The present-day atmospheric dust deposition process in the South China Sea

Journal Pre-proof The present-day atmospheric dust deposition process in the South China Sea Shuhuan Du, Rong Xiang, Jianguo Liu, Paul Liu, G.M. Ariful Islam, Muhong Chen PII:

S1352-2310(20)30003-0

DOI:

https://doi.org/10.1016/j.atmosenv.2020.117261

Reference:

AEA 117261

To appear in:

Atmospheric Environment

Received Date: 5 June 2019 Revised Date:

31 December 2019

Accepted Date: 4 January 2020

Please cite this article as: Du, S., Xiang, R., Liu, J., Liu, P., Islam, G.M.A., Chen, M., The present-day atmospheric dust deposition process in the South China Sea, Atmospheric Environment (2020), doi: https://doi.org/10.1016/j.atmosenv.2020.117261. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Author Contribution Statment Shuhuan Du: Writing- Original draft preparation; Rong Xiang: Methodology; Jianguo Liu: Sample collecting; Paul Liu: Writing- Reviewing and Editing; G. M. Ariful Islam: Software; Muhong Chen: Supervision.

1

The present-day atmospheric dust deposition process in the South China Sea

2

Shuhuan Dua,b*, Rong Xiang a,b, Jianguo Liua,b, Paul Liuc, G. M. Ariful Islama,b,d, Muhong

3

Chena,b

4

a

Chinese Academy of Sciences, 510301 Guangzhou, China

5 6

b

c

Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh NC 27695, USA

9 10

Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, 510301 Guangzhou, China

7 8

Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology,

d

University of Chinese Academy of Sciences, 100049 Beijing, China

11 12 13

* Corresponding author. Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences. Guangzhou, 510301, China.

14

E-mail addresses: [email protected] (S. Du)

15

ABSTRACT

16

Modern dust plays essential roles in marine and climate processes, which bring continental

17

material to the ocean and sensitivity in marine ecosystems. However, the atmospheric dust

18

deposition process has rarely been studied in the South China Sea (SCS). Here, we present 51

19

atmospheric dust samples, collected along the SCS, to investigate the grain size distribution,

20

depositional flux, and features revealed by scanning electron microscopy, combined with 5-day

21

back trajectories to indicate the present-day dust deposition process for the first time. The grain

22

size distribution and depositional flux of aerosol samples illustrate the seasonal trend, coarser

23

particle and higher flux mass in winter than summer, reflected in average grain size (5.75µm

24

during winter and 3.62µm from summer) and 1.4 times depositional flux in former than that in

25

summer, both are related to the transport pathway and power of the East Asian monsoon.

26

Modeled 5-day back trajectories of dust samples suggest a southwesterly transport pathway in

27

summer and the Southeast Asian monsoon as a possible source of the dust loading, while the 1

28

northeast winds drove the aeolian dust transport during the winter monsoon from the Asian

29

continent. Meanwhile, westerly circulation conveys the fine particles (~0.63 µm) as the stable

30

terrigenous component into the SCS, deposited through the entire dust deposition process from

31

the atmosphere and water to the surface sediment. Furthermore, the surface of quartz particles

32

from atmospheric dust shows the unique structure in the aeolian environment as a reference to

33

distinguish the different continental components in the sediments. This study provides new

34

insights into the present-day dust deposition process in the SCS, significantly extending the

35

current understanding of the relationship between atmospheric dust and the marginal sea.

36 37

Keywords: atmospheric dust; grain size; transport process; SEM; South China Sea

38 39

1. Introduction

40

Modern atmospheric dust generated by wind (Shao, 2008), with high sensitivity to climate

41

and weather processes (Bryant et al., 2007; IPCC, 2007), not only feeds back atmospheric energy

42

balance, precipitation, and sea surface temperature (Maher et al., 2010; Prospero and Lamb,

43

2003; Stuut et al., 2008) but also provides nutrient and essential elements for terrestrial and

44

marine ecosystems (Bishop et al., 2002; Tsuda et al., 2003). When dust is deposited in the ocean,

45

the input of dust-related micronutrients increase the oceanic primary production, leading to

46

increased carbon fluxes, and the ballasting of marine snow aggregates and fecal pellets with dust

47

mineral leads to increased densities and sinking velocities (Jickells et al., 2005; Martin, 1990).

48

Therefore, atmospheric dust is an essential part of the Earth system, contributing significantly to

49

global climate, carbon, and biogeochemical cycles (Jickells et al., 2005).

50

Dust is an essential component in the climate system because large amounts (~500 to ~4400

51

Tg yr-1) are emitted globally (Huneeus et al., 2011). Without consideration of the contribution of

52

human activity, current dust aerosols emissions originating from natural sources alone are 1840

53

Tg yr-1 (Tegen et al., 2004). In China, approximately half of the modern Asian dust transported

54

results in sediment in the China Sea regions and across the North Pacific every year (Zhang et

55

al., 1997; Arimoto et al., 1996). These sediments deposited in oceans can be used to reconstruct

56

the past and predict the present climate and environmental changes. The physical and chemical

57

characteristics of mineral dust in sediment core records can be used as a qualitative proxy for the

58

paleoenvironmental conditions (Rea, 1994). A late Pleistocene and Holocene record of aeolian 2

59

deposition in the northwest Pacific Ocean provides a history of the aridity of the Asian source

60

region and information on the changing latitude and intensity of the zonal westerlies (Rea and

61

Leinen, 1988). High-resolution dust flux records in the central equatorial Pacific share similar

62

patterns, characteristic of the glacial-interglacial cycles in ice volume, confirming a coherent

63

response to global climate forcing on long timescales (Jacobel et al., 2016). The dust during the

64

geological periods was used as an indicator of the intensity of East Asian monsoon in the South

65

China Sea (Wang et al., 2003; Wan et al., 2007). Wang et al. (2003) found a sharp increase in the

66

particle size of terrigenous debris after 2.5 Ma, reflecting a strengthening of dust transport, which

67

supported the strengthening of East Asian monsoon at 3.2 to 2.0 Ma. The terrigenous deposition

68

grain size results indicate that approximately 20% of dust particles are transported from inner

69

Asia by the winter monsoon in the core ODP 1146 (Wan et al., 2007).

70

The research of dust in the South China Sea (SCS) is mainly focused on the reconstruction

71

of ancient oceans, using the terrestrial debris in the sediments as an indicator of the winter

72

monsoon. However, there are different transport process components in the terrigenous matter.

73

Most of the terrestrial debris of SCS is fluvial sediment, which constitutes ~80% of the total SCS

74

surface sediments (Huang and Wang, 2006). The other significant source of land-based input is

75

atmospheric dust, which is transported by the winter wind, the direct evidence of the East Asian

76

winter monsoon. The difference in the transport process of terrigenous sediments indicate the

77

climate conditions, tectonic activity, and specific lithological character of the physical and

78

chemical weathering processes on land (Liu et al., 2009), with critical importance for

79

paleoenvironment reconstruction and climate explanation (Clift et al., 2014; Liu et al., 2010;

80

Wan et al., 2007). At the same time, different transport environments form unique quartz surface

81

structural characteristics (Du et al., 2016). Quartz is one of the components of mineral dust, with

82

stable physical and chemical properties, reflecting the source region climate information and the

83

dynamic characteristics. Terrigenous sediments (quartz) input the marginal sea in different

84

environments, record the information of the transport process, as evidence to distinguish the

85

different terrigenous components. When the quartz surface structural characteristics of modern

86

dust are recognized, dust component in the sediment can be extracted as a quantitative proxy to

87

reconstruct the evolution process of winter monsoon in the SCS.

