea level (Fig. 2B). It
is a small mountainous river, with 48% of its drainage basin above
1000 m, 32% between 100 and 1000 m, and 20% below 100 m (Liu
et al., 2009a). Consequently, the riverbed gradient is 1:15 in the upper
reaches, 1:100 in the middle reaches, and 1:1000 in the lower reaches,
with an average of 1:150 (Fig. 2C, Liu et al., 2009a).
Located ~ 1 km from the mouth of the GPR, the head of the GPSC
cuts across the narrow Gaoping shelf and slope, and merges into the
northern end of the Manila Trench about 260 km away (Yu et al.,
2009,Fig. 1C). The morphology of the canyon is closely affected by the
intrusions of mud diapers in the upper reaches and thrust faulting in
the middle and lower reaches of the canyon (Chiang and Yu, 2006).
High and steep walls characterize the head region of the canyon (Yu
et al., 1993). The canyon has relief exceeding 600 m, with a cross-
sectional geometry varying from V-shaped to broadly U-shaped +/−
irregular troughs (Chiang and Yu, 2006, 2011). The formation of this
submarine canyon was controlled by the tectonic evolution of the arc-
continent collision between the Chinese continental margin and the
Luzon volcanic arc, which generated many structural deformations in
the Taiwan accretionary wedge, including the GPSC (Liu et al., 1997).
2.2. Strong forcing: typhoons and earthquakes
In addition to tectonic deformation, typhoons and earthquakes are
two major factors that affect the system (Liu et al., 2013). On average,
approximately four typhoons pass through Taiwan per year (Liu et al.,
2013). Among the historical typhoons, 23% were from the western
Pacific Ocean, and 13% were from the SCS (Liu et al., 2013). Typhoons
can affect the system when still hundreds of kilometers away by bring-
ing marine-sourced foraminifera into the upper reaches of the GPSC.
298
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During the proximal phase, when typhoons induce river floods, high
waves and storm surges are typically present in thecoastal areas and lo-
cally sourced sediments from the river flood and sea-floor resuspension
dominate the sediment fluxes in the system (Liu et al., 2006). Typhoons
are a significant factor not only as a geological agent that affects sedi-
ment generation and burial throughout the system on millennium
time scales and longer, but also as a major forcing agent moving sedi-
ment from the terrestrial river catchmentto the deep sea via the subma-
rine canyon on time scales of hours and days (Liu et al., 2012; Hsu et al.,
2014). This review considers how several recent typhoons, including
Kalmaegi (2008), Morakot (2009), and Fanapi (2010) affected the
source-to-sink processes in the GPR–GPSC system (Liu et al., 2013).
The influence of earthquakes has been minor in the sub-aerial GPR
catchment area in recorded history compared to other areas in
Taiwan. However, a major earthquake in 2006 triggered significant
gravity flows in the nearby Fangliao Submarine Canyon and the lower
GPSC and Manila Trench (Hsu et al., 2008; Carter et al., 2012; Su et al.,
2012). Associated with the event, ~22 subsea communication cables
were broken, implying the tremendous power of the gravity flows.
Hung and Ho (2014) reported elevated concentrations and inventories
of suspended matterand trace metals in both Fangliaoand Gaoping can-
yons after this earthquake, which could be caused by the earthquake-
triggered gravity flows.
2.3. A highly perturbed GPR catchment: complex signal generation
Landsides/debris flows triggered by earthquakes and typhoons epi-
sodically increase the sediment supply from upland catchments in the
GPR drainage basin (Hsieh and Chyi, 2010; Liu et al., 2013). Fan terraces
are a very common landscape form at the confluences of the major and
secondary tributaries along the Qishan and Laonong Rivers (Hsieh and
Chyi, 2010; Hsieh and Capart, 2013), and the stratigraphic sequences
of these fans attest to the mass-wasting process that erode the moun-
tains and aggrade the river channels and riverbanks (Hsieh and Chyi,
2010). An event that took place b200 yr BP, for example, caused the
riverbed to elevate 120 m (Hsieh and Capart, 2013). Because of the ep-
isodic nature of the mass wasting, both the aggradation and subsequent
incisionprogress rapidly,on time scales of less than a few hundred years
(Hsieh and Chyi, 2010). There is recurring aggradation-incision in the
history of tributary-fan development throughout the upper and
middle-reaches of the GPR catchment, thus the high spatial and tempo-
ral variability in river morphology indicates that sedimentation to and
in the river is dynamic and complex. Although typhoons and earth-
quakes are key driving forces in the sediment supply (Liu et al.,
2013), there are other factors such as aggradation-incision that con-
trols the ‘fluvial behavior’of each tributary (Hsieh and Chyi, 2010;
Hsieh and Capart, 2013). These in turn, form internal sources and
sinks on centennial to millennium time scales within the river
basin.
In 5–10 August 2009, a super typhoon (Morakot) hit Taiwan (Chien
and Kuo, 2011). In about three days, it brought cumulative precipitation
of 1677 mm in the GPR basin, which is equivalent to the annual rainfall
in an ordinary year (Tsou et al., 2011). A catastrophic landslide in
the upper reaches of the Qishan River on 9 August buried the Xiaolin
Village and caused more than 400 casualties (Supplemental Fig. S1A).
The Xiaolin Village was located at the foot of an ancient colluvium
deposit that had accumulated over 120 m thick (Hsieh et al., 2012)
(Supplemental Fig. S1B). The erosion left a concave-shaped slope
(Supplemental Fig. S1C). Underlying the erosional slope of the Xiaolin
landslide, the prevailing appearance (dated 21, 14.9, 13.7 and 12.0 ka)
of mass-wasting sequences thicker than 20 m also suggests frequent
recurrences of landslides/debris flows in this area (Hsieh et al., 2012),
which would episodically generate enormous amount of sediment in
the source-to-sink system.
Fig. 1. A 3-D topographic mapof the GPR basin and the upperand middle reaches of theGPSC (A). The smallermap of Taiwan shows the areaincluded in the 3-D map (B).The top ographic
changes fromnear the mountainoussource area to the middle reaches of the canyon areshown by the plot of elevationsalong the thalweg of the riverand the submarine canyon(C). The
red arrowindicates the confluence point of the riverand submarine canyon. The deployment locations of four sediment trap moorings (T6KP,T7KP, T10KP3, T10KP4)discussed in the text
are indicted. The locations of three long piston cores (OR1-811 K08P, OR1-820 35, OR1-811 K31A) discussed in the text are also indicated.
276 J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

Another more common type of disturbance brought on by typhoons
on the time scale of da ys, is flooding of in the Gaoping fluvial plain south
of the confluence of the Qishan, Laonong, and Ailiao rivers (Fig. 2B).
River floods often cause erosion of the topsoil and destructions of
man-made structures. For example during Typhoon Morakot, five of
the nine bridges crossing the GPR were destroyed (Supplemental
Fig. S2A, B). River dikes collapsed in many places, roads were washed
away, and buildings were destroyed. Disturbances in the GPR affected
Fig. 2. (A) Thetopographic map of Taiwan showing thearea of the GPR basin (grey).(B) An enlarged topographic map of theGPR basin, showing the maincourse of the GPR and the three
major tributaries: Qishan River, Laonong River, andAilao River. The diamond symbols show the locations of the elevations plotted in (C) along the thalweg of the main courseof the river
system. The source area is near Mt. Jade, the highest peak of the Central Range.
277J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

