Clay-mineral compositions of sediments in the Gaoping River-Sea system: Implications for weathering, sedimentary routing and carbon cycling

    Clay-mineral compositions of sediments in the Gaoping River-Sea system: Implications for weathering, sedimentary routing and carbon cycling Yangyang Wang, Daidu Fan, James T. Liu, Yuanpin Chang PII: DOI: Reference:

S0009-2541(16)30557-5 doi: 10.1016/j.chemgeo.2016.10.024 CHEMGE 18117

To appear in:

Chemical Geology

Received date: Revised date: Accepted date:

7 June 2016 21 August 2016 15 October 2016

Please cite this article as: Wang, Yangyang, Fan, Daidu, Liu, James T., Chang, Yuanpin, Clay-mineral compositions of sediments in the Gaoping River-Sea system: Implications for weathering, sedimentary routing and carbon cycling, Chemical Geology (2016), doi: 10.1016/j.chemgeo.2016.10.024

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ACCEPTED MANUSCRIPT Clay-mineral compositions of sediments in the Gaoping River-Sea system: implications for weathering, sedimentary routing and carbon cycling

School of Geoscience, Yangtze University, Wuhan430100, China

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Department of Oceanography, National Sun Yat-sen University, Kaohsiung, 80424, Taiwan

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State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China

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Yangyang Wanga, Daidu Fana,b,*, James T. Liuc, Yuanpin Changc

* Corresponding author at: School of Ocean and Earth Science, Tongji University, Shanghai 200092, China

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E-mail addresses: [email protected] (D. Fan).

Abstract

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Small mountainous rivers (SMRs) play a crucial role in the global sediment and carbon cycle

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by rapid transfer of huge particulate and dissolved loads into the sea. Gaoping river-sea system is a

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good representative of SMRs in Oceania, having an extreme high denudation rate (15,000 t km-2 yr-1) and a high burial efficiency of terrestrial particulate organic carbon (POC) in the Gaoping

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submarine canyon (GPSC). However, it is contentious whether silicate weathering or POC burial plays more important role in long-term carbon cycling in such a high active orogen. Clay minerals

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of fluvial sediments not only act as an important carrier of POC, but also encode valuable information of weathering regimes on land and dispersal routes in the seas. To uncover these informations, surface and core sediments are systematically collected from the Gaoping river-sea system for clay-mineral analysis. Comparative studies are carried out on the basis of new results and vast published data to better understand weathering processes and products in Oceanian islands and Southeast Asian continent. We find that clay-mineral compositions in the Gaoping River (GPR) and other Taiwan rivers are a legacy from provenance rocks in the alpine reach, characterized by predominant illite and chlorite and limited compositional change throughout the river courses. These sharply contradict the downstream increase in compositional complexity (rich 1

ACCEPTED MANUSCRIPT in kaolinite and smectite) and chemical weathering intensity in Southeast Asian large rivers because of strong hydrolysis and active sediment recycling at the lowland reach with extensive

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floodplains. High consistency of clay-mineral compositions is also observed in the interior and

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exterior of GPSC. Careful examination shows that a slight offshore decrease in cumulative illite and chlorite content is accompanied by a slight increase in exotic clay-mineral inputs by ocean circulations. The combination of this finding with previously reported data on the offshore change

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in δ13Corg and Δ14Corg values suggests that petrogenic POC predominates in the river-dominated

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Gaoping continental margin with high preservation efficiency due to its tight bandage with clay minerals. Moreover, a tardy response of chemical weathering fluxes to high elevated physical

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erosion rates is remarkably examined in the Oceanian and global river gauging data. We therefore

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postulate that rapid transfer and high burial efficiency of terrestrial POC in the tectonically active

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high-standing islands and their adjacent continental margins play much more important role in the

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long-term carbon cycle than silicate weathering.

Key words: Mountainous river; submarine canyon; clay mineral; weathering; ocean circulation; carbon cycle

1. Introduction

Weathering and erosion on the orogen are the paramount process on the Earth’s surface by carving landscape, influencing marine biogeochemical cycle, and regulating long-term global climate. Dissolved and particulate loads of large rivers draining the Himalayan-Tibetan orogen 2

ACCEPTED MANUSCRIPT have been extensively studied for clues to the Cenozoic uplift-cooling hypothesis (Raymo and Ruddiman, 1992; Lupker et al., 2013). However, the hypothesis has recently been questioned to

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enhance CO2 sequestration by accelerating silicate weathering in the rapidly denudated orogen

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(Gaillardet and Galy, 2008; Gabet and Mudd, 2009; Willenbring and von Blanckenburg, 2010). As shown by both analytical and modeling data, increasing physical erosion significantly enhances chemical weathering by the exposure of fresher mineral surfaces with higher reactivity at lower

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denudation rates (100-102 t km-2 yr-1) (Gaillardet et al., 1999b; Millot et al., 2002; Riebe et al.,

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2004; West et al., 2005), but the linear relationship breaks down at higher denudation rates (103-104 t km-2 yr-1) (Gabet and Mudd, 2009). Recently, flux of terristrial organic carbon (TOC)

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and its burial in the marine setting have caught increasing attention, because they may exert larger

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Galy et al., 2007, 2015).

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impact on the Neogene carbon cycle than silicate weathering (France-Lanord and Derry, 1997;

The watersheds of small mountainous rivers (SMRs) have the highest denudation rate in the

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world, consequently attracting increased concern of their role in global sediment and carbon cycle (Milliman and Syvitski, 1992; Lyons et al., 2002; Dadson et al., 2003; Drenzek et al., 2009; Walsh and Nittrouer, 2009; Hilton et al., 2012, 2014; Liu et al., 2013; Bao et al., 2015; Kuehl et al., 2016). Besides coastal rivers on active margins, SMRs are typically referred to those draining the high-standing islands in Oceania (Milliman and Syvisky, 1992). These rivers have an average annual sediment yield approaching 3,000 t km-2 yr-1, 1~2 orders of magnitude higher than craton and large mountainous rivers. They supposedly contribute nearly 40% of global sediment discharge into coastal oceans, with their total watersheds accounting for less than 2.5% of the global land area (Milliman and Syvisky, 1992; Milliman and Farnsworth, 2011). SMRs usually 3

ACCEPTED MANUSCRIPT have minor estuaries and narrow shelves, fascinating sediment bypass to abyssal basins. Rapid sediment transfer in these dispersal systems also favors TOC escape from respiration in transit,

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being efficiently buried to enter the long-term carbon cycle (Kao and Liu, 1996; Komada et al.,

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2005; Drenzek et al., 2009; Hilton et al., 2011; Blair and Aller, 2012; Kao et al., 2014; Bao et al, 2015; Liu JT et al., 2016).

Among these active high-standing islands, Taiwan stands out for extreme uplift and

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denudation rates (5-7 mm yr-1), a surpassing high mean sediment yield (~9,500 t km-2 yr-1), and

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the uppermost efficiency of sediment delivery from the mountain peak (~4,000 m) to the ocean deep (Liu, 1982; Dadson et al., 2003; Kao and Milliman, 2008; Hilton et al., 2011; Liu et al., 2013,

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2016a). Based on river gauging data, Taiwan rivers collectively supplied ~380 Mt yr-1 of

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suspended sediment to the ocean, accounting for 1.9% of the global flux inappropriately from

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0.024% of the land surface (Dadson et al., 2003; Kao and Milliman, 2008; Liu JT et al., 2016). Additionally, Taiwan was estimated to deliver 1.6-1.8 Tg C yr-1 of TOC to the ocean, with an

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uppermost burial efficiency of 70-85% by sharp contrast with <15% in large river systems (Galy et al., 2007; Hilton et al., 2011; Kao et a., 2014). This is attributable to rapid transfer of rock-derived sediments by episodic hyperpycnal currents from the upland reach, through the narrow lowland-reach-estuary-and-shelf zone, to the deep-sea setting with limited oxygen-exposure time (Hilton et al., 2011; Kao et al., 2014; Liu JT et al., 2016). Most of fluvial sediments produced by hillslope mass-wasting in response to frequent earthquakes and heavy rainstorms are rapidly removed by typhoon-generated floods into the sea (Hovius et al., 2000; Hartshorn et al., 2002; Milliman and Kao, 2005; Kao and Milliman, 2008; Liu JT et al., 2013, 2016). Chemical weathering intensity is therefore considered to be low in such a rapid sediment 4

ACCEPTED MANUSCRIPT transfer system, ignorable in comparison with the extreme high physical erosion rate (Dadson et al., 2003). Weak chemical-weathering regime is further attested by low CIA (chemical index of

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alteration of silicate rocks) values of river sediments (Selvaraj and Chen, 2006; Bi et al., 2015).

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However, water-chemistry analyses suggested a topmost chemical weathering regime in Taiwan (Li, 1976; You et al., 1988; Chung et al., 2009; Calmels et al., 2011). The difference in weathering regimes indexed by river sediment and water chemistry needs moderation.