88

Therefore, quantitative proxy data of modern atmospheric dust are required to explain the

89

variability in dust mobilization, transport, and deposition in the sediments. This information is 3

90

needed to facilitate the paleoclimatic interpretation of sedimentological dust records. The

91

northern SCS is characterized as a sink area of Asian dust (Lin et al., 2007; Wang et al., 2011);

92

unfortunately, there are few studies on modern atmospheric dust in the SCS.

93

In this study, we present the depositional flux, grain size distribution, and results from

94

scanning electron microscopy (SEM), from meteorological data and the air particle five-day

95

back-trajectory data of 51 atmospheric dust samples, collected from 1.78° N–23.09° N (latitude)

96

to 105.48° E–119.80° E (longitude) over different seasons along the South China Sea from 2010

97

to 2014, in combination with surface sediment and sediment trap samples collected in the SCS.

98

We use this data to reveal the progression of the modern dust deposition in the South China Sea,

99

including (1) dust grain size contribution mode in different seasons and weather conditions, (2)

100

atmospheric dust transport pathways and heights, and (3) features of atmospheric dust revealed

101

by SEM that distinguish the terrigenous components in the sediments.

102

2. Materials and Methods

103

2.1 Study site

104

The SCS is a marginal sea located in the far western tropical Pacific and the eastern Indian

105

Ocean. It is connected with the Pacific Ocean through the Taiwan and Bashi Straits in the

106

northeast and the Indian Ocean through the Sunda Shelf in the south, with an area of

107

approximately 3.5 million km2 (Fig. 1).

108

The East Asian monsoon is the dominant climate feature in the SCS, with seasonal

109

alternation of prevailing winds forcing annual precipitation and runoff regimes (Webster, 1994;

110

Wang et al., 2003). The summer monsoon is accompanied by continental heating and the

111

development of low pressure over central China, leading to moderate southwesterly winds across

112

the SCS. In contrast, the winter monsoon is followed by continental cooling and the development

113

of the Siberian-Mongolian anticyclone high pressure over northern Asia, resulting in northeast

114

winds passing through the SCS. Furthermore, the sea surface circulation is affected strongly by

115

the East Asian Monsoon, through which surface water of the tropical Indian Ocean flows

116

northward into the SCS and then into the Pacific, mostly through the Bashi Strait in summer.

117

Meanwhile, the northeast wind drives the tropical and subtropical Pacific waters along with the

118

colder water of the longshore current to the SCS through the Bashi and Taiwan Straits and then

119

across the Sunda Shelf into the Indian Ocean during the winter period (Wang et al., 1995).

120

Nowadays, the active winter monsoon lasts nearly six months (November to April; Chu and 4

121

Wang, 2003), as a prevailing northeaster carrying dust mixed with anthropogenic aerosols during

122

the winter monsoon season (Lin et al., 2007). On the other hand, the weaker summer monsoon

123

lasts about four months (mid-May to mid-September; Chu and Wang, 2003), as the smoke

124

particles associated with biomass burning in Borneo and Sumatra are transported to the southern

125

SCS (Lin et al., 2007).

126

The terrigenous materials in the SCS are mostly transported there by rivers, such as the

127

Rejang, Mekong, Hong Ha, Pearl River, and Hanjiang, which contain ~80% of the total SCS

128

surface sediments (Huang and Wang, 2006). However, the contribution of terrigenous material

129

from atmospheric dust should not be ignored. Wan et al. (2007) discovered that the dust particles

130

could account for ~20% of the terrigenous deposition during the intense winter period in the

131

northern SCS. As the sink area of Asian dust, the ecosystem of SCS responds significantly to

132

atmospheric input (Wang et al., 2011).

133

2.2 Materials

134

Airborne atmospheric dust samples (N = 51, Table 1 and 2) were collected from ~23° N to 1° N,

135

~105° E to ~119° E in the SCS (Fig. 1) using the KB-100 TSP large flow dust collector

136

(Qingdao JinShida Electronic Technology Co. Ltd.), placed on the top deck of a scientific

137

research ship. In order to avoid the impact of external pollution from the research vessel, such as

138

fuel combustion and dust on the deck, using wind speed and direction sensor signal to control the

139

collection from windward from the bow. When the condition (wind speed and direction) meet

140

the requirement set, keeping to collection the dust sample, whereas in a state of waiting. Each

141

sample was collected in 1–2 d with a vacuum-cleaner engine sucking 1.05 m3 air per min

142

through letter-size (250 × 250 mm) filters. The modern atmospheric dust samples analyzed in the

143

present study were exposed to different prevailing wind periods along the SCS collected during

144

four cruises (2010, 2011, 2012 and 2014) by R/V Shiyan 3 of the South China Sea Institute of

145

Oceanology (SCSIO), Chinese Academy of Sciences, including 35 samples in the winter

146

monsoon and 16 samples in the summer monsoon regular season (Table 1). These airborne dust

147

samples were also collected under different depositional conditions—5 samples under typhoon

148

conditions, 10 under wet deposition, and the other 36 samples under dry deposition (Table 2 and

149

Table 3).

150

The sediment trap (SMD-26S) sample from XS1 (17124.50N, 110155.00E, water depth

151

1690 m) (Fig. 1) was deployed at a water depth of 1500 m, with a collection area of 0.5 m2 and a 5

152

sample duration of 14–16 d (two samples per month). The sampling cups were filled with in situ

153

filtered seawater (0.45 mm filter) collected from trap locations, to which 3.3 g/L HgCl2 was

154

added before deployment to prevent decomposition of organic material. Samples in polyethylene

155

bottles (250 mL) were kept at 4 °C and transported to the laboratory of the South China Sea

156

Institute of Oceanology (SCSIO), Chinese Academy of Sciences (CAS) for grain size and total

157

particle flux (TPF) measurements. TPF was determined by measuring the dry weight of the

158

materials on an analytical balance and using the sampling area of the trap and the exposure time

159

to calculate the flux (mg/m2/d).

160

The surface sediments D21-7 (17.69° N, 110.00° E, water depth 1740 m) and 11E406

161

(18.74° N, 119.74° E, water depth 3415 m) were collected using box samples from the northern

162

SCS and taking the first 2-cm depth.

163

2.3 Grain size

164

For analysis, all atmospheric samples should first be separated from the glass-fiber filter.

165

Then, 20 ml of deionized water is added to vibrate in an ultrasonic cleaner for 15 min, and this

166

process is repeated two or three times to gather the dust sample from the filter successfully. On

167

mineral dust particles, the secondary components, such as sulfates and nitrates, exist in the forms

168

of SO and NO , respectively (Liao et al.,2003). Both SO and NO are water solubility (Seinfeld

169

and Pandis, 2006), when molecules of sulfates and nitrates in the ultrasonic step associated with

170

water molecules, playing little impact on grading analysis of dust. Based on the Stokes principle,

171

samples were allowed to stand for 1–2 d to fully precipitate the dust particles, letting the particles

172

concentrate and enrich for laboratory analyses. For the grain size analysis, the dust sample was

173

first passed through a 100-µm sieve to remove the glass fiber film that might fall off during the

174

separation process, according to previous studies, most dust particles in the air are not greater

175

than 50 µm in diameter (Pye, 1987), and then measure the laser particle size after ultrasonic

176

vibration.

-2 4

3

-2 4

3

177

Dust, trap sediment, and surface sediment sampled for grain size analysis were measured

178

with the Malvern Mastersizer 2000 at the SCSIO, CAS. This instrument was used to measure and

179

calculate the frequency distribution of particle diameters, providing 100 grain size classes from

180

0.02 to 2000 µm. Every subsample was measured thrice and was averaged. The measurement

6

181

repeatability is 0.5% for a single sample, and the reproducibility is better than 2% for duplicate

182

samples.

183

2.4 Scanning electron microscopy

184

Scanning electron microscopy (SEM) was carried out at the SCSIO, CAS, with the Japanese

185

Hitachi S3400 scanning electron microscope. The SEM is used to scan the quartz particle only.