the characteristics of the fluvial sediment signal during the floods (Liu
et al., 2013), which will be elaborated on in later Sections 4.1.2 and 5.4.
2.4. An effective conduit for land-sea exchanges: the Gaoping Submarine
Canyon
The sediment discharged by the GPR to the open shelf and slope ac-
counts for less than 20% of the river's sediment load (Huh et al., 2009a).
A larger amount of the sedimentis carried by gravity flows, in the form
of hyperpycnal turbidity currents, via the GPSC, to the deeper South
China Sea (Carter et al., 2012; Huh et al., 2009b; Liu et al., 2006, 2012,
this volume). Polycyclic aromatic hydrocarbon (PAH) concentrations
and compositional patterns show that riverine particulates are mostly
directed to the NW-shelf and/or the GPSC (Fang et al., 2007). The
GPSC also acts as a major sink for riverine trace metals (Hung and
Hsu, 2004; Liu et al., 2009b). But studies show that the GPSC is actually
a two-way conduit for land-sea exchanges of water masses and
suspended particles (Liu et al., 2002, 2006; Liu and Lin, 2004; Lin et al.,
2005). While terrigenous material is transported down the canyon to-
wards the deep sea by gravity flows, marine-sourced biogenic material
is transported landward via the canyon conduit by tide-dominated pro-
cesses (Lin et al., 2005). Nevertheless, lithogenic sediments dominate
suspended particles in the head region of the canyon (Liu et al., 2009b).
3. Approach and methods
The objectives of field monitoring and analysis of water and sedi-
ment in the GPR–GPSC system are firstly to address diversity in the
source (terrestrial vs. marine), origin (lithogenic vs. biogenic), and
history (modern vs. fossil) of the sediment. Consequently multiple
geochemical and isotopic analyses were used to extract the geochemical
characteristics (e.g., organic material, trace metals) of the sediment.
Furthermore, tracers that can identify the origin of particles, such as
biomarkers, PAHs, and foraminifers were used.
210
Pb
ex
was employed
as an indictor of steady settling in sediment in cores and sediment
trap samples on the centennial time scale, and the AMS
14
C was used
to determine the longer chronology of material in sediment cores and
particles captured in non-sequential sediment traps.
Secondly, to address relationships betweenphysical forcing and sed-
iment responses, instrumented taut-line moorings were configured
with CTDs, current meters, and sediment traps. To establish the cou-
pling between processes in the water column and the seabed, mooring
deployments were accompanied by water-column sampling of the
suspended-sediment and concentrations, by sampling of the surficial
sediment on the seafloor, and by coring at the deployment site. Concur-
rently, the hydrography at the deployment site was surveyed over tidal
cycles. Results from eight mooring deployments are included in this
review.
4. The environmental units in the Gaoping River–Gaoping
Submarine Canyon system
4.1. The fluvial system
4.1.1. Hydrological cycles and episodic events
Due to the influence of typhoons and the monsoon climate, rainfall
in the GPR basin is highly seasonal, most of which (70%) occurs during
the flood season from June to September (Huh et al., 2009a; Milliman
and Kao, 2005). Consequently, 91% of the annual discharge occurs in
the flood season (Liuetal.,2002). Typhoon-related episodes of high
runoff are accompanied by larger suspended-sediment discharge (Liu
et al., 2013). Based on a 41-year long monthly discharge record, the
temporal fluctuations of river runoff show important periodicities of
about less than 12 years, 1 year, and 0.5 years. The 12-yr periodicity is
close to the 11-yr cyclicities in the solar activities that affect decadal
global climatic patterns in precipitation (Prokoph et al., 2012; Wang
and Su, 2013). The monsoon climate, on the other hand, brings distinct
dry and flood seasons over the annual hydrological cycle. The passing of
typhoons mostly occurs in the summer and early fall, potentially bring-
ing massive rainfall.
4.1.2. Sediment generation and delivery
The geology, topography, climate, and human activities in the river
catchment lead to significantly higher physical and chemical weathering
rates than the world average (Hung et al., 2004). Studies show that the
sediment yield from the GPR basin (15 kg/m
2
/yr) is higher than
Taiwan's overall average (10 kg/m
2
/yr) (Dadson et al., 2003), and
much higher than the mean value of global small mountainous rivers
(3 kg/m/yr) (Milliman and Syvitski, 1992). The rate of chemical erosion
in the GPR basin is estimated to be 1.3 kg/m
2
/yr, which is also much
higher than the world average of 33–40 g/m
2
/yr (Hung et al., 2004).
During the flood season, on average, the river runoff and the sediment
load are two to three orders of magnitude higher than those observed
in the dry season, respectively (Hung et al., 2004). In general, the river
discharges high sediment loads and strong geochemical signals to the
sea in the flood season and during typhoon events.
The transport and delivery of sediment particulate C, N, and bio-
markers differs substantially between dry and flood seasons. During the
flood season, episodic floods from heavy monsoon rains or typhoons
critically influence not only the annual load of the suspended-sediment
but also POC and PN values. The concentrations of POC (PN) are closely
correlated (r N0.92, p b0.003) with suspended-sediment concentrations,
which in turn are dependent on water discharge during the flood season.
This implies that they have the same source (Hung et al., 2012). The daily
river transport of POC is estimated to be 5.98 × 10
6
g C/d in the dry season
and 1.25–276 × 10
6
g C/d in the flood season. In 2007, the estimated
annual load of suspended-sediment, POC and PN (based on discharge-
weighted concentration) were 3.7 × 10
7
metric tons/yr, 226 Gg C/yr
and 15 Gg N/yr, respectively (Hung et al., 2012). In the very dry year of
2002, the estimated suspended-sediment, POC and PN load were
2.14 × 10
6
metric tons/yr, 9.51 Gg C/yr and 1.53 Gg N/yr, respectively
(Hung, unpublished data). Therefore, the load of river-borne materials
in any particular year depends highly on the river discharge, which in
turn, largely depends on the occurrence of typhoons in that year.
In the flood season, the suspended-sediment load was largely
composed of the 10–63 μm fraction except possibly during an event.
For example during Typhoon Pabuk, the 63–153 μmfractiondominated
the suspended-sediment load. The grain-size distribution of POC gener-
ally matches that of suspended-sediment. However, during Typhoons
Wutip and Sepat, the 63–153 and 2.7–10 μm fractions dominated the
suspended-sediment load, probably representing the two pathways of
POC in the GPR basin (Hung et al., 2012). As noted elsewhere, the
coarse-grained fraction of POC, N25 μm in the case of the Eel River and
N63 μm in the case of the Amazon River, is derived mainly from plant
debris. The fine-grained fraction of POC, b4μm in the case of the Eel
River and b63 μm in the case of the Amazon River, is derived from the
soil (Blair et al., 2003). Similar relationships are expected in the GPR
basin.
Based on linear regression analyses, Liu et al. (2009a) found that
of the five grain-size classes in suspension in the lower GPR (N500,
250–500, 63–250, 10–63, and 1.2–10 μm), all except for the N500 μm
grain-size class are carriers of POC and PN in the surface water, with
Fig. 3. The sampling sitesand sample types forbiomarkers in twoseasons are markedand color-coded in(A) and (B). Seasonalmean distributionpatterns of carbonnumbers derivedfrom
n-alkanes(C15–C35) of sediment and soilsamples in (C) summer(flood season), and (D) winter (dry season). The mean compositions of carbon numbers aredistinctive among therock,
agricultural soil, and suspended particles (E). The calculated Carbon Predominance Index (CPI) along the GPR in (F) summer and (G) winter, respectively (0 km is located at the river
mouth).
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279J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

clearly terrestrial POC and PN in the 63–250, 10–63, and 1.2–10 μmsize
classes. In contrast, in the near-bedwater, only the finest size class (1.2–
10 μm) carries terrestrial POC and PN. On the riverbed, the coarsest size-
classes of the total and lithogenic part of the sediment N473 and 249–
473 μm are terrestrial. The finest two size-classes of b10 and 10–
63 μm are of marine origin. These findings indicate that under non-
typhoon conditions the GPR is exporting large-sized suspended terres-
trial organic particles to the sea in the surface water and importing
fine-grained marine organic particles as a result of estuarine circulation,
potentially trapping these materials on the riverbed (see next section).
Under typhoon conditions, landslides and erosion of forested hill
slopes in the GPR basin release large amount of woody debris in the
form of driftwood that ranges from tree trunks to branches and twigs
(Liu et al., 2013). Thus the driftwood is another pathwayfor modern ter-
restrial organic carbon (OC) to escape to the sea (Liu et al., 2012, 2013).
Hilton et al. (2011a) used biomarkersto distinguish the POC sources
in the highly disturbed GPR basin, and found that they are mainly
derived from bedrock erosion. Fifty-four samples including riverbed
sediment, forest soil, agricultural soil, suspended particles, and rocks,
were taken from the GPR basin in dry and flood seasons for biomarker
studies (Fig. 3A,B). The mean distributions of carbon numbers derived
from n-alkanes (C15–C35) show cleardistinction between the sediment
and soil in different seasons (Fig. 3C, D). In forest soil samples, similar to
agricultural soil (Fig. 3E), the odd-number carbons are predominantly in
higher molecular n-alkanes, meaning that hydrocarbons in the soil are
mainly generated by vascular plants with less degradational influence
(Fig. 3C, D). However, in riverbed sediment, there is no odd-number
predominance, and the patterns are similar to those of rock samples
(Fig. 3E). This discrepancy between the soil and river sediment is caused
by different contributing sources, e.g. recently produced material (soils,
standing biomass, and modern woody debris) and fossil OC (Sparkes
et al., 2015). Thus, soil apparently contains fresh organic carbon from
vascular plants (Brassell et al., 1986; Eglinton and Hamilton, 1967),
whereas riverbed sediment is composed of reworked organic carbon
(petrogenic alkanes). The carbon number composition of the riverine
suspended particles suggests high dilution from degraded hydrocar-
bons released from sedimentary rocks by incision and landslides
(Hilton et al., 2011b). Such petrogenically-sourced alkanes are predom-
inant in sediments in the GPR basin and in suspended particles in the
GPR.
Fig. 4. Salt-wedge influence on sediment dynamics as shown by the along-channel grain-size composition of the unaltered riverbed sediment (A), the lithogenic fraction of the riverbed
sediment, (C) the distribution of the medium-silt fraction and TOC, As, and Hg, and (D) the salinity structure that shows the boundary of the salt-wedge intrusion. The red dashed line
indicates the boundary between the salt-wedge and fluvial regimes (taken from Liu et al., 2009a).
280 J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

