Two distinct types of turbidity currents observed in the Manila Trench, South China Sea

Abstract

Sediment gravity flows are the most direct and efficient transport mechanisms for moving terrestrial sediments into deep oceans. Scarcity of firsthand measurements, however, has hindered the quantitative, even qualitative characterization of such flows. Here we present a unique year-long data record from ~4000 m depth in the Manila Trench that captured two very different gravity flows in terms of their hydraulic and sedimentary properties. The first flow was of slow speed (~40 cm s⁻¹) and long duration (~150 h), thus nicknamed ‘Tortoises’, and carried very fine sediment with low concentration (~0.01%). The fast (~150 cm s⁻¹) but short-lived (~40 h) flow, nicknamed ‘Hares’, carried much coarser sediment with higher concentration (~1.2%). Clay mineral compositions suggest that the ‘Tortoises’ originated from upstream canyon wall slumping, whereas the ‘Hares’ was likely submarine canyons southwest of Taiwan Island due to typhoon. Grain size is a key factor in determining evolution of turbidity currents.

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ARTICLE
Two distinct types of turbidity currents observed in
the Manila Trench, South China Sea
Meng Liu1, Zhiwen Wang2, Kaiqi Yu1& Jingping Xu 1,3 ✉
Sediment gravity flows are the most direct and efficient transport mechanisms for moving
terrestrial sediments into deep oceans. Scarcity of firsthand measurements, however, has
hindered the quantitative, even qualitative characterization of such flows. Here we present a
unique year-long data record from ~4000 m depth in the Manila Trench that captured two
very different gravity flows in terms of their hydraulic and sedimentary properties. The first
flow was of slow speed (~40 cm s−1) and long duration (~150 h), thus nicknamed ‘Tortoises’,
and carried very fine sediment with low concentration (~0.01%). The fast (~150 cm s−1) but
short-lived (~40 h) flow, nicknamed ‘Hares’, carried much coarser sediment with higher
concentration (~1.2%). Clay mineral compositions suggest that the ‘Tortoises’originated from
upstream canyon wall slumping, whereas the ‘Hares’was likely submarine canyons south-
west of Taiwan Island due to typhoon. Grain size is a key factor in determining evolution of
turbidity currents.
https://doi.org/10.1038/s43247-023-00776-8 OPEN
1Department of Ocean Science and Engineering, Southern University of Science and Technology, 1088 Xueyuan Rd., Shenzhen 518055 Guangdong, China.
2National Marine Environmental Monitoring Center, 42 Linghe Str., 116023 Dalian, China. 3Southern Marine Science and Engineering Guangdong Laboratory
(Guangzhou), 523936 Guangzhou, China. ✉email: xujp@sustech.edu.cn
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Oceanic turbidity currents are sediment-laden, gravity-
driven, turbulence-supporting underflows primarily
active at river delta fronts and in submarine canyons1–3.
They are known to transport large quantities of terrigenous
materials (e.g., sediments and organic carbon) into the deep4,5.
Turbidity currents’power and unpredictability make them diffi-
cult to measure directly in the field. Their flow properties, e.g.,
speed, thickness, sediment concentration, grain size etc., are most
commonly either evaluated from laboratory experiments6,7or
inferred from turbidite deposits at outcrop or in the deep sea8.In
general, turbidity currents can be placed broadly into two dif-
ferent categories: surge-like flows with peak velocity waxing and
then waning rapidly, and a prolonged quasi-steady flow that, as
its name suggests, continues for much longer time. Both types of
flows were well investigated in laboratory flumes9,10 or numerical
modelling11,12, but it is not until the Acoustic Doppler Current
Profiler (ADCP) became available, detailed hydraulic and sedi-
mentary properties of field scale turbidity currents were directly
measured13. The Monterey Submarine Canyon and Congo Sub-
marine Canyon, two locations with very different river discharge
and sediment sources, produced probably the most complete field
data for the surge-like and prolonged quasi-steady flows14,15. Yet,
it remains a difficult task to make a direct comparison study of
the two flows because of the different context within which these
two types of flows have been observed. This paper presents results
from a field investigation where both types of flows were directly
measured on the same subsurface mooring in the Manila Trench.
Here, we describe the detailed measurements and characteristics
of the two types of flows. We then discuss the influence of tidal
currents and grain size of particles on the evolution of the tur-
bidity currents, infer their provenances and triggers, and calculate
the sediment flux into the deep part of the Manila Trench. Lastly
we describe methods of flow measurement and sediment sam-
pling in the field, and subsequent analysis.
The Manila Trench. Located in the northeastern part of the
South China Sea, the Manila Trench, is tectonically part of the
subduction system where the Sunda plate in Eurasia goes beneath
the Luzon volcanic island arc16,17. The trench begins at the
confluence of three submarine canyons (Gaoping, Penghu, and
South Taiwan Shoal) and continues to the south for more than
350 kilometers, with a maximum water depth of nearly 5400 m
(Fig. 1). Until recently, direct measurements of turbidity current
in the Manila Trench are still very limited, even though existing
geologic records, such as sediment waves around the trench18–20
and core logs in the trench21, have revealed the extensive and
frequent occurrence of gravity-induced flows. Heavy rainfalls,
brought by frequent typhoons22, and the high weathering and
denudation rate generates huge flux of freshwater and sediments
from Taiwan’s rivers into the ocean23. Liu et al. 24 discovered that
hyperpycnal plumes in the Gaoping Canyon could ignite turbidity
currents, which were later found to correlate well with typhoons
passing through the region several times per year25. Also, fre-
quent earthquakes of various magnitude exacerbate the occur-
rence of landslides, terrestrial or submarine, that (1) markedly
increase the sediment supply into the Manila Trench, and (2)
directly triggers turbidity currents into the Manila Trench. A
series of subsea cable breakage in the Gaoping Canyon and
Manila Trench also documented rapid (5–16 m s-1, particularly
5–8 m s-1 in the Manila Trench), and long-runout (>300 km),
sediment-laden flows associated with earthquakes and
typhoons26–28. None of the above studies, however, was designed
to investigate the detailed hydraulic and sedimentary properties
such as flow structures and grain size distribution inside the
turbidity currents.