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As the largest river in Taiwan by the catchment size, Gaoping River (GPR) annually carries

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7.4 km3 of water and 49 Mt of suspended sediment into the sea (Dadson et al., 2003; Kao and Milliman, 2008). Fluvial sediments are generally flushed on land and dumped into the Gaoping

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Submarine Canyon (GPSC) in forms of episodic hyperpycnal flow (suspended sediment

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concentration > 40 kg m-3) and turbidity current, concentrating on a few hours or days during

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typhoon strikes in each wet season (Kao and Milliman, 2008; Liu JT et al., 2013, 2016). Characterized by its huge amount and high efficiency of sediment delivery, Gaoping dispersal

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system has been extensively studied through multi-disciplinary methods in the recent decade (Hung and Hsu, 2004; Fang et al., 2007; Chung et al., 2009; Hung et al., 2009; Hsu F-H et al., 2014; Liu JT et al., 2016). According to a sediment budget balance model using radionuclide data, Gaoping continental margin is considered a minor sink of fluvial sediment, with more than 80% of the flux being assumably transported to the Malina trench (Huh et al., 2009). This sedimentation model needs further amendment because of its strong bias toward the downcanyon sediment routing process during short-lived high-energy events, and little concern is given to the net upcanyon sediment transport at the lower boundary layer by internal tides (Hsu RT et al., 2014; Liu JT et al., 2016), and the contour-parallel conveying along the slope by ocean circulations 5

ACCEPTED MANUSCRIPT (Wang and Chern, 1987; Hsu F-H et al., 2014; Nan et al., 2015). Clay-mineral assemblages are considered a good indicator of weathering behaviors on land

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and dispersal routes in the sea (Biscaye, 1965; Chamley, 1989; Petschick et al., 1996; Gingele et

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al., 2007; He et al., 2013; Liu Z et al., 2016). Taiwan-sourced clay-mineral assemblage differs significantly from those of Asian continental rivers, been widely applied to trace sediment distribution patterns in the East China Sea (ECS) and the northern South China Sea (SCS) (Liu Z

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et al., 2008, 2016; Xu et al., 2009; Wan et al., 2010). Clay minerals are the predominant

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composition of suspended sediment in the Gaoping river-sea system, with >90% of particles being fine silts and clays under non-typhoon conditions (Liu JT et al., 2016). They are also considered to

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play an important role in protecting organic and pollutant matter from oxidation as carriers and

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loads during transit (Fang et al., 2007; Hung et al., 2009; Hsu F-H et al., 2014). In this study, a set

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of surface and short-core sediments from the Gaoping dispersal system are examined for clay minerals. Intercomparison is carried out: (1) to specify the weathering pattern in the active

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high-standing island, (2) to trace the sediment dispersal and reworking behaviors both by episodic extreme events and ordinary ocean circulations in the Gaoping continental margin, and (3) to discuss the role of high-standing Oceanian islands in the global carbon cycling.

2. Materials and methods

2.1 Geographic setting

2.1.1 Terrestrial setting Taiwan was formed by oblique collision between the Luzon arc and the Asian continental margin over the past 5 Ma (Teng, 1990). The rising central mountain belt approaching 4000 m 6

ACCEPTED MANUSCRIPT height is a natural divide for steep-sloped rivers flowing westward into the Taiwan Strait and eastward into the Pacific (Fig. 1a). The mean annual precipitation is over 2,500 mm, and heavy

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rainfalls usually occur during typhoon strikes with an annual average of 4 landfalls (Liu et al.,

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2013). Together with frequent earthquakes and steep gradient, heavy precipitation by typhoons often produces rapid mass-wasting and hyperpycnal flows (Dadson et al., 2003; Milliman and Kao, 2005; Kao and Milliman, 2008; Liu JT et al., 2013, 2016).

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The GPR drains the southwestern part of Central Range with a total trunk length of 171 km

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and a catchment area of 3256 km2 (Fig. 1a). Over 90% of the annual river runoff occurs in the flood season from June to October, typically concentrated on a few hours to days during typhoon

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activities. It is therefore not surprising to have hyperpycnal flows during the episodic floods (Liu

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JT et al., 2002, 2016). The mean sediment yield is roughly 15,000 t km-2 yr-1 over the drainage

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basin, much higher than Taiwan’s overall average (Dadson et al., 2003; Kao and Milliman, 2008). The GPR has two major tributaries: Qishan and Laonong Rivers. Their drainage divide

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roughly runs along the bedrock boundary between Miocene epimetamorphic rocks (Lushan formations) in the east and Miocene sandstone and shale (Nanchang and Nankang & Shihti formations) in the west (Fig. 1b). The Lushan formation is majorly composed of slate and phyllite. Moreover, large area of Eocene epimetamorphic rocks are outcropped at the headwaters and upland reaches of Laonong River and its tributaries draining the Yushan and Beinanshan mountains with their highest elevations of 3952 m and 3295 m, respectively. They are Pilushan, Shihpachungchi, Tachien, and Hsitsun & Chiayang formations, composing of slate, phyllite and quartzite, with slight higher metamorphic grades than the Lushan Fm. Additionally, some basaltic lava and Paleozoic-Mesozoic schist and meta-limestone are outcropped at the upland reach of 7

ACCEPTED MANUSCRIPT Ailiao River, a tributary of Laonong River (Chen, 2000; Chung et al., 2009). The Qishan River draining the southwest slope of Yushan Mount with its headwater at 2700 m has a much gentler

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relief than the Laonong River. Approximately 2/3 of river sediments come from the Laonong

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hinterland, and the remnant from the Qishan hinterland, proportional to their drainage area of 2014 km2 versus 842 km2 (WRA, 2004). Coarse sediments begin settling at the lower reaches of Qishan and Laonong, producing alluvial terraces at the toe of hills and subaerial fans in the coastal plain

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(Fig. 1b).

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2.1.2 Oceanic setting

Gaoping continental margin is characterized by a narrow shelf (~10 km), broad slope and

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deeply-incised canyons (Fig. 1c, d). The GPSC extends ~260 km from its head located roughly 1

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km seaward of the GPR mouth to the seaward terminus, joining the Manila Trench in a water

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depth ~3600 m (Yu et al., 2009). It acts as a trap and conduit for both land- and marine-borne matters (Liu JT et al., 2002, 2016). Internal tides are highly intensified due to the canyon

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topography. Tidal currents increase toward the bottom boundary with a maximum observed velocity of over 100 cm s-1 (Lee et al., 2009). Tidally-driven residual flows are directed landward, resulting in an upcanyon net transport of suspended particles in the bottom nepheloid layer (Hsu RT et al., 2014). While, tidal oscillations are occasionally interrupted by typhoon-induced hyperpycnal events in the wet season, which may last a few hours to days, producing exclusively down-canyon turbidity flows and huge sediment transport into the deep sea (Liu JT et al., 2002, 2016; Hsu RT et al., 2014). Outside the submarine canyon, Gaoping shelf is evidently reworked by strong waves, resulting in offshore and alongshore sediment transports (Liu et al., 2002). Gaoping slope is highly 8

ACCEPTED MANUSCRIPT influenced by the Kuroshio intrusion and SCS warm currents, which are significantly modulated by seasonal monsoon activities (Fig. 1c). During the summer monsoon, the northeast-flowing SCS

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warm currents along the shelf edge of the northwestern SCS bifurcate when approaching Taiwan.

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The southeast branch carries ~77% of the total volume to flow off the southwestern Taiwan, and the remnant 23% enters the Taiwan Strait through the Penghu Channel (Wang and Chern, 1992; Hsu F-H et al., 2014). The Kuroshio intrusion into the SCS takes three major paths through the

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southern part of Luzon Strait: leaking, looping and leaping patterns (Hu et al., 2000; Nan et al.,

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2015). Additional intrusion path is identified to enter in the northern part of the strait and form a cyclonic circulation before returning to the mean path (Caruso et al., 2006). Intrusions evolve

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between the different types with varied strengths over different time scales. The leaking (leaping)

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path dominates in winter (summer), and the looping path appears southwest of Taiwan more

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frequently in winter than other seasons (Nan et al., 2015). It is also significant interannual and decadal variability of the Kuroshio intrusion (Caruso et al., 2006; Nan et al., 2015).

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2.2 Sampling and experiments

A set of riverine and marine sediment samples were collected from the Gaoping dispersal system for clay-mineralogy analyses. Totally 23 riverine samples were taken during a field campaign on May 11-12 (2013), consisting of two suspended-sediments, 15 surface-sediments, one terrace-soil, and 5 sedimentary-rocks (Fig. 1, Table 1). The suspended-sediment samples were achieved by filtering subsurface riverwater through a 0.45 μm membrane of cellulose acetate in the field. Surface sediments were collected by scooping the top 2 cm of the river bank deposits using a clean polyethylene spade. Marine samples comprise 23 surface-sediment samples from box cores and 21 samples from 7 piston cores. They were retrieved by R/V Ocean Researcher-1 9

ACCEPTED MANUSCRIPT (OR-1) during three cruises, including OR1-789 in the period of March 29 to April 2 (2006), OR1-791 on April 11-15 (2006), and OR1-915 from September 27 to October 3 (2009). Piston

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cores were dated using the non-destructive gamma spectrometry (Huh et al., 2009), and the results

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are publicly available at http://dmc.earth. sinica.edu.tw/Contributor/Huh/Hub_et_al2007a/. In addition, 10 and 8 riverine sediment samples were respectively collected from the Zhuoshi and Lanyang Rivers during a field campaign from May 16 to 19 in 2013, for a comparative study.