The Carbon Predominance Index (CPI) indicates the ‘freshness’of
the OC. Larger values of CPI (usually N4) denote fresh and well-
preserved n-alkanes with less diagenetic change and a smaller content
from petrogenic sources (Powell et al., 1978; Powell and Mokirdy,
1973). On the other hand, petrogenic input and recycled organic matter
(OM) have a CPI value close to unity (Pendoley, 1992). Along the GPR,
the spatial distribution of CPI reveals that petrogenic hydrocarbons
dominate river sediments regardless of the season (Fig. 3F, G).
Therefore, there is a disassociation between the biomarkers in the soil
and in the river sediment throughout the GPR system due to the over-
whelming signals from the recycled OC sources in the system. This
will be further discussed in 4.4.
4.2. The estuary
4.2.1. The estuarine filter related to the salt-wedge
In the flood season, tidally driven salt-water intrusion is the major
factor influencing the hydrodynamics of the lower reaches of the GPR,
which in turn, affects the down-river and water-column distribution
of suspended and riverbed sediments (Liu et al., 2009a). The intrusion
front of the salt-wedge driven by the tide creates a dynamic barrier
(Fig. 4). On its landward side, the riverbedsubstrate is composed of sed-
iment mostly coarser than 249 μm. Within the salt-wedge the riverbed
substrate is finer, consisting mostly of mud (b63 μm) as in other
systems (Bianchi, 2007; Carlin et al., 2015; Verlaan, 2000). The barrier
creates a filter on the riverbed immediately seaward of the intrusion
front, trapping higher percentages of clay-sized sediment and associated
TOC and metal species (Figs. 4, 5,Hung et al., 2009; Liu et al., 2009a). The
estuarine filter also separates the OM of terrestrial and marine sources in
the suspended and riverbed sediments. Within the salt-wedge the major
contributor of riverbed TOC is clay-sized marine sediment carried
upstream by the intruding seawater (Fig. 5). The terrestrial POC is a
minor contributor to the riverbed TOC.
4.2.2. Flocculation
Liu et al. (2009a) attribute the coarse-grained composition of the
riverbedsediment immediately landward of the salt-wedge to the effect
of flocculation (Fig. 5). The grain-size changes between riverbed and
suspended-sediments are coupled. The abundances of most suspended
and riverbed sediments in matching size-classes have a reciprocal
relationship (negative feedback) through resuspension and deposition
at the sediment-water interface. Only the grain-size classes of 62–
473 μm on the riverbed and that of 63–250 μminsuspensionareposi-
tively co-varying (both increase and decrease together) (Liu et al.,
2009a). Unlike other grain-size classes in suspension and on the river-
bed, this grain-size class has no statistically significant correlation
with the salinity, suggesting it is neither marine nor terrestrial (Liu
et al., 2009a). It consists largely of a transient floc population that is
formed and destroyed over the course of a tidal cycle. Additionally as
part of the same study, but not presented in Liu et al. (2009a),peak
values of both the concentration and the mean grain size of the
suspended sediment at the river mouth were found to coincide with
minimum values in salinity. This suggests the presence of short-lived
flocs during the ebbing tide when the salinity was the lowest, which
corroborates well with the presence of the floc group.
4.3. The Gaoping Submarine Canyon
The collision between the continental margin of the northern South
China Sea (part of the Eurasian plate) with the Luzon volcanic arc (Hsu
and Sibuet, 2004; Lallemand and Tsien, 1997)hashadsignificant influ-
ence on the seafloor structure and topography, including the formation
of the GPSC (Liu et al., 1997). The collision area today is an accretionary
prism/wedge (Chemenda et al., 2001; Huang et al., 1997) or an under-
filled foreland basin (Yu, 2004), where sediments from the Taiwan
orogen accumulate.
Both structural and sedimentary processes have affected the geo-
morphology of the GPSC (Chiang and Yu, 2006). In the upper sinuous
part of the canyon, intrusions of mud diapirs have complicated the
cut-and-fill process and have strongly affected the canyon morphology
(Fig. 6). In the middle and lower reaches of the canyon, thrust faulting
created two prominent bends in the course of the canyon (Fig. 1).
Base tilting has also affected the morphology of the lower canyon
(Chiang and Yu, 2006).
Fig. 5. Schematic 3-D plots of the river-salt wedge dynamics. (A) The basic setting showing the steep slope of the riverbed and the shallow water depth. The red columns represent
the double pilings of the Xuangyuan Bridge (insert). (B) During flood, the river water slides over the intruding salt-wedge with little mixing. The landward end of the salt-wedge
forms a dynamic barrier that facilitates the deposition of coarse sediment on its landward side and the trapping of fine-grained sediment on its seaward side (the estuarine filter).
(C) During ebb,the salt-wedge withdraws to the river mouth, and river-borne sediments are transported to the sea relatively unimpeded.
281J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

Based on geomorphology and seismic surveys, Chiang and Yu
(2008) hypothesized that gravity flows were the main control in the
formation and maintenance of meanders of the canyon. For example,
the sinuosity of the upper reaches of the canyon was likely caused by
erosion of the seafloor due to gravity flows and slumping of the canyon
walls (Fig. 6B, C, D, E), which could also trigger gravity flows. Identified
boulders on the canyon floor in a 3-D image from multi-beam bathy-
metric surveys (Fig. 6D) support the presence of gravity flows
(Shanmugam, 2006). Additionally, in a distal meander of the canyon,
overspilled sediments at the outer bend of levees and a terrace of flat
stratified facies at the inner bend also suggest active sedimentary pro-
cesses in the canyon (Chiang and Yu, 2008, 2011).
4.3.1. Physical oceanography of the canyon
A branch of Kuroshio Current flows NW parallel to the coast through
this area,bringing water masses and marine substances from the Pacific
and the South China Sea (Liu et al., this volume). The observed
temperature-salinity properties at the head region of the GPSC shows
that the canyon is filled with three types of waters: 1) the effluent
from the GPR, 2) Kuroshio Current water, and 3) South China Sea
Fig. 6. (A) Detailed bathymetry of the upper reaches of the GPSC based on the composite of 3 multi-beambathymetric surveys conductedby Taiwan Ocean ResearchInstitute (TORI). The
two blackdashed lines indicatethe locations of two seismiccross-sections(B, C). Localizedcanyon-rim slumping (D) and canyon-wall landslide (E) are indicated by red arrowson two 3-D
topographic plots. (B is taken from Chiang and Yu, 2011; C is taken from Chiang and Yu, 2006; D and E are taken from Yeh et al., 2013).
282 J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

water (Liu et al., 2002, 2006). The canyon interior is stratified mainly
due to the temperature gradient (Liu et al., 2002), and slopes of
isopycnal surfaces in the canyon are controlled by the tidal phase (Liu
et al., 2002).
Tidal currents generally dominate the flow field in many submarine
canyons, and internal tides may be an order of magnitude more
energetic than the barotropic tidal currents (Kunze et al., 2002). Obser-
vations of currents and hydrographic profiles in the GPSC (Wang et al.,
2008) show that in the lower part of the water column the major axis
of the tidal currents is aligned with the thalweg of the canyon (Liu
et al., 2010). Tidal energy is channeled from the shelf landward with a
beamlike internal wave, guided by bottom topography. The semidiurnal
internal tide (M
2
) dominates with an intensity increasing with depth
(bottom trapped) and is also increasing toward the canyon head
(Wang et al., 2008). The water mass displacement associated with the
internal tidal current may have had a major effect on the sediment
transport (Xu et al., 2002; Liu et al., 2012). The internal tide can be easily
generated and amplified with the isotherm displacement over 200 m
and thus plays an important role in mixing, leading to downward
mixing of river discharged materials over a tidal cycle. Note, the internal
tidal energy flux in the GPSC is 3–7 times greater than that in the
Monterey Canyon (Lee et al., 2009a,b). The isopycnal disturbances and
associated high suspended-sediment concentrations near the canyon
floor suggests that turbulent mixing and breaking of internal tides
play a role in sediment transport processes in the canyon (Lee et al.,
2009b; Liu et al., 2002).
In 2008, two sediment trap moorings were deployed in deeper
water (~ 650 m) than previous deployments in the GPSC (Fig. 7): 1)
January–March (T6KP) and 2) July–September (T7KP) (Figs. 1, 7). The
observations from these two moorings reveal twofundamental patterns
of the flow field and the associated sediment dynamics in the GPSC
(Huh et al., 2009b; Liu et al., 2012). In each mooring dataset there is a
clear demarcation between the shelf and canyon flow regimes (Fig. 8).
On the shelf, the impinging flow of the Kuroshio Current and wind-
generated currents dominate the sub-tidal flow field, and these
are superimposed by tidal oscillations (Liu et al., 2006)(Fig. 8A, B).
Under normal conditions the flow in the canyon interior shows a two-
layered structure affected by the internal tide (Fig. 8A) (Liuetal.,
2006, 2009c; Wang et al., 2008), which is better revealed by the mea-
surements of an upward-looking long-range ADCP (Huh et al., 2009b).
However, during the passing of four typhoons in the 2-month deploy-
ment of T7KP (Liu et al., 2012), the two-layer structure was replaced
by a thick layer of landward flow almost filling the entire water column
of the canyon interior, with a thin layer of seaward flow close to the
canyon floor (Fig. 8B).
4.3.2. Benthic nepheloid layer
The benthic nepheloid layer has been observed in the head region of
the GPSC throughout a year (Liu et al., 2002, 2010). The benthic
nepheloid layer was as thick as 100 m and the suspended-sediment
concentration reached 30 mg/l. The thickness and the suspended-
sediment concentration of the benthic nepheloid layer are controlled
by the tidal current in the course of a semi-diurnal tidal cycle (Liu
et al., 2010).
Previous sediment trap moorings in 2000, 2002, and 2004 produced
month-long time series records in the benthic nepheloid layer near the
canyon floor of 1) along-canyon velocity, 2) water temperature, and
3) the volume concentration of suspended particles in various grain-size
classes (Fig. 7). Data show that the benthic nepheloid layer is strongly
modulated by the tides at semidiurnal, quarter, and sixth diurnal frequen-
cies and spring-neap cycles (Liu and Lin, 2004; Liu et al., 2006, 2010). In
Fig. 7. The deployment locations of 6 taut-line sediment trap moorings and one shipboard
hydrographic profiling station in different years (Liu et al., 2013) are plotted over the
bathymetric map shown in Fig. 6.
Fig. 8. Flowregimes above the canyonand in the canyon interior, differentiatedas shown by the progressivevector plotsof each bin in the ADCP measurements at the samelocation (Fig. 7)
between 10 Jan. -20 March, 2008 (A) and 7 July–11 Sept., 2008 (B). (A) is taken from Huh et al. (2009b) and (B) is the re-plot of the data reported in Liu et al. (2012).
283J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