The Manila Trench observation program started in September
2019 when four subsurface moorings (S1–S4) were deployed
along the trench (Fig. 1a). All moorings were recovered in August
2020. After all the data were downloaded and new batteries
installed, two of those four moorings (S1 and S2) were redeployed
at their original locations respectively. The redeployed moorings
had the same configurations as before except that one turbidity
sensor was added on each mooring at 72 MAB. These two
moorings were scheduled for recovery in early August of 2021 but
were delayed for a week till mid-August because of Tropical
Storm Lupit. S2 was successfully recovered, but S1 encountered
technical problems and remained on the seafloor. No turbidity
current signal was found in any sensors of the four moorings
during the 2020 deployment, probably because no typhoons made
landfall in Taiwan in this period, and there were no earthquakes
within the Gaoping drainage. Consequently, here we only present
the data from the S2 mooring of the 2021 deployment. The S2
mooring, at 3808 m water depth, recorded data for nearly
12 months.
Results
Characteristics of the turbidity currents observed in the Manila
Trench. Two turbidity current events, occurred in April and
August respectively, and can be clearly identified in the field data
(Supplementary Fig. 1). The turbidity current in April 2021
(hereafter E1, Fig. 2) began at 23:10 on March 31 (Beijing Time,
UTC +8) and lasted for about 6.3 days (150 h). Notably, all
measured parameters clearly show that E1 pulsated several times
in tidal frequencies: the ebb tidal phase (flowing down trench)
correlated well with increasing current velocity, water tempera-
ture, and the thickness of turbidity current (Fig. 2d). The max-
imum speed of E1, 44.5 cm s−1, occurred during the second pulse,
i.e., 9 h after the flow’s arrival at the mooring (Fig. 2a). The
acoustic backscatter intensity (a proxy of turbidity, Fig. 2b) and
the measured turbidity from the Seaguard recording current
meter system (RCM) at 12 MAB (Fig. 2c) correlated well with the
pulsating velocities. However, the turbidity measurements from
the turbidity sensor (RBR-TU) at 72 MAB only gradually
increased as E1 phased out (Fig. 2c), suggesting a long-time lag
before sediment particles from the turbidity current diffused
upward to the RBR-TU sensor. The temperature slowly increased,
also modulated by tide, by as much as 0.08 °C, against a back-
ground fluctuation of ~0.02 °C (Supplementary Fig. 1e), before
returning to the pre-event value at the end of E1 (Fig. 2e).
The turbidity current in August 2021 (hereafter E2, Fig. 3)
began at 14:12 on August 10 and lasted for 1.67 days (40 h).
Pressure data measured by the Conductivity–Temperature–Depth
recorder (CTD) at 17 MAB jumped instantaneously when E2
arrived (Fig. 3a), indicating that the mooring, at least the section
below the sediment trap (see Fig. 1d), was severely tilted by the
flow. The tilt angle reached the maximum value of 43° more than
2 h later (at 16:51) when the pressure perturbation was about 4.6
dbar (Fig. 3a). After the peak of the flow passed, the mooring
remained tilted for nearly 8 h. Even after the mooring returned
upright, the stabilized pressure was 0.3 dbar greater than the pre-
E2 value, suggesting that the mooring might have been dragged to
a slightly deeper location by the E2 flow. The instantaneous
increase of the along-trench velocity (Fig. 3b) and backscatter
intensity (Fig. 3c) from the ADCP marked the arrival of E2 at
14:12 August 10. While the backscatter intensity reached its
maximum value at the beginning of the flow, the along-trench
velocity did not. Rather, it stayed at an elevated speed of
20–60 cm s−1within a thin layer near the canyon seafloor for
more than 40 min before it rose dramatically to a maximum
speed of 145.1 cm s−1at 15:06 (Fig. 3b). Surge-type turbidity
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Fig. 1 Mooring locations in the Manila Trench. a Bathymetric map of the northeast South China Sea showing the mooring locations. The white dashed
lines indicate the Manila Trench and three submarine canyons upstream. The red stars denote the mooring locations (S1–S4). The area inside the yellow
rectangle is zoomed in (b). The green dots are the tracks of Tropical Storm Lupit happened in August 2021, and the corresponding wind speeds are shown.
The magenta dots show the earthquake source centers before occurrence of the turbidity currents. bMulti-beam bathymetric measurements showing the
seafloor topography covering the mooring site S2 (indicated by the red star). The blue square indicates the box sampling site X02. And the black line
indicates the location of cross section of (c). cCross-trench section of the thalweg channel at the mooring sites S2 (the red star). dSchematic diagram of
the mooring. The depth, height in meters above the seabed (MAB), and the corresponding instruments are indicated. The detailed information of
instruments is introduced in materials and methods.
Fig. 2 Plots of the time-series measurements for the turbidity current E1 at the mooring site S2. a Along-trench current velocity (cm s−1) and (b) net
acoustic backscatter intensity (counts) (averaged over four beams) recorded by the downward-looking ADCP. The influence of water attenuation and
spherical spreading had been corrected. The area under the black dashed line was affected by the side lobe interference. cTime-series of turbidity
measured by the RBR-TU at 72 MAB (blue line) and by the turbidity sensor of RCM at 12 MAB (orange line). dAlong-trench current velocity (cm s−1)
measured by the RCM at 12 MAB (orange line), along-trench current velocity at 12 MAB recorded by the downward-looking ADCP (dark blue line), and
predicted along-trench tidal current velocity (black line). eTime-series of temperature measured by CTD at 17 MAB (blue line) and by the sensor of RCM
at 12 MAB (orange line). The red dashed lines indicate the beginning and end of the turbidity current.
