Hsiung et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-020-00396-2(2021) 8:2

Progress in Earth andPlanetary Science

PAPER WITH FULL DATA ATTACHEDOpen Access

X-ray fluorescence core scanning, magnetic
signatures, and organic geochemistry
analyses of Ryukyu Trench sediments:
turbidites and hemipelagites
Kan-Hsi Hsiung1* , Toshiya Kanamatsu1, Ken Ikehara2, Kazuko Usami3, Chorng-Shern Horng4, Naohiko Ohkouchi1,
Nanako O. Ogawa1, Saneatsu Saito1 and Masafumi Murayama5

Abstract
The southwestern Ryukyu Trench represents the ultimate sink of sediments shed from Taiwan into the Philippine
Sea, which are mainly transported to the trench by turbidity currents via submarine canyons. Here, we present
trench turbidites intercalated with hemipelagites in a gravity pilot core and a piston core acquired on the Ryukyu
Trench floor at 6147 m water depth. We performed X-ray fluorescence core scans (ITRAX profiles), magnetic
measurements, and organic geochemistry analyses to discriminate turbidites from hemipelagites. We identified 36
turbidites (0.9–4.2 cm thick) based on visual core descriptions and Ca/Fe ratios in the ITRAX profiles. Three of theseturbidites show magnetic signatures indicating the presence of pyrrhotite and peaks in the magnetic susceptibility
profile, suggesting that Taiwan-sourced sediments are transported to the Ryukyu Trench by long-runout turbidity
currents. Pyrrhotite is also present in hemipelagites of the upper part of the retrieved cores, indicating a dominant
sediment source in Taiwan over the last several thousand years. Ca/Fe and Zr/Rb ratios in the ITRAX profiles markdistal turbidites (about 1–3 cm thick), and Zr/Rb peaks mainly reflect grain size changes. Detailed analyses of arepresentative turbidite show good correlation between Ca/Fe and Zr/Rb peaks with upward-coarsening and upwardfining trends that delimit the turbidite. Sedimentary organic matter in hemipelagites is characterized by higher totalorganic carbon and total nitrogen contents and higher δ13C values than that in turbidites. Our multi-proxy approach
employing high-resolution XRF core scans to differentiate turbidites from hemipelagites contributes to establishing acomprehensive view of modern trench sedimentation from Taiwan to the southwestern Ryukyu Trench.
Keywords: Distal turbidites, ITRAX, Pyrrhotite, Ryukyu Trench, Taiwan

1 Introduction
Oceanic trenches are the most significant elongate depressions on the seafloor along subduction zones andare generally thousands of kilometers long, up to 8 km
deep, and tens to hundreds of kilometers wide (Jarrard
1986; Stern 2002). In the source-to-sink scheme (e.g.,
Sømme et al. 2009; Covault et al. 2011), trenches* Correspondence: hsiung@jamstec.go.jp
1
Yokosuka Headquarters, Japan Agency for Marine-Earth Science andTechnology, 2-15, Natsushima-cho, Yokosuka-city, Kanagawa 237-0061, Japan
Full list of author information is available at the end of the article

represent the ultimate sinks of regional sediment transport
along convergent margins. There are two fundamental transport routes for sediment filling the trench floor: generally,
trench-filling sediments are transported downslope from adjacent forearc ridges by gravity-driven sediment flows,
whereas axial sediment transport plays an important role indelivering terrigenous sediments to the trench floor (e.g.,
Hsiung et al. 2015; Malatesta et al. 2013). Some large deepsea channels along trench axes, such as the middle Chile
Trench and the Nankai and Zenisu Troughs (Thornburg
and Kulm 1987; Le Pichon et al. 1987; Wu et al. 2005), also

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Hsiung et al. Progress in Earth and Planetary Science(2021) 8:2

indicate active sediment transport on the trench floors. Sediments accumulated within the trench tend to form a wedgeshaped package of deposits (i.e., the trench wedge) that increases in thickness toward the accretionary prism (Daviset al. 1983). Trench-fill deposits are commonly characterizedby fine-grained turbidites, which result from turbidity currents within submarine canyons and channels of the trench(Stow and Shanmugam 1980). The Japan Trench, however,lacks a major canyon system; there, turbidity currents are
triggered by large earthquakes and related tsunamis (Ikehara
et al. 2016, 2018). However, reported ages of turbidites andhemipelagic muds in core samples from the landward slopeof the Japan Trench are variable (Usami et al. 2018; Ikehara
et al. 2020). Advanced understanding of discrepancies ofsedimentation rates from distinct trenches are important toimprove the knowledge of trench sedimentation.
The southwestern Ryukyu Trench near Taiwan (Fig. 1)
is an ideal location for studying trench deposits within asource-to-sink framework because of the distinct sediment transport routes from their terrestrial source in
Taiwan to their ultimate sink in the trench. Hsiung et al.
(2017) suggested that deposits in the western end of the
Ryukyu Trench record the accumulation of sediments
derived from the distal source in Taiwan, transported
mainly by turbidity currents down the Hualien and Taitung Canyons. To date, those trench turbidite deposits
have mainly been interpreted on the basis of seismic facies characteristics and major sedimentary features, butthese interpretations lack constraints from core samples(Hsiung et al. 2017). The main objective of this study istherefore to verify the provenance of Ryukyu Trench
sediments based on analyses of core samples acquired by
the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) in 2015. These newly acquired cores
were characterized by integrating X-ray fluorescence(XRF) core scanning, magnetic measurements, and organic geochemistry. We use our analyses to understandthe origins of turbidites (particularly distal turbidites) in
Ryukyu Trench deposits, which contain crucial information on far-field sediment transport. Our results contribute to establishing a comprehensive view of modern
trench sedimentation by documenting the efficient sediment transport from Taiwan to the southwestern Ryukyu Trench. Besides, radiocarbon dating provides age
model constraints for the Ryukyu Trench floor.
1.1 Regional setting of the southwestern Ryukyu Trench

The oceanic crust of the northern Huatung Basin andwest of the Gagua Ridge is actively subducting northward
beneath the Ryukyu Trench (Schnürle et al. 1998). The
southwestern Ryukyu Trench is indented by the northernpart of the Gagua Ridge and is characterized by a curved,
fan-shaped trench floor (Fig. 1). Bathymetry and seismicreflection profiles show the tilting trench wedge in the