186

First, the quartz is extracted, as sodium pyrosulfate (Na2S2O7) is used for melt removal of other

187

minerals besides quartz and feldspar in the sample. Then, fluosilicic acid (H2SiF6) is added to

188

remove the feldspar mineral to extract pure quartz (Jackson, 1981).

189

2.5 Back trajectories

190

Five-day back trajectories are analyzed to provide insight into the sources and transport

191

pathways of atmospheric dust in the SCS, obtained from the Hybrid Single-Particle Lagrangian

192

Integrated Trajectory (HY-SPLIT4) model of the National Oceanic and Atmospheric

193

Administration (NOAA). The meteorological data and model are taken from the ARL-NOAA

194

server (http://www.arl.noaa.gov/HYSPLIT_data2arl.php). This model is widely used in the

195

modeling and prediction of dust diffusion paths (Waisel et al., 2008). We chose six samples,

196

collected under different weather conditions (dry, wet, and typhoon) and prevailing winds

197

(winter and summer monsoon) in the same scientific voyage.

198

3 Results

199

3.1 Grain size distribution

200

The grain size distributions of atmospheric dust samples from the SCS exhibit a non-unimodal

201

distribution (Fig. 2), it differing from the wind-blown sediments with a well-sorted unimodal

202

distribution (Krumbein and Pettijohn, 1938; Stuut et al., 2005). The dust size modal ranges from

203

approximately 0.3–68 µm, mainly focus on the 0.63-20µm range, including an accumulation

204

mode (0.1 < D < 0.5 µm) and coarse mode particles (D > 1.0 µm) (Seinfeld and Pandis, 2006).

205

The mean geometric median diameter (Dg) of atmospheric dust in the SCS is ~5 µm, similar to

206

other modern ocean area deposits (Stuut et al., 2005; Skonieczny et al., 2013). Combined with

207

the result from a sieving blank example shows all the particles >300µm (Fig. 2d), indicates the

208

necessity to remove the fibers by sieving.

7

209

As shown in Figure 2, there is more than one peak in the modal distribution, many samples

210

show tri-modal distribution, one stable peak appears at 0.63 µm, the other distinct peak is present

211

at 3 µm and 6 µm during the summer monsoon (Fig. 2a) and winter monsoon (Fig. 2b),

212

respectively, the third frequency peak show giant grains up to 60 µm in some summer dust

213

samples (Fig. 2a), and a frequency peak of 20-30µm common present in the winter monsoon

214

grain size mode (Fig. 2b). Therefore, the atmospheric dust samples show a seasonal distribution

215

trend, with an average grain size of 3.62 µm in the summer monsoon and 5.75 µm in the winter.

216

Besides, the grain size distribution is impacted by weather conditions. Under different weather

217

conditions (fine, rainy, and typhoon), three dust samples collected in the same voyage show

218

different grain size distributions (Fig. 2c). Similar grain size distributions are evident in fine and

219

rainy conditions; however, the frequency of 0.63 µm particles and finer particles in general

220

(peak: 4 µm) is higher under wet conditions than under dry conditions (peak: 5 µm). Meanwhile,

221

during extreme typhoon weather conditions, particles of up to 60 µm are present owing to strong

222

wind.

223

Interestingly, the depositional flux of dust also undergoes seasonal change. The total particle

224

flux of trap sediment in the northern SCS indicates a higher flux (average: 138 mg m-2 d-1, 95%

225

confidence interval from 54 mg m-2 d-1 to 169 mg m-2 d-1) during the winter than that during the

226

other seasons (average: 92 mg m-2 d-1, 95% confidence interval from 64 mg m-2 d-1 to 111 mg m-2

227

d-1; Liu et al., 2014). Maximum dust production for the SCS occurs in the spring, and minimum

228

dust production in the summer, with the flux mass in the winter monsoon season (Table 3)

229

approximately 1.4 times higher than that during the summer monsoon (Table 2).

230

The grain size result of the trapped sediment and surface sediment is connected with that of

231

atmospheric dust, although the particle distributions look quite different (Fig. 3). Comparing the

232

grain

233

15.91° N/110.67° E), trap sediment (D21-7, 17.69° N, 110.00° E, water depth 1740 m), and

234

surface sediment samples (XS1, 17.40° N, 110.92° E, water depth 1690 m) in the northern SCS,

235

there is a clear typical frequency peak around 0.63 µm, while the maximum frequency peak

236

becomes gradually coarser from dust to trap to surface sediments (Fig. 3).

237

3.2 SEM

size

distribution

from

the

dust

(14SCS07,

from

13.99° N/113.03° E

to

8

238

The atmospheric dust in the SCS is transported and deposited through long distances,

239

influenced by the East Asian Monsoon. The surface of quartz particles forms the unique surface

240

structural characteristics of the aeolian environment. The results of the SEM images of dust

241

quartz surface features present well-rounded (Fig. 4a), U-/dish-shaped (Fig. 4b), and meander

242

ridge (Fig. 4c) features, which are the classic quartz surface features under an aeolian

243

environment (Powers, 1953). These features of the dust sample were also distinguished in a

244

surface sediment sample (11E406, 18.74° N, 119.74° E, water depth 3415 m; Fig. 4d–f) from the

245

northern SCS. However, the SEM images of quartz surface features from an aeolian environment

246

are markedly different from those in a fluvial deposition environment. Fig. 4g–i show the SEM

247

quartz features from terrigenous detritus transported by the river of 11E406, exhibiting poor

248

roundness (Fig. 4g), V shape (Fig. 4h), and conchoidal fracture (Fig. 4i) in the same surface

249

sediment sample.

250

3.3 Back trajectories and transport pathway

251

Using the HY-SPLIT 4 model of NOAA (HY-SPLIT model available from NOAA Air

252

Resources Laboratory READY at http://www.arl.noaa.gov/ready/hysplit4.html), 5-day back

253

trajectories at the 0, 1000, and 3000 m levels were calculated under different prevailing wind and

254

weather conditions (Fig. 5). From these calculations, it appears that for the samples collected

255

during the summer monsoon (Fig. 5a–c), the dust was mainly transported to the SCS by

256

southwest airflow at low levels (0–1000 m) and easterly flow at higher levels (3000 m), under

257

fine (Fig. 5a), rainy (Fig. 5b), or typhoon (Fig. 5c) conditions. Comparing the sample collected

258

under the winter monsoon (Fig. 5d–f), low-level northeast prevailing winds are primarily

259

responsible for the transport of dust, while an easterly trend is evident at higher levels. Back

260

trajectories clearly illustrate that the dust in the SCS was transported by the East Asian monsoon,

261

primarily through the low-level wind. In contrast, the upper level (3000 m) 5-day back

262

trajectories for dust samples show an easterly trend.

263

4. Discussion

264

The atmospheric dust samples were collected from ~23° N to 1° N, ~105° E to ~119° E in

265

the SCS, some dust samples were collected cover long distance more than eight latitudes (e.g.,

266

14SCS09, 14SCS11, Table 2), some samples collected in one latitude (e.g., 14SCS04, 14SCS05,

267

Table 2) during the same voyage, rarely difference reflects in the mass flux under dry deposition 9

268

(Table 2), however, some difference present in the grain size composition. Comparison of the

269

proportion of grain size >10 µm between 14SCS11and 14SCS05, there are 22.28% and 36.55%,

270

respectively, the former was collected cover the longest distance and far away from the land or

271

inland, while the later was collected near the Indochina Peninsula, which provides more coarser

272

particles.

273

The atmospheric dust samples in the northern SCS with mean Dg of ~5µm, similar to other

274

marine dust deposit, however, all grain size distribution exhibit more than one frequency peak

275

(Fig. 2), reflecting in one stable frequency peak at ~0.63 µm and a variable maximum peak

276

appearing at ~3.6 µm in summer and ~5.7 µm in winter monsoon under normal weather

277

conditions, indicating that sedimentary dust comes from different transport pathways.