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currents typically have their maximum speed at the head or the
beginning of a Eulerian time-series data6,29, thus the nearly
40 min of low velocity stage is rather unusual (see Discussion
section). It is also worth noting that the small zone of low acoustic
backscatter intensity inside the body of E2 from 15:00 to 19:00 on
August 10 (Figs. 3c and 4c). This is probably caused by the high
suspended sediment concentration above this zone which
hindered the downward-looking ADCP’s acoustic
penetration30,31. Only the RBR-TU at 72 MAB recorded turbidity
during E2 because the RCM ran out of battery. Similar to what is
seen during E1, there is a long lag for the E2 turbid plume to
reach the sensor at 72 MAB (Fig. 3d). The measured maximum
Fig. 3 Plots of the time-series measurements of the turbidity current E2 at the mooring site S2. a Pressure perturbation (dbar) measured by CTD at 17
MAB. bAlong-trench current velocity (cm s−1) and (c) net acoustic backscatter intensity (counts) (averaged over four beams) recorded by the downward-
looking ADCP. dTime-series of turbidity measured by the RBR-TU at 72 MAB. eAlong-trench current velocity (cm s−1) at 12 MAB (dark blues line)
recorded by the downward-looking ADCP and predicted along-trench tidal current velocity (black line). fTime-series of temperature measured by CTD at
17 MAB. The red dashed lines indicate the beginning and end of the turbidity current.
Fig. 4 Sediment concentration profiles of the turbidity currents E1 and E2 at the mooring site S2. a Profiles of the inverted SSC for E1. The area under the
black dashed line was affected by the side lobe interference. The yellow line indicates the local tidal oscillation (positive value is ebb phase). bPlots of the
inverted SSC at 7, 12, 22, 42 and 62 MAB for E1, and the calculated SSC from RBR-TU at 72 MAB and RCM-TU at 12 MAB. Different colors represent the
SSC in different heights those are indicated by white dashed lines in (a). cInverted SSC profiles for E1. dPlots of the inverted SSC at 7, 12, 22, 42 and 62
MAB for E2, and the calculated SSC from RBR-TU at 72 MAB. Different colors represent the SSC in different heights those are indicated by white dashed
lines in (c). Only the SSC after E1 and E2 happening was shown.
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turbidity in E2 (1333.2 FTU), however, is one order of magnitude
greater than in E1. The sudden change in temperature around
14:57 also clearly marked the E2’s arrival (Fig. 3f). The
temperature fluctuated around 0.03 °C above pre-E2 normal for
nearly 4 days, long after the along-trench velocity had
diminished.
Suspended sediment concentration of the turbidity currents.
Suspended sediment concentrations (SSC) were estimated from
optical (turbidity sensor) and acoustic (ADCP’s backscatter
intensity) data (see Methods section). The good correlation
coefficients (>0.7) between the optically and acoustically con-
verted SSC (Supplementary Fig. 2) suggest that these converted
SSC are generally reliable even though the acoustic conversions
are more dependent on particle sizes than the optical ones. E1’s
maximum SSC (~0.01% or 0.26 g l−1) was recorded at the lowest
possible ADCP bin at 7MAB when the flow first arrived (Fig. 4a,
b). Sensors at different heights ranging from 7 to 72 MAB
recorded the sudden jump of turbidity almost simultaneously
(Fig. 4b), suggesting that a thick, albeit slow, turbid plume
engulfed the whole mooring. Close to the bed (indicated by
ADCP at 7 MAB and RCM at 12 MAB; Fig. 4b), the SSC was
relatively stable over the initial several days from April 1 to April
4—varying around 0.10 g l−1roughly in a frequency of local tidal
oscillation. This variation with tide is more apparent in the upper
part of the flow (Figs. 2,4b). In contrast, E2’s SSC is much
greater: the recorded maximum concentration at 7 MAB is ~1.2%
(31.0 g l−1), nearly 130 times larger than that of E1. Secondly, the
fast but thinner flow arrived at the lower part of the mooring (7
MAB) nearly 4 h before the whole mooring was submerged by the
turbidity current (72 MAB, Figs. 3,4). The front of the flow
arrived at 15:00 on August 10, well corroborated by the SSC
(Fig. 4c, d) and the measured maximum velocity (Fig. 3b, e). The
near-bed maximum concentration decayed rapidly over 10 h and
then became relatively stable hovering around 1 g l−1for the next
3 days. Because of the poor signal-to-noise ratios mainly due to
the side-lobe effect of the downward-looking ADCP, is 7 MAB.
Thus the true SSC in the bottom 7 meters of the flow could be
higher than the maximum SSC used in the above statistics.
Properties of sediment particles collected inside the turbidity
currents. Both sediment traps collected sediment particles but the
volume in the trap at 15 MAB is much greater than the one at 73
MAB (Fig. 5): the dry weights are respectively 1704 g and 32 g. In
comparisons, the two sediment traps on a similar mooring placed
at the same site for 1 year during a previous deployment
(2019–2020) collected 6.1 g (15 MAB) and 7.4 g (73 MAB) of
sediment. Given the fact that no turbidity current occurred
during the 2019–2020 deployment, the large amount of sediment
in the two traps during the 2020–2021 deployment can be
interpreted as the result of the two turbidity currents in that
period. It is also reasonable to designate the discontinuity in the
trap sediment as the boundary between the two flows (Fig. 5b):
accumulation below the unconformity resulted from E1 and
accumulation above the discontinuity resulted from E2. Knowing
the duration of the two flows and the respective dry weights of
sediment collected during the two flows, the deposition rates
during the two flows could be estimated: 312 and 5351 g m−2d−1.
These were respectively 0.97 × 103and 1.7 × 104times greater
than the average deposition rate of the previous deployment
(2019–2020) when no turbidity current occurred. The sediment
flux of E1 and E2 are 0.039 Mt d−1(millionmetric tons per day)
and 5.1 Mt d−1, respectively. Although the transport rate of E2 is
nearly 130 times than E1, considering the long duration of E1, the
gross amount transported by E2 (8.5 Mt) is only 35 times than E1.
And the sediment load of E1 and E2 accounts for about 34.3% of
the yearly average sediment transport by turbidity currents in
Gaoping Canyon (25.5 Mt, though the actual turbidity flux should
be higher due to a deficiency of basal observations)25, and 17.8%
of the yearly average sediment flux of Gaoping River (49 Mt)32.
In addition to the volume difference between the sediment
accumulations from the two flows, other properties of the
sediment particles are also visually different. Firstly, the sediment
from E1, about 6 cm thick, has a yellowish-brown color. This is in
strong contrast to the greyish-black color for the sediment from
E2 that is about 25 cm thick. When subsampled, at 1 cm interval
(From the bottom up, these are Layer #1-#31; Fig. 5b), for grain
size and mineral analyses, the size differences between the
sediment grains from the two deposits are very obvious (Note
that because the interface between the two deposits in the trap
was tilted, layers #7 and #8 each contains sediment particles from
both flows. Each sample was manually partitioned to E1 and
E2 sediment particles for grain size analysis. Layers #1–#8 from
E1 are composed of nearly 90% of silt and <5% of sand. The
highest clay content in the middle of the E1 deposit reached
nearly 10%. The median/mean grain size of the major portion of
the E1 deposit (layers #1–#6) is about 25 μm (Fig. 5c), but the fine
portion of layers #7 and #8 are much finer, with a median size of
only 9 μm. Because this is very close to the grain size of (1)
samples from the upper sediment trap at 73 MAB, (2) trap sample
from previous year when no turbidity current occurred, and (3)
bed surface sample from outside of the trench (Fig. 5c), it
probably represents normal pelagic settling rather than turbidity
current E1.