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western end of the Ryukyu Trench (Fig. 2), implying sediment deposition rather than erosion in this area. The connection between the Hualien Canyon, east of Taiwan, andthe Ryukyu Trench is a major seaward transport route for
orogenic sediments shed from the island of Taiwan(Hsiung et al. 2017). The sediment flux delivered to theocean from eastern Taiwanese rivers has been estimatedto be about 44 Mt/year (Dadson et al. 2005) to 79.4 ±13.8 Mt/year (Kao and Milliman 2008). Large volumes oforogenic sediment eroded from the Taiwan orogenic belt
are transported seaward and deposited as a ~ 4 km-thicksequence in the confined Huatung Basin. The major sediment transport routes from Taiwan to the southwestern
Ryukyu Trench floor have been determined from bathymetric and seismic reflection data (Hsiung et al. 2017).
The different water depths in the Huatung Basin (~ 4000m) and the West Philippine Sea Basin (~ 6300 m) imply
that the Gagua Ridge is a natural boundary preventing thetransport of sediments farther eastward (Van Avendonk
et al. 2014; Lehu et al. 2015).
North of the Ryukyu Trench, the Ryukyu Island Arc
was formed as a consequence of the subduction of thePhilippine Sea Plate beneath the Eurasia Plate (Karig
1973; Angelier 1986). The Ryukyu subduction system extends 2200 km southwest from Kyushu Island, Japan, tonorthern Taiwan and includes the Ryukyu Trench, theYaeyama accretionary prism, several forearc basins, theRyukyu Islands, and several backarc basins (Dominguez
et al. 1998; Schnürle et al. 1998). The forearc basins aresemi-isolated intra-slope basins that parallel the southwestern Ryukyu Trench. The Yaeyama Ridge, the frontal
part of the accretionary prism created by subduction in
the southwest Ryukyu Trench, forms a linear boundary
separating the accretionary prism from the forearc basins. The Ryukyu Trench wedge is mainly fed from two
directions: (1) Taiwan-sourced sediments from the westand (2) sediments eroded from the Yaeyama Ridge and
Ryukyu island arc to the north (Hsiung et al. 2017). The
southwestern Ryukyu Trench floor can be regarded as
the ultimate sink for sediments derived from the
Yaeyama Ridge and is isolated from the Ryukyu forearc
basins. Because there are no obvious submarine canyons
or channels providing sediment pathways across the
Yaeyama Ridge and forearc basins, the sediment flux to
the Ryukyu Trench floor from the Ryukyu Islands andYaeyama Ridge is expected to be small compared to thatfrom Taiwan. The southwestern Ryukyu Trench wedge
is about 0.8–1.0 km thick (1 s two-way travel time), as
estimated from seismic reflection profiles (Fig. 2b). The
braided channel along the trench axis indicates sedimenttransport by axial flows (Fig. 2a). Specifically, the southwestern Ryukyu Trench has received a large amount of
sediment from Taiwan since its uplift began about 8.0–
6.5 Ma (Lin et al. 2003; Sibuet and Hsu 2004). The

Hsiung et al. Progress in Earth and Planetary Science(2021) 8:2

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Fig. 1 Regional bathymetric map of the southwestern Ryukyu subduction zone between Taiwan and Iriomote Island. The southwestern Ryukyu
Trench (shaded orange) terminates at Gagua Ridge, which separates the Huatung and West Philippine Sea basins. Several large canyons (yellow
broken lines) incise the narrow slope off eastern Taiwan and extend into the Huatung Basin. CR, coastal range; HB, Hoping Basin; NB, Nanao Basin;
ENB, East Nanao Basin; HTB, Hateruma Basin; Ir, Iriomote Island; Is, Ishigaki Island; HLC, Hualien Canyon; TTC, Taitung Canyon; and Lu and La,
Lutao-Lanyu Ridge (i.e., the Luzon arc). The square indicates the area shown in Fig. 2a, and the red solid line the location of seismic reflection
profile line 89 (Fig. 2b)

geophysical aspects of the southwestern Ryukyu subduction zone, particularly its perpendicular link with the
Taiwan collision zone, have been intensively studied(e.g., Lallemand et al. 1997, 2013). In contrast, sedimentation along the southwestern Ryukyu Trench floor has
not been well studied. The southwestern Ryukyu Trenchis thus a suitable study area for examining distal turbidite transport and emplacement along subduction zones.

2 Samples and methods
2.1 Bathymetry, piston core, and radiography

Swath bathymetric data used in this study were collected
in 2015 during JAMSTEC cruises YK15-01 by R/V Yokosuka and KR15-18 by R/V Kairei (JAMSTEC 2015,2016). To better determine the course of the braided
axial channel, the high-resolution bathymetric map was

redrawn using GeomapApp (Fig. 2a). Cores were drilled at
6147 m water depth at 23° 02′ 31″ N, 123° 07′ 42″ E, behind the channel levee on the trench floor near the end of
the trench, during cruise KR15-18 (Fig. 2a). An 8 m-longpiston coring system with 7.8 cm-diameter core liners wasused to collect piston core PC04 (length 532 cm) andgravity pilot core PL04 (length 76.5 cm). Based on thecorer setup, the bottom of PL04 corresponds to < 20 cm
above the top of PC04. Both cores were split laterally onboard and physically described. X-radiographs were takenof slab samples of PC04 (1 cm thick and 20 cm long) toobserve detailed sedimentary structures (Fig. 3).
2.2 XRF core scanning analysis

The elemental compositions of the cores were loggedusing an ITRAX XRF Core Scanner (ITRAX hereafter)Hsiung et al. Progress in Earth and Planetary Science(2021) 8:2

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Fig. 2 a Detailed bathymetry of the southwestern Ryukyu Trench. Cores PL04 and PC04 (red circle) were drilled in the north levee of the axial
channel of the Ryukyu Trench floor. b Seismic reflection profile line 89 runs roughly N-S through the study area (red solid line in Fig. 1). Depth isrepresented as two-way travel time (TWT)at Kochi Core Center, Japan. The ITRAX X-ray beam
(0.2 × 20 mm) is used to irradiate (excite) samples togenerate radiographic images and energy dispersive
XRF spectra (Croudace et al. 2006; Löwemark et al.
2019). The ITRAX data are normally output as counts
and can be considered semiquantitative relative toelemental intensities. PC04 was scanned at a resolution of 0.10 cm, and the reported elemental ratios
were calculated as five-point running averages; PL04
was scanned at 0.05 cm resolution, and the reported
elemental ratios were calculated as ten-point runningaverages (Fig. 3).
2.3 Magnetic parameters

Samples for magnetic analyses were packed in 7 cm3plastic cubes at 2.2 cm intervals spanning the entire
cores and analyzed using a KLY-4 magnetic susceptibility meter (AGICO Inc.). A total of 145 cubes from

PC04 and 33 cubes from PL04 were measured at JAMSTEC Yokosuka headquarters, Japan. Magnetic hysteresis loops and direct-current demagnetizations were
measured by a vibrating sample magnetometer (VSM)at the Institute of Earth Sciences, Academia Sinica,
Taiwan, and the JAMSTEC Yokosuka headquarters, respectively. A total of 38 dry bulk samples from separate
layers at different core depths were selected and storedin capsules (ca. 0.2 g each) from PL04 and PC04. Hysteresis loops were measured on each capsule at an applied field of 0.5 T. Hysteresis parameters including thesaturation remanent magnetization (Mr), saturation
magnetization (Ms), coercivity (Hc), and coercivity of
remanence (Hcr) were calculated. Magnetic parameters,
including magnetic susceptibility (MS), the minimum
principal axis of the anisotropy of magnetic inclination(K3), Hc, Hcr, Mr, and Ms, are plotted against core
depth (Fig. 4).