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Clay minerals were identified by X-ray diffraction (XRD) using a PANalytical diffractometer

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at the State Key Laboratory of Marine Geology (Shanghai) on oriented mounts of non-calcareous clay-sized (<2 μm) particles (Liu et al., 2008). Three XRD runs were performed following air

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drying, ethylene-glycol solvation for 12 h, and heating at 490 °C for 2 h. Identification of clay

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minerals was made mainly according to the position of the (001) series of basal reflections on the

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three XRD diagrams. Relative percentages of the four main clay-mineral groups were estimated by weighting integrated peak areas of characteristic basal reflections (smectite plus mixed-layers:

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15-17 Å, illite: 10 Å, and kaolinite/chlorite: 7 Å) on the glycolated curve using the MacDiff software. Relative proportions of kaolinite and chlorite were determined using the ratio of 3.57/3.54 Å peak areas. Replicate analyses of the same sample reported a relative error of 2% in the dating results. Moreover, some mineralogical characters of illite were determined on the glycolated curve to decode hydrolysis strength. Illite chemistry index refers to a ratio of 5 Å and 10 Å peak areas. Illite crystallinity can be calculated by the IB (integral breadth) or the FWHM (full width at half maximum) of the glycolated 10 Å peaks. Lower IB/FWHM values represent higher crystallinity,

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ACCEPTED MANUSCRIPT characterizing weak hydrolysis in the provenance area with a dry and cold climate (Chamley, 1989;

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Liu et al., 2008; He et al., 2013).

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2.3 Comparison studies

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A huge mass of clay-mineral data are available for comparison studies in the SCS and its inflowing rivers (Liu Z et al., 2016 and references therein). However, great attention should be

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paid to the inconsistency between different analytical procedures and parameterization methods. Except the Changjiang River, all published clay-mineral data referred in this study were generated

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using the same analytical method following Liu et al. (2007), and the weighting factors introduced

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by Biscaye (1965) were not used in calculating relative percentages of four clay minerals. These

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data include analytical samples from Taiwan rivers and the Gaoping continental margin (Liu et al., 2008), Luzon rivers (Liu et al., 2009), and the rivers of Mekong, Red and Pearl (Liu et al., 2007).

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However, the upper and middle reaches of these rivers were not sampled and analyzed because they focused on establishing individual endmember compositions for each river to trace their

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dispersal patterns in the SCS. He et al. (2013) reported detailed clay-mineral compositions throughout the Changjiang River system but using the weighting factors of Biscaye (1965). These compositional data were therefore excluded from the comparison study, but their illite crystallinity and chemistry-index data were quoted to discuss weathering regimes changing from the upper to lower reaches. In addition, clay-mineral compositions in a Holocene core from the Changjiang Subaqueous Delta, determined by the same method as this study, were referred to as the endmember of the Changjiang derived sediment entering the sea (Fan, unpublished data). The above-mentioned dataset were all archived in the supplementary data Table S1.

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ACCEPTED MANUSCRIPT 3. Results

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3.1 Rivers

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Clay-mineral compositions of surface sediments in the GPR consist of predominantly illite

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(48-70%, averagely 58%) and chlorite (30-46%, averagely 39%) (Fig. 2a, Table 1). Smectite is absent or its percentage is below the measurement error (2%). Kaolinite content is remarkably

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different in the two tributaries that the Qishan has an average of 7% comparing to being negligible

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(averagely <1%) in the Laonong. The clay-mineral compositions in the surface sediment take after their source rocks in two different tributaries, respectively. So do illite indices. Both source rocks

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and riverine sediments in the Qishan tributary have slightly higher values of illite crystalline and

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chemistry indexes than those in the Laonong. After the confluence, the Gaoping mainstream has

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mixed clay-mineral compositions but is biased toward the Laonong tributary. The Zhuoshui and Lanyang Rivers have similar clay-mineral compositions and illite indices

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as the GPR (Table 1, Fig. 3). Moreover, the Zhuoshui River has on average a little higher illite content than the Lanyang River (64-65% vs. 55-60%). Kaolinite is almost not detectable in the both rivers. Smectite is very scarce in the Zhuoshui River with a mean percentage of 2-3%, but none in the Lanyang River.

3.2 Gaoping continental margin

3.2.1 Surface sediments Clay-mineral compositions of surface sediments on the Gaoping shelf and slope are similar to those of the GPR, consisting of predominantly illite (51-66%, averagely 62%) and chlorite (29-39%, averagely 36%), with scarce kaolinite (0-6%, averagely 1%) and smectite (0-14%, 12

ACCEPTED MANUSCRIPT averagely 1%). They also have smaller illite chemistry index (0.34-0.38, averagely 0.35) and FWHM values (0.14-0.18°Δ2θ, averagely 0.16°Δ2θ), corresponding to those of the GPR (Fig. 2b;

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Table S1). However, we call for attention to the lower GPSC and the middle to lower continental

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slope (>600 m) where 6 out of 11 samples have a detectable smectite contents (≥1%). Among of them, two samples (L39 and L36) stand out for their smectite content reaching 12% and 14%, respectively. These samples may also have higher kaolinite content than others, typically in the

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northwestern part of continental slope (Fig. 2b). The similar distribution pattern was also found in

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the core sediments and the published data of Liu et al. (2008). 3.2.2 Core sediments

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According to clay-mineral compositions, 7 analyzed cores can be subdivided into two groups

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(Table 2). Group A (K1, L9, L24, L17, and L18) has little downcore variations in their

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clay-mineral compositions, in which kaolinite and smectite are also scarce with their total content less than 2%. In contrast, Group B (L6 and K38) shows clear downcore variations in clay-mineral

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compositions, generally compromising more than 3% of kaolinite and smectitie, with a maximum of 23%. The Group-B cores were sampled from the middle and lower slope, while the Group-A cores from the continental shelf, the upper slope and the canyon. The spatial distribution pattern of clay-mineral compositions in the core sediments mimics the surface sediments, and their downcore variations potentially encode temporal variations in sediment-dispersal controlling factors.

4. Discussion

4.1 High consistency of clay-mineral compositions in the Taiwan river systems 13

ACCEPTED MANUSCRIPT The GPR system is an excellent testing ground for the issue of inheritance and admixture of provenance signals along the river course. The two tributaries Laonong and Qishan run across two

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different terrains with diverse provenance rocks (Fig. 1b). Although they both compromise

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predominantly illite and chlorite, the Laonong bedrock (epimetamorphic rocks) has a higher illite content and little kaolinite to differ from the Qishan bedrock (sedimentary rocks) that averagely contains 7% kaolinite. In the ternary compositional diagram, samples of terrace soil, suspended

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and surface sediments are clustered into two distinct groups strictly following with their bedrocks

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at the tributaries (Fig. 3b). So do illite indices (Table 1). Moreover, Selvaraj and Chen (2006) reported very similar elemental compositions between provenance rocks and riverine sediments in

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the GPR together with similar low CIA values. It is therefore easy to conclude that riverine

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sediment compositions are tightly controlled by their source rocks due to low weathering intensity.

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However, the influence of climate and topography cannot be excluded for the presence of kaolinite at the Qishan watershed, which has on average lower elevation, gentler relief, and slightly warmer

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climate than the Laonong River (WRA, 2004). This is also witnessed by slight higher illite-crystallinity and chemistry-index values of surface sediments at the Qishan River than the Laonong River, which are 0.19 versus 0.15 ºΔ2θ and 0.39 versus 0.35, respectively (Table 1). Previous statistics show that a higher value of the two indices mirrors stronger hydrolysis in the provenance area with a warmer and more humid climate (Chamley, 1989; Liu et al., 2008; He et al., 2013). After confluence, the Qishan signal can be detected at the upper mainstream section by the weak presence of kaolinite, together with slightly reduced illite content and a slight increase in illite indices. However, the kaolinite presence almost vanishes toward the rivermouth, where both 14

ACCEPTED MANUSCRIPT clay-mineral compositions and illite indices take after the Laonong River (Figs. 2b, 3b, Table 1). Dilute effect is attributed to the signal attenuation in that the Laonong River generally contributes

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over two third of sediment discharges to the Gaoping mainstream, consistent with a much larger

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drainage basin area and a higher sediment yield in the steeper-sloped Laonong highland than the Qishan river (WRA, 2004).

The dilute effect should also account for other Taiwan rivers in terms of provenance signal

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mixing along the river course. As shown by the Zhuoshui and Lanyang Rivers, the mainstreams

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have similar clay-mineral compositions with their major tributaries (Table S1), although they both have a few small tributaries. Most of Taiwan rivers have their major tributaries winding in the

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steep-sloped highland, where the outcropped pre-Tertiary Tananao schist and Paleogene

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epimetamorphic rocks have tiny difference in clay-mineral compositions, and the sediments are

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mainly produced by hillslope mass-wasting and fluvial bedrock erosion with low chemical weathering intensity (Hovius et al., 2000; Hartshorn et al., 2002; Dadson et al., 2003; Milliman

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and Kao, 2005; Liu JT et al., 2013, 2016). Small tributaries draining the lowland contribute minor sediments to the mainstreams. These factors cumulatively produce almost the same clay-mineral compositions at the deltas of Taiwan rivers, featured by the predominant illite and chlorite suite with minimal change in their percentages (Fig. 2a, Table 1). The accumulative content of illite and chlorite reaches 97%, 99% and 100% at the deltas of Zhuoshui, Gaoping and Lanyang Rivers, respectively. The consistent characteristic has been widely used to trace Taiwan-sourced sediment distribution in the ECS and the SCS (Liu Z et al., 2008, 2016; Xu et al., 2009; Wan et al., 2010).