Layers #7–#28 are composed of much greater proportion of
sands and thus considered to be from turbidity current E2. The
first five E2 layers (#7-#12) contain nearly 50% of sand (Fig. 5d).
Above these very coarse layers the content of sand gradually
decreases (and the content of silt gradually increases). Overall, the
median and mean grain size of layers #7–#28 decrease from
approximately 65 to 30 μm, displaying the distinct normal
grading of grain size distribution that is common in turbidite.
Layers #29–#31 are similar to the finer portion of layers #7–#8,
with clay content of >10%, representing typical pelagic particles.
Other particle parameters such as sorting (Fig. 5c) reflect the two
very different settling process between E1 and E2. The much
higher sorting of E1 and the very top portion of the E2 deposit
resulted from fine and quite uniform size of particles settling in a
weaker hydrodynamic environment than that in the beginning of
E2. In contrast, the much coarser grains with poorer sorting
indicate rapid deposition indicative of more energetic
gravity flows.
Clay minerals in sediments collected at 15 MAB were mainly
composed of illite and chlorite, accounting for more than 90%
(Fig. 5e). The kaolinite content in E1 sediment (5–9%) are
consistently higher than in E2 sediment (less than 5%). Smectite
was only present in the bottom three quarters of E1 sediment
(layers #1–#4), comprising 5–9% of total clay minerals, strongly
suggesting that E1 came from a different source than E2. The
averages of δ13C values of particulate organic carbon were
−24.6‰in E1, slightly more depleted than the average δ13C
values of Trap-73, Trap-2020 and Substrate (around −23.5‰)
(Fig. 5f). The sudden drop of δ13C values at the beginning of E2
(from −24.0‰to −25.5‰indicate a more terrestrial source of
the sediment particles.
Discussion
Contrasting flow structures. A striking difference between the
two flows is their arrival at the mooring, as clearly demonstrated
in Fig. 4, where the first 4 days of converted sediment
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concentration profiles, in both backscatter intensity maps and line
plots of SSC at several heights above bed, are plotted on an equal
time interval. Turbidity current E1 arrived at sensors of all heights
ranging from 7 to 72 MAB, almost synchronously, with merely
15 min delay to reach the highest sensor. This flow resembled a
thick plume of a powder snow avalanche of low density and slow
speed, that engulfed the mooring instantaneously. Turbidity
current E2 resembled a flash flood in a stream with rising water
level; It took 50 min to increase in thickness by 15 meters (from 7
to 22 MAB), 44 min to grow additional 20 meters (from 22 to
42 MAB), and another 136 min to overtop the sensor at 72 MAB.
In another word, the head of E2 had traveled more than 20 km
further down trench by the time the top of the flow finally
reached highest sensor (72 MAB) on the mooring. These above
contrasting behaviors could have resulted from different degrees
of mixing with the ambient water in the flow front of E1 and
E233–37.
Some turbidity currents with high velocity is more likely to
have a fast-moving head with a dense basal layer whose SSC is
high (generally > 10%), but how high is still an open
question14,37–39. Given its weak velocity and low SSC, turbidity
current E1 certainly does not fit the above characteristics. Instead,
E1 is likely a fully dilute turbulence-supported flow without a
powerful basal layer. As for turbidity current E2, whose highest
concentration at 7 MAB was ~1.2% (lower than 10%), it is
qualitatively similar to flows that contains a dense near-bed dense
layer38,39. The SSC of this layer, assuming a thickness of few
meters14, probably be much >1.2%, a value obtained from 7
MAB. In addition, both the multi-beam bathymetric measure-
ments (Fig. 6a) and the ADCP beam’s acoustic footprints (Fig. 6b,
c) indicated that the mooring was on the east wall of the trench,
~500 m away from and 25 m above the trench thalweg (Fig. 1c).
We argue that, for the first ~40 min, between the time when
ADCP beam 1 and beam 4 first detected high acoustic intensity
signals at 14:12 and the time when significant increase of flow
speed occurred (Fig. 6, Supplementary Fig. 5), the fast but thin
(<25 m) turbidity current E2 was restricted at the bottom of the
trench thalweg. The SSC of this flow front may thus have been
significantly greater.
Sediment sources and possible triggers. Clay mineralogy is often
used as an indicator of sediment provenance or origin40,41.At
least two distinct patterns can be drawn from the clay mineral
composition of the sediment trap samples (Fig. 5e): (1) Illite and
chlorite are abundant throughout the trap samples, including
deposit from both turbidity currents E1 and E2; (2) Smectite only
exists in sediment deposit from E1 where content of kaolinite is
also higher. While illite and chlorite are the two most abundant
clay minerals in the South China Sea region42, smectite is believed
to be uniquely sourced by the volcanic rocks in the Luzon arc43.
In general, chemically-weathered smectite is discharged from the
Luzon rivers and transported northward by the NW Luzon
Coastal Current, then westward mainly via mesoscale eddies
produced by the westward South China Sea Branch of the Kur-
oshio and Taiwan Warm Current32,44.
The presence of smectite in the mostly fine particles of
E1 sediment, along with E1’s weak flow velocity, strongly suggest
that this turbidity current is likely to have originated from trench
wall slumping analogous to the dilute flow monitored in
Monterey Canyon caused by a similar mechanism45. Collapse
of mud drapes on the trench walls upstream of the mooring site
could create dilute sediment clouds that move down the trench as
gravity flows. Lack of momentum, due low density and small
gradients, would allow the currents to be modulated by tides
(Fig. 7a, details in next section). However, what triggered such a
slump or slumps is still unclear—there was no record of either
storms or earthquakes in the region prior to the turbidity current
E1. The exact location of the slump is also unknown.