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Fig. 3 Core photographs, radiographs, lithological logs, and Itrax geochemical data (Ca, Fe, Zr, Rb, Ca/Fe, and Zr/Rb) of pilot core PL04 and piston
core PC04. Sedimentary unit 1, 0–62.0 cm depth in PL04; sedimentary unit 2, 62.0–76.5 cm depth in PL04 and 0–55.0 cm depth in PC04;
sedimentary unit 3, below 55.0 cm depth in PC04. Core PC04 shows basal flow-in below 322.0 cm depth

A total of 30 discrete samples (ca. 0.2 g each) weredried and stored in capsules and analyzed with aquantum design superconducting quantum interference
vibrating sample magnetometer (SQUID-VSM) at theInstitute of Earth Sciences, Academia Sinica. SQUIDVSM analyses provide rapid identification of pyrrhotite
in bulk samples on the basis of its low-temperature magnetic transition at 34 K. To detect low-temperature magnetic transitions in the core samples, their remanent

magnetizations were measured during cooling from
300 to 5 K in a zero field in the SQUID-VSM. At 5 K,
a 5 T DC field was momentarily applied to impart asaturation isothermal remanent magnetization (Hsiunget al. 2017). The SQUID-VSM patterns and the Day
plot (Day et al. 1977) of hysteresis parameters arepresented in Fig. 5. The presence/absence of pyrrhotite with depth in the core is marked by closed/open
diamonds in Fig. 4.

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Fig. 4 Magnetic hysteresis parameters (Hc, Hcr, Mr, and Ms) from vibrating sample magnetometry (VSM), and magnetic susceptibility (MS) and K3
inclination (K3) measured using a KLY-4 magnetic susceptibility meter. Solid diamonds in the MS column indicate the presence of both pyrrhotite(transition at 34 K) and magnetite (119 K) as determined from SQUID-VSM (see Fig. 5a, b); open diamonds indicate magnetite only (see Fig. 5c, d).
Peaks in the MS profile that correlate with identified turbidites are labeled. Increasing disturbance of PC04 by coring is noticeable below
262 cm depth

2.4 Sediment particle observations and grain size analysis

Regarding the preservation of the core surface, we choseturbidite T25 as a representative of the pyrrhotitebearing turbidites (i.e., T25, T27, and T33). To determine grain size variations in coarse-grained layers, weselected turbidite T25 (a ~ 3 cm-thick silty sand layer)and ~ 1 cm-thick sections of clay sediments directly

above and below it for grain size analyses by aMorphologi-G3 particle characterization system (M-G3)
at JAMSTEC Yokosuka Headquarters, Japan.
The sample set included 10 specimens (at 0.5 cm intervals, ~ 13 mm3 each) from 207.0 to 212.0 cm depth in
PC04, which were prepared and dried in a freeze-dryer for
dispersion analyses. We used the software of the M-G3 toHsiung et al. Progress in Earth and Planetary Science(2021) 8:2

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Fig. 5 SQUID-VSM magnetic susceptibility analyses of bulk marine sediments from a, b 209.5 and c, d 66.5 cm depth in PC04 during a, c coolingand b, d warming over temperatures from 5 K to room temperature. Both heating and cooling rates were 3 K/min. The magnetic susceptibility
scale (emu) on the left of each plot is reported as ΔM/ΔT on the right. Peaks at 119 K and 34 K indicate the presence of magnetite (mt) andpyrrhotite (po), respectively, in the bulk sediments. Pyrrhotite peaks are observed throughout PL04 and in three turbidite beds in PC04. e Day plot
(Day et al. 1977) of hysteresis magnetization and coercivity parameter ratios. The clastic mud beds and silty-sand laminae samples plot in thepseudo-single domain (PSD) and multi-domain (MD) fields, indicating that most of the magnetic particles are uniformly fine-grained

count particles and scan particle images. The M-G3 wasoperated in its automatic standard mode (Malvern Instruments Ltd. 2017; Becker et al. 2018). The M-G3 can detect
dry particles with circle-equivalent diameters (CE) of
0.54–1000 μm (Malvern Instruments Ltd. 2017). Because
most of the sediments are fine particles, the × 20 lens was
selected for particle morphological analyses. We analyzed
more than 200,000 particles from each sample to achieve
statistically representative populations. After particle analyses, we calculated statistical parameters including aspectratio (the ratio of width to height) and circularity (thecircumference of a circle of equivalent area divided by theactual perimeter). The grain size scale and classification
scheme are based on Blott and Pye (2012).
2.5 Radiocarbon dating and organic geochemistry

Eight horizons were selected from PC04 to determine
radiocarbon ages from the dried bulk sediments. Radiocarbon dating was performed by accelerator mass spectrometry at Beta Analytic Co., Ltd. Each dried sample(~ 10 mg) of finely powdered sediment was washed with
1 N HCl at 80 °C to remove carbonates. Eighteen further

Hsiung et al. Progress in Earth and Planetary Science(2021) 8:2

horizons were selected from specific mud and silty sandbeds to analyze total organic carbon (TOC, wt.%), total
nitrogen (TN, wt.%), and the stable carbon isotopic composition of TOC (δ13C, ‰ vs VPDB). Determinations of
TOC, TN, and δ13C were performed using a highsensitivity elemental analyzer-isotope ratio mass spectrometer (EA-IRMS) comprising a modified elemental
analyzer (Flash EA1112), a continuous flow interface(ConFloIII), and an isotope ratio mass spectrometer(Delta plus XP IRMS; all from Thermo Finnigan,
Bremen, Germany) as described in Ogawa et al.
(2010). Each sediment sample was treated with 0.1 M
HCl prior to EA-IRMS analysis to remove carbonates. C and N abundances and C isotopic compositions were calibrated against the interlaboratoryreference material L-tyrosine (BG-T; N = 7.7 wt.%, C= 59.7 wt.%, δ13C = –20.83 ± 0.13‰), the δ13C valueof which was determined based on multiple international standards (Tayasu et al. 2011). The quantitative and analytical errors on TOC, TN, and δ13C
were ± 0.25 μg C (1.4–9.6 μg C), ± 0.026 μg N (0.18–
1.2 μg N), and ± 0.25‰ (1σ, n = 8, 1.4–9.6 μg C), respectively. The TOC/TN ratio was calculated as theatomic ratio, and propagated errors ranged from ±0.29 to ± 0.83.