4.2 Contrasting weathering regimes between small and large high-mountainous rivers

Clay-mineral compositions of riverine sediments, a good proxy for weathering intensity in 15

ACCEPTED MANUSCRIPT the watershed, are mainly controlled by lithology, climate, tectonic and topographic settings, vegetation, and soil development (Chamley, 1989; Bi et al., 2015). Kaolinite is preferentially

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formed in warm and humid climates with intensive chemical weathering, consequently high

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abundant at low latitudes, while chlorite and illite formation is increased at higher latitudes where chemical weathering is significantly reduced by cold and dry climates (Chamley, 1989; Stumpf et al., 2011). Kaolinite is highly abundant in the Southeast Asian mega-rivers (Fig. 3a), respectively

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accounting for 24-41% and 17-38% in the Mekong and Red Rivers, and being the most abundant

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clay mineral (30-67%, averagely 46%) in the Pearl River (Table S1). The low-relief and stable morphology in the Pearl River basin favors long-continued and intense hydrolysis to have higher

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kaolinite abundance than the other two rivers (Liu et al., 2007). More intense chemical weathering

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in the Pearl River basin is also attested by higher illite indices than the Mekong and Red Rivers

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(Fig. 4a). Values of illite crystallinity (calculated by the FWHM) and chemistry index are usually between 0.24 and 0.42°Δ2θ (averagely 0.30°Δ2θ) and between 0.45 and 0.80 (averagely 0.62) in

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the Pearl River, while they are respectively between 0.11 and 0.23°Δ2θ (averagely 0.20°Δ2θ) and between 0.27 and 0.63 (averagely 0.40) in the Red River, and between 0.19 and 0.21°Δ2θ (averagely 0.20°Δ2θ) and between 0.46 and 0.90 (averagely 0.47) in the Mekong River (Table S1) (Liu et al., 2007). In comparison, the GPR watershed is located at the similar latitude as the Pear River with a warm and humid weather system influenced by typhoons and monsoon cycles. However, the predominant illite and chlorite suite with scarce kaolinite denotes low chemical weathering intensity in the GPR watershed, together with low values of illite indices (Figs. 3a, 4a). The illite chemical index above 0.4 corresponds to Al-rich illite altered by strong hydrolysis, and below 0.4 16

ACCEPTED MANUSCRIPT represents Fe- and Mg-rich illite derived from physical erosion (Liu et al., 2008; He et al., 2013). The illite chemical index of GPR sediments concentrates between 0.29 and 0.41, indicating a

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strong physical erosion environment. In addition, higher FWHM values represent less crystallinity

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of illite, resulting from a strong hydrolysis process (Chamley, 1989; Liu et al., 2008; He et al., 2013). GPR sediments have generally much lower FWHM values than the Asian mega-rivers, further attesting its strong physical weathering setting. Other Taiwan rivers have similar

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clay-mineral assemblages and illite indices with the GPR, indicating analogous low chemical

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weathering intensity in their watersheds, although the island has a tropical and subtropical climate. Besides the low-reach and delta section (Figs. 3a, 4a), a basin-scale comparison is carried out

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between the Gaoping and Changjiang Rivers (Fig. 4b). The Changjiang River drains the

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southeastern margin of Tibetan Plateau with a headwater elevation of ~5,400 m, having a

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catchment area of 1.8×106 km2 and a total trunk length of ~6300 km, which are roughly 550 and 37 times those of the Gaoping River, respectively. Consequently, the Changjiang River has much

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more diverse sediment sources and complex mixing behaviors in the routing course. Tributaries in the western and northern Changjiang watersheds have higher illite and lower kaolinite contents than those in the southern flank in response to their climate and relief disparities (He et al., 2013). Along the mainstream, illite content decreases slightly from the upper to the lower reaches, accompanying with a slight increase in kaolinite content. The disparity is more remarkable in terms of illite indices between the northern and southern tributaries, also between different reaches of the mainstream (Fig. 4b). Except for higher illite-crystalline values, the upper reach and northern tributaries of the Changjiang River have similar values of illite chemistry index to the GPR, which are generally below 0.4, denoting strong physical erosion. In contrast, the southern 17

ACCEPTED MANUSCRIPT tributaries (Xiangjiang and Ganjiang) have much higher values of illite indices (Fig. 4b), because their watersheds have relative warmer and more humid climate together with low-relief and stable

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morphology. The illite indices have much larger values at the middle and lower reaches than the

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upper reach of the Changjiang mainstream, attributed to the confluence of southern tributaries and recycled sediments from the extensive floodplain at the middle and lower reaches. A downstream increasing intensity of chemical weathering is also evident from CIA values in that they are

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generally below 65 (weak weathering) in the upper reach, but between 65-85 (intermediate

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weathering) for the middle and lower reaches of the Changjiang mainstream (Yang et al., 2004; Wu et al., 2011; Shao et al., 2012; He et al., 2013; Bi et al., 2015).

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In short, SMRs in Taiwan are in sharp contrast with large mountainous rivers in Asia by

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virtue of their high consistency in clay-mineral compositions and low values of illite indices.

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Sediment chemistry is majorly determined by source rocks in SMR watersheds with minor climate influence due to strong physical erosion. To a certain extent, the upland weathering signal of large

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mountainous rivers is analogous to SMRs in terms of relatively low chemical weathering intensity. However, the upland signal is subject to severe modulation by the lowland weathering processes for large mountainous rivers, where strong hydrolysis and active sediment recycling take place at the extensive floodplain favorable for soil development.

4.3 Relationship between physical and chemical weathering in tectonically active Islands

Chemical and physical weathering rates are respectively ~700 and ~15000 t km-2 yr-1 in the GPR watershed according to the river gauging data (Dadson et al., 2003; Milliman and Farnsworth, 2011). They are roughly 20 and 100 times higher than the global mean values of ~39 and ~145 t km-2 yr-1, derived from total dissolved-solid and suspended-sediment fluxes into the oceans 18

ACCEPTED MANUSCRIPT (Milliman and Farnsworth, 2011). The global mean chemical denudation flux was estimated of 24

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SMRs draining oceanic islands were included (Gaillardet et al., 1999b).

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t km-2 yr-1 based on the data of 60 largest rivers, and it was expected to significantly enlarge if

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Total dissolved solids calculated from the sum of major ions in the Gaoping riverwater range from 223 to 318 mg l-1, ranking the highest in the world (Chung et al., 2009). Other Taiwan rivers also have highly elevated physical and chemical weathering rates in comparison with the global

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dataset of West et al. (2005) (Fig. 5a). This intuitively implies that rates of chemical weathering

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and physical erosion are coupled to some extent in such a highly active orogenic island, because more and higher reactive fresh-minerals are exposed by rapid denudation to accelerate weathering

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(Gaillardet et al., 1999a, b; Lyons et al., 2005). However, great attention must be paid to the

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positive effects of fresher minerals being highly modulated by the negative effects of shorter

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residence time of sediments in alpine catchments with thinner regolith and faster delivering processes (West et al., 2005; Gabet and Mudd, 2009).

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Further investigations show that the ratio of Wchem (chemical weathering rate) to Wmech (mechanical weathering rate) decreases as the Wmech value increases (Fig. 5b). The relationship of Wchem/Wmech  1 is examined merely at the lower denudation rates, usually below 102 t km-2 yr-1 in stable cratons and a few submontane settings (West et al., 2005; Millot et al., 2002; Riebe et al., 2004). The Wchem/Wmech ratios are generally below 0.1 for the denudation rates higher than 102 t km-2 yr-1. They range from 0.253 to 0.02 for Taiwan rivers with an average of 0.087 (0.047 for the GPR), comparable with the average ratio of 0.026 for the global alpine catchments. The transit from a strong to a faint coupling corresponds to the change from a transport-limited weathering regime in stable craton catchments to a kinetic-limited weathering regime in active alpine 19

ACCEPTED MANUSCRIPT catchments (West et al., 2005; Gabet and Mudd, 2009). This can be further explained by that the function to accelerate weathering through exposing more and fresher minerals by rapid erosion is

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greatly offset by the decrease in the total volume of exposed minerals due to thinner regolith in the

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alpine catchments. Together with small residence time of fluvial sediments in the routing course, the minerals may leave the alpine catchments with minor alternations by weathering reactions (Gabet and Mudd, 2009).