In contrast, turbidity current E2 is characterized by its larger
velocities and coarse sediments. In addition, the absence of
Fig. 5 Properties of sediments collected from the 2020 deployment, the 2021 deployment and the substrate at the mooring site S2. a The gray-scale
values of 3–7 cm of the sediment at 15 MAB are indicated by the blue line. bThe gray scale maps are computed-tomography (CT) images of sediment
trapped from E1 to E2. The coarser the grain size, the lighter is the color of the CT images. The blue circled numbers denote the positions of cross-section
photos. cMedian and mean grain size and sorting of the collected sediment. dThe content of clay, silt and sand, (e) proportion of the clay minerals, and (f)
the content of δ13C relative to PDB. In the legends, sediment properties from layers #7-#8’denotes the finer sediment samples of layers #7-#8 (hereafter
‘_F’). Layers #7-#8’located in the oblique interface between the deposits from the turbidity currents E1 and E2, containing the sediment from E1 to E2. The
‘Substrate’in legends indicates the substrate sediment sampled in 2020 at the site X02 (see the location in Fig. 1b). The ‘Trap-2020’and ‘Trap-73’indicate
the sediment trapped during 2019–2020 and at 73 MAB during 2020–2021, respectively.
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Fig. 6 The arrival process of the turbidity current E2 at the mooring site S2. a Multi-beam bathymetric measurements around the mooring site S2 from
Fig. 1b. bThe vertical distance from the transducer to the seafloor in different directions (−180°–180°) measured by an individual ADCP beam (Beam 3).
The center of the ADCP beam’s acoustic footprint locates on the right side of the trench floor. The red lines are the positions of four beams according to
the initial heading of ADCP during E2, which is indicated in (e). The black and dark blue lines are the major flow direction of Periods 1 and 2 indicated in (d).
cNet acoustic backscatter intensity (counts) from bin 50 (12 MAB) to 70 of four beam recorded by the downward-looking ADCP. The maximum intensity
values indicate the seabed position. The red dashed rectangle indicates the arrival time of the turbidity current E2. dCurrent velocity vectors at 12 MAB
measured by the down-looking ADCP at the initial stage of E2. The light yellow and green rectangles indicate Period 1 and Period 2, respectively. eThe
postures include the heading, pitch, and roll measured by the down-looking ADCP.
Fig. 7 The schematic diagrams of the turbidity currents E1 and E2 (not to scale). a The formation process of the turbidity current E1. The sediment from
mass wasting moves down the trench under gravity and is promoted by tidal currents. bThe flow thickness of turbidity current E2 is gradually thickening
when the front of E2 gets through the mooring site S2.
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smectite in the E2 sediment, and the more negative δ13C values
(Fig. 5f) of particulate organic carbon in E2 (−24.6‰) than in E1
(−24.0‰), both suggesting that E2’s sediment was more likely
from a fresh source with abundant terrestrial organic matter such
as Taiwan’s rivers46. Previous studies have shown that frequent
passage of tropical storms often bring very high precipitation to
Taiwan, and consequently dramatic increase of sediment
discharge into the South China Sea47,48. For instance, after the
typhoon Hagibis in June 2014, the Gaoping River’s discharge
increased, and a turbidity current signal was detected by a
mooring deployed in Gaoping Canyon25. When Typhoon
Soudelor passed through southern Taiwan in 2015, a turbidity
current was detected not only in Gaoping Canyon25 but also in
the Manila Trench28, and finally its front velocity reached
5.6 m s−1near the mooring site S2. Not only was the intensity
and track of tropical storm Lupit of August 2022 comparable to
Typhoon Hagibis of 2014, it also rapidly increased the water and
sediment discharge of the Gaoping River to the ocean, reaching
maximum values of 6980 m3s−1and 3.21 g l−1respectively
(Supplementary Note 1 and Supplementary Fig. 6). Thus, it is
reasonable to argue that turbidity current E2 resulted from the
enhanced discharge of terrestrial sediment from Taiwan rivers
into, first the several canyons SW of Taiwan, then the head of the
Manila Trench. Except the hyperpycnal flow, it also might be
helpful for triggering the turbidity current that the direct
sediment resuspension in the shallow shelf and transport to the
canyon head by typhoon-induced waves and currents49.
In summary, sediment slumping from the trench wall or
upstream canyon triggered by earthquakes or sediment instability
is one major source of turbidity currents. A second source, likely a
more important one, is typhoon-generated flows from Gaoping or
other submarine canyons southwest of Taiwan Island, that are fed
by frequent landslides, terrestrial floods and combinations of
storm waves and currents during the annual typhoon season.
Tidal modulation of weak turbidity current E1. For a weak
turbidity current such as E1 whose speed is in the same order of
magnitude of the local tidal current, its flow structure as well as
the resulting deposit are inevitably modulated by the internal tide.
The velocity and flow thickness of the turbidity current E1
exhibited periodic pulsation resembling tidal oscillations that
happened to be in a transition from semidiurnal to diurnal
(Fig. 2a, d). Model predictions showed that the local tide was at its
neap phase when E1 arrived at the mooring, but the predicted
tidal current in the along canyon direction grew from ~5 cm s−1
at the beginning of E1 to ~12 cm s−1when E1 diminished several
days later. The flooding (ebbing) phase of tidal current acted like
a headwind (tailwind) that reduced (enhanced) the speed and
decreased (increased) the flow thickness of turbidity current E1.
This explains why E1’s peak velocity was recorded 9 h after the
initial arrival of the turbidity current. Despite the tidal modula-
tion, turbidity current E1 still moved suspended sediment
downcanyon. The footprint of E1, estimated by the cumulative
product of velocity and time, is about 140 km long. In another
words, the body of turbidity current E1, at its maximum, stret-
ched along the trench for 140 km.
E1 was inferred to originate from an upstream trench wall
slumping (i.e., mass wasting). These collapsed materials moved
down the trench wall by a gravity flow in direction nearly
orthogonal to the trench strike (Fig. 7a). After a head-on collision
with the trench bottom and opposing trench wall, substantial
sediment was suspended to form a turbid cloud. This cloud,
initially with low or zero initial along-trench momentum, slowly
moved down the trench, driven by gravity. The headwind
(tailwind) effect of tidal currents on turbidity current inevitably
changed the flow’s sediment carrying competence as well as
capacity. Researchers have used a Rouse-based criterion
(B ¼Ws=u) to determine whether a sediment particle of settling
velocity Ws, should stay in suspension or fall to the bed given a
shear velocity of u*50,51. A value of B ¼0:3 was also found to
provide a best fit to data for equilibrium open channel flows52.
Here, the grain size data of Layer #1-#6 sediment (Fig. 5c) were
used to calculate the average D
50
(median grain size), D
75
, and
D
90
. Their corresponding settling velocities were 0.028, 0.068, and
0.148 cm s−1respectively. These settling velocities and the
computed shear velocity in the flow (see Material and Method
section for details) were used to obtain the Rouse-based criterion
B. In the first 5 days of the E1, the computed B values were always
smaller than 0.3 if D
50
was used, independent of tidal phases
(Fig. 8a). For D
75
and D
90
size particles, while the B values during
ebb tide (tailwind) were still <0.3, it became significantly greater
than the critical value (0.3) during peak flood tide (headwind).
For instance, the B values for D
90
and D
75
during the peak of
several flood tides reached 0.6 and 0.4 respectively. The 12 h
settling distances for the D
50
,D
75
, and D
90
particles are ~12, 29
and 64 m, respectively. the same order of magnitude as the
Fig. 8 Tidal modulation of weak turbidity current E1. a Variations of the Rouse-based criteria B computed from different grain sizes (D
50
,D
75
, and D
90
).
The orange line and blue line are the vertical maximum velocity (u
M
) and tidal current (positive value is ebb phase). bThe sketch map of tidal modulation of
weak turbidity current. The shadow indicates the depth-averaged flow thickness. The brown and black dots denote the fine and coarse particles,
respectively. cand dConceptual vertical velocity and sediment concentration profiles of E1 during the flood (red) and ebb (blue) phases.
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changes of flow thickness (10–60 m). In addition, the estimated
bulk Richardson number (Ri) increased during the flooding and
decreased during the ebb (Supplementary Fig. 7). It suggests that
the weak-turbulence current during flood was unable to support
the sediment particles in suspension53, so the larger particles,
though fewer and fewer in the upper part of the flow, would fall
toward the bottom and effectively reduce the thickness of the
turbidity current (Figs. 4a and 8b). During the next ebb tide, some
of these particles that had fallen into the lower part of the flow or
even onto the seafloor would be lifted/resuspended by the
enhanced turbulent shear resulted from the stronger flow velocity,
and the thickness of the turbidity current also increased (Figs. 4a
and 8c, d).
Evidence of tidal modulation was also found in the sediment
trap collection during E1. The alternating gray scale highs and
lows in the CT images of the sediment trap (layers #1–#8) are
believed to have resulted from differential deposition between
tide-enhanced and retarded phase of the turbidity current
(Fig. 5a). There are only 6 gray-scale couplets (borrowing a term
from classical tidal sedimentology), about half of the total number
of tidal cycles during the lifespan of E1. This perhaps indicates
that the differential sedimentation pattern is only discernable for
the tidal cycles in the early days of E1 when sediment
concentration is very likely greater.
‘Tortoises’or ‘Hares’determined by particle sizes. Table 1lists
the measured and computed parameters of the two turbidity
currents, E1 and E2. Two proverbial characters, Tortoises and
Hares, represent the two flow types well: slow, steady, and long-
lasting for the former; fast, rapid-decaying, and short-lived for the
latter.
As seen in the previous section, E1 was tidally modulated for
several days. The peak current velocity and SSC, both occurred at
each ebb tide, decreasing only slightly over that time (Fig. 2d and
Fig. 4b). The long duration of E1 was primarily owing to its slow
speed, but the stop-and-go movement caused by tidal modulation
probably also contributed. E1’sflow properties such as large
initial thickness (rapidly thickening and well mixed) and low SSC
(dilute) of fine sediment particles within each ebb tide episode are
similar to the “entirely slow, dilute, and well mixed”turbidity
currents observed in Bute Inlet37, except E1 lasted much longer
duration (~150 h vs 2 h), probably due to its much finer-grained
sediment particles (9–25 μm vs ~200 μm)54. Prolonged turbidity
currents such as E1 have also been reported in Congo Canyon55.
The 170 h lifespan of the Congo Canyon flow was believed to
have been assisted by a fast erosive zone at the front that
apparently caused flow stretching and the fine grains in the flow.
This flow stretching mechanism was not present in E1. There is
one property that is shared by these two long duration flows: very
fine particles in suspension inside the flows (9–25 μm vs 4.23 μm).
The fast, thin, and short-lived E2 is an entirely different animal
(metaphorically). When it arrived at the mooring, its flow speed
first accelerated fast but then decelerated exponentially. The same
flow structure as E2 were also observed in Monterey Canyon,
Bute Inlet and Whittard Canyon14,37,56, which were mostly
characterized by a fast, dense, thin, and stratified head. Compared
with these turbidity currents, E2’s slightly finer sediment
(10–65 μm vs ~200 μm) and lower SSC (1.2%) had probably
contributed to its longer duration (~40 h vs ~2 h).
Sediment grain sizes clearly played a key role in defining the
different characteristics of the two turbidity currents, conforming
with findings from other field observations29,55, laboratory
experiments57–59, and numerical simulations11,60. The interplay
between hydraulic shear and particle settling velocity governs
both the density structure and evolution of turbidity currents61.
Conclusions
Frequent occurrence of sediment gravity flows out of the Gaoping
River and into the Gaoping Canyon have been reported24,25, but
how often those flows reach the deeper water of the Manila
Trench is unclear. This field study has shown that two different
turbidity currents passed the mooring site at 3800 m water depth,
both in the second year of a 2-year investigation. Grain size and
clay mineral analyses indicated that turbidity current E2 in Aug.
2021, which was characterized by coarse sediment with high
content of illite and chlorite but no smectite and of flowing fast
and thin thus nicknamed ‘Hares’, was originated from the
Gaoping river/canyon system. Turbidity current E1 in April 2021,
containing fine sediment with smectite and flowing thick and
slow thus nicknamed ‘Tortoises’, had a very different source
which we propose to be the slumping of trench wall material.
Notably, tidal modulation in the deep trench had likely prolonged
the lifespan of this ‘Tortoises’flow. Those turbidity currents
increase our understanding of the transport of terrigenous
material to the deep waters. The rivers from Taiwan Island deliver
a large amount of sediment and organic carbon due to the unique
geographical and climatic conditions47. Turbidity currents
transport those material (including carbon) into the deep sea.