the course of a semidiurnal cycle, the flood (up-canyon) current brings
colder and more saline offshore water containing higher suspended-
sediment concentrations that enters the canyon near the canyon floor
(Fig. 9A, B). This causes the thickness of the benthic nepheloid layer
to increase. During the ebb phase, warmer and less saline coastal water
displaces the colder and more saline water near the canyon floor. The
suspended-sediment concentration immediately near the canyon floor
increases in response to the maximum flood and ebb currents of the M
2
Fig. 9. Shipboard measurements of the tidally modulated benthic nepheloid layer as shown by the vertical chang es over a 25-h period of (A) salinity, (B) temperature, and (C) light transmission.
The simultaneously measured flow field is represented by the magnitude and horizontal orientations (north is to the top of the plot) of the arrows (taken from Liu et al., 2010).
284 J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

tide (Fig. 9C). In the benthic nepheloid layer, suspended-sediment trans-
port is also strongly affected by non-linear processes such as the genera-
tion of M
4
tide (the overtide of M
2
), as indicated by the increased values of
the amplitude ratios of M
4
/M
2
for the suspended-sediment grain-size
classes (Liu et al., 2010) and by the interaction between the internal
tide and the canyon topography (Lee et al., 2009a).
4.3.3. Dynamics and sources of the settling particles
Liu and Lin (2004) show that river plume dynamics and the coastal
wind field largely control the delivery of river and shelf-sourced terrig-
enous fine-grained sediment to the canyon under non-flood conditions.
Based on theoretical settling velocities of siliciclastic spherical particles
and assuming a still and stratified canyon interior, Liu and Lin (2004)
found that sand-sized particles fall through the canyon within an hour
while the clay-sized particles take over a month to settle. Consequently,
the ‘behavior’of suspended particles of different sizes in the canyon
interior can be differentiated into a coarse-grained group (sand) that
forms the ‘rapid-settling’population and a fine-grained group (mud)
that forms the ‘background’population. The coarse-grained group is
delivered directly to the canyon by the river effluent and by wave
resuspension of the shelf substrate (Liu et al., 2012). Both lithogenic
and non-lithogenic particles contribute to high mass fluxes including
all sediment types (exceeding 700 and 800 g/m
2
/d) in the lower part
of the canyon (Liu and Lin, 2004; Liu et al., 2009c).
With lower settling velocities, fine-grained sediment remains in the
water column longer and is transported farther, causing the observed
downward fining of suspended-sediment in the canyon (Liu and Lin,
2004; Liu et al., 2009c). Because of the longer residence time of fine-
grained sediment in the water column, there are also downward
decreases of fine-grain affiliated non-lithogenic components in the
suspended-sediment in the water column such as TOC and PAH due to
decay, decomposition, and mineralization (Liu et al., 2009c).
4.3.4. Sedimentation: tide-dominated versus gravity-flow dominated
regimes
Time-series data of water-borne properties and sediment captured
by the sediment traps on T6KP and T7KP moorings provide clear insight
into the dynamics of settlingparticles in the GPSC (Huh et al., 2009b; Liu
et al., 2012, 2013). The two moorings had similar configurations, in
which identical non-sequential sediment traps were used. They were
also deployed near the same location, which makes them ideal to
show contrast in the canyon hydrodynamics and sediment dynamics
(see Sections 5.2,5.3).
The T6KP mooring was deployed in the winter/dry season when
tides were the dominant forcing in the GPSC. The spring-neap tidal
cycles were well reflected in the textural composition and the mean
grain size of the sediment captured by the sediment trap (Fig. 10A, F).
The data show that higher energy during the spring tide caused
increases in the coarse-grained faction (sand), which resulted in larger
mean grain size (Fig. 10A). Conversely, the fine fraction was higher
during the neap tide. Since the amount of clay controls the texture of
the captured particles in the trap, the down-trap watercontent basically
follows that of the clay content (Fig. 10B). These fine-grained particles
are major carriers of bio-geochemical signals. For example, the TOC
and TN mimics that of the clay fraction in the sediment trap samples
(Fig. 10C, D). Liu et al. (2009c) also reported similar trends.
Fig. 10. Findings from the sediment trap mooring(T6KP) betweenJanuary and March 2008 thatshow tidal-regimecontrolled dynamicsof settling particles in theGPSCover5spring-neap
cycles as revealed by the down-trap changesof (A) composition of the coarse and fine fractionsand mean grain-size,(B) water content and clayfraction, (C) TOC and clay fraction, (D) TN
and clay fraction, (E) cumulation rate (half-tone diamonds indicate the locations and time of timer discs) and mass flux (horizontal bars) between adjacent timer-discs, and the
(F) corresponding along-canyon flow. The red arrows point to the time of the spring tide in the canyon.
285J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

The T6KP mooring encompassed five spring-neap cycles (Fig. 10F),
which generally recorded an increase in the mass flux from a neap to
the ensuing spring tide (Fig. 10E). Liu et al. (2009c) also documented
the close co-varying relationship of the mass flux and coarser grain-
size composition to the spring-neap cycle. During this deployment,
most of the captured sediment was mud, of which the clay content
was as high as 30% (Fig. 10B). This suggests the tidal regime provides
a stable hydrodynamic environment for steady sedimentation of
hemipelagic mud (Huh et al., 2009b), which is the normal sedimenta-
tion pattern in the GPSC. The sedimentation of mud leads to the domi-
nant presence of mud on the seafloor of the GPSC (Liu et al., 2002,
2009c).
Being deployed in the summer/flood season, T7KP (Fig. 11A) experi-
enced an energetic regimeof hyperpycnal events followed by the quies-
cent normal tidal regime (Liu et al., 2012). This is clearly reflected in the
grain-size texture and the mean grain size of the captured sediment
(Fig. 11B). In the sediment trap sediment, Liu et al. (2012) identified
sediment carried by two hyperpyncal turbidity current events, each
characterized by an inverse graded layer overlain by a graded layer,
then capped with a thin layer of fine-grained sediment (Fig. 11B). This
is hypothesized to be the waxing and waning phases of the turbidity
currents. However, the inverse-to-normal grading could also be caused
by an increase in suspended-sediment concentration in the flow
(Talling, 2014). The grain-size composition is much coarser during this
period of typhoon influence. The normal tidally influenced pattern
returns in the sediment in the upper part of the sediment trap
(Fig. 11B). Three spring tides could be identified by the periodic pattern
of higher sand content and a greater mean grain size shown by the
captured particles (Fig. 11B).
Comparing the texture in the upper and lower part of the sediment
trap (Fig. 11B), it is clear that sediments in the lower trap were coarser
and had a higher settling velocity, pointing to a more energetic regime
(Fig. 11C). The estimated mass flux during the 16-h period of the two
hyperpycnal turbidity currents was 2 orders of magnitude higher than
that measured during the spring tide (Liu et al., 2009c). This dataset
suggests that the tide-dominated regime represents the hemipelagic
background, in which the fine-grained sedimentation occurs. In con-
trast, the hyperpycnal turbidity currents triggered by the hyperpycnal
plumes at the river mouth during episodic river floods appear to be
the main agent (based on the mass flux) for coarse-grained sediment
deposition and sediment transport in the canyon.
4.3.5. Tracers for terrestrial- and marine-sourced particles
Ubiquitous contaminants such as PAHs are mainly transported via
atmospheric wet/dry deposition, air-water exchange and fluvial deliv-
ery in coastal areas. Nonetheless, they can also be transported through
oceanic circulation and long-range atmospheric transport (Dachs
et al., 1996; Fang et al., 2009, 2012; Huang et al., 2012; Lai et al., 2014;
Lin et al., 2013; Sato et al., 2008). As thesecompounds are particle reac-
tive, the specific patterns of their presence in particles can help trace
particulate transport within a dispersal system (Fang et al., 2009;
Huang et al., 2012; Lin et al., 2013). Several diagnostic ratios related to
their provenance have been widely used to distinguish the possible
sources of these particles (Fang et al., 2003, 2007, 2009; Huang et al.,
Fig. 11. Findings from the sediment trap mooring (T7KP) between July and September 2008 that show captured signals of two typhoon-triggered hyperpycnal turbidity current events.
(A) Schematic drawing of the non-sequential sediment trap showing the level of collected sediment in yellow, physical locations of the 1st three timer-discs and their corresponding
normalized down-trap depths (pointed by the arrows), and the location of the sediment ‘interface’as indicated by the circled alphabet ‘K’for Typhoon Kalmaegi. (B) The mean grain-
size (bluecircles) plottedover the cumulativepercentage of clay, silt, andsand. The red and light purple arrows represent inferred waxingand waning stages of two hyperpycn altu rbidity
currents.(C) Settling velocity. (D) Total carbon(TotalC) as expressed by thesummation of TOC and TIC. (E) C/N ratio by weight. (F) Mass flux. The right/left-hand pointing red triangles
indicate the down-trap depths of the 5 time-points that delineate the intervals for estimating the mass fluxes. The blank segment indicates the space where the funnel of the trap is
threaded into the PVC pipe below which the sediment sample was greatly disturbed in the disassembly process after retrieval (taken from Liu et al., 2012).
286 J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