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Taiwan rivers are the typical representative to have thinner regolith and shorter residence

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time in the catchments. Fluvial sediments are majorly produced by hillslope mass-wasting, being efficiently cleared by hyperpycnal flows shortly after the slope failures to leave valley floors little

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mantled (Hovius et al., 2000; Hartshorn et al., 2002). Different from the mega-river systems, the

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upland-derived sediments are little modified at the lowland catchments due to small extents and

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poor development of soil at the narrow floodplains of small mountainous rivers. This is consistent with fluvial sediments being predominantly composed of rock fragments and unaltered primary

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minerals (e.g., illite, chlorite and feldspar), and having low αNa (averagely 2.6 and 7.0 for Taiwan rivers (Bi et al., 2015) and Global large rivers (Gaillardet et al., 1999a), respectively), CIA, and illite-index values to denote low weathering intensity in Taiwan watersheds (Selvaraj and Chen, 2006; Liu et al., 2008; Bi et al., 2015). The tardy response of chemical weathering rates to the highly elevated physical erosion rates (> 103 t km-2 yr-1), however, does not equate to having low weathering fluxes in the active alpine catchments. Both long-term river gauging inventories and temporary water-chemistry data show Taiwan and New Zealand rivers have the highest weathering flux in the world (Jacobson and Blum, 2003; Chung et al., 2009; Milliman and Farnsworth, 2011). The contribution from soil 20

ACCEPTED MANUSCRIPT weathering should be gradually reduced as soil profiles become thinner in rapider denudation zones (Gabet and Mudd, 2009; West 2012). In contrast, bedrock weathering can be highly boosted

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through exposing fresh rock surfaces by erosion and prolonging water-rock interactions by slow

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percolation of surface runoff in the highly fractured rocks and fragmented rock debris, which are very common in active mountain belts (Calmels et al., 2011; Emberson et al., 2016). Both temperate and runoff are indicated to exert significant impact on weathering flux in the rapidly

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denudated and kinetic-limited catchments through modeling analyses (West, 2012; Maher and

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Chamberlain, 2014). A slight difference in weathering intensity between Qishang and Laonong drainage basins indicates the sensitivity of chemical weathering to the climate. However, it is

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worth noting that high dissolved concentrations in the tectonically active alpine rivers may

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predominantly come from carbonate weathering. This is the case for Taiwan and New Zealand

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rivers where hydrothermal calcite is highly abundant in the metamorphic bedrock of uplifted islands, whose weathering rate was assumed roughly 350 times faster than plagioclase (Jacobson

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and Blum, 2003; Jacobson et al., 2003). Chung et al. (2009) reported that 79 to 95% of Ca and Mg ions in the GPR riverwater were derived from carbonate weathering. Therefore, the correction of river dissolved concentrations from non-silicate inputs such as carbonate and evaporite dissolution is vital to address the long-term relationship between weathering and carbon cycling (Gaillardet et al., 1999a, b; Millot et al., 2002; Jacobson et al., 2003). We also note that silicate weathering flux can be still very high after correction. The silicate cation denudation flux in the Liwu River (Taiwan) was reported to approach 18 t km-2 yr-1, which is so far a highest measured value for the felsic lithology in the world (Calmels et al., 2011).

4.4 Sediment dispersal patterns and associated dynamics in the Gaoping continental margin 21

ACCEPTED MANUSCRIPT Fine-grained sediments sourced from the GPR and other Taiwan rivers are featured by the predominant illite (57%) and chlorite (40%) suite with scarce kaolinite (2%) and smectite (1%).

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They are remarkably different from other terrigenous sediments debouching into the northern SCS

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(Fig. 3a, Table S1). Thereinto, Luzon riverine sediments are characterized by the predominant smectite (87%) with minor chlorite (7%), kaolinite (5%) and scarce illite (1%), and the Pear River sediments are rich in kaolinite (46%) with moderate abundance in illite (26%) and chlorite (25%)

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but scarce in smectite (3%) (Liu et al., 2007, 2009). The characteristics have been extensively used

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to trace dispersal routes of terrigenous sediments and their mixing patterns in the northern SCS (Liu Z et al., 2008, 2016). Sediment-trap experiment showed Taiwan-sourced sediments to be

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potentially transported southwestward as far as to the Xisha Trough (~111°E), contributing

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50-70% of the trapped inorganic substances (Liu et al., 2014). Clay-mineral analyses of IODP

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Cores 1144-1148 demonstrated that the timing of Taiwan taking over the Pearl River to be the predominant sediment source of the northern SCS was dated back at least to 3 Ma (Wan et al.,

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2010). We here intend to discuss the controlling factors and mechanisms on the sediment dispersal and reworking in the Gaoping continental margin. 4.4.1 Surface sediments We plot our clay-mineral composition data of surface sediments together with those of Liu et al. (2008) using the same laboratory-analytical and data-processing methods (Figs. 6a-d). All samples were analyzed by XRD using a PANalytical diffractometer at the State Key Laboratory of Marine Geology (Shanghai), and relative percentages of clay minerals were estimated on the glycolated curve using the MacDiff software. We identify the down-canyon sediment transport path by relatively higher chlorite-content patches within or connected to the canyon belt (Fig. 6b), 22

ACCEPTED MANUSCRIPT which has higher C/N ratio ranging from 9 to 11 (Fig. 6f), denoting predominance of terrestrial POC (Hsu H-F et al., 2014). Another striking pattern is the contour-parallel zonation distribution

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of clay minerals with a clear boundary roughly along the 800-m isobath. Besides the continental

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shelf, there are two higher illite-content patches between the 200-m and 800-m isobaths (Fig. 6a), coincident with two deposition centers with a sedimentation rate higher than 0.3 mm yr-1 flanked the middle canyon (Fig. 6e) (Huh et al., 2009). Below the 800-m isobath, a few patches with

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kaolinite (smectite) content larger than 5% (3%) are present, arrayed roughly parallel to the

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contourline but dissected into segments near the GPSC (Figs. 6c, d). Additional sediment sources should be involved because kaolinite and smectite are both very scarce in the GPR and other

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Taiwan rivers.

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The GPSC is generally known as a high efficient route to transfer terrigenous sediments into

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the deep sea, considering the canyon head almost annexing the river mouth (Liu JT et al., 2016). Huge sediment transfer from land to ocean is highlighted by episodic sediment-gravity flow in the

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river and surface plume and bottom hyperpycnal flow in the canyon belt. The canyon interior also serves as a sediment trap, where surface sediments have identical clay-mineral suite as the GPR, together with higher C/N ratio (>9, Fig. 6f) and lighter δ13C value (<-24‰) indicating a predominance of terrestrial origin (Hsu F-H et al., 2014). The transient depositional rates can be very high induced by the episodic turbidity current (Liu JT et al., 2016), but the long-term sedimentation rates derived from 210Pb and 137Cs inventories are generally low (< 0.2 cm yr-1) in the canyon (Huh et al., 2009). The big gap between different-term sedimentation rates is partly attributed to strong reworking by intense internal tides within the canyon and sediment redistribution by ocean circulations out of the belt. The episodic hyperpycnal flow and turbidity 23

ACCEPTED MANUSCRIPT current take play merely a few hours or days during typhoon strikes in each wet season (Kao and Milliman, 2008; Liu JT et al., 2013, 2016), while internal tides occur incessantly with peak

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velocities over 100 cm s-1 (Lee et al., 2009; Hsu RT et al., 2014). The sediment reworking is

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evident in the grain-size distribution at the middle canyon with two circular patches having coarse-grained sediments at the center (Fig. 6e). They are coincidently flanked by a pair of mud patches with 210Pb-based sedimentation rates larger than 1 cm yr-1 at the center, together with

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higher illite and organic-carbon contents and lighter δ13C value, hinting a destination of

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fine-grained sediments resuspended from the middle canyon (Huh et al., 2009; Hsu F-H et al., 2014).

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The contour-parallel arraying patches with higher kaolinite and smectite contents at the lower

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slope are inferred to suffer significant reworking by ocean circulations. The anticyclonic and

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cyclonic looping branches of Kuroshio potentially entrain smectite-rich Luzon-sourced sediments (Fig. 3a), and the SCS warm current conceivably transports kaolinite-rich Pear-River borne

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sediments to the destination (Fig. 1c). The similar mechanism has been widely involved to explain long-distance transport of Luzon and Taiwan sourced sediments to the northern slope of the SCS by the leaking branch of Kuroshio, also known as the SCS Branch of Kurosio (Liu Z et al., 2008, 2016; Liu et al., 2014). 4.4.2 Core sediments Except K1, profiles of 210Pbex show exponential or quasi-exponential decreases with depth for partial or entire sections of dated cores (Fig. 7). Sedimentation rates were derived for the six cores from the slope of the regression line fitting the depth trend of 210Pbex on a semi-log plot, ranging from 0.07 to 0.57 cm yr-1. We also note that the topmost layer of most of the cores has 24

ACCEPTED MANUSCRIPT anomalously low 210Pbex activities. These layers were also dated with appreciable 7Be by Huh et al. (2009). Their thicknesses change from 12 cm in the shallow shelf offshore to <2 cm in the lower

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slope. The cores were taken by the cruises OR1-789 and OR1-791 in March and April, 2006, a

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few months after the typhoon Haitang in July, 2005. These evidences point to the origin of these layers to be fresh flood deposition by the typhoon Haitang (Huh et al., 2009). Mean deposition time of each 2-cm segment sampled for clay-mineral analysis was deduced from the 210Pbex

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derived sedimentation rates for the six cores (excluding K1) and summarized in the last column of

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Table 2. The flood deposition layer at K1 is roughly 12 cm thick, denoting severe erosion and rapid deposition induced by the typhoon in the shallow shelf. Below this layer, the 210Pbex

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activities are extremely low to fluctuate near the regional background value, indicating the

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deposits potentially formed ~100 years ago (Table 2).