Applying the average organic carbon concentration of 0.44% for
the sediment carried by the hyperpycnal turbidity currents from
Taiwan Island24, the estimated organic carbon transport into the
deep sea by the turbidity currents in this study was roughly
3.85 × 104t, accounting for nearly 16% of the annual organic
carbon load in the Gaoping Canyon47,62. But carbon transport
and burial into the deep sea is more than just the strait shots by
those ‘Hares’flows to move terrestrial carbon. Rather, it’s more
like a cascading process in which ‘Tortoises’also play an
important role in transporting marine carbon, typically resulted
from marine particle settling, given that these flows have much
longer lifespan and finer material composition. The two modes of
transport form an effective route for the organic carbons of
continental margins, both autochthonous (marine) and alloch-
thonous (terrestrial), into the deep ocean for eventual burial. Not
only that, they have important implications for the transport of
pollutants (i.e., microplastics) and the warning of geological
hazards.
Methods
Field data collection. The S2 substrate mooring was at 3808 m water depth and
recorded data for a whole year (from 2020 to 2021). High-resolution multi-beam
bathymetric data that were collected during the cruise on August 2022 clearly
show the seafloortopographyaroundthemooringsiteS2,includingthelong-
itudinal seafloor slope of the canyon (0.4% or 0.23°), the width of the trench
thalweg (about 2.5 km), the average slope of canyon walls (left: 8% or 4.6°, right:
3% or 1.7°), and the depth of the channel thalweg (80 m) (Fig. 1b, c). A down-
looking Workhorse Marine 300 kHz acoustic Doppler current profile (ADCP)
was mounted at 65 MAB, to record a vertical profile of flow speed and direction
Table 1 Key parameters of the two turbidity currents.
Tortoises (E1) Hares (E2)
Peak velocity (cm s−1) ~50 ~150
Flow duration (hours) 150 40
Initial flow thickness (m) >30 ~10
Volume concentration (%) 0.01 1.20
Bulk Richardson number 0.2–0.3 >1
Median grain size (μm) 54.9 19.8
Mean clay content (%) 7.6 1.6
Sources Luzon Island Taiwan Island
Sediment transport rate (Mt d−1) 0.039 5.1
Sediment transport flux (Mt) 0.243 8.5
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every 60 s from an ensemble of five 12 s pings, with a spatial resolution (ADCP
bin size) of 1 m (Fig. 1d and Supplementary Table 1). A Seaguard recording
current meter system (RCM) placed at 12 MAB was set to record flow speed,
flow direction, water temperature, conductivity, and turbidity. An SBE 37-SM
Conductivity–Temperature–Depth recorder (CTD) was deployed at 17 MAB,
with a sampling interval of 1 min. Two Anderson-type sediment traps, each
composed of a fiberglass cone (90 cm in length, 26.8 cm in diameter) and a clear
acryliclineartubeinsideaPolyvinylChloride(PVC)sedimentprotection
cylinder (150 cm in length), were placed respectively at 15 and 73 MAB on the
mooring. Each sediment trap was equipped with an intervalometer (Timer) that
dispensed Teflon disks, as time marks, into the liner tube at a preset interval of
15 days. A turbidity sensor (RBR-TU) was deployed at 72 MAB, with a sampling
interval of 1 min. The accuracy of flow measurements for ADCP was 0.5% ± 1%
for RCM. The CTD’s precision was 0.0028 °C for temperature, 0.003 millisie-
mens (mS cm−1) for conductivity, and 0.1% of the full-scale range for pressure
(about 7 m for CTD used in the study). The seafloor sediment near the mooring
site S2 was sampled by box corer in 2020 compared to the sediment in the
sediment traps. All raw measurements were shown in Supplementary Fig. 1.
Typhoon and earthquake records (Supplementary Table 2) are from the
Typhoon Network of the Central Meteorological Observatory (NMCS, http://
typhoon.nmc.cn) and the China Earthquake Networks Center (CENC, https://
news.ceic.ac.cn), respectively. Bathymetry data of the South China Sea is from the
GEBCO Gridded Bathymetry Data (https://download.gebco.net/).
Vertical velocity structure of turbidity currents. The measurements of flow
velocities were rotated 159° clockwise to the along- and cross-trench components,
and the down-trench direction is positive (Supplementary Fig. 3). In a typical
velocity profile of the turbidity currents, the lower part below the velocity max-
imum is the wall region, where the velocity distribution exhibits a logarithmic
relationship known as the ‘law of the wall’, and the upper part above the velocity
maximum is referred to as the jet region, where the vertical distribution of velocity
is nearly Gaussian relation. The vertical velocity profiles of E1 and E2 were plotted
in Supplementary Fig. 4.
In addition, the normalized velocity profiles, using the [u/U, z/h] scheme where
the Uis the depth-averaged velocity, his the depth-averaged thickness of the flow, z
is the corresponding height of each layer velocity (u), were compared with other
field measurements63. The depth-averaged thickness (h) and velocity (U) were
calculated using the moment equations64. The vertical velocity distribution of a
turbidity current essentially represents the balance between the logarithmic
boundary layer and the Gaussian outer layer6,65.
Inversion of suspended sediment concentration. Applying the relationship
between backscatter voltage (V
rms
) and the SSC proposed by Thorne and
Hurther66, the structures of the SSC of the turbidity currents were inverted from
the acoustic backscatter intensity (E) measured by the 300 kHz ADCP:
SSC rðÞ¼ VrmsφrðÞr
KtKs

2
e4αωr
ðÞ ð1Þ
V
rms
is calculated by the acoustic backscatter intensity according to
Vrms ¼10KcE=20,K
c
is a measured constant for each of the transducers; φ(r) is a
correction for transducer’s near field, r is the distance from the measuring point to
the transducer; K
t
should be constant and describes the sensitivity of the individual
transducers; K
s
is the parameter of the scattering properties of sediment with mixed
mineralogy, which depends on the particle size, particle distribution characteristic
and backscattering of suspended particles. Here the properties of particles collected
in sediment trap at 15 MAB were used. α
ω
is the absorption of sound by the
properties of seawater, which depends on the measured temperature, salinity, depth
and the assumed pH of 8 during the turbidity currents E1 and E2. The SSC derived
by inversion of bin 50 (at 12 MAB) and bin 1 (the top layer at 62 MAB
approximately represented the SSC at 72 MAB) was compared to the correlated
SSC at 12 and 72 MAB during E1 and E2 (Supplementary Fig. 4), which was
calculated from the turbidity measured by RCM and RBR-TU with a correlation
relationship between the turbidity measured by RCM and in situ SSC from Zhang
et al:25,67. SSC (mg l−1)=0.96 × turbidity (FTU).