2011; Lai et al., 2011, 2013; Lin et al., 2013; Page et al., 1999; Savinov
et al., 2000; Yunker et al., 2002). Three diagnostic ratios, representing
petrogenic, biogenic and pyrogenic origins, have been effective in differ-
entiating and characterizing sediments among offshore, nearshore and
inner harbor samples in the Gaoping system, respectively (Fang et al.,
2003). Enrichment of pyrogenic and petrogenic PAHs in sediments
exhibit spatial dilution by biogenic PAHs, which can be used to infer
their transport and origin.
Before PAHs were used as provenance tracers in southwest Taiwan,
seasonal and spatial variations of PAH fingerprints in sediments were
explored (Fang et al., 2007; Jianget al., 2009). The sources of particulate
PAHs in the water column of the GPR are predominantly petrogenic in
the flood season but pyrogenic in the dry season. Samples used to
characterize PAH compounds in the water column were collected
from two taut-line sediment trap moorings (T4KP) deployed at the
upper rim and near the floor of the canyon (Liu et al., 2006, 2010).
PAH diagnostic ratios and statistical tools (e.g., principal component
analysis) were applied to distinguish sources of PAHs and trace the
transport path of the land-derived particles from the GPR (Fang et al.,
2009). During the sampling period, both traps were significantly tilted
by the tidal currents and fluctuated vertically (Lee and Liu, 2006; Liu
et al., 2009b). The trap at the canyon rim experienced greater vertical
movements, thus their particle characteristics (e.g., particulate organic
carbon content, particle mass, and fine particle fraction) varied more
than those from the trap near the canyon floor. Nonetheless, interesting
observations were drawn from the study. Hourly depth variations of the
tilted sediment trap array were echoed by the corresponding total PAH
concentrations. The PAH composition of the collected particles was re-
lated to the flow direction and speed, reflecting alternating seaward
(down-canyon) or landward (up-canyon) sources.
A second example illustrates the usefulness of PAHs to resolve the
effects of a typhoon on particulate transport (Liu et al., 2012). Trap sam-
ples collected from T7KP provided a high resolution PAH profile that
traced Typhoon Kalmaegi-induced particle movement (Liu et al.,
2013). The PAH composition in the trap samples allowed the typhoon's
effect to be broken into stages including; 1) marine signature before any
typhoon; 2) signature of resuspended-sediments caused by typhoon-
induced currents and waves; 3) pyrogenic particles from northwestern
shelf caused by down-canyon flows; and 4) strong pyrogenic signatures
caused by two hyperpycnal turbidity currents, suggesting the delivery
of terrestrial particles. The anthropogenic PAHs discharged to the
canyon head in the wake of the typhoon floods was indicated by low
perylene ratios and these PAHs indicated a very clear petrogenic signal,
representing the background composition of the GPR watershed.
In contrast to the PAHs, foraminiferal shells provide evidence for
landward transport of biogenic particles of marine origin in the GPSC.
During the distal phase of Typhoon Kai-tak (2000), the concentration
of both benthic and planktonic foraminifera increased 2 to 4-fold in
sediment trap samples (T1KP mooring) in the upper water column
(104 mab, meter above the seafloor) and near the floor (54 mab) of
the GPSC (Lin et al., 2005; Liu et al., 2006). The presence of shallow-
water living benthic foraminiferal shells indicated landward transport
in the canyon (Lin et al., 2005). The benthic and planktonicforaminiferal
shells discovered in sediment traps on two moorings (T4KP) at the
upper rim and near the floor of the canyon in 2004, also indicated lateral
(cross-shelf) and up-canyon transport of biogenic marine particles (Liu
et al., 2009b). These studies confirm that foraminiferal shells are not
only useful tracers, but also important components of sinking particles
in the GPSC (Liu et al., 2009b). Together, the tracers indicate two-way
transport of particles in the GPSC.
5. The source-to-sink sediment processes in the GPR–GPSC system
In a conceptual model for the particle dynamics in the head region of
the GPSC, Liuetal.(2009c)depict the diverse source and nature of
particles, their complex delivery pathways to the canyon, and the sedi-
mentation process in the canyon (Fig. 12). Generally, particles delivered
to the canyon consist of three major types: 1) lithogenic/siliciclastic
particles which are mostly sourced from the GPR and delivered by
river plumes and from resuspension of shelf sediments; 2) biogenic
Fig. 12. A conceptual model illustrating particle dynamics in the head region of the GPSC. The thick dashed line depicts the upper and lateral boundaries of the canyon. The bands of dif-
ferent colorsin the background indicate the stratified water columnespecially in the interior of the canyon. Thelegends in the first panel indicate particles from thefluvial and reworked
shelf sources. Th e second panel represents particles of biogenic origins. The legends in the third panel include proc esses related to gravity-flow-caused erosion and deposition
(hyperpycnites and turbidites). The meanings of the callouts are as follows: A: Fine-grained sediment is dispersed by the river plume on the shelf. B: Sediment coarser than very-fine
silt descends into the canyon, a process that is modulated by the tide. C: Particles settling into the canyon are from diverse sources of terrestrial sediment, reworked sediment from the
shelf floor, and biogenic particles related to marine foraminifers. D: Authigenic particles related to primary production on the shelf. E: Progressive change in physical and geochemical
properties of sinkingparticles. F:Particles from resuspensionfrom the canyon floor,entrainment fromcanyon walls, and along-canyontransport contribute to the formation of the benthic
nepheloidlayer. G: Planktonic and benthicforaminifersare significant biogenic particles transported fromdistal sources andfrom settling fromabove. H: Erosionof the substrateby gravity
processes. I: Deposits by gravity flows such as turbidites and hyperpycnites. (Taken from Liu et al., 2009c).
287J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

particles related to primary production on the shelf, and planktonic and
benthic foraminifera settling into the canyon from the shelf and/or
transported up-canyon in its interior; and 3) reworked sediment due
to failure of the canyon walls and entrainment from the canyon floor
by gravity flows. Moreover, recent studies reveal that the river-
sourced sediment, especially during floods, also contains terrestrial OC
of multiple phases (Liu et al., 2012, 2013; Hsu et al., 2014; Sparkes
et al., 2015). Transport agents including free settling, will be discussed
in Sections 5.2 and 5.3. The estimated mass flux near the canyon floor
is 2–7 times higher than that measured at the upper rim of the canyon
(Liu et al., 2009c), suggesting that the extra sediment flux near the can-
yon floor could not have come from the upper water column in the can-
yon. Therefore, lateral transport along the canyon thalweg must be
important in the lower part of the water column, probably within the
benthic nepheloid layer of the canyon as has been documented in
other canyon systems around the world (Arzola et al., 2008; Drexler
et al., 2006; Puig et al., 2014; van Weering et al., 2002).
The physical and geochemical nature of particles is transformed as
they settle through the water column in the canyon. Typically, there is
a downward fining trend in the size composition of suspended particles
as the average percentage of clay-to-fine-silt particles (0.4–10 μm)
increases from 22.7% in the surface water in the canyon to 56.0% in
the bottom water near the canyon floor. Conversely, the average
percentage of the sand-sized (N63 μm) particles decreases with depth
from 32% in surface water of the canyon to 12% in the bottom water
near the canyon floor. As a result, the substrate of the canyon floor is
composed largely of lithogenic hemipelagic mud. Along with this down-
ward fining trend is the vertical decrease in the water column of the
concentration of suspended non-lithogenic substances such as TOC
and PAH, despite their affinity to fine-grained particles.
The downward change in the physical nature of the sinking particles
is also reflected in the porosity and bulk (solids and voids) density of
particles of three grain-size classes at the surface and near the bottom
in the canyon. Hsu and Liu (2010) attribute this vertical variability
to 1) flocculation, 2) biogenic processes, and 3) terrestrial-sourced
sediment from the GPR effluent.
5.1. Fine-grained sediments as carriers of geochemical signals through the
system
Fine-grained sediments are important vehicles for carrying
geochemical signals along source-to-sink pathways of the GPS–GPSC
system. An illustration comes from the combined studies of Liu et al.
(2009a) and Hung et al. (2009) in which physical and geochemical
properties of sediment on the riverbed of the GPR and the floor of the
GPSC were investigated for the spatial variability along the river–
canyon conduit (Fig. 13). The estuarine filter related to the salt-wedge
creates a trap that retains higher levels of clay content on the riverbed
that results in the elevation of all the measured geochemical variables
of TOC, TN, Fe, Al, As, Hg, and Mn (Fig. 14,Liu et al., 2009a).
The spatial variability along the river–canyon pathway in the 8
geochemical variables was reanalyzed using the multivariate analysis
technique of EOF (Empirical Orthogonal/Eigen Function) analysis. The
results show that the first eigenmode explains over 52% of the overall
correlation (standardized variance), indicating a high correlation. The
sign of the eigenvectors of the first mode of all the variables is the
same, meaning they all co-vary coherently (Fig. 15A). Clay particles
are widely known as an efficient carrier for most particle-reactive
trace metals and organics. Consequently, the eigenweighting curve of
the first mode, which shows the spatial pattern of the mode, almost
mimics the distribution of the clay content (Fig. 15B). This means that
the distributions of the 8 geochemical variables are primarily controlled
by the abundance of the clay (Fig. 15A).
The second mode explains over 24% of the correlation. In this mode,
As, Al, Fe, and Mn co-vary with clay because Fe–Mn oxides/hydroxides
are often coated on, or associated with, clay particles, leading to good
correlations among them (Fig. 15A) This suggests that the Fe–Mn
oxides/hydroxides are important in carrying metals, particularly for As
(Hung et al., 2009). On the other hand, the organics and Hg co-vary in
an opposite fashion to clay since Hg is abundant in the organic/sulfide
fraction (Hung et al., 2009). Those sediments enriched in sulfide are
likely derived from relatively reduced conditions with OM as a driver.
Therefore, the eigenweightings of this mode become negative in the
Fig. 13. Sampling stationsfrom the lower reachesof the GPR to the upper reachesof the GPSC for a study of the geochemical controlof organics and tracemetals in the surficial sedimentin
the GPR–GPSC system in 2003 and 2004 (after Hung et al., 2009).
288 J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