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Compared to the present deposition, five short cores in the Group A (K1, L9, L24, L17, and L18) show little downcore change in clay-mineral compositions. They were sampled from the

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canyon interior, shelf and upper slope, where sediment is inferred to entirely source from the GPR at present, so should it be in the past. The other two cores from the middle and lower slopes demonstrate higher kaolinite and smectite contents in the early deposits than the present, with a maximum smectite content of 23% at the intermediate layer of L6 (Table 2). Kaolinite and smectite are indicated to respectively source from the Pearl-River and Luzon borne sediments, carrying to the destination by the branch currents of SCS Warm Current and Kuroshio Current. These circulations are therefore coarsely postulated to have been significantly strengthened at least in some intervals of the past 3 centuries from sparse core data. Decadal variations in the strength of Kuroshio intrusion into the SCS and its cyclonic looping branch off the southwest Taiwan were 25

ACCEPTED MANUSCRIPT also reported based on comprehensive analysis of satellite data, in situ hydrographic measurements and modeling results, but they only went back to the last 2-3 decades (Caruso et

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al., 2006; Nan et al., 2015).

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4.5 Implications for the carbon cycle

The orogeny has been widely considered as a climate regulator through consumption of CO2

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by rock weathering and delivering particular organic carbon (POC) by rivers to the ocean. First, the coupling between chemical and physical denudation rates has been examined to exist in both

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large and small river systems (Gaillardet et al., 1999b; Millot et al., 2002; Lyons et al., 2005).

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Although the chemical denudation yields are only a small fraction (in most instances less than 1%,

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Fig. 5) of total denudation yields, the absolute values are conceivably very high in the active tectonic regions (Gaillardet et al., 1999a, b; Lyons et al., 2005). The extrapolation strongly

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supports the “Raymo hypothesis” (Raymo and Ruddiman, 1992). Furthermore, recent investigations showed that export yields of both biospheric and petrogenic POC are positively

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related to the suspended sediment yield (SSY). Burial efficiency of terrestrial POC in marine sediments is also highly determined by the SSY, which can be up to three times higher at high-SSY regions (Galy et al., 2007, 2015; Hilton et al., 2011, 2012; Kao et al., 2014). In the G-B river system, POC burial may scavenge a factor of 10 more CO2 than by chemical weathering, challenging the “Raymo hypothesis” (Galy et al., 2007; Gaillardet and Galy, 2008). Finally, the CO2 sequestration mechanism should be deliberately examined considering the importance of different processes (the consumption of CO2 by silicate and carbonate weathering, and the balance between organic matter burial and oxidation of sedimentary organic matter) at their relevant time scales (Gaillardet and Galy, 2008). 26

ACCEPTED MANUSCRIPT Taiwan has been intensively studied recently because it serves as an efficient factory in producing and delivering ample inorganic and organic terrestrial material from land to ocean.

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According to secular river-gauging data, Taiwan rivers carry 190-380 Tg yr-1 suspended load and

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12 Tg yr-1 dissolved load into the sea, accounting 1-2% and 0.32% of the global fluxes, respectively (Dadson et al., 2003; Milliman and Farnsworth, 2011). POC flux from Taiwan rivers was estimated to be 1.6-1.8 Tg yr-1 (Hilton et al., 2011, 2012; Kao et al., 2014; Bao et al., 2015),

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accounting for ~1% of the global total, which has been estimated to be 170-200 Tg C yr-1 (Ludwig

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et al., 1996; Schlünz and Scheider, 2000; Galy et al., 2007). “Disproportionate” is a high frequently used word to describe the nature of Taiwan rivers for high yields and fluxes of

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terrestrial material from ~0.024% of the global land area (Table 3). This is ascribed to rapid uplift

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of the central mountainous range outcropped with high fragile metamorphic and sedimentary

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rocks, together with frequent earthquakes and torrential typhoon rain (Dadson et al., 2003; Milliman and Kao, 2005; Kao and Milliman, 2008; Bao et al., 2015; Liu JT et al., 2013, 2016).

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Other SMRs are considered to have similar extremely high yields and fluxes as those in Taiwan (Milliman and Farnsworth, 2011; Blair and Aller, 2012), consequently exerting significant impacts on the global carbon cycle. Both physical and chemical weathering rates in Taiwan watersheds are highest in the world (Fig. 5), but Wchem is not much elevated in comparison with a few large rivers (Milliman and Farnsworth, 2011). The correlation is further examined by a log-log plotting of Wchem vs. Wmech and best-fitting analysis as suggested by Millot et al. (2002). The equation of the line shown in Fig. 8 for Taiwan rivers is Wchem = 41.3Wmech0.29. Power function is Wchem = 3.9Wmech0.47 for New-Zealand rivers (Lyons et al., 2005), and Wchem = 0.39Wmech0.66 for Canadian shield rivers 27

ACCEPTED MANUSCRIPT (Millot et al., 2002). It is indicated that a tenfold increase in the physical erosion rate will result in a 4.6-fold, 2.4-fold, or 1.9-fold increase in the chemical weathering rate for Canadian shield rivers,

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New Zealand or Taiwan alpine rivers, respectively. The decrease in exponent values with

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increasing Wmech values attests the intuitive view of Gaillardet et al. (1999a): “The more active physical erosion is, the less intense weathering reactions are”. The less weathered sediments in Taiwan rivers are in consistent with lower CIA, αNa and illite-index values due to kinetic-limited

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weathering in active alpine catchments (West et al., 2005; Gabet and Mudd, 2009). Dissolved

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loads in the rivers of Taiwan and other Oceanian islands are not examined to be high abnormally increased (Milliman and Farnsworth, 2011). Together with their tiny watersheds, chemical

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weathering in the Oceanian islands are consequently considered to have minor contribution to the

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global carbon sequestration, in sharp contrast with their total contribution of ~40% of the global

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sediment flux.

Moreover, river dissolved loads can be sourced from rain, atmosphere, pollutant, and rock

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weathering. According to Chung et al. (2009), dissolved loads in the GPR are majorly sourced from weathering of silicates and carbonates, with the latter contributing >80% of Ca and Mg ions. The similar conclusion was also drawn from study of New Zealand rivers (Jacobson and Blum, 2003; Jacobson et al., 2003). Carbonate weathering contributes nil to long-term (geological) CO2 consumption because it is balanced by carbonate precipitation in the ocean (Gaillardet and Galy, 2008). Gaillardet et al. (1999b) extrapolated that only 26% of global dissolved material (~2.13 Pg yr-1) originates from silicate weathering, with the remnant majorly from carbonate and evaporite dissolution. These clues lead to a conclusion that chemical weathering in the orogen should play a much less important role in regulating secular climate change as presumed (France-Lanord and 28

ACCEPTED MANUSCRIPT Derry, 1997; Jacobson and Blum, 2003; Galy et al., 2007, 2015). High sediment yields and fluxes in SMRs are always accompanied by high loads of fossil

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(petrogenic) POC. It is attributed to that carbonaceous epimetamorphic and sedimentary rocks are

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high abundant in the uplifted terranes. For example, schist, slate, shale and marine sandstone are extensively outcropped in the central mountainous range of Taiwan, with an average POCfossil content of ~0.4% (Chung et al., 2009; Hung et al., 2009; Hilton et al., 2011). They are fragile for

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physical erosion in nature. Clastic fragments are usually produced by slope failures in the steep

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alpine watersheds and transported rapidly into the sea by typhoon-induced hyperpycnal flows (Hovius et al., 2000; Hartshorn et al., 2002; Dadson et al., 2003; Milliman and Kao, 2005).

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Refractory POCFossil can be high efficiently delivered into the sea with <15% loss, because it is

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highly protected from oxidation by a tightened bandadge with clay minerals, together with

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prevalence of mechamical abrasion and short residence time in the river transit (Hilton et al., 2011). This is in sharp contrast with large river systems with extensive floodplains, where more

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than 50-85% of POCfossil can be oxidized (Galy et al., 2008; Bouchez et al., 2010). Taiwan rivers was estimated to deliver 1.3 Tg yr-1 POCfossil into the sea (Hilton et al., 2011), and accumulatively 40 Tg yr-1 POCfossil was supposed to be delivered by SMRs draining the world’s active margins into the sea (Blair et a., 2003; Drenzek et al., 2009), accounting >20% of the global POC flux. Small mountainous rivers are important not only for the POCfossil transfer (Kao and Liu, 1996; Blair et al., 2003; Komada et al., 2005; Drenzek et al., 2009; Hilton et al., 2011), but also in the export of non-fossil (biospheric) organic carbon (Hilton et al., 2008, 2012; Bao et al., 2015). A significant correlation was observed not only between lignin and suspended sediment concentrations (Bao et al., 2015), but also between POCnon-fossil and suspended-sediment yields in 29

ACCEPTED MANUSCRIPT the Taiwan rivers (Hilton et al., 2012). The annual lignin flux from Taiwan was estimated to be 19.7 Gg yr−1, with an annual yield being 10–100 times higher than those of large rivers (Bao et al.,

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2015). The POCnon-fossil flux was estimated to be 0.5 Tg yr-1 for Taiwan (Hilton et al., 2012), and

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37 ± 25 Tg yr-1 for Oceanian SMRs (Bao et al., 2015; Kao et al., 2014), the latter accounting for ~20% of the global POCnon-fossil output (Blair and Aller, 2012; Galy et al., 2015). High POCnon-fossil yields in SMRs are also determined by predominant physical erosion in the watersheds and rapid

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transfer by hyperpycnal flows during transit (Hilton et al., 2012; Bao et al., 2015; Galy et al.,

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2015).