278

Meanwhile, under different weather conditions, wet deposition catches further finer particles

279

than that obtained in dry weather; dust collected in typhoon conditions captures more material

280

due to strong wind and rainfall, particularly giant grains from proximal deposits. Moreover,

281

particles larger than 10 µm represents approximately 45% and approximately 27% of the

282

deposits during the winter monsoon and summer monsoon, respectively, suggesting that

283

sedimentation is dynamic with a seasonal distribution trend. Furthermore, this seasonal trend is

284

also reflected in sedimentary flux variability, as the flux mass in winter is about 1.4 times higher

285

than that during the summer monsoon (Table 2 and 3). This result is similar to the northern SCS

286

dust flux estimate, in which the seasonal flux in winter monsoon constitutes about 60% of the

287

annual dust flux (17.68 gm-2) (Wang et al., 2012).

288

The transport path and deposition of atmospheric dust of SCS are significantly affected by

289

wind; however, the grain size distribution differs from that of the well-sorted unimodal wind-

290

blown sediment (Stuut et al., 2005). The back trajectory results can be combined with the

291

different transport altitudes and paths through which air masses are transported, overlaid to

292

produce a deposit of atmospheric dust in the SCS. Back trajectories simulation can be used as a

293

rough estimate of where the air masses originate from and the transport altitude level. This

294

method has been frequently used to determine the source of aerosols (Caquineau et al., 2002;

295

Schefuß et al., 2003; Stuut et al., 2005). The back trajectories at the 0, 1000, and 3000 m levels

296

(Fig. 5) of this study show that the prevailing winds shift significantly from southwest in summer

297

(Fig. 5a–c) to northeast in winter (Fig. 5d–f) at the low-latitude level (0 and 1000 m), suggesting

298

that the East Asian monsoon governs the seasonal trend of the dust samples. Previous studies 10

299

regarding dust transport during the geological periods in the SCS reflect the dominance and

300

presence of intensity change of the East Asian monsoon (Wang et al., 2003; Wan et al., 2007).

301

The annual cycle of the mean wind stress field shows the large-scale characteristics shared

302

among the different wind fields; the southwest summer monsoon generally has weak wind stress

303

and uniform wind curl, while the northeast winter monsoon has consistent speed, uniform

304

direction, and significant wind stress curl (Chao et al., 1995; Caruso et al., 2006).

305

The back trajectories of dust samples suggest that the dust transported to the SCS is

306

modified by the marine environment, changing the transported pathway in summer and winter

307

monsoon seasons through the marine boundary layer (0–300 m) to a different direction (Huang

308

and Mao, 2015). During the winter monsoon period (Fig. 5d–f), aeolian dust driven by northeast

309

winds and winter monsoon transport can drop from 6000 m to 0 m in 4–5 d. Thus, the sharp

310

winter wind may carry the coarser fraction of the settling dust from the Asian continent,

311

reflecting in the dust grain size composition of 20-30µm mode only present in the winter

312

monsoon (Figure 2b). On the other hand, southwesterly winds prevail in the summer (Fig. 5a–c),

313

with maximum heights broadly lower than in the winter monsoon. Most of the winds are

314

transported in the marine boundary layer, except under rainy conditions, which explains the finer

315

mode exhibited by dust samples collected under wet conditions. Moreover, from the back

316

trajectories in the summer monsoon period, dust from the Asian continent can be considered

317

negligible, whereas the densely populated and industrialized areas in Southeast Asia could have a

318

significant impact on the dust loading over the SCS (Lin et al., 2007; Wang et al., 2011).

319

It is interesting to note that a stable frequency peak (~0.63 µm) occurs in all atmospheric

320

dust samples of this study, which with a higher proportion in wet condition than dry deposition

321

(Figure 2c). According to the aeolian dust component of sediments from the North Pacific Ocean

322

originates from the arid interior of Asia and has a single fine component size range of 0-10µm

323

(Rea and Hovan, 1995), is principally transported by long-term suspension over a more massive

324

vertical range and deposited either by rainfall wash out or through attachment to bigger grains

325

(Pye, 1987). This stable background dust component was suggesting a transport path different

326

from the change monsoon path that prevails at the relatively low-latitude level. In consideration

327

of a persistent easterly wind throughout the different season at 3000 m level (Fig. 5), indicates

328

that this fine component could be transported on a relatively high-latitude level. The westerly has

11

329

been working as the planetary circulation system in the middle latitude of the northern

330

hemisphere (Chen et al., 1991), carrying continental material from Asia across the ocean at 350–

331

600 hPa. While the air moves southward in the northern hemisphere, subsidence occurs until the

332

air is entrained in the northeast trade wind flow (Merrill et al., 1985), depositing the stable

333

terrigenous component through the entire transport and sedimentation process in the SCS. The

334

fine particles (~0.63 µm) not only exist in the aerosol sample but also appear in the trap and

335

surface sediments (Fig. 3), indicating that terrigenous matter constitutes a stable component in

336

the SCS. This stable component has also been reported as a monsoon precipitation index in the

337

Bohai Sea, China (Du et al., 2016).

338

However, the observed difference between atmospheric dust, trap sediment, and surface

339

sediment in this study was significant. Except the fine terrigenous component (~0.63 µm)

340

transported by the westerly circulation, the central peak of the grain size distribution, increasing

341

gradually from atmospheric dust (~4 µm) to trap (~7 µm) to surface sediment (~10 µm) (Fig. 3),

342

shows that the decrease in the atmospheric component correspondingly follows top-down during

343

dust deposition. Prior studies have noted the complexity of the sediment sources and transport in

344

the marginal sea sediments. These sources include terrigenous clastics, biogenic materials, and

345

self-produced materials in the early diagenetic process (Zheng et al., 2008; Liu et al., 2010),

346

while transport and power cover the surface ocean current, bottom current, and turbidity,

347

including the dust deposition (Zheng et al., 2008). During the sedimentation process in the SCS,

348

river-borne terrigenous sediments constitute ~80% of the total SCS surface sediments (Huang,

349

2004); the coarse particle is transported by the river predominantly, sometimes through the

350

turbidity. Fig. 3 shows a significant peak at 400–600 µm in the grain size distribution of surface

351

sediment, which probably corresponds with the turbidite current transport in the Qiongdongnan

352

Basin (He et al., 2013; Su et al., 2014).

353

The terrigenous component in the sediment of the marginal sea is transported primarily by

354

the river, with a small percentage from wind-driven processes, indicating the remarkable

355

importance of the reconstruction of the paleoceanography and East Asian monsoon evolution in

356

the SCS. As mentioned in the literature review (Rea and Leinen, 1988; Rea, 1994; Wang et al.,

357

2003; Wan et al., 2007; Jacobel et al., 2016), dust deposition records not only provide

358

information on the changing latitude and intensity of the zonal westerlies but also serve as an

359

indicator to reflect the intensity of the East Asian monsoon in the SCS. However, most of the 12

360

research using terrigenous clastics indicate the winter monsoon without extraction of the dust

361

component, and the river-borne and wind-driven components exhibit significantly different

362

characteristics, which are useful in explaining the monsoon evolution and environmental change.

363

Therefore, it is necessary to distinguish the river-borne and wind-driven component in the

364

sediment.