3 Results
3.1 Sedimentary units determined by core description
and ITRAX elemental variations

Cores PL04 and PC04 are mainly composed of homogeneous gray mud layers intercalated with coarser layers ofsilty sand. The coarse-grained layers are olive-black incolor and mostly < 2 cm thick. These coarse-grained
layers have sharp basal contacts, and some have crosslaminations and faint parallel laminations. Most coarsegrained layers are recognizable by their coarser grain size
and upward-fining trend. Most of their basal contacts
are easily identifiable by a change in grain size, whereassome of their upper contacts are indistinct. No foraminifera or fragments thereof were identified in either core.
Based on deep-sea turbidite classification criteria (e.g.,
Shanmugam 2000; Patton et al. 2015) and deep-water
sediment facies (Stow and Shanmugam 1980; Stow andSmillie 2020), the silty sand beds are interpreted as turbidites and the mud layers as hemipelagites.
We were unable to obtain ITRAX profiles at 0.0–27.9
cm, 83.1–91.3 cm, and 163.9–169.1 cm depth in PC04
because of the soupy sediments. Variations of the elemental intensities were carefully compared with depth in
the cores (Fig. 3). Ca (Fig. 3) and Sr intensities (notshown) are positively correlated with turbidite occurrence, and Zr is weakly correlated with Ca and Sr. Fe,
Rb (Fig. 3), Ti, and K (not shown), on the other hand,are negatively correlated with turbidite occurrence. In

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hemipelagites, Ca, Fe, and Rb intensities show minor
fluctuations, whereas K and Ti show greater fluctuations. Other elements such as Si and Al did not show
any significant correlation with these intercalated
layers. Previous studies on marine sediments havedemonstrated that the Ca/Fe ratio is a useful proxy
for terrigenous components in sediments, and Zr/Rb
is often positively correlated with mean grain size
(Croudace et al. 2006; Rothwell and Croudace 2015).
Therefore, we selected Ca, Fe, Zr, and Rb as target elements to determine the boundaries of sedimentary
units and coarse-grained layers (Fig. 3). The sedimentary units in cores PL04 and PC04 were determined mainly based on visual lithological descriptions
and Ca variations.
We distinguished four major sedimentary units. Sedimentary unit 1, found only in PL04, extends from the
top to 62.0 cm depth in the core and consists of subparallel beds of hemipelagites interbedded with thin turbidite layers (Fig. 3). Thin turbidite layers (< 1 cm) laminate
within the hemipelagites. The average Ca/Fe and Zr/Rb
ratios in this unit are about 0.06 and 0.90, respectively(Fig. 3).
Sedimentary unit 2 is characterized by mounded orcontorted interbeds in radiographs at 62.0–76.5 cm
depth in PL04 and 0.0–55.0 cm depth in PC04. This unit
contains more silt layers interbedded with the hemipelagites than unit 1, and the silt layers are in olive-graycolor. The exact boundary at 55.0 cm depth in PC04 wasdetermined based on the radiographs and ITRAX profiles (Fig. 3). Sedimentary structures include crosslaminations, faint parallel laminations, and several tubular burrows. One sand layer at ~ 8–11 cm depth in PC04
(no available XRF data) can be correlated with a MS
peak at ~ 8 cm depth in the core (turbidite T0 in Fig. 4).
In this unit, Ca/Fe varies from 0.02 to 0.10 and Zr/Rbfrom 0.60 to 1.40 (Fig. 3).
Sedimentary unit 3 extends from 55.0 to 322.0 cm depth
in PC04 and consists of laminated fine-grained sands intercalated with gray hemipelagites. Bioturbations are observed at 55–75 cm depth in the core. Thirty-six coarsegrained layers were determined as turbidites, hereafter
T1–T36. Turbidite thicknesses range from ~ 0.9 to 4.2
cm. All 36 turbidites are correlated with Ca/Fe peaks. Of
these, 21 are correlated with Zr/Rb peaks. Baseline Ca/Fe
ratios in hemipelagites are ~ 0.02, and peak Ca/Fe values
range from 0.05 to 0.18 in turbidites. Baseline Zr/Rb ratios
in hemipelagites are ~ 0.70, and peak Zr/Rb values range
from 0.90–1.59 in turbidites (Fig. 3). Magnetic susceptibility in unit 3 is less consistent with turbidites than in units
1 and 2: 6 of 7 MS peaks can be correlated with certain
turbidites in unit 3 (Fig. 4). Disturbance of the sediments
by the coring of PC04 becomes more pronounced below
262 cm depth in the core (Fig. 4).

Hsiung et al. Progress in Earth and Planetary Science(2021) 8:2

Sedimentary unit 4 extends from 322.0 to 532.0 cm
depth in core PC04 and contains exclusively gray mud.
Sediments in this unit show marked vertical streaking incontrast to the horizontal layers observed throughout
the rest of the section, indicating substantial disturbanceduring coring. Elemental profiles are consistently uniform, and Ca/Fe and Zr/Rb ratios average ~ 0.02 and ~0.80, respectively.
3.2 Magnetic parameters

Analytical results of magnetic parameters comprise (1)K3 and MS, (2) the hysteresis loop from VSM measurements (Hc, Hcr, Mr, and Ms), and (3) SQUID-VSM
measurements (Fig. 4). The first two sets of magnetic
measurements were performed throughout the cores
PL04 and PC04. After the detailed contacts between turbidites and hemipelagites had been determined, 30 samples from separate layers of the two sediment types (10turbidites and 20 hemipelagites) were analyzed by
SQUID-VSM.
K3 directions are roughly the same in undisturbed sediments, and gradually change below 262 cm depth in
PC04 (i.e., in sediments disturbed by coring). No obvious
MS peaks are observed in PL04, and eight are observedin separate sections of PC04 (Fig. 4). Hysteresis loop results show subtle variations. In the Day plot (the ratio of
Mr/Ms plotted against the ratio of Hcr/Hc; Day et al.
1977; Dunlop 2002), the samples plot in the pseudosingle-domain (PSD) and multi-domain (MD) fields (Fig.
5e), indicating the fine grain size and uniformity of thecores. Hc and Hcr decrease gradually from the top to
the bottom of the cored sections, and there are no obvious variations of Mr and Ms with depth in the cores(Fig. 4).
In SQUID-VSM measurements, low-temperature magnetic transitions at 34 K indicate the presence of monoclinic pyrrhotite in our samples (Fig. 5a, b; Rochette
1987; Dekkers et al. 1989), and those at 119 K indicate
the presence of magnetite or magnetic clay minerals(Fig. 5c, d; Abrahams and Calhoun 1953). Pyrrhotite +magnetite signatures (solid diamonds, Fig. 4) are ubiquitous from the top of PL04 to 9.2 cm depth in PC04 (i.e.,unit 1 and upper unit 2). Below 9.2 cm depth in PC04,
pyrrhotite + magnetite signatures occur only in turbidites T25, T27, and T33, at 209.5 cm, 234.0 cm, and261.0 cm depth in PC04, respectively (Fig. 4). These
three turbidites are also correlated with MS peaks. Magnetite signatures (i.e., in the absence of a pyrrhotite signature) occur in hemipelagites and other turbidites(open diamonds, Fig. 4).
3.3 Turbidite observation and grain size analyses

To further understand the characteristics of turbidites
with high MS and pyrrhotite signatures, ten specimens

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from Turbidite T25 (207.0–212.0 cm depth in PC04)
were selected for grain size analyses based on the visualcore description and Ca/Fe and Zr/Rb ratios. Dispersed
sediment particles from these ten samples were counted
under a microscope with a × 20 objective lens (Fig. 6).
The sample from 209.75 cm depth in the core, a representative example of a turbidite (Fig. 6a, b), shows finesand-sized grains and translucent minerals. The sample
from 207.25 cm depth is representative of hemipelagites(Fig. 6c, d), which are markedly finer. More than 200,000 particles were detected and photographed from eachsample. Particle sizes mostly range from 0.54 to 63.0 μm
in CE diameter. The volume percentages of sand, silt,
and clay in these samples are reported in Table 1 alongside mean circularities and aspect ratios. The average
circularity in this sample set is 0.704, and the average aspect ratio is 0.653. The volume percentage of sand (>63 μm) only exceeds ~ 50% in the samples from 209.25
and 209.75 cm depth in the core, which are identified assilty sand.
3.4 Radiocarbon dating, δ13C, TOC, and TN

We performed radiocarbon dating and organic geochemistry analyses to compare turbidites and hemipelagites in core PC04, as identified based on the Ca/Fe and
Zr/Rb proxies. The eight determined radiocarbon ages
range from 17,680 to 26,710 14C yr BP in conventionalage (Fig. 7, Table 2), including one turbidite (T0, 9.2 cm
depth in PC04, 25,980 14C yr BP). The ages of the sevenanalyzed hemipelagites are not consistent with theirstratigraphic depths; nonetheless, the youngest (17,680
14
C yr BP, 195.0 cm depth in the core) is used to constrain the regional age model (Fig. 7). In PC04, we selected ten turbidites (T0, T4, T7, T9, T11, T20, T25,
T27, T33, and T36; red triangles in Fig. 7) and eighthemipelagites (black circles in Fig. 7) for δ13C, TOC, andTN analyses. These 18 samples show δ13C values from− 27.3‰ to − 21.8‰, TOC concentrations from 0.18 to
0.43 wt.%, and TN concentrations from 0.037 to 0.088
wt.%. TOC/TN ratios range from 4.64 to 6.75.

4 Discussion
4.1 Thin-bedded turbidites on the trench floor

Grain size variations in fine-grained sediments can bepredicted based on multi-elemental XRF analyses(Bertrand et al. 2015; Liu et al. 2019). However, thesetrends are difficult to observe in very thin-bedded
turbidites (1–3 cm thick). Because most of the 36
identified turbidites in PC04 are less than 2 cm thick,we selected turbidite T25 (2.9 cm thick with a MS
peak and magnetic pyrrhotite signatures) for grain
size analysis (Fig. 8). Intensity variations of Ca, Fe,Zr, and Rb, and particularly Ca/Fe and Zr/Rb, effectively trace sedimentary facies changes in PC04. Ca

Hsiung et al. Progress in Earth and Planetary Science(2021) 8:2

Page 10 of 17

Fig. 6 Characteristic particles in samples from a, b 209.75 cm depth (turbidite T25, silty sand) and c, d 207.25 cm depth in PC04 (silt) observed
using Morphologi-G3. Mean grain sizes and sand percentages of each 0.5 cm-thick sample are shown in Fig. 8c and Table 1

variations mainly indicate Ca content, and negative Fe
peaks may indicate increased water content (Fig. 3).
The decreased Fe, a common phenomenon in upper
XRF-scan profiles or organic-rich sediments, is likely
due to the closed-sum effect of increased Ca (e.g.,
Löwemark et al. 2011).
Deep-sea sediments usually contain both terrigenous
and biogenic components. The water depth at the studysite (6147 m) is below the carbonate compensation depth,
which may play an important role in determining the biogenic carbonate content of the sediments. Ca intensities
show that the carbonate contents of turbidites are consistently higher than those of hemipelagites. Furthermore, Ca

and Sr intensities co-vary in PC04. Because Sr is fixedalong with Ca by calcifying organisms, it is used in marine
environments as a marker of sediments of strictly biogenic
origin (Zaragosi et al. 2006). Therefore, the co-variation of
Ca and Sr implies that Ca is mainly sourced from biogenic
carbonates, with a minor contribution from terrigenous
sources. The lack of biogenic material (e.g., foraminifera
or fragments thereof) in both cores indicates that the seafloor in the southwestern Ryukyu Trench is below the carbonate compensation depth.
Compared to other elements, Ca and Fe variations
show relatively small fluctuations in hemipelagites. Ca
and Fe intensities are negatively correlated, and Ca/Fe

Table 1 Grain size analysis of turbidite T25 in core PC04
Core depth (cm)

Diameter (μm)

Mean circularity

Mean aspect ratio

Sand (%)

Silt (%)

Clay (%)

Category

207.25

11.68

0.708

0.649

15.28

84.69

0.03

Silt

207.75

9.57

0.739

0.656

9.16

90.77

0.07

Silt

208.25

11.51

0.701

0.653

13.32

86.65

0.03

Silt

208.75

16.76

0.693

0.666

29.51

70.48

0.01

Sandy silt

209.25

31.72

0.670

0.636

65.57

34.42

0.00

Silty sand

209.75

29.12

0.699

0.641

59.02

40.98

0.00

Silty sand210.25

14.83

0.679

0.637

46.60

53.39

0.01

Sandy silt

210.75

12.72

0.738

0.668

33.54

66.43

0.02

Sandy silt

211.25

12.48

0.704

0.668

21.01

78.97

0.02

Silt

211.75

15.06

0.710

0.656

20.15

79.84

0.01

Silt

Hsiung et al. Progress in Earth and Planetary Science(2021) 8:2

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Fig. 7 Geochemical results from core PC04, including radiocarbon ages, stable carbon isotopic compositions (δ13C), total organic carbon contents(TOC), and total nitrogen contents (TN). Eight radiocarbon ages were determined from bulk sediments at locations marked by red arrows to theleft of the section. Ten samples from turbidite beds (red triangles) and eight from hemipelagic sediments (black circles) in specific layers wereselected for δ13C, TOC, and TN analyses

peaks mark very thin-bedded turbidites in sedimentary
unit 3 (Fig. 3). Except for eight peaks, MS variations are
small in both cores (Fig. 4), indicating a purely depositional environment without hydrothermal or geothermal
effects. Six MS peaks occur at the depths of turbidite
layers showing extremely low Fe intensities (Fig. 4),
which can be explained by the closed-sum effect with
the Ca intensity.
Zr mainly occurs in zircon, which is usually enriched incoarse sediment fractions and indicates the presence of
detrital grains. Considering Zr intensities alone, the peaks
correlated with turbidites are not very sharp. However,
Zr/Rb provides a useful grain size proxy (Dypvik andHarris 2001) because Zr is present mainly in coarser
grains and Rb in clays. Negative Rb peaks are sharp andclear (Fig. 3), indicating decreased clay percentages in theturbidites (Rothwell and Croudace 2015). Therefore, theobserved Zr/Rb peaks indicate increased abundances ofcoarse grains, and not necessarily terrigenous sediments.
Although Ca/Fe peaks are strongly correlated with Zr/Rb
peaks (correlation coefficient r = 0.80) in PC04, 15 of thevery thin-bedded turbidites (< 2 cm thick, Ca/Fe < 0.1)
show only Ca/Fe peaks; the grain size variations in theseturbidites may be so subtle that Zr/Rb variations are inconspicuous. This indicates that co-occurring peaks of
Ca/Fe and Zr/Rb reflect coarser grained turbidites,
whereas Zr/Rb peaks are not observed in muddier
turbidites.
Sedimentary units 1–3 show slight differences in the
Zr/Rb vs. Ca/Fe plot (Fig. 9a, b). Elemental ratios of unit
2 show wider distributions than those of unit 1 (Fig. 9a).
Sedimentary unit 3 is separated into turbidite beds (redtriangles) and hemipelagites (gray dots; Fig. 9b), demonstrating the homogeneity of elemental ratios in unit 3
hemipelagites. The identification of turbidite peaks basedon Ca/Fe and Zr/Rb in units 1 and 2 is difficult becauseof the strong variations in Ca content (Fig. 3); therefore,turbidites in these upper sections were mainly determined based on visual core descriptions and radiographs. The basal contact of each turbidite in unit 3 iseasily identified as a change to coarser grain size basedon Ca/Fe and Zr/Rb. When plotted with symbol size