Both biospheric and petrogenic POC can be high efficiently buried in the continental margins

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off SMRs due to high sedimentation rates (Kao et al., 2014; Galy et al., 2015). Marine burial of

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biospheric POC represents a net sequestration of contemporary atmospheric carbon, also a major

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carbon sink over geological timescales. Although petrogenic POC transfer from the rock reservoir to marine sediments disconnects from the atmosphere, its oxidation during transit releases CO2 to

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the atmosphere, serving as a carbon source. Consequently, the fate of riverine POC (both biospheric and petrogenic) is vital in the global carbon cycling (Galy et al., 2015). Hyperpycnal flows and rapid deposition in the narrow continental margin off SMRs favor terrestrial POC burial. The burial efficiency of 70-85% has been proposed for small river systems, by sharp contrast with <15% in large river systems (Galy et al., 2007; Hilton et al., 2011; Kao et al., 2014). Based on dual organic carbon isotope (δ13Corg and Δ14Corg) data of trap, surface and core sediments, the preservation of terrestrial POC (POCfossil + POCnon-fossil) in the upper Gaoping Canyon was supposed nearly 100%, but decreased to 70% downward and outside the canyon (Kao et al., 2014). The mean values of δ13Corg and Δ14Corg are -24.9‰ and -730‰ for a short core 30

ACCEPTED MANUSCRIPT (OR1-K1) in the upper canyon with the cumulative illite and kaolinite content of 99%, while they are -22.9‰ and -667‰ for a short core (OR1-K11A) in the lower canyon with the cumulative

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illite and kaolinite content of 95%. In the northwestern Gaoping continental margin, δ13Corg and

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Δ14Corg increase from -23.3‰ and -669‰ at the shelf to -23.0‰ and -501‰ at the lower slope (Kao et al., 2014), accompanying with a decreasing trend of the cumulative illite and kaolinite content from 99% to 94%. Relatively negative δ13Corg and Δ14Corg values are very close to those of

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fluvial sediments in Taiwan (Kao et al., 2014), denoting a predominance of terrestrial POC source,

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similar to what clay-mineral compositions tell us. A slight offshore increase in δ13Corg and Δ14Corg values denotes gradual loss of terrestrial POC by increased substitution of marine POC, also

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analogue to offshore increase in the cumulative kaolinite and smectite content which indicates the

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input of other sourced sediments by ocean circulation as discussed in the previous section. Their

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synchronous variations are determined by the predominance of petrogenic POC in the sediment, which is tightly banded with clay minerals (Blair et al., 2003; Blair and Aller, 2012).

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Considering a mean preservation efficiency of 70% for terrestrial POC and a total suspended-sediment flux of 380 Mt yr-1 from Taiwan, and biospheric and petrogenic POC contents of 0.15% and 0.29% in the sediment, annual burial fluxes were therefore extrapolated to be 0.5–0.6 TgC yr-1 of POCnon-fossil and 0.9–1.1 TgC yr-1 POCfossil (Kao et al., 2014). Cumulatively, they represent 2.7% of annual terrestrial POC burial flux (58 TgC yr−1) in the oceans (Burdige 2005; Blair and Aller, 2012), high disproportional to the small percentage (0.024%) of Taiwan watersheds in the global total. If the finding from Taiwan is a good representative of the Oceania, the Oceanian watersheds (<2.5% of the global watershed surface) might disproportionally contribute ~16% (8-11 TgC yr−1) of annual terrestrial POC burial flux in the global ocean (Kao et 31

ACCEPTED MANUSCRIPT al., 2014). The above estimation of terrestrial POC burial efficiencies and fluxes should be

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overestimated to some extent, because the sampling locations are generally constrained to the

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fluvial-sediment conveying belts in the seas with high sedimentation rates (Kao et al., 2014). As discussed in the previous section, sediment reworking and redistribution are obvious in the Gaoping continental margin (Fig. 6). Deposition in the Gaoping submarine canyon (GPSC) is

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predominated by river-borne sediments, but it is not excluded from reworking by strong internal

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tides (Lee et al., 2009; Hsu RT et al., 2014). A pair of mud patches flanking the GPSC at the upper slope have highest sedimentation rates and consist of predominantly terrestrial sediments with

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identical clay-mineral suite and organic compositions as the GPR (Fig. 6), but they should be

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majorly formed by complex interactions of waves, internal tides and ocean circulations after

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typhoon-induced hyperpycnal flow events. Reworking should significantly lower terrestrial POC burial efficiency. Except for the canyon interior and the rapid depositional mud patches,

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sedimentation rates in the Gaoping continental margin are generally lower than 0.2 cm yr-1 (Huh et al., 2009). It was estimated that merely 20-23% of riverine sediments and ~13% of terrestrial POC have been trapped in the Gaoping continental margin (Huh et al., 2009; Hsu F-H et al., 2014). The remnant riverine sediments should be transported out of the system, but low trapping efficiency of terrestrial POC should be partly due to oxidation by sediment reworking. For example, a few contour-parallel depositional patches at the lower continental slope have higher kaolinite and smectite contents, indicating exotic sediment sources from the Pear River and Luzon rivers by ocean circulations. They were also examined to have lower C/N ratio, heavier δ13C value, and higher Δ14C value, indicating an increasing replacement by marine-sourced POC (Fig. 6; Hsu F-H 32

ACCEPTED MANUSCRIPT et al., 2014). This highlights that the present constraint on the fate of terrestrial POC in the seas is still loose, meriting further study. It is noteworthy that given a much lower burial efficiency (e.g.,

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Blair and Aller (2012) proposed the global average of 20-44%), the burial of terrestrial POC in the

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seas should still sequestrate more CO2 into the long-term carbon cycle than chemical weathering in the watersheds of SMRs, strongly supporting the similar conclusion from the study of the G-B

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river system (Galy et al., 2007) and the global representative river systems (Galy et al., 2015).

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5. Conclusions

Clay-mineral compositions of detrital sediment hold valuable information on the weathering

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regime on land and dispersal route in the ocean. Our investigation of fluvial sediment in the GPR

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and other Taiwan rivers demonstrates that SMRs have astonishingly consistent clay-mineral

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compositions in terms of extreme abundance of illite and chlorite and acute deficiency of kaolinite and smectite, along with low illite-index values. The nature is a legacy from source rocks

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outcropped in the alpine watersheds of Taiwan, where sediments are majorly produced by hillslope mass-wasting because of fragile metamorphic and sedimentary rocks with steep slopes, frequent earthquakes and torrential stormrains. Limited compositional change along the river courses is due to shorter residence time in transit that sediments are rapidly flushed into the estuaries by episodic hyperpycnal flows. This is in sharp contrast to large mountainous rivers in Southeast Asia, where sediments are rich in secondary clay minerals (kaolinite and smectite), together with higher illite-index values. A downstream increase in chemical weathering intensity is clearly exhibited because strong hydrolysis and active sediment recycling take place at the

33

ACCEPTED MANUSCRIPT lowland reaches with extensive floodplains. So the upland provenance signal is severely modulated by the lowland climate signature in the large rivers.

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Deposition in the Gaoping continent margin is primarily sourced from Taiwan rivers as

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indicated by clay-mineral compositions. Together with published data from the literatures, we highlight the role of sediment reworking by strong internal tides and contour-parallel conveying processes by ocean circulations. Additional sediment sources are identified to derive from Luzon

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rivers and the Pear River, entrained to the destination majorly by the branch currents of Kuroshio

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intrusion into the northern SCS. Sparse core data preliminarily demonstrate the secular change in the intrusion-current strength. Except for a pair of mud patches flanking the submarine canyon at

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the upper continental slope, sedimentation rates and POC% decrease away from the GPSC. At the

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lower continental slope where exotic clay minerals are accumulated, the content of terrestrial POC

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decreases slightly as marine POC increases. The coupling of chemical weathering and physical erosion has been examined among Taiwan

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rivers as reported to be universal in large rivers. However, the ratio of Wchem (Chemical weathering rate) to Wmech (mechanical weathering rate) decreases as the Wmech increases. It is roughly a unit for some rivers draining stable cratons or submontane settings with a denudation rate lower than 102 t km-2 yr-1, but sharply decreases to 0.087 for Taiwan rivers with an average denudation rate of 104 t km-2 yr-1. Different power laws of Wchem and Wmech are examined between craton (0.66 exponent) and alpine rivers (0.29-0.47 exponent), denoting a tardy response of chemical weathering fluxes to high elevated physical erosion rates. The transit is contingent on the change from a transport-limited weathering regime for craton rivers to a kinetic-limited weathering regime for alpine rivers, where the function to accelerate weathering by exposing more 34

ACCEPTED MANUSCRIPT and fresher minerals is largely offset by the decrease in total volume of exposed minerals due to thinner regolith (Gabet and Mudd, 2009). Furthermore, selective dissolution of hydrothermal

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calcite characterizes Taiwan and other Oceanian islands due to overwhelming advantage of

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carbonate to silicate weathering.