365

In this study, the results of SEM images show the quartz surface features characteristic of

366

dust, such as good roundness (Fig. 4a), U-shaped (Fig. 4b), and meander ridge (Fig. 4c), under

367

an aeolian environment (Powers, 1953). Quartz is the most resistant to alteration during

368

transport, sedimentation, and weathering processes (Xiao et al., 1995); however, different

369

transport environments form unique surface structural characteristics, serving as an excellent

370

method to distinguish the different terrigenous components. The SEM results in the surface

371

sediment (11E406, 18.74° N, 119.74° E, water depth 3415 m) from the northern SCS reveal a

372

dust different from the river-borne component (Fig. 4); quartz features of terrigenous clastic

373

transported by fluvial processes exhibit angular (Fig. 4g), V-shaped (Fig. 4h), and conchoidal

374

fracture (Fig. 4i) features, different from the dish-shaped (Fig. 4d–e) and meander ridge (Fig. 4f)

375

features of dust. Therefore, the SEM results can be used to recognize these two different

376

terrigenous components in the same sample. The dust component in core sediments is obtained

377

as an index of winter monsoon in the SCS, and quantitative statistics could be used to reconstruct

378

the East Asian monsoon evolution.

379

5. Conclusions

380

In this paper, the present-day dust deposition process in the northern SCS was studied for the

381

first time. Based on 51 atmospheric dust, sediment trap, and surface sediment samples in the

382

SCS, the grain size distribution mode and mass flux are revealed and combined with 5-day back

383

trajectories to indicate the various present-day dust transport pathways and heights. In addition,

384

the surface of quartz particles from atmospheric dust shows a unique structure in the aeolian

385

environment. SEM is considered to be an effective method to distinguish the terrigenous

386

components in the sediment in the SCS.

387

The atmospheric dust samples with a mean geometric median diameter of ~5 µm and usually

388

exhibits two frequency peaks for normal weather conditions, one variable maximum frequency

389

peak appearing at ~3.6 µm in summer, and a ~5.7 µm peak in winter monsoon, illustrating a 13

390

seasonal trend of coarser particles collected in winter than summer. The dust grain size

391

distribution is susceptible to the alteration from wind strength and dust availability. Wet

392

deposition catches finer particles than the dry state, while under the typhoon conditions, because

393

of strong wind and rainfall, more material is captured, particularly giant grains from proximal

394

deposits. In addition, one stable frequency peak at ~0.63 µm throughout the year shows a

395

different provenance without seasonal change.

396

The combination of modeled 5-day back trajectories of dust samples in different collection

397

periods and weather conditions reveal more than one transport pathway, and altitude impacts the

398

dust loading in the SCS. As the prevailing wind, the East Asian monsoon controls dust transport

399

and deposition, as coarser particles with more mass flux are deposited during the winter

400

monsoon period, transported by the northeast winds from the Asian area. On the other hand,

401

southwesterly winds drove the aeolian dust in summer, and the Southeast Asian monsoon could

402

have a significant impact on dust loading. Therefore, the monsoon is the main transport pathway

403

and power of the atmospheric dust in the SCS, also responsible for the seasonal grain size change

404

and mass flux. The present study also enhances understanding of the westerly circulation impact

405

on the dust, confirming the findings of different transported pathways and height on the transport

406

and sediment process in the SCS.

407

SEM images of atmospheric dust reveal the features of the quartz surface under an aeolian

408

environment, in which its unique structure differs from the terrigenous clastic transported by

409

fluvial processes, becoming a reference to distinguish both terrigenous components in the

410

sediments. The dust component can be extracted to establish as a winter monsoon index. Further

411

research might explore the core sediments, using quantitative statistics to analyze the dust

412

component during the sedimentation period to reconstruct the East Asian monsoon evolution.

413

Overall, our results from the present-day dust provide essential implications on dust,

414

including source origins, transport pathways, and heights, and deposition process from the

415

atmosphere and water to the surface sediment in the northern SCS. Furthermore, this study also

416

provides new insights to distinguish the dust component from the terrigenous clastic particles,

417

which would be useful in the effort to reestablish the evolution history of the East Asian winter

418

monsoon in the SCS.

14

419

Acknowledgments

420

This work was funded by the Natural Science Foundation of Guangdong Province (No.

421

2018A0303130156), Key Laboratory of Ocean and Marginal Sea Geology, Chinese Academy of

422

Sciences (No.OMG2019-06) and Innovative Development Fund projects of Innovation Research

423

Institute on the South China Sea Ecological and Environmental Engineering, Chinese Academy

424

of Sciences (No.352ISEE2018PY02). We specially thank the anonymous reviewers for their

425

constructive reviews of this paper.

426

References

427

Arimoto, R., Duce, R.A., Savoie, D.L., Prospero, J.M., Talbot, R., Cullen, J.D., Tomza, U.,

428

Lewis, N. F., Ray, B. J., 1996. Relationships among aerosol constituents from Asia and the

429

North Pacific during PEM-West A. Journal of Geophysical Research 101, 2011-2023.

430

https:// doi.org/10.1029/95JD01071

431

Bishop, J.K.B., Davis, R.E., Sherman, J.T., 2002. Robotic observations of dust storm

432

enhancement of carbon biomass in the North Pacific. Science 298, 817-821.

433

https://doi:10.1126/science. 1074961

434

Bryant, R.G., Bigg, G.R., Mahowald, N.M., Eckardt, F.D., Ross, S.G., 2007. Dust aerosol

435

emission response to climate in southern Africa. Journal of Geophysical Research, 112,

436

D09207. https://doi:10.1029/ 2005JD007025

437

Caquineau, S., Gaudichet, A., Gomes, L., Legrand, M., 2002. Mineralogy of Saharan dust

438

transported over northwestern tropical Atlantic Ocean in relation to source regions. Journal

439

of Geophysical Research 107(D15), 4251. https://doi: 10. 1029/ 2000 JD000247

440

Caruso, M.J., Gawarkiewicz, G.G., Beardsley, R.C., 2006. Interannual variability of the

441

Kuroshio intrusion in the South China Sea. Journal of Oceanography 62, 559-575.

442

https://doi.org /10.1007/s10872-006-0076-0

443

Chao, S.Y., Shaw, P.T., Wang, J., 1995. Wind relaxation as possible cause of the South China

444

Sea

445

https://doi.org/10.1007/BF02235940

446 447

Warm

Current.

Journal

of

Oceanography

51,111-132.

Chen, L.X., Zhu, Q.G., Luo, H.B., 1991. East Asian Monsoon (in Chinese), Beijing: China Meteorology Press, 1-262.

15

448 449

Chu, P.C., Wang, G., 2003. Seasonal variability of thermohaline front in the central South China Sea. Journal of Oceanography 59, 65-78. https://doi.org/10.1023/ A:1022868407012

450

Clift, P.D., Wan, S.M., Blusztajn, J., 2014. Reconstructing chemical weathering, physical erosion

451

and monsoon intensity since 25 Ma in the northern South China Sea: a review of competing

452

proxies. Earth-Science Reviews 130, 86-102.https://doi.org/10.1016/j.earscirev.2014.01.

453

002

454

Du, S.H., Li, B.S., Li, Z.W., Chen, M.H., Zhang, D.D., Xiang, R., Niu, D.F., Si.,Y.J.,2016. East

455

Asian monsoon precipitation and paleoclimate record since the lastinterglacial period in the

456

Bohai Sea coastal zone, China. Terrestrial, Atmospheric and Oceanic Sciences 27,825-836.

457 458

Fang, G., Fang, W., Fang, Y., Wang, K., 1998. A survey of studies on the South China Sea upper ocean circulation. Acta Oceanography of Taiwanica 37 (1), 1-16.

459

He, Y.L., Xie, X.N., Kneller, B.C., Wang, Z.F., Li, X.S., 2013. Architecture and controlling

460

factors of canyon fills on the shelf margin in the Qiongdongnan Basin, northern South

461

China

462

https://doi.org/10.1016/j.marpetgeo.2012.03.00 2

Sea.

Marine

and

Petroleum

Geology

41,

264-276.