submarine canyon region (Fig. 15B), which is caused by the decomposi-
tion, mineralization, and degradation of the OM in the GPSC (Liu et al.,
2009b; Hung et al., 2009). In summary, fine-grained particles (finer
than medium silt) are the primary control of geochemical signals not
only in suspended (Fig. 4) and settling (Fig. 10) sediments, but also in
the substrate (Fig. 14) throughout the river–canyon system.
5.2. The river–canyon transport pathway under normal conditions
The two-month deployments of T6KP and T7KP provide valuable
contrasts in the net sediment transport in the near-bottom part of the
GPSC under normal and hyperpycnal conditions. The T6KP dataset
only shows net displacement of water; suspended sediment transport
Fig. 14. The surficialsediment datasetof Hung et al. (2009) re-plotted to show thedistributions ofclay, TOC, TN, Fe, Al, (upper panel)As, Hg, and Mn (lower panel)along the river–canyon
conduit. Stations designated by numbers were located in the river, the smaller the number, the closer to the river mouth. The along-canyon stations are designated by alpha-numerals,
from b1 (at the head of the canyon) to 38. The two red arrows point to the locales of the estuarine filter (Fig. 5B) in both dry and wet seasons. The dashed black line separates the
river and canyon regimes.
Fig. 15. EOFanalysis results of theco-variability in theriver-to-canyon geochemicalproperties of the sediment samplesshowing (A) groupingsof the first three eigenmodesaccording to
the sign of the eigenvector of each mode, and (B) the spatial characteristics of each mode according to the eigenweightings. The red arrows point to the locales of the estuarine filters in
both seasons. The percentage of clay ineach sample is plotted to illustrate the effect of gain-size.
289J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

should theoretically show an identical pattern (Fig. 16A). Duringthe de-
ployment in the non-typhoon season, the net water displacement was
up-canyon, superimposed by tidal oscillations, both of which followed
the orientation of the canyon axis at the mooring site (Figs. 7, 16A).
This net transport pattern is mainly controlled by the tidal regime.
5.3. The river–canyon transport pathway under hyperpycnal conditions
The T7KP mooring revealed that the normal up-canyon directed
tidal regime was interrupted by a typhoon-triggered down-canyon
hyperpycnal regime (18–25 July), followed by a transitional period of
recovery (25 July–24 Aug.). Eventually, the normal up-canyon tidal
regime resumed (Fig. 16B; Liu et al., 2012, 2013).
Liu et al. (2012) provide the most comprehensive observations of
not only the flow field of two passing hyperpycnal turbidity currents
in the GPSC, but also the warm water and the terrestrial sediment
that originated from Typhoon Kalmaegi-triggered floods in the GPR
(Fig. 17). The estimated mass flux of these two sequential events in
16 h was 198.2 kg/m
2
/d. The results further allow for an estimation of
the sediment carried by the two hyperpycnal turbidity currents of
2.59 × 10
6
t.
Using discharge and suspended-sediment content from the Water
Resources Agency, Ministry of Economic Affair, the sediment load in
2008 was 35.13 × 10
6
t, which is comparable previous studies
(Dadson et al., 2003; Hung et al., 2012; Liu et al., 2008). Based on the av-
erage concentration of 0.58% for POC in GPR sediment (Kao et al., 2006),
the average OC exported by the GPR in 2008 was roughly 24.4 × 10
4
t,
comparable to that estimated by Hung et al. (2012). Using measured
TOC (0.44%) and total carbon (0.83%) in the sediment carried by the
two hyperpycnal turbidity currents (Liu et al., 2012),their total amounts
are estimated, respectively (Table 1). The two hyperpycnal turbidity
currents carried about 18% and 13.7% of the suspended-sediment and
OC load exported by the GPR floods on July 18, 2008, respectively
(Table 1). However, these low percentages are not surprising because
the sediment trap mooring only captured the upper two thirds of the
passing turbidity currents (Liu et al., 2012). Considering that the higher
suspended-sediment concentration in the lower part of the turbidity
currents was missing in the observation, the sediment load is likely
underestimated. Further studies are needed on sediment gravity flows
with improved observational tools.
After the passing of Typhoon Fanapi in 2010, Hsu et al. (2014)
captured the wake of hyperpycnal turbidity currents in the GPSC using
two taut-line moorings (T10KP3, 4; Fig. 7). Mooring T10KP4 was config-
ured with a non-sequential sediment trap similar to that used by Liu
et al. (2012). Although Hsu et al. (2014) missed the beginning of the
hyperpycnal events, they did capture the warm water and the sediment
carried by the hyperpyncal flow (Fig. 18). Their data showed a decreas-
ing mass flux from 56.8 kg/m
2
/d at the beginning to 1.24 kg/m
2
/d at the
end of the deployment over 5 daysand 13 h. Although the hyperpycnal
turbidity currents in the canyon were triggered under the same peak
value of 20 kg/m
3
for the suspended-sediment concentration in the
GPR during Typhoons Kalmaegi and Fanapi, the mass fluxes in the
hyperpycnal turbidity currents after Typhoon Fanapi were much
lower than during Kalmagei. This is probably because the Kalmaegi
observation included two entire hyperpycnal events (Liu et al., 2012)
whereas the Fanapi observation (Hsu et al., 2014) only included the
wake of the hyperpycnal events.
In the Fanapi case, the average TOC in the sediment carried by the
hyperpycnal flows was 0.44%, of which 70–90% was of terrestrial origin
(Hsu et al.,2014). During TyphoonKai-tak (in 2000) the TOC contents in
captured sediment trap samples at the upper rim and near the floor of
the GPSC were also about 0.4%, which is lower than 0.7% at the upper
Fig. 16. (A) Progressive vec tor plot of the net water displacement measured at the sediment trap mooring (T6KP). The beginning and end time poi nts of the record are marked.
(B) Progressive vector plot of the net sediment transport observed at the sediment trap mooring (T7KP) (taken from Liu et al., 2013). The plot shows the normal tidal regime in the
beginning of the record, which was then interrupted by the typhoon-induced hyperpycnal regime, followed by a transitional/recovery period, and at the end, the normal tidal regime
returned.
290 J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

rim and 0.5% near the seafloor for non-typhoon periods (Liu et al.,
2006). Liu et al. (2006) attribute this decrease to a dilution effect caused
by the increase of terrestrial lithogenic particles during the typhoon.
However, this was not the case with the sediments carried in the head
of a hyperpycnal turbidity current during Typhoon Kalmaegi (in
2008), when TOC content was as high as 1.92% (Liu et al., 2012). Since
Typhoon Kalmaegi was the first typhoon in 2008, the initial river floods
appear to have flushed out higher percentages of fresh organic matter
(standing biomass) in the fluvial system, which was then carried by
the hyperpycnal plume into the canyon and captured by the sediment
trap. To summarize, both accounts of hyperpycnal turbidity currents
in the GPSC confirm the river–canyon pathway for the delivery of
terrestrial sediment and OC to the deep sea during typhoons.
In the GPSC, typhoon-related hyperpycnal turbidity currents
originate in the fluvial environment, so the water they carry is warmer
than the ambient water near the canyon floor (Kao et al., 2010).
Consequently, from a temperature viewpoint, these flows are catego-
rized as ‘warm-water turbidity currents’(Fig. 19A-C). On the other
hand, earthquakes (Talling et al., 2013) can also trigger mass failures,
leading to the development of turbidity currents (Hsu et al., 2008;
Carter et al., 2014). The water temperature of the turbidity currents of
this nature should be the same or close to the ambient temperature of
the canyon, forming ‘cold-water turbidity currents’(Fig. 19D–F). In
any case, either of these two mechanisms could generate gravity flows
that have high speeds and long run-outs and are able to cause
geohazards such as breaking subsea communication cables laid across
the GPSC (Carter et al., 2012, 2014).
Since both warm- and cold-water turbidity currents are gravity-
driven, the turbidites they deposit should have similar physical charac-
teristics and typically consist of coarse-grained sediments with high
amounts of lithogenic components (Liu et al., 2009c). A study has
shown that turbidites deposited from warm-water turbidity currents
Table 1
Comparison of sediment and carbon loads and percentages of the two hyperpycnal turbidity currents and the GPR in 2008 (Liu et al., 2012).
Category Sediment Load × 10
6
(t) Total Carbon Load × 10
4
(t) Organic Carbon Load × 10
4
(t)
GPR in 2008 (Liu et al., 2012) 35.13 –24.4
GPR in 2007 (Hung et al., 2012) 37.0 35.1 22.6
GPR on July 18, 2008 14.40 –8.35 (0.58% TOC content, based on Kao et al., 2006)
41.0% of 2008 –34.2% of GPR in 2008
Turbidity currents between 08:00–24:00 July 18, 2008 2.59 2.15 1.14
18.0% of GPR on July 18 –13.7% of GPR on July 18
7.4% of GPR in 2008 6.1% of GPR in 2007* 4.5% of GPR in 2008
Note: All percentages are based on the river loads in 2008.
Fig. 17. Observations of two hyperpycnal turbidity currents in July 2008 by T7KP that include: (A) the tidal and non-tidal parts of the along-canyon velocity recorded by a single-level
acoustic current meter at 56 mab(meter above bed); (B) contoured temperature and the acoustic backscatter measured on the mooring; (C) the vertical profile of the non-tidal along-
canyon velocity measured by the RDI(LADCP) on a nearby mooring. The down-canyonflows are in red and up-canyon flowsare in blue. The color-coded diamond time-markers indicate
the beginning of the waxing phase (red), the transition fromwaxing to waning phases (purple), and theend of the waning phase (lightblue) of the first turbiditycurrent (taken from Liu
et al., 2012).
291J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

Fig. 18. Evidence of a river-flood relatedhyperpycnal event in the GPSCas shown by (A) the temperature reversals at thetwo moorings (T10KP3, 4), and (B) the vertical structureof the
non-tidalflows observed at thetwo moorings. The positive and blue sticks indicatethe up-canyon directedflows and the negativeand red sticks indicatedown-canyon directed flows. The
black dashed line separates the hyperpyncal and tidal regimes of the flow field (taken from Hsu et al., 2014).
Fig. 19. Schematic drawings of the genesis of the ‘warm’and ‘cold’gravity flows in the GPSC. (A) During episodic flood events the GPR effluent containing high concentration of suspended-
sediment forms the hyperpycnal plume and plunges into the head of the GPSC. (B) The hyperpycnal process ignites gravity flows. (C) Hyperpycnal turbidity currents carry warm water and
a large amount fresh terrestrial sediment down the canyon. (D) Earthquakes or other marine processes cause the failure of canyon walls and slopes. (E) Slumping occurs and generates debris
flows. (F) Through a hydraulic jump, the debris flows transform into turbidity currents carrying the ambient seawater and reworked marine sediment down the canyon (Talling et al., 2013).
292 J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

Fig. 20. (A) Mapshowings locationsof 5 gravity cores along thethalweg of the upper reaches of the GPSC takenshortly after TyphoonMorakot. The core labelsmatch the locationson the
map (B, C, D, E, F). For eachcore, the followingdown-core variablesare shown in sequence:the mean grain-sizeplotted over the photoof the core, cumulativeplot of the sand, silt, and clay
and water content, the Terrestrial Fraction (F
t
) and the C/N ratio, and the
210
Pb
ex
. Core K1 h as additional three data points of A MS
14
C age (provided by Robert Sparkes) plotted over the
210
Pb
ex
curve.
293J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

potentially contain fresher sediment from the terrestrial environment,
having high values of terrestrial OC and water content and low to very
low levels of
210
Pb
ex
activity (Liu et al., 2009c). Conversely, turbidites
deposited from cold-water turbidity currents are mainly reworked
coarse marine sediment, having marine-sourced OC and lowwater con-
tent, and potentially very low levels of
210
Pb
ex
activity (Liu et al., 2009c).
5.4. Internal sources and sinks: the new is old
The GPR–GPSC system is an active sediment dispersal system that
links the mountainous river catchment to the deep sea via a submarine
canyon. Typical sediment processes operate over tidal, sub-tidal to sea-
sonal time scales. However, episodic typhoon and earthquake events
dominate the system and move large amounts of sediment down the
canyon and to the South China Sea (Carter et al., 2012).
Typhoon Morakot devastated the GPR catchment and left deposi-
tional layers not only in the GPSCbut also in the nearby Fangliao Subma-
rine Canyon (Hale et al., 2012). Several lines of evidence in five cores
from the upper GPSC taken after this typhoon show flood-triggered
hyperpycnal turbidites (Fig. 20). Notably in Core K1 located right at
the head of the canyon, where plunging of the hyperpycnal plume
probably took place, is a turbidite layer that contained over 80% of
sand and high amounts of terrestrial OM (with TOC exceeding 1.0%),
terrestrial fraction (F
t
) exceeding 80%, a C/N ratio exceeding 14 (Liu
et al., 2013; Sparkes, 2012; Sparkes et al., 2015), and very low
210
Pb
ex
(Fig. 20B). Three AMS
14
C dated samples show the ages for the core ma-
terial are 8.2,11.2, 12.7 ka. (data provided by Robert Sparkes) in the tur-
bidite, mid-core, and the core-top, respectively. The turbidite was likely
fresh terrestrial sedimentdelivered by Morakot-related river floods, yet
the OC it contained is over 8200 yr old. The ensuing deposited sediment
Fig. 20 (continued)
294 J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

was medium silt, still containing over 70% of terrestrial OM, and is even
older. Furthermore, the upward linear increase of
210
Pb
ex
activity in the
top 5 cm of the core probably reflects the scavenging effect due to
steady sedimentation of terrestrial sediment after the floods. The old
age of freshly delivered terrestrial sediment points to multiple sources
for the sediment, particularly the OC from the terrestrial system
(Hilton et al., 2011a; Sparkes et al., 2015). Typhoon-induced mass
wasting and debris flows in the river catchment apparently ‘released’
the old sediment including fossil OC and mixed it with the fresh sedi-
ment that contained the terrestrial non-fossil carbon (Sparkes et al.,
2015) during the transport process. This turbidite was probably depos-
ited at the beginning of the river floods, with a higher amount of eroded
topsoil, making it younger than the overlying sediment. As the typhoon
progressed, older fan terraces, tributary fans, and ancient colluvium
began to be incised, introducing older and finer sediment into the
river flow which was then deposited in the wake of the peak floods
forming the finer sediment deposited above the turbidite in K1
(Fig. 20). This phenomenon highlights the fact that fresh sediment
newly delivered to the sea from highly disturbed river catchments con-
tains old sediment with fossil OC (Sparkes, 2012) from internal sinks
and deep erosions of the hill slopes, landslides, and fan terraces. This
freshly delivered sediment that contains terrestrial fossil OC from the
fluvial system is different from the reworked sediment from the seafloor
found in sediment records that bears marine signals (including marine
OC), which had already been delivered and deposited in the offshore
part of the system (Kao et al., 2014).
Liu et al. (2002) found high abundance of very-coarse and coarse
sand in the surficial sediment on the north flank of the canyon head.
Recent multi-beam bathymetric surveys show gullies on the canyon
wall at this location (pointed by the red arrow in Fig. 20A), which are
likely erosional features from descending/plunging hyperpycnal
plumes. The presence of the turbidite in Core K1 suggests that there
might be another internal sink of temporarily deposited coarse-
grained terrestrial sediment at the head of the canyon. These deposits
may have been flushed down the canyon by the second flow described
by Carter et al. (2012), or later by other hyperpycnal events (Puig et al.,
2014).
After Typhoon Morakot, hyperpycnal turbidity currents may have
been ignited at the canyon head multiple times and then moved down
the canyon over a period of a few days (Kao et al., 2010; Carter et al.,
2012). Due to topographically induced secondary flows and tidal energy
distribution at the first meander of the canyon thalweg, erosion of the
canyon floor and wall at the core site K12A is suspected to have caused
the high percentage of sand at this site (Fig. 20C, Liu et al., 2010). Addi-
tionally, the lower F
t
and C/N and high
210
Pb
ex
values, suggest that the
substrate sediment at this location is probably a mixture of marine
and terrestrial material, including sediment from hemipelagic settling
(Kao et al., 2014). After passing the first canyon meander, acceleration
of the turbidity current due to the increased slope likely caused erosion
at the location of Core K25B (Fig. 20D). In Cores K8 and K8X, turbidites
are present showing higher values of F
t
and C/N, and low
210
Pb
ex
values
(Fig. 20E, F). These two cores also contain coarse woody debris and the
highest presence of terrestrial biomass of any of the Morakot cores
(Sparkes, 2012; Sparkes et al., 2015). At the sites of Cores K8 and K8x,
the canyon topography probably caused the turbidity current to decel-
erate (Fig. 20A), thus becoming depositional (Sequeiros et al., 2009).
Fig. 20 (continued)
295J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