Notably, both the grain size and concentration influence the correlation
relationship between the measured turbidity and SSC68,69. Because the turbidity
recorded by RBR-TU and RCM are not exceeding 1500 FTU, the relationship
between the measured turbidity and SSC is still linear. Under the same turbidity,
the bigger particle size corresponds to a larger SSC. The particles at 72 MAB in E2
are finer than those trapped at 15 MAB in E2, but coarser that those at 72 MAB in
E1. The particles at 12 MAB in E1 also are coarser than those at 72 MAB in E1.
This linear relation was adopted in the transformation of the turbidity recorded by
RCM, which was deployed nearly 500 m vertically from the thalweg, and SSC
during the long-term monitoring of turbidity currents in the Gaoping Canyon. So,
the true SSC in the bottom of turbidity currents E1 and E2 may be underestimated.
Sediment analysis. Sediment samples collected in two sediment traps were pro-
cessed following the protocol for typical sediment cores24,70. They were first
scanned at 0.5-mm resolution using an x-ray computed-tomography (CT) device
developed at the Shenzhen Institute of Advanced Technology, Chinese Academy of
Sciences. The material in the traps was pushed up and out with a 1-cm stepper and
resampled into plastic bags and stored in a refrigerator. There are a total of
31 samples from the sediment trap at 15 MAB. These samples, along with other
sediment samples either from the first deployment or seafloor samples at locations
nearby the mooring site, were then analyzed for grain size using a laser particle size
analyzer (Mastersizer 3000), and for clay minerals using a Rigaku SmartLab (9 kW)
diffractometer. Stable isotopes of total organic carbon (δ13C) were measured using
an isotope ratio mass spectrometer (Thermo Science Delta Plus, USA) connected
online to an elemental analyzer (Carlo Erba Instruments Flash 1112, USA).
Estimation of suspended sediment flux (SSF). The SSF of the turbidity currents
was calculated with the following equation:
SSF ¼ZZt
0
SSC uLdz ð2Þ
where Z
t
is the top of the flow where the velocities vanish or are close to zero, Lis
the averaged width of the trench. Because the SSC and current velocity near the
seabed at the mooring site S2 and from the thalweg to the mooring site S2 were
unknown, they were replaced by the SSC and current velocity of the distinguished
lowest layer. It should be noted that this replacement may result in the SSF being
underestimated.
Estimations of bulk Richardson number (Ri). Following Park et al. 71, the bulk
Richardson number (Ri) was calculated from
Ri ¼RCgh
U2ð3Þ
Where Ris the submerged specific gravity (1.65), Cis the depth averaged sediment
concentration, gacceleration of gravity (9.81 m s−2), h is the depth averaged
thickness, Uis the depth averaged velocity.
Estimations of settling and shear velocities. With known median sediment
grain size (D
50
), the settling velocities (W
s
) are calculated using Stokes’s equation:
Ws¼RgD
50

2
18v
ð4Þ
Where νis the kinematic viscosity of the seawater (here set to 1.6 × 10−6m2s−1).
Here we did not take the viscous characteristics of sediment into consideration
because the turbulence in flow might break the flocculation. The turbulent shear, as
expressed by the shear velocities (u*), are determined using:72
u¼uMkln ZM
0:1D90

1
ð5Þ
Where u
M
is the maximum velocity, Z
M
is the height where u
M
was found, D
90
is
the sediment grain size at the 90th percentile.
Data availability
Typhoon and earthquake records are from the Typhoon Network of the Central
Meteorological Observatory (NMCS, http://typhoon.nmc.cn) and the China Earthquake
Networks Center (CENC, https://news.ceic.ac.cn), respectively. Bathymetry data of the
South China Sea are from the GEBCO Gridded Bathymetry Data (https://download.
gebco.net/). Atmospheric pressure data are from the National Centers for Environmental
Prediction (NCEP, https://rda.ucar.edu/datasets/ds094.1/). The data of rainfall, river
discharge, sediment content and load of the Gaoping River are from Hydrological Year
Book of Taiwan (http://gweb.wra.gov.tw/). Data reported in this study are publicly
available at https://doi.org/10.5281/zenodo.7690065. Further inquiries can be directed to
the corresponding author.
Received: 4 November 2022; Accepted: 24 March 2023;
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant
Nos. 41720104001) and the Southern Marine Science and Engineering Guangdong
Laboratory (Guangzhou) (Grant Nos. GML2019ZD0210) received by J.X.. The last
recover of mooring was conducted onboard of R/V “DONGFANGHONG 3”imple-
menting the open research cruise NORC2021-05 supported by NSFC Shiptime Sharing
Project. We thank Wei Zhao, Chun Zhou, and Baoduo Wang for their help in deploying
and recovering the moorings. We thank Yongshuai Ge for his help in acquiring the CT
images of sediment. We thank Yunpeng Lin and Wenpeng Li for their help in measuring
the sediment properties. We thank Fukang Qi, Yuping Yang and Hanying Cao for their
help in bathymetric survey.
Author contributions
M.L. performed the data analysis and wrote the paper, assisted by J.X., and with com-
ments from other authors. J.X. conceived and designed the field experiment and con-
tributed to interpretation of the data. M.L. deployed and recovered the moorings on
research cruises in 2019, 2020, and 2021. Z.W. helped to collect data on research cruises
in 2020 and assisted with data analysis and visualization. K.Y. assisted with data visua-
lization and improvement of the paper.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information The online version contains supplementary material
available at https://doi.org/10.1038/s43247-023-00776-8.
Correspondence and requests for materials should be addressed to Jingping Xu.
Peer review information Communications Earth & Environment thanks Ben Kneller,
Chris Stevenson and the other, anonymous, reviewer(s) for their contribution to the peer
review of this work. Primary Handling Editors: Olivier Sulpis, Joe Aslin and Clare Davis.
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ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-023-00776-8
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