463

Huang, H.J., Mao, W.K., 2015. The South China Sea Monsoon Experiment-Boundary Layer

464

Height (SCSMEX-BLH): Experimental Design and Preliminary Results. Monthly Weather

465

Review 143, 5035-5053. https://doi.org/10.1175/MWR-D-15-0067.1

466 467

Huang, W., 2004. Sediment Distributional Patterns and Evolution in the South China Sea since the Oligocene. Doctoral Dissertation. Shanghai: Tongji University. 113 pp.

468

Huang, W., Wang, P.X., 2006. Sediment mass and distribution in the South China Sea since the

469

Oligocene. Science in China Series D: Earth Sciences 49 (11), 1147 -1155. https://doi.org

470

/10.1007/s11430-006-2019-4

471

Huneeus, N., Schulz, M., Balkanski, Y., Griesfeller, J., Kinne, S., Prospero, J., Bauer, S.,

472

Boucher, O., Chin, M., Dentener, F., Diehl, T., Easter, R, Fillmore, D., Ghan, S., Ginoux,

473

P., Grini, A., Horowitz, L., Koch, D., Krol, M.C., Landing, W., Liu, X., Mahowald, N.,

474

Miller, R.L., Morcrette, J.J., Myhre, G., Penner, J.E., Perlwitz, J.P., Stier, P., Takemura, T.,

475

Zender, C., 2011. Global dust model intercomparison in AeroCom phase I. Atmospheric

476

Chemistry and Physics 11, 7781-7816. https://doi.org/10.5194/acp-11-7781-2011

477

IPCC, 2007. The Physical Science Basis. Contribution of Working Group I to the Fourth

478

Assessment Report of the Intergovernmental Panel on Climate Change. (Eds.) Solomon, S., 16

479

Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor M., Miller H.L.,

480

Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 996

481

pp.

482

Jackson, M.L., 1981. Oxygen isotopic ratios in quartz as an indicator of provenance of dust, in

483

Desert Dust: Origin, Characteristics, and Effect on Man. (Eds.) Pewe, T. L., Special Paper

484

186, Geological Society of Amercian, Boulder, CO. 27-36 pp.

485

Jacobel, A.W., McManus, J.F., Anderson, R.F., Winckler, G., 2016. Climate-related response of

486

dust flux to the central equatorial Pacific over the past 150 kyr. Earth and Planetary Science

487

Letters 457, 160-172. http://dx.doi.org/10.1016/j.epsl.20 16 .09.042

488

Jickells, T.D., An, Z.S., Andersen, K.K., Baker, A.R., Bergametti, G., Brooks, N., Cao, J.J.,

489

Boyd, P.W., Duce, R.A., Hunter, K.A., Kawahata, H., Kubilay, N,, laRoche, J., Liss, P.S.,

490

Mahowald, N., Prospero, J.M., Ridgwell, A.J., Tegen, I., Torres, R., 2005. Global iron

491

connections between desert dust, ocean biogeochemistry, and climate. Science, 308, 67-71.

492

https://doi: 10.1126/science.1105959

493 494

Krumbein, W. C., Pettijohn, F. J., 1938. Manual of Sedimentary Petrography, Appleton Century Crofts, New York, 549 pp.

495

Liao, H., Adams, P. J., Chung, S. H., Seinfeld, J. H., Mickley, L. J., Jacob, D. J., 2003.

496

Interactions between tropospheric chemistry and aerosols in a unified general circulation

497

model.

498

doi:10.1029/2001JD001260.

Journal

of

Geophysical

Research

108(D1),

4001.

https://

499

Lin, I.I., Chen, J.P., Wong, G.T. F., Huang, C.W., Lien, C.C., 2007. Aerosol input to the South

500

China Sea: results from the moderate resolution imaging spectro-radiometer, the quick

501

scatterometer, and the measurements of pollution in the troposphere sensor. Deep-Sea

502

Research II 54(14-15), 1589-1601. https://doi.org/ 10.1016/j.dsr2.2007.05.013

503

Liu, J.G., Clift, P.D., Yan, W., Chen, Z., Chen, H., Xiang, R., Wang, D.X., 2014. Modern

504

transport and deposition of settling particles in the northern South China Sea: sediment trap

505

evidence

506

https://doi.org/10.1016/j.dsr. 2014.08.005

adjacent

to

Xisha

Trough.

Deep-Sea

Research

I

93,

145-155.

507

Liu, J.P., Xue, Z., Ross, K., Wang, H.J., Yang, Z.S., Li, A.C., Gao, S., 2009. Fate of sediments

508

delivered to the sea by Asian large rivers: long-distance transport and formation of remote

17

509

alongshore clinothems. The Sedimentary Record 7 (4), 4-9. https://doi.org/10.2110/sedred.

510

2009.4.4

511

Liu, Z.F., Colin, C., Li, X.J., Zhao, Y.L., Tuo, S.T., Chen, Z., Siringan, F.P., Liu, J.T., Huang,

512

C.Y., You, C.F., Huang, K.F., 2010. Clay mineral distribution in surface sediments of the

513

northeastern South China Sea and surrounding fluvial drainage basins: source and transport.

514

Marine Geology, 277, 48-60. https://doi.org /10.1016/j.margeo.2010.08.010

515

Maher, B.A., Prospero, J.M., Mackie, D., Gaiero, D., Hesse, P.P., Balkanski, Y., 2010. Global

516

connections between aeolian dust, climate and ocean biogeochemistry at the present day

517

and

518

https://doi.org/10.1016/j. earscirev.2009.12.001

519 520 521 522 523 524 525 526

at

the

last

glacial

maximum.

Earth-Science

Reviews

99,

61-97.

Martin, J.H., 1990. Glacial-interglacial CO2 change: The iron hypothesis. Paleoceanography and Paleoclimatology 199(5), 1-13. https://doi.org/10.1029 /PA005i001p00001 Merrill, J.T., Bleck, R., Avila, L., 1985. Modeling atmospheric transport to the Marshall Islands. Journal of geophysical research 90, 12927-12936. Powers, M.C., 1953. A new roundness scale for sedimentary particles. Journal of Sedimentary Petrology 23, 117-119. https://doi: 10.1306/D4269567-2B26-11D7-8648 000102C1865D Prospero, J.M., Lamb, P.J., 2003. African droughts and dust transport to the Caribbean Climate change implications. Science 302, 1024-1027. https://doi: 10.1126/ science.1089915

527

Pye, K., 1987. Aeolian Dust and Dust Deposits. Academic Press, London, 334pp.

528

Rea, D.K., 1994. The palaeoclimatic record provided by eolian deposition in the deep sea: the

529

geologic history of wind. Reviews of Geophysics 32, 159-195. https://doi.org/10.1029/93R

530

G03257

531

Rea, D.K., Hovan, S.A., 1995. Grain size distribution and depositional processes of the mineral

532

component of abyssal sediments: lessons from the North Pacific. Paleoceanography 10,

533

251-258.

534

Rea, D.K., Leinen, M., 1988. Asian aridity and the zonal westerlies: Late Pleistocene and

535

Holocene record of eolian deposition in the northwest Pacific Ocean. Palaeogeography,

536

Palaeoclimatology, Palaeoecology 66, 1-8. https://doi. org/10.1016/0031-0182(88)90076-4

537

Schefuß, E., Ratmeyer, V., Stuut, J.B.W., Jansen, J. H. F., Damste. J. S. S., 2003. Carbon isotope

538

analyses of n-alkanes in dust from the lower atmosphere over the central eastern Atlantic,

18

539

Geochimica et Cosmochimica Acta 67(10), 1757-1767. https://doi.org/10.1016/S0016-

540

7037(02)01414-X

541 542

Seinfeld, J. H., Pandis, S. N., 2006. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, John Wiley & Sons, 2nd edition.

543

Shao, Y., 2008. Physics and Modelling of Wind Erosion. Kluwer Academic Publishers, London.