Hsiung et al. Progress in Earth and Planetary Science(2021) 8:2

Page 12 of 17

Table 2 Radiocarbon ages from core PC04 compared to previous age results from the Ryukyu ArcCore

Depth (cm)Conventional radiocarbon age

Water depth (m)Material/sediment type

Reference

KR1518-PC04

7.0–11.525980 ± 100

6147

Bulk/turbidite

This study

29.0–31.0

26710 ± 110

RN93-PC11a

RN93-PC12a
GH08-2004a

Bulk/hemipelagite

72.0–74.0

20500 ± 70

Bulk/hemipelagite

106.0–108.0

18600 ± 60

Bulk/hemipelagite

145.0–147.0

21990 ± 80

Bulk/hemipelagite

194.0–196.0

17680 ± 60

Bulk/hemipelagite

242.0–244.0

21940 ± 80

Bulk/hemipelagite

292.0–294.0

24860 ± 100

Bulk/hemipelagite

16.0–18.01410 ± 70

156.0–158.0

13280 ± 110

28.0–30.0

3360 ± 80

136.0–138.0

13420 ± 110

19.3–21.4

3530 ± 40

23.6–25.7

4540 ± 170

2540

Foraminifera

Ujiié and Ujiié (1999)

Foraminifera
2160

Foraminifera

Ujiié and Ujiié (1999)

Foraminifera
1166

Foraminifera

Kubota et al. (2015)

Foraminifera

40.5–42.7

6450 ± 40

Foraminifera

43.0

7300

Ash

67.3–69.5

8265 ± 100

Foraminifera

80.7–82.9

9320 ± 40

Foraminifera

127.6–129.8

11660 ± 50

Foraminifera

134.3–136.0

12265 ± 110

Foraminifera

142.9–145.2

12940 ± 70

Foraminifera

172.8–175.1

14955 ± 115

Foraminifera

225.6–227.9

18780 ± 100

Foraminiferaa

Ryukyu Arc core

proportional to turbidite thickness (0.9–4.2 cm), mostof the thicker turbidites (> 2 cm) plot above the general turbidite regression line and have smaller Zr/Rb
peak values (Fig. 9c). Generally, thicker turbidites
show larger Ca/Fe and Zr/Rb ratios. Turbidite T25(~ 2.9 cm thick) has the highest Ca/Fe value (red circle, Fig. 9c), and turbidite T27 (the thickest observed, 4.2 cm thick; green circle, Fig. 9c) has one of
the highest Zr/Rb ratios. Both T25 and T27 show
MS peaks and magnetic pyrrhotite signatures.

Fig. 8 Detailed analytical results for turbidite T25 (207.0–212.0 cm depth in PC04): a photograph, b X-ray fluorescence image, c grain size analysis(mean grain diameter, percentage sand, and grain size distribution) of 0.5 cm-thick samples, d Ca/Fe ratios, e Zr/Rb ratios, and f Ca/Fe vs. Zr/Rb.
Raw data (thick gray line curve) and 5-point running averages (black dots) are shown in d and e. Upward-coarsening then upward-fining trends
from A to D in T25 show good correlation with the Ca/Fe and Zr/Rb variations. The highest Ca/Fe (C) and Zr/Rb values (B) are slightly offset,
which can be observed as a loop in (f)Hsiung et al. Progress in Earth and Planetary Science(2021) 8:2

Page 13 of 17

Fig. 9 Plots of Ca/Fe vs. Zr/Rb for a sedimentary units 1 (black circles) and 2 (blue squares), b hemipelagites (gray circles) and turbidites ofsedimentary unit 3 (red triangles), and c 36 individual turbidites (maximum values). Regressions for sedimentary units 1 and 2 are shown as blackand blue lines, respectively, in a. Symbol sizes in c represent the thickness of each turbidite bed. Turbidite T25 (red circle, 2.9 cm thick) has thehighest Ca/Fe value of all analyzed turbidite beds. Turbidite T27 (green circle) is the thickest observed turbidite bed (4.2 cm). Regressions forturbidites and hemipelagites are shown as red and gray lines, respectively, in c

Our interpretation of Ca/Fe ratios differs from that for
shallow marine sediments. For example, the Ca/Fe ratio
is usually enhanced in coarse-grained, carbonateenriched materials (Fralick and Kronberg 1997; Marshet al. 2007). Lehu et al. (2015) reported that Ca concentrations are higher in some turbidite layers and lower inothers. Therefore, because shallow marine sediments
may comprise both biological and non-biological Cabearing materials, Ca/Fe may not be applicable as an indicator of turbidites in shallow marine environments.
Ca/Fe is useful as an indicator of turbidites in PC04
because the Ca-bearing components of the sediments
mainly comprise biological materials. Even the lack offoraminifera or fragments, the co-variation of Ca and Sr
intensities imply that biogenic source is dominated in
PC04 rather than terrigenous. TOC contents are

relatively low in both turbidites and hemipelagites ofPC04 (Fig. 10) compared to those of shallow marine sediments. The PC04 turbidite is characterized by relatively
low TOC content, high TC content, and high Ca intensity. It can be expected that the turbidites enriched in
terrigenous components tend to dilute the TOC content.
Therefore, Ca/Fe should only be applied as an indicatorof turbidites under consideration of the carbonate
sources.
4.2 The presence of pyrrhotite in turbidites

Seven of the identified turbidites (T0, T5, T7, T9, T25,
T27, and T33) coincide with MS peaks (Fig. 4), three of
which (T25, T27, and T33) also coincide with magnetic
pyrrhotite signatures. Because they are interbedded with
pyrrhotite-absent turbidites, these three pyrrhotite-

Fig. 10 Organic geochemistry results from hemipelagites (gray dots) and turbidites (red triangles) are grouped based on their stable carbon
isotopic compositions (δ13C) and total organic carbon (TOC) and nitrogen (TN) contents. a TN vs. TOC, b δ13C vs. TOC/TN, and c δ13C vs. TOC/TN
shown at a broader scale and interpreted in terms of particulate organic carbon (POC) provenance after Lamb et al. (2006). C3 and C4 plants
represent the fields of terrestrial plants