The low values of F
t,
C/N, and
210
Pb
ex
are also likely to have been
contributed to by internal sinks from canyon wall slumping
(Fig. 6B-E). Using elemental and isotopic analyses, Sparkes et al.
(2015) confirmed that terrestrial OC (fossil and non-fossil) domi-
nat ed the sediment in Morakot canyon cores. In the down-canyon trans-
port by hyperpycnal turbidity currents, reworked marine sediment
(resuspended from the shelf and from erosion of the canyon wall) includ-
ing marine-sourced OC was also incorporated into the deposits (Sparkes
et al., 2015).
Complementing the presence of fossil OC in terrestrial sediment
freshly deposited in the Morakot cores, fossil OC contained in suspended
terrestrial sediment was also captured in the non-sequential sediment
trap immediately after Typhoon Fanapi (Hsu et al., 2014). The AMC
14
C
ages of three sediment samples at the bottom, middle, and the top of
the trap, were 7.8, 6.4, and 8.1 ka, respectively (data provided by George
Burr), and corresponding F
t
values were 82.2%, 92.2%, and 94.4%, respec-
tively. Since sediment captured by the sediment trap was from the wake
of hyperpycnal turbidity events (Hsu et al., 2014), the upward increase in
the F
t
values suggests that the weakened turbidity current reduced
mixing with the ambient canyon water. This caused less dilution
by marine-sourced OC in the captured terrestrial sediment, causing the
upward increase in F
t
values in the trap. There is no doubt that the
typhoon-related events eroded modern OC and remobilized fossil OC
in sediment from internal sinks in the GPR basin and transported them
to the deep sea along the GPR–GPSC dispersal system via turbidity
currents (Kao et al., 2014; Sparkes et al., 2015).
5.5. Sediment record signatures of the dual sedimentation pattern along the
canyon conduit
Steady tidal oscillations and episodic sediment-gravity flows leave
two distinct types of sedimentary signatures in seafloor deposits of
the canyon conduit. Trends similar to those observed in the sediment
trap moorings (Huh et al.,2009b and Liu et al., 2012) werealso observed
in longer piston cores taken at three locations: 1) proximal to the head
in upper reaches (OR1-811 K08P), 2) at the transition between the
upper and middle reaches (OR1-820 35), and 3) more distally, at the
transition between the middle and lower reaches (OR1-811 K31A) of
the GPSC (Fig. 1). 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.
Based on the core descriptions, the sediments at all three sites
consist largely of fining-upward deposits, with coarser sandy units
interspersed with bands of finer muddy units (the extent, depending
on location within the GPSC). The coarser sandy-silt layers are dark
greenish grey, while the finer clayey-silt layers are olive grey. Sporadic
dark olive brown layers also occur. The coarser sandy units most likely
reflect sediment-gravity flow deposits, while the finer muddy units rep-
resent hemipelagic deposits (Rendle-Buehring et al., 2009). The finer
muddy units in the other two cores are dominated by sediments that
are comparable to two pelagic, undisturbed long cores (ORI 799-G24
and ORI 732-8G) located outside the GPSC in the northeastern area of
the South China Sea (Rendle-Buehring et al., 2008).
Core OR1-811 K08P is dominated by thick, very coarse, sandy turbi-
dite deposits interspersed with minor thin bands of hemipelagic mud
(Fig. 21). These findings suggest that this location along the canyon
conduit forms a depocenter and a major sink for very coarse turbidite
deposits left by turbidity currents generated farther up-canyon
(Rendle-Buehring et al., 2009).
The lithology of Core OR1-820 35 consists mainly of clayey-silt
intermingled with thin bands of coarser (sandy-silt) turbidite deposits
(Fig. 22A, B). Many of these turbidites have the typically sharp lower
boundary defined by a contrast in color, texture and grain size when
compared to the hemipelagic background mud below(Fig. 22C). Graded
laminations in the turbidites are also apparent in the X-radiograph
(Fig. 22B). The fine-grained nature of the coarser deposits suggest that
only the finer sediments carried in turbidity currents reach this location
far down the conduit.
At the most distal Core OR1-811 K31A, the lithology is again dominat-
ed by fine-grained silty-clay and hemipelagic sediments. Interspersed in
the hemigelagic sediments are minor intermittent fine-grained
(b100 μm) thin turbidites (Fig. 23A). Due to the very fine-grained nature
of these turbidites, they are more visible in the X-radiographs
(Fig. 23B). The results suggest that only the very low energy (end)
phase of the turbidity currents deposit sediment this far down the
GPSC.
The sedimentary lithology observed in these three cores shows that
grain size decreases down-canyon in both the fine-grained hemipelagic
sediments and in the coarse-grained gravity flow deposits. This demon-
strates the down-canyon decrease in the strength of the fluvial-fed
sediment load and/or the episodic sediment-gravity flows (Talling
et al., 2013). These results probably indicate that the upper reaches of
the GPSC constitute a sink for the high-energy, coarser turbidites,
while the middle and lower reaches of the canyon form a sink for the
progressively lower-energy, thinner and finer-grained turbidites that
are deposited in the more distal reaches of the canyon conduit. Farther
away from the head of the canyon, the fine-grained mud becomes
Fig. 21. Photo of Core OR1-811 K08P.
296 J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

the dominant lithology in the more distal gravity flow deposits.
These fine-grained mud deposits in the canyon are comparable to
the hemipelagic sediments on the Gaoping slope (Sparkes et al., 2015)
that ultimately form the deposits on the abyssal plain and in the
Manila Trench in the South China Sea (the ultimate sink for sediments).
6. Summary and future work
The GPR–GPSC dispersal system is an interesting natural laboratory
because it consists of a high-yield river basin that is dynamic in its
sedimentary behavior and a submarine canyon whose sediment
dynamics are controlled by tidal oscillations under normal conditions
and by gravity flows under typhoon-induced hyperpycnal conditions
or triggered by earthquakes. A river–canyon pathway is formed by
complex mechanisms to deliver sediment of terrestrial, marine, and bio-
genic material with diverse physical and geochemical properties into
the canyon. The submarine canyon is a two-way conduit facilitating
land-sea interactions. Under normal tidal influence, the net sediment
transport direction is up-canyon. Under episodic typhoon conditions,
the net sediment transport is down-canyon by energetic gravity flows
that deliver terrestrial sediment and carbon to the deep sea. There are
two modes of sedimentation pattern in the canyon. The hemipelagic
mud is formed in the tidal regime and turbidity currents in the
hyperpycnal regime form the coarse- to fine-grained turbidite
sequences. The internal sources and sinks in both the river basin and
canyon conduit complicate the chronological, geochemical and
sedimentary signals in the system. This is a highly complex sediment-
routing system in terms of physical, geochemical, and sedimentary
signals that requires an interdisciplinary approach to understand the
interplay among them.
To further advance our understanding of the GPR–GPSC system or
similar systems in the future, there are two challenges. The first lies in
improving the observational methods for capturing passing turbidity
currents in their entirety (the flow and suspended sediment structures
in space and time, and the material they carry) and the corresponding
signature they leave on the seafloor. The second challenge is to improve
our ability to model, in a prognostic fashion, the generation (start from
the plunging process of the hyperpycnal plume at the head of the
canyon) and propagation of turbidity currents down the canyon, with
particular attention to the processes of sediment entrainment and
deposition at the flow-seabed interface, which determines the fate of
the turbidity current.
Acknowledgments
All the authors are grateful to the R.O.C. Ministry of Science and
Technology (formerly the National Science Council-NSC) for the finan-
cial support to the FATES research program (Fate of Terrestrial/Non-
terrestrial Substances in the Kaoping Submarine Canyon, 2003–2006;
Fate and Transport of Terrestrial/Non-terrestrial Substances in a
Collision Margin of Arc and Continental, 2006-2009; and Fate of
Terrestrial/Non-terrestrial Sediments in High Yield Particle-Export
River-Sea Systems, 2009-), and to the individual authors. The authors
also acknowledge the use of R/V Ocean Researcher I and III in the field-
work, which is vital in the data acquisition and the success of the
research. JTL and RHR-B benefited from the support of the NSC-DAAD
(German Academic Exchange Service) Personnel Exchange Program.
We benefited from discussions with Robert Sparkes. We thank Peter
Talling and another anonymous reviewer and the guest editors J.P.
Walsh and Patricia Wiberg for their helpful comments and suggestions
to improve the manuscript.
Fig. 22. Photoof Core OR1-820 35 (A), imagesof the X-ray radiograph of the segmentsof the core (B), and an enlargedphoto showing a turbidite layer andthe hemipelagic mud aboveand
beneath it (C).
297J.T. Liu et al. / Earth-Science Reviews 153 (2016) 274–300

Appendix A. Supplementary data
Supplementary data associated with this article can be found in the
online version, at http://dx.doi.org/10.1016/j.earscirev.2015.10.012.
These data include the Google map of the most important areas de-
scribed in this article.
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