544

Skonieczny, C., Bory, A., Bout-Roumazeilles, V., Abouchami, W., Galer, S. J. G., Crosta, X.,

545

Diallo, A., Ndiaye, T., 2013. A three-year time series of mineral dust deposits on the West

546

African margin: Sedimentological and geochemical signatures and implications for

547

interpretation of marine paleo-dust records. Earth and Planetary Science Letters 364, 145-

548

156. https://doi.org/10.1016/j.epsl. 2012. 12.039

549

Stuut, J.B., Zabel, M., Ratmeyer, V., Helmke, P., Schefuß, E., Lavik, G., Schneider. R., 2005.

550

Provenance of present-day eolian dust collected off NW Africa. Journal of Geophysical

551

Research 110(4), 1-14. https://doi.org/10.1029/2004JD0 05161

552 553

Stuut, J.B.W., Mulitza, S., Prange, M., 2008. Challenges to understanding past and future climate in Africa. EOS 89, 196.

554

Su, M., Xie, X.N., Xie, Y.H., Wang, Z.F., Zhang, C., Jiang, T., He, Y.L., 2014. The

555

segmentations and the significances of the Central Canyon System in the Qiongdongnan

556

Basin, northern South China Sea. Journal of Asian Earth Sciences 79, 552-563.

557

https://doi.org/10.1016 /j.jseaes.2012.12.038

558

Tegen, I., Werner, M., Harrison, S., Kohfeld, K., 2004. Relative importance of climate and land

559

use in determining present and future global soil dust aerosol emission. Geophysical

560

Research Letters 31, L05105. https://doi:10.1029/2003 GL019216

561

Tsuda, A., Takeda, S., Saito, H., Nishioka, J., Nojiri, Y., Kudo, I., Kiyosawa, H., Shiomoto, A.,

562

Imaik., O.T., Shimamoto, A., Tsumune, D., Yoshimura, T., Aono, T., Hinuma, A.,

563

Kinugasa, M., Suzuki, K., Sohrin, Y., Noiri, Y., Tani, H., Deguchi, Y., Tsurushima, N.,

564

Ogawa, H., Fukami, K., Kuma, K., Saino, T., 2003. A mesoscale iron enrichment in the

565

western Subarctic Pacific induces a large centric diatom bloom. Science 300, 958-961.

566

https://doi: 10.1126/science.1082000

567

Waisel, Y., Ganor, E., Epshtein, V., Stupp, A., Eshel, A., 2008. Airborne pollen, spores, and dust

568

across the East Mediterranean Sea. Aerobiologia 24,125-131. https://doi.org/10.1007/

569

s10453-008-9087-1 19

570

Wan, S.M., Li, A.C., Clift, P.D., Stuut, J.B.W., 2007. Development of the East Asian monsoon:

571

mineralogical and sedimentologic records in the northern South China Sea since 20 Ma.

572

Palaeogeography, Palaeoclimatology, Palaeoecology 254, 561-582. https://doi.org/10.1016

573

/j.palaeo.2007.07.009

574

Wang, B., Clemens, S.C., Liu, P., 2003. Contrasting the Indian and East Asian monsoons:

575

implications on geological timescales. Marine Geology 201, 5-21. https://doi.org/10.1016/

576

S0025-3227(03)00196-8

577

Wang, P., Wang, L., Bian, Y., Jian, Z., 1995. Late Quaternary paleooceanography of the South

578

China Sea: Surface circulation and carbonate cycles. Marine Geology 127, 145-165. https://

579

doi.org/10.1016/0025-3227(95)00008-M

580

Wang, S.H., Hsu, N.C., Tsay, S.C, Lin, N.H., Sayer, A.M., Huang, S.J., Lau, W.K.M., 2012. Can

581

Asian dust trigger phytoplankton blooms in the oligotrophic northern South China Sea?

582

Geophysical Research Letters 39, L05811. https://doi:10. 1029 /2011GL050415, 2012

583

Wang, S.H., Tsay, S.C., Lin, N.H., Hsu, N.C., Bell, S.W., Li, C., Ji, Q., Jeong, M.J., Hansell,

584

R.A., Welton, E.J., Holben, B.N., Sheu, G.R., Chu, Y.C., Chang, S.C., Liu, J.J., Chiang,

585

W.L., 2011. First detailed observations of long-range transported dust over the northern

586

South

587

https://doi.org/10.1016/j.atmosenv.2011.04.077

588 589

China

Sea.

Atmospheric

Environment

45,

4804-4808.

Webster, P.J., 1994. The role of hydrological processes in ocean-atmosphere interactions. Reviews of Geophysics 32, 427-476. https://doi.org/10.1029/94RG01873

590

Xiao, J.L., Poter, S.C., An, Z.S., Kumai, H., Yoshikawa, S., 1995. Grain size of quartz as an

591

indicator winter monsoon strength on the Loess Plateau of central China during the last

592

130,000 yrs. Quaternary Research 43, 22–29.

593

Zhang, X.Y., Arimoto, R., An, Z.S., 1997. Dust emission from Chinese desert sources linked to

594

variations in atmospheric circulation. Journal of Geophysical Research 102(D23), 28041-

595

28047. https://doi: 10.1029/97JD02300

596 597

Figure caption

598

Fig. 1. Positions of samples sites in the South China Sea. The red lines indicate the dust samples,

599

orange and green points report trap and surface sediment samples, respectively. Monsoon winds

600

after Webster (1994), grey dotted line with arrow, grey line with arrow indicate the Winter 20

601

monsoon and Summer monsoon, respectively; surface current after Fang et al. (1998), pink

602

dotted line with arrow, blue dotted line with arrow indicate the winter ocean circulation and

603

summer ocean circulation, respectively.

604 605

Fig. 2 The grain size distribution of dust samples from the SCS, a) during summer monsoon

606

season; b) during winter monsoon season; c) under different weather conditions; d) for the blank

607

sample with no sieving.

608 609

Fig. 3 Comparison of the grain size distribution of dust sample with sediment trap and surface

610

sediment samples collected at adjacent locations. Dust sample (14SCS07) collected from

611

13.99° N/113.03° E to 15.91° N/110.67° E, sediment trap sample collected at a water depth of

612

1500 m layers at XS1 (17.40° N, 110.92° E, water depth 1690 m), and surface sediment sample

613

D21-7 (17.69° N, 110.00° E, water depth 1740 m) from the northern SCS.

614 615

Fig. 4 SEM images of quartz surface features from atmospheric dust sample (a–c) and surface

616

sediment sample (d–i, 11E406) in the northern SCS, a) sub-round to round; b) U-shaped; c)

617

meander ridge; d)–e) dish-shaped; f) meander ridge; g) angular; h) V-shaped; i) conchoidal

618

fracture.

619 620

Fig. 5 The simulated 5-day back trajectories at 0, 1000, and 3000 m levels were calculated under

621

different prevailing wind and weather conditions. The red, blue, and green lines represent the

622

trajectory at 0, 1000, and 3000 m altitudes, respectively. a)–c) samples collected under the

623

summer monsoon; d)–f) samples collected under the winter monsoon. For further air movement

624

trajectory information, please see http://www.arl.noaa.gov/ready/hysplit4.html.