Physical erosion in the active orogen accelerates both chemical weathering rates and POC yields. A brief review of dissolved loads and POC contents in the rivers of Taiwan and Oceania is

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presented. The fluxes of dissolved solids are not examined to be abnormally elevated from Taiwan

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and other Oceanian islands. However, yields and fluxes of both biospheric and petrogenic POC are highly accelerated by rapid physical erosion. Hyperpycnal flow and high suspended sediment

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concentration favor terrestrial POC transfer and burial in the oceans. Annual terrestrial POC burial

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flux has been estimated 8-11 TgC yr-1 (~1.5 TgC yr-1) from Oceanian islands (Taiwan),

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accounting for ~16% (2.7%) of the global total, high disproportional to their land percentage of 2.5% (0.024%) in the world’s watershed area. Even modulated to a lower burial efficient

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percentage, carbon sequestration through POC burial still outpace significantly silicate weathering in the Oceanian rivers. The results highlight the role of POC transfer and burial over silicate weathering in the active orogens on the long-term carbon cycle and climate change as previous assumptions (France-Lanord and Derry, 1997; Galy et al., 2007, 2015).

Acknowledgement We are grateful to Ray T. Hsu, Rick J. Yang and other Taiwan friends for assisting in the fieldtrips and sediment pretreatment. We thank Yanli Li at the State Key Laboratory of Marine Geology, Tongji University for XRD analyses reported in this study. This work was funded by the National 35

ACCEPTED MANUSCRIPT Natural Science Foundation of China (NSFC-41476031), National Programme on Global Change and Air-Sea Interaction (GASI-GEOGE-03), and the Special Research Fund for the Doctor

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Program of Higher Education in China (20130072130003). We greatly appreciate Prof. Jerome

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Gaillardet and two anonymous reviewers for the very insightful suggestions on the previous version.

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with emphasis on the Gaoping (Kaoping) Submarine Canyon. J. Mar. Syst. 76, 369–382.

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ACCEPTED MANUSCRIPT Figure captions

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Fig.1. Overview and detailed maps of the sampling locations in the Zhuoshui and Lanyang Rivers,

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and the Gaoping river-sea system. (a) Taiwan topographic map, (b) geological map of the Gaoping

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River watershed (modified from Chen, 2000), (c) major ocean circulations in the northeast SCS including NW Luzon Coastal Current (pink short dash line), SCS Warm Current (pink long dash

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line), and four types of Kurashio intrusions (leaking: orange solid, leaping: magenta solid, anticyclonic looping: yellow dash, and cyclonic looping: red solid) (after Caruso et al., 2006; Nan

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et al., 2015), (d) bathymetric map of the Gaoping continental margin.

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Fig. 2. Clay-mineral compositions of surface sediments, suspended sediments (SS), terrace soil

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and bedrock in the GPR. Digit in the circle denotes the number of samples being included to

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calculate mean clay-mineral compositions.

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Fig.3. Ternary diagram of the major clay-mineral groups illite+chlorite, kaolinite, and smectite.

Fig. 4. Correlations of illite chemistry index with illite crystallinity of fluvial sediments at the low reach or delta of the rivers (a), and different reaches and tributaries of the GPR and Changjiang River (b). Data source: Red River, Mekong River and Pearl River from Liu et al. (2007), Changjing River from He et al. (2013).

Fig. 5. Correlations of the Wmech (mechanical weathering rate) vs. Wchem (chemical weathering rate) and the Wmech/Wchem ratio of SMRs in Taiwan derived from the river gauging data collection of Milliman and Farnsworth (2011) and the global river dataset of West et al. (2005).

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ACCEPTED MANUSCRIPT Fig. 6. Sediment transport patterns along the submarine canyon and parallel to the contour lines demonstrated by spatial variations of (a) illite%, (b) chlorite%, (c) kaolinite%, (d) smectite%, (e)

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mud% (gray scale) and sedimentation rates (yellow line) (after Huh et al., 2009), and (f) TOC%

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(gray scale) and C/N ratio (pink line) (after Hsu F-H et al., 2014) in surface sediments of the Gaoping continental margin. The down-canyon dispersal route is shown by relatively higher chlorite content in the canyon belt (b), together with relatively higher C/N ratio (f).

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Contour-parallel distribution patterns are demonstrated by: (1) two higher illite-concentration

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patches at the upper slope (-200 ~ -800 m) (a) together with higher C/N ratio, mud content and sedimentation rate (e), and (2) some higher kaolinite and smectite content patches at the middle

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and lower slope (c, d) together with relatively lower C/N ratio.

Fig. 7. Profiles of 210Pbex of seven short cores from the Gaoping continental margin. Arrows point

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to the lower boundary of flood depositions by the typhoon Haitang in 2005. Short-core informations are given with their names, geographic locations, core lengths, thicknesses of

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Haitang-induced rapid deposits, and mean sedimentation rates in terms of cm yr-1.

Fig. 8. Log-log plotting of chemical and physical erosion rates in Taiwan and New-Zealand Alpine rivers, the latter data source from Lyons et al. (2005). Best fitting lines are shown together with that of Canadian shield rivers (Millot et al., 2002).

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 7

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Fig. 8

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Table 1

compositions: 2%) Sample

Sample

type

number

Illite

Bedrock

2

50

Qishan

Terrace soil

1

53

Qishan

Surface sediment

4

Laonong

Bedrock

3

Laonong

Suspended sediment

2

Laonong

Surface sediment

Gaoping

Surface sediment

Gaoping*

Kaolinite

Smectite

Illite crystallinity

Illite chemistry

(°Δ2θ)

index

41

7

1

0.18

0.41

41

5

1

0.18

0.35

7

0

0.19

0.39

62

38

0

0

0.17

0.34

58

42

0

0

0.17

0.32

7

62

37

1

0

0.15

0.35

4

58

41

1

0

0.15

0.36

Surface sediment

11

56

44

0

0

0.17

0.33

ZS River

Surface sediment

4

60

38

0

2

0.14

0.34

ZS Delta

Surface sediment

6

55

41

1

3

0.13

0.36

Surface sediment

1

51

44

1

3

0.12

0.30

Surface sediment

5

65

35

0

0

0.18

0.36

Surface sediment

3

64

36

0

0

0.17

0.36

LY River LY Delta

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ZS Delta*

D

42

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52

TE

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Qishan

Chlorite

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Location

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Clay mineral compositions (%) and illite indices of sediments in the three Taiwan rivers (uncertainty for

* noting data source from Liu et al. (2008), ZS and LY shorten for Zhuoshui and Lanyang rivers, Illite crystallinity being calculated by the

FWHM method, see Supplementary data Table 1 for detail.

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Table 2

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Clay mineral compositions (%) and illite indices in the seven short cores collected from the Gaoping continental

margin Core number

Location

Depth

(water

(cm)

Illite

Chlorite

61

Canyon

20-22

61

(144 m)

42-44

61

NW shelf

OR1-789

OR1-789

L18

L9

L6

K38

crystallinity

Chemistry

deposition

(°Δ2θ)

index

time (AD)

0.14

0.34

2005

38

1

0

0.15

0.38

Pre-1900?

39

0

0

0.14

0.34

Pre-1900?

39

0

1

0.15

0.36

2005

38

3

1

0.15

0.35

1985

32-34

58

40

1

1

0.16

0.39

1945

0-2

61

38

0

1

0.15

0.36

2005

shelf

10-12

59

41

1

0

0.15

0.34

1995

(82 m)

20-22

59

39

2

0

0.16

0.32

1976

0-2

61

39

0

0

0.15

0.34

2001

shelf

16-18

60

39

1

0

0.17

0.36

1909

(129 m)

30-32

60

39

1

0

0.17

0.34

1825

Upper

0-2

64

36

0

0

0.15

0.36

2005

slope

24-26

57

37

3

2

0.16

0.34

1966

(491 m)

48-50

61

39

0

0

0.16

0.34

1922

Middle

0-2

58

37

2

3

0.15

0.36

1994

slope

6-8

46

28

3

23

0.14

0.37

1907

12-14

55

35

3

7

0.16

0.35

1805

Lower

0-2

60

37

1

1

0.15

0.36

2005

slope

20-22

56

35

4

5

0.16

0.35

1894

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SE

(845 m) OR1-791

Mean

58

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OR1-791

L17

60

Illite

16-18

(130 m) OR1-791

0-2

Illite

0

39

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0-2

D

L24

Upper

TE

OR1-791

K1

Smectite

0

depth) OR1-789

Kaolinite

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Cruise

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53

33

5

9

0.15

0.38

1775

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(1263m)

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Table 3

Watershed size

Water discharge

Suspended load

(106 km2)

(103 km3 yr-1)

(Pg yr-1)

105a

36a

19a

Taiwan rivers

0.025a

0.055a

0.19a -0.38d

Percentage

0.024%

0.15%

1-2%

Dissolved load

POCfossil

POCnon-fossil

(Pg yr-1)

(Pg yr-1)

(Pg yr-1)

3.8a

43b,c

157b,c

0.012a

1.3e,f

0.5g,h

0.32%

3.0%

0.3%

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Global rivers

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Disproportionate contribution of Taiwan rivers to the global inorganic and organic fluxes into the ocean

a: Milliman and Farnsworth, 2011; b: Galy et al., 2015; c: Blair and Aller, 2012; d: Dadson et al., 2003; e: Hilton et al., 2011; f: Kao et al.,

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2014; g: Hilton et al., 2012; h: Bao et al., 2015.

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