Hsiung et al. Progress in Earth and Planetary Science(2021) 8:2

bearing turbidites must have been generated by three individual long-runout turbidity currents and likely transported sediments from Taiwan to the Ryukyu Trench
floor via submarine canyons (Horng et al. 2012; see next
subsection). These pyrrhotite-bearing (thus Taiwansourced) turbidites represent a small fraction of the sediments but not the percentage in these cores.
Regarding the sample preservation of T25, T27, andT33, we chose pyrrhotite-bearing turbidite T25 as a representative sample for detailed grain size analyses (Fig. 8).
The mean grain diameter and sand percentage curves
show maxima at 209.25–209.75 cm depth in the core, andthe circularity, aspect ratio, and grain size distribution define a well-sorted layer at 209.25–210.25 cm depth (Fig.
8c, Table 1). The grain size trend of T25 also coincides
with peaks in the Ca/Fe and Zr/Rb profiles: the highest
Ca/Fe and Zr/Rb values within T25 are labeled C and B,
respectively, in Fig. 8. The plot of Ca/Fe vs. Zr/Rb shows
the upward-fining section as a tilted loop from A to D
(Fig. 8f), implying the gradational nature of the bed andthe asymmetry of the Ca/Fe and Zr/Rb peaks. We notethat the peak grain size occurs above the peaks in elemental ratios (B and C in Fig. 8d and e), which might be dueto the sampling interval: grain size analyses were performed at 0.5 cm intervals, a much coarser resolution thanthe ITRAX profile (0.10 cm intervals). Nonetheless, thegrain size distribution, circularities, and aspect ratios show
an abrupt change from 210.75 to 210.25 cm depth, andthis abrupt change at ~ 211.0 cm depth is better correlated
to the peaks in elemental ratios (Fig. 8c–e). In fine-grained
sediments, homogeneous particles in well-sorted turbidites might slightly enhance ITRAX peaks. Therefore, the
ITRAX peaks in this depth interval are mainly correlatedwith grain size, and ITRAX peaks may generally be enhanced in well-sorted turbidites.
Figure 8f elaborates the upward-coarsening then upwardfining trend of turbidite T25, as well as the offset between
the maximum grain size and elemental ratios. Indeed, theslight 0.2 cm offset of the Ca/Fe and Zr/Rb peaks manifests
as a counterclockwise loop in the Ca/Fe vs. Zr/Rb plot.
Similar slight offsets are observed in the ITRAX records ofother turbidites thicker than 2 cm in PC04. Thisphenomenon should be further explored at adjacent sites.
In summary, our analyses of turbidite T25 demonstrate that Zr/Rb peaks indicate increased abundances ofcoarse particles. High-resolution ITRAX data are usefulfor identifying very thin-bedded turbidites and sedimentary facies, especially in fine-grained trench sediments(e.g., Pérez et al. 2019).

Page 14 of 17

predominantly magnetite and maghemitized magnetitewith minor amounts of hematite (Kawamura et al.
2008), which is not the case in southwestern Ryukyu
Trench floor sediments (Hsiung et al. 2017). In this region, the presence of pyrrhotite with a magnetic signature strongly indicates that some portion of thesediments was sourced from metamorphic rocks of the
Taiwan orogenic belt (Horng et al. 2012). Pyrrhotitebearing turbidites T25, T27, and T33 can thus beregarded as direct evidence that long-runout turbidity
currents can feasibly transport Taiwan orogenic sediments to the Ryukyu Trench floor. In a holistic view, intense erosion of the uplifted Taiwan orogen due to theongoing arc-continent collision between the Luzon Arc
and the Asian continent produces a large amount of sediments that are transported seaward (Dadson et al.
2005). Indeed, sediments derived from the Taiwan orogenic belt are transported to the slope off eastern
Taiwan and the Huatung Basin through major submarine canyons such as the Hualien, Chimei, and Taitung
Canyons (e.g., Lehu et al. 2015). Furthermore, thecanyon network connecting the Hualien Basin to the
Ryukyu Trench is an effective pathway delivering terrestrial sediments from Taiwan to the southwestern Ryukyu
Trench (Hsiung et al. 2017).
The continuous magnetic pyrrhotite signatures observed in hemipelagites of PL04 and the uppermost partof PC04 (Fig. 4) may indicate the ongoing contributionof terrestrial sediments transported from Taiwan Island(Fig. 2b). Considering globally decelerating sea-level riseduring the Holocene (Stanley and Warne 1994) and the
mid-Holocene warm period roughly 6000 years ago, thecontinuous pyrrhotite signatures observed in the upper
part of PC04 (Fig. 4) may speculatively be linked to
Holocene climate change. Another interpretation is thatthe volume of orogenic sediments shed from Taiwan islarge and easily distributed to the southwestern Ryukyu
Trench. However, the simple presence or absence of
pyrrhotite cannot be used to quantify sediment contributions, and the sampling distribution and small sample
size (~ 0.2 g) in this study may not be of sufficient resolution to capture the pyrrhotite signatures of thinnerturbidites. Although this study area may also receive
sediments from the Ryukyu island arc and forearc regions, the absence of thick turbidite sequences and ash
layers in PL04/PC04 implies a distal sedimentary environment far from the Ryukyu Islands to the north, andthus a dominant terrestrial source (Taiwan) to the west.
4.4 Age model and relict organic carbon

4.3 Source-to-sink: from the Taiwan orogenic belt to theoceanic trench

Previous investigations have shown that the magnetic
carriers in central Ryukyu Trench sediments are

We obtained non-chronological radiocarbon ages fromthe bulk core sediments, implying that those sediments
contain some amounts of relict organic carbon. Basedon the age of 25,980 14C yr BP obtained for turbidite T0

Hsiung et al. Progress in Earth and Planetary Science(2021) 8:2(7–11.5 cm depth in PC04), such relatively old ages may
be affected by relict organic carbon (Table 2). Such carbon recycling along the seafloor requires that turbidity
currents entrain organic carbon from bedload sediments
during transport. Based on the youngest age obtained incore PC04 (17,680 14C yr BP at 194.0–196.0 cm depth,
Table 2), the sedimentation rate in the southwestern
Ryukyu Trench can be roughly constrained to ~ 1.1 mm/
year. Radiocarbon ages measured from foraminifera in
the nearby Ryukyu Arc indicate sedimentation rates of~ 1.2 mm/year (Kubota et al. 2015) and 1.0–1.2 mm/year(Ujiié and Ujiié 1999). The similarity of the sedimentation rates in the southwestern Ryukyu Trench and the
Ryukyu Arc indicates that bulk radiocarbon ages areminimally disturbed by relict organic carbon. The water
depth and lack of foraminifera in cores PL04 and PC04
indicate that the seafloor in the southwestern Ryukyu
Trench is below the carbonate compensation depth.
Although the radiocarbon ages obtained for the trench
sediments are varied, sedimentation rates are strongly affected by sediment source. For example, the sedimentation rate variability from ~ 0.01 to ~ 2.96 mm/year(Ikehara et al. 2018) has been documented in the Japan
Trench (> 7500 m water depth). Because there are no
major canyon systems along most parts of the Japan
Trench, high primary (diatom) productivity in the Sanriku area provides the dominant influx of fresh biogenic
particles there, determining the high sedimentation
rates. Despite the relatively old age model for PC04, thehigh frequency of very thin-bedded turbidites interbedded with hemipelagites likely contributes to the observed
deposition rate.
4.5 Organic geochemistry of turbidites and hemipelagites
on the Ryukyu Trench floor

Organic geochemistry analyses (δ13C, TOC, and TN) areuseful for determining terrestrial sediment sources indeep-sea deposits, especially in this semi-isolated depositional environment (Lamb et al. 2006). For example,
bulk organic δ13C values and TOC/TN ratios have beenused to reveal sediments sources from estuarine environments (Yu et al. 2010). TOC/TN ratios are often used todifferentiate marine from terrestrial organic matter because higher TOC/TN ratios reflect larger contributionsof lignin phenols from terrestrial environments (Perdueand Koprivnjak 2007; Fernandes et al. 2011). As turbidites may contain sediments from both marine and terrestrial sources, we attempted to discriminate turbidites
from hemipelagites based on organic geochemical analyses of bulk sediments from eight hemipelagites and ten
turbidites (Fig. 7). The ten turbidite samples have TOC
contents of 0.18–0.36%, TN contents of 0.037–0.074%,and δ13C values of − 27.3 to − 21.8‰ (Figs. 7 and 10).
Compared to these turbidites, the hemipelagites are

Page 15 of 17

characterized by higher average TOC (0.39%) and TN
contents (0.081%) and slightly heavier average δ13C
values (− 22.7‰). Both the high TOC contents and low
TOC/TN ratios of the homogeneous hemipelagites in
PC04 show a differentiation from the turbidites. Indeed,
the low Ca intensities observed in the hemipelagites ofPC04 (Fig. 4) indicate that most organic carbon resides
in hemipelagites rather than turbidites. The turbidite
containing a small amount of terrigenous component
tend to dilute the TOC content. The TOC contents, TN
contents, TOC/TN values, and δ13C values of Taiwaneserocks, sediments, and rivers are ~ 0.42%, ~ 0.07%, 5.8–
6.5, and about − 25‰, respectively (Kao and Liu 2000;
Kao et al. 2014). Therefore, PC04 turbidites show awider range of values than Taiwan-sourced sediments.

5 Conclusions
This study presented very thin-bedded turbidites intercalated with hemipelagites in cores PL04 and PC04, obtained
at 6147 m water depth on the Ryukyu Trench floor. We
subdivided these cores into four sedimentary units basedon visual core descriptions and variations of Ca intensities
in ITRAX scans. Two sediment types, hemipelagites andturbidites, were identified mainly based on ITRAX Ca/Feand Zr/Rb ratios. The contacts of 36 turbidites (0.9–4.2 cm
thick) were determined based on Ca/Fe peaks. Three ofthese turbidites also presented MS peaks and magnetic signatures of pyrrhotite, which we interpret as evidence oflong-range sediment transport from Taiwan to the Ryukyu
Trench floor by long-runout turbidity currents travelingthrough submarine canyons. Detailed grain size analyses ofa relatively thick turbidite showed good correlation between
elemental ratios (Ca/Fe and Zr/Rb) and the upwardcoarsening and upward-fining units that delimit the bottom
and top of turbidites, respectively. These results suggestthat both Ca/Fe and Zr/Rb can be used to identify distal
turbidites (about 1–3 cm thick), with Zr/Rb ratios mainlyreflecting grain size changes in deep-sea sediments below
the carbonate compensation depth. Some very thin-bedded
turbidites (< 2 cm thick, Ca/Fe < 0.1) were identified basedon their Ca/Fe value, but we did not observe coincident Zr/
Rb peaks; grain size variations in such thin turbidites may
be so subtle that Zr/Rb variations are inconspicuous. We
observed pyrrhotite magnetic signatures in hemipelagites
from the top of PL04 to 9.2 cm depth in PC04, indicatingthat Taiwan-sourced sediments may have been consistently
dispersed as far as the southwestern Ryukyu Trench overthe last several thousand years. Sedimentary organic matter
was characterized (δ13C, TOC, and TN) to further discriminate turbidites from hemipelagites. The results concluded
that hemipelagites are characterized by low Ca intensities,
high TOC and TN contents, and heavy δ13C values,
whereas turbidites show high Ca intensities and peak Ca/Fe
values.

(2021) 8:2

Page 16 of 17

Abbreviations
14
C yr BP: Radiocarbon years before the present; JAMSTEC: Japan Agency for
Marine-Earth Science and Technology; R/V: Research vessel; SQUIDVSM: Superconducting quantum interference vibrating sample
magnetometer; XRF: X-ray fluorescence; TOC: Total organic carbon; TN: Total
nitrogen

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Hsiung et al. Progress in Earth and Planetary Science

Acknowledgements
We thank the anonymous reviewers for their careful reading of ourmanuscript and their constructive comments. We gratefully recognize theefforts of Captain Takafumi Aoki, the crew of R/V Kairei, and the marine
technicians of Marine Work Japan Ltd. during the KR15-18 survey. The ITRAX
measurements were performed as part of the cooperative research programof the Center for Advanced Marine Core Research, Kochi University, Japan(No. 16A036 and 16B032).
Authors’ contributions
KHH, TK, KI, and KU worked together during the cruises and collaborated toconstruct this manuscript. KHH proposed the topic and conceived anddesigned the study. CSH helped perform SQUID-VSM analyses. N Ohkouchi
and N Ogawa helped analyze and interpret the δ13C, TOC, and TN data. SS
and MM assisted with ITRAX analyses. All authors read and approved thefinal manuscript.
Funding
Cruises YK15-01 and KR15-18 were supported by the research project for
Compound Disaster Mitigation on the Great Earthquakes and Tsunamis
Around the Nankai Trough Region of the Japanese Ministry of Education,
Culture, Sports, Science, and Technology, Japan. This core analysis was
supported by the Japan Agency for Marine-Earth Science and Technology,
Japan.
Availability of data and materials
The datasets supporting the conclusions of this article are available in the
JAMSTEC repository.
Competing interests
The authors declare that they have no competing interest.
Author details
1
Yokosuka Headquarters, Japan Agency for Marine-Earth Science andTechnology, 2-15, Natsushima-cho, Yokosuka-city, Kanagawa 237-0061, Japan.
2
Geological Survey of Japan, National Institute of Advanced Industrial
Science and Technology, Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567,
Japan. 3Safety and Environment Analysis Unit, Energy Consulting
Department, Japan NUS Co., Ltd., Nishi-Shinjuku Prime Square 5F, 7-5-25
Nishi-Shinjuku, Shinjuku-Ku, Tokyo 160-0023, Japan. 4Institute of Earth
Sciences, Academia Sinica, 128 Academia Road, Section 2, Nankang, Taipei
11529, Taiwan. 5Faculty of Agriculture and Marine Science, Kochi University,
200 Monobe, Nankoku-shi, Kochi 783-8502, Japan.
Received: 17 February 2020 Accepted: 2 December 2020

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