625 626

Table caption

627

Table 1 Sampling periods and number collected in the different prevailing wind in the South

628

China Sea

629

Table 2 Airborne dust sampling situation during summer monsoon prevailing wind period of

630

South China Sea

21

631

Table 3 Airborne dust sampling situation during winter monsoon prevailing wind period of South China

632

Sea

22

Table 1 Sampling periods and number collected in different prevailing wind in the South China Sea Prevaili ng wind Winter monsoo n

Summe r monsoo n

Sampling period 2010: 11/07-11/08;11/08-11/09;11/10-11/11;11/11-11/12;11/13-11/14;11/15-11/16; 11/16-11/17;11/17-11/18;11/18-11/19;11/19-11/20;11/21-11/22;11/22-11/23; 1/25-11/26 2011: 11/29-11/30;11/30-12/01;12/08-12/09;12/09-12/10;12/10-12/11;12/11-12/14; 12/14-12/15;12/16-12/17;12/19-12/20;12/20-12/21;12/21-12/22;12/22-12/25;12/25-12/3 0;12/30-12/31 2012: 12/31-01/01;01/01-01/02;01/02-01/03;02/23-2/24;02/24-02/25;04/18-04/19; 04/19-04/20; 04/20-04/21 2011: 08/25-08/25;08/26-08/27; 08/28-08/29; 08/30-08/31 2014: 08/27-08/29;08/29-08/31;08/31-09/02;09/02-09/04;09/05-09/07;09/07-09/09;09/09-09/1 1;09/12-09/14;09/19-09/21; 09/22-09/24; 09/25-09/27; 09/27-09/29

Samp le numb er 13 14 8 4 12

Table 2 Airborne dust sampling situation during summer monsoon prevailing wind period of South China Sea Sample No.

Sample hour/h

11SCS01

12

11SCS02

24

11SCS03

24

11SCS04

24

14SCS01

48

14SCS02

48

14SCS03

48

14SCS04

48

Start

End

21.58°N /118.53°E 20.05°N /119.35°E 20.10°N /115.84°E 20.30°N /114.65°E 23.09°N /113.41°E 18.94°N /114.04°E 15.45°N /111.99°E 12.55°N /111.54°E

20.05°N /119.35°E 20.10°N /115.84°E 20.30°N /114.65°E 20.92°N /113.40°E 18.94°N /114.04°E 15.45°N /111.99°E 12.55°N /111.54°E 12.48°N /114.04°E

Flux/m3

Weather

Grain size (>10µm)

Sample No.

Sample hour/h

753

Fine

29.4%

14SCS05

48

1507

Fine

32.8%

14SCS06

48

1174

Fine

36.6%

14SCS07

48

1126

Rain

37.6%

14SCS08

48

1026

Fine

21.25%

14SCS09

48

1030

Fine

38.72%

14SCS10

48

1030

Fine

30.86%

14SCS11

48

1030

Fine

35.28%

14SCS12

48

Start

End

12.48°N /114.01°E 12.99°N /110.51°E 13.99°N /113.03°E 15.91°N /110.67°E 18.17°N /109.46° E 8.00°N /111.01°E 11.12°N /112.66°E 20.83°N /117.58°E

12.99°N /110.51°E 13.99°N /113.03°E 15.91°N /110.67°E 17.23°N /109.51°E 10.00°N /110.67°E 11.12°N /112.66°E 20.83°N /117.58°E 22.01°N /113.89°E

Flux/m3

Weather

Grain size (>10µm)

1030

Fine

36.55%

1030

Fine

35.27%

1028

Rain

7.69%

1032

Typhoon

26.18%

1026

Fine

33.57%

1028

Fine

32.97%

1029

Fine

22.28%

1029

Fine

16.58%

Table 3 Airborne dust sampling situation during winter monsoon prevailing wind period of South China Sea Sample No.

Sample hour/h

10SCS01

24

10SCS02

24

10SCS03

24

10SCS04

24

10SCS05

24

10SCS06

24

10SCS07

24

10SCS08

24

10SCS09

24

10SCS10

24

10SCS11

24

10SCS12

24

10SCS13

24

11SCS05

24

11SCS06

24

11SCS07

24

11SCS08

24

11SCS09

24

Start

End

15.00°N /112.99°E 11.99°N /112.99°E 9.39°N /113.28°E 6.99°N /112.99°E 5.99°N /109.49°E 10.00°N /110.99°E 9.85°N /113.99°E 9.99°N /117.00°E 14.37°N /118.74°E 20.36°N /119.80°E 18.00°N /115.50°E 17.99°N /111.20°E 18.33°N /110.33°E 22.59°N /113.75°E 17.50°N /113.00°E 5.98°N /112.35°E 6.01°N /109.94°E 6.06°N /106.74°E

11.99°N /112.99°E 9.79°N /112.93°E 6.99°N /112.99°E 6.01°N /112.50°E 9.01°N /109.25°E 9.85°N /113.99°E 9.99°N /117.00°E 14.37°N /118.74°E 20.36°N /119.80°E 17.90°N /117.98°E 17.99°N /111.20°E 18.33°N /110.33°E 22.32°N /113.77°E 17.50°N /113.00°E 12.97°N /113.00°E 6.01°N /109.94°E 6.06°N /106.74°E 4.21°N /106.07°E

Flux/m3

Weather

Grain size (>10µm)

Sample No.

Sample hour/h

2995

Rain

42.1%

11SCS10

72

3009

Rain

40.4%

11SCS11

24

2967

Fine

32.3%

11SCS12

24

2981

Fine

42.6%

11SCS13

24

2958

Fine

43.6%

11SCS14

26

3009

Rain

55.3%

11SCS15

24

2996

Fine

46.9%

11SCS16

72

3010

Fine

47.1%

11SCS17

120

2996

Fine

36.0%

11SCS18

24

3009

Fine

39.4%

12SCS01

24

2980

Fine

40.3%

12SCS02

24

3019

Fine

34.9%

12SCS03

24

2999

Fine

38.4%

12SCS04

24

1347

Fine

50.1%

12SCS05

24

1301

Rain

43.8%

12SCS06

24

1258

Fine

46.6%

12SCS07

24

970

Fine

54.2%

12SCS08

24

1255

Fine

47.5%

Start

End

2.58°N /105.48°E 2.56°N /106.49°E 9.02°N /111.12°E 3.93°N /109.07°E 1.78°N /108.40°E 2.85°N /108.23°E 7.88°N /113.01°E 9.57°N /112.94°E 9.69°N /113.21°E 13.99°N /112.52°E 13.99°N /112.52°E 13.99°N /112.52°E 22.91°N /113.56°E 22.91°N /113.56°E 14.89°N /112.35°E 18.77°N /113.35°E 22.91°N /113.56°E

2.56°N /106.49°E 7.42°N /108.58°E 9.57°N /112.94°E 1.78°N /108.40°E 2.85°N /108.23°E 7.88°N /113.01°E 9.57°N /112.94°E 9.69°N /113.21°E 13.99°N /112.52°E 13.99°N /112.52°E 13.99°N /112.52°E 13.99°N /112.52°E 22.91°N /113.56°E 18.75°N /113.48°E 18.77°N /113.35°E 22.91°N /113.56°E 22.91°N /113.56°E

Flux/m3

Weather

Grain size (>10µm)

3613

Fine

36.6%

1286

Typhoon

48.5%

1287

Typhoon

51.3%

1247

Rain

50.8%

1242

Rain

36.4%

1201

Fine

52.8%

3296

Typhoon

42.7%

5070

Typhoon

40.2%

1074

Fine

47.7%

1159

Fine

58.2%

1117

Fine

52.7%

1120

Fine

41.9%

1218

Fine

34.5%

1499

Fine

49.2%

1477

Rain

53.0%

1498

Rain

39.9%

1498

Fine

23.1%

it ra

Ba

ish

iS tr

ai

t

Ta i

w

an

St

Continent of Asia

N

Hainan 11E406

CS 11 S

Sunda Shelf

10SCS07

16

10SCS06

E

•

Present-day dust grain size distribution, seasonal trend change due to wind strength and dust availability in the South China Sea

•

Modeled 5-day back trajectories show transport pathway and possible source in different prevailing monsoon wind for dust deposition

•

Scanning electron microscope as a reference to distinguish continent components from terrigenous sediments.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: