Chinese Science Bulletin

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Nd isotopic variations and its implications in the recent sediments from the northern South China Sea

SHAO Lei1†, QIAO PeiJun1, PANG Xiong2, WEI GangJian3, LI QianYu1, MIAO WeiLiang1 & LI Ang1

1 State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China; 2 CNOOC Ltd. Shenzhen, Guangzhou 510240, China; 3 Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

The chondrite-normalized REE distribution patterns in recent sediments from the northern South China Sea and surrounding areas are similar, with the HREE values being almost equal to or slightly higher, and the LaN/LuN ratio being lower, than PAAS standards. However, samples from the Xijiang River, a major branch of the Pearl River system, show opposite trends, i.e., with slightly lower HREE values and higher LaN/LuN ratio than PAAS. The distribution of Nd isotopes in sediments from the northern South China Sea was controlled by the Pearl River and the inshore area of South China, respectively. The volume of εNd(0) from inshore areas of southern China is apparently higher than that from the Pearl River, and εNd(0) values in offshore sediments and Taiwan Island are between these values. The results clearly show that εNd(0) volume in the northeastern South China Sea is higher than in the southwest part of the northern South China Sea, indicating different source rock types. The main source rock of the Pearl River sediment is carbonate types and affects sediment deposition in the southwest of the northern South China Sea, while the source rock in the inshore area of southern China, mainly Meso-zoic and Cenozoic granites, contributes to sediments in the northeastern area of the northern South China Sea. Due to different source rock types, the HREE values and the volume of εNd(0) in recent sediments supplied by the inshore area of southern China are higher than those from the Pearl River. Over 80% of the rapidly accumulated sediments on the northern slope of the South China Sea in the southeast of Dongsha Islands area were derived from Taiwan Island and from inshore areas of South China, with only less than 20% from the Pearl River.

South China Sea, Nd isotope, provenance, marine sedimentology, sedimentary geochemistry

The northern South China Sea (SCS) is one of the most active sedimentary areas in the region with extremely complex geological activities. The shallow shelf devel-oped from the oldest river in East Asia―the Pearl River, and sediment drift sequences with high sedimentation rates of up to 470 m/Ma on the slope and deep water fan sediments in the deep-sea have been found[1―8]. Espe-cially, the recent discoveries of deep-sea gas field and gas hydrate resource make this area one of the most im-portant research regions in the world. Sedimentary stud-ies in the area began in the 1980s. Particularly through the 1994 Sino-German program ‘Sonne 95’ voyage, in systematic deep-sea sampling, we have found a high

sedimentation rate drift in the southeast of the Dongsha Islands[1]. The average sedimentation rate for the drift in the last 40000 years is about 33 cm/ka, based on the δ 18O record and AMS 14C dating[2]. ODP Leg 184 drilled 5 sites in this area in 1999, and the drilling record from Site 1144 on the sediment drift reveals the average sedimentation rate of 49 cm/ka for the last 1.05 Ma[3], the highest sedimentation rate record found so far in the

Received June 2, 2008; accepted September 2, 2008; published online October 15, 2008 doi: 10.1007/s11434-008-0453-8 †Corresponding author (email: lshao@mail.tongji.edu.cn) Supported by National Natural Science Foundation of China (Grant Nos. 40276019, 40238060 and 40621063), and National Basic Research Program of China (Grant No. 2007CB819501)

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SCS. However, seismic profiles from Site 1148, in the southwest of Dongsha Islands, display that sediments at the site were drifted from the Pearl River delta, under the control of a Pearl River source[4―7] with the average sedimentation rate of only 7 cm/ka[3], which is similar to the value from the deep ocean. All these records demon-strate the complexity of deposition in this area of the northern SCS.

The likely input sources of the recent terrigenous sediments in the northern SCS include the Red River, Hainan Island, Pearl River, Hanhe River, Taiwan Island, Luzon, etc. Due to the separation by Hainan Island, sediments from the Red River are mainly deposited in the northwestern SCS. Areas to the east Hainan Island are mainly affected by input sources of the Pearl River, Hanhe River and Taiwan Island, while sediments carried by ocean currents through the Taiwan Strait and Bashi Channel are also deposited in the northern SCS[9―11]. The questions are, therefore, what are the main charac- teristics of sediment compositions in different input sources? How are sediments from different sources dis- tributed in the northern SCS? Where did the sediment of the high deposition rate drift in the southeastern Dong- sha Islands come from? These questions need to be an- swered by detailed sedimentological studies. Above all, the difference between sediments from different sedi-

mentary sources and their distribution extents in the SCS are the key issues of the SCS sedimentology. Because the deep water sediment consists mainly of terrigenous clay sediments, geochemical analyses used in this paper will be better than other methods to decipher their provenance.

1 Material and methods

Thirty-nine samples were collected from modern clay sediments of main rivers around the northern SCS and cored sediments of over the last 2 Ma at different water depths from offshore (Figure 1). Site 1144 of ODP Leg 184 is located on the southeast of the Dongsha Islands (20°3.18′N, 117°25.14′E, 2037 m water depth). Most of the Site 1144 sediments are dull gray-green clay, from which ten samples covering the last 1 Ma were collected. Site 1148 is located in the southwestern Dongsha Islands (18°50.17′N, 116°33.94′E, 3294 m water depth), and three samples of the uppermost terrigenous clay repre-senting the last 1.7 Ma were collected[12]. Three samples from the ancient Pearl River delta (Lh1, PY1―2) repre-senting the 2 Ma deposit were taken. Meanwhile, four samples (Z1―2, Z3―4) from the main branch of Pearl River-Xijiang River, five samples from the Lingdingy- ang Sea (L1―5), three samples from the Hanhe and

Figure 1 Location of the studied area and samples analyzed in the northern South China Sea.

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Minhe Rivers (H1, H2, M1), five samples from the Quaternary deposit of western Taiwan Island (T1―5) and five surface samples from the southwestern shelf of Taiwan Island (T6―10) were also analyzed for com-parison (Figure 1).

Samples were heated first and dried at low tempera-ture (~50℃), crushed, and iglossed for 30 min at 350℃ to remove the organic matter and the interlayer water of clay minerals. CaCO3 was removed by adding 0.1 mol/LHCl, in order to avoid the interference of authi-genic components on the analytic results. Samples were then dissolved with HF+HNO3 and analyzed by induc-tively coupled plasma emission mass spectrometry (ICP-MS). Each sample required 6 run of analyses and was calibrated with international rock standards (e.g., GSR-1, JSD-1), and checked with drift control samples, duplicate samples, and blank samples. Accuracy and precision were estimated and monitored by the control of samples and duplicates, with RSD less than 2%. All samples were prepared and analyzed, respectively, in the State Key Laboratory of Marine Geology, Tongji Uni-versity and in the Laboratory of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.

2 Results and discussion

The sedimentary input sources of the northeastern SCS mainly include the coastal area of South China, Taiwan Island and the Pearl River source area over the Yunan Guizhou Plateau. The stratigraphy of western Taiwan Island is similar to that in the coastal area of South China[13]. However, extensive Mesozoic magmation in coastal areas of South China does not exist in west Tai-wan Island, where only some Late Cenozoic volcanic sediments developed, suggesting a different sedimentary source region from the coastal South China. Whether there is any difference between the components of source rocks from coastal areas of South China and from the inner South China plate is a primary focus of re-search of sediments from the northern SCS. Furthermore, sedimentation rates at Site 1144 varied in the last 1.05 Ma, when the content of terrigenous clasts also changed with time. Three major stages can be recognized on a descending order: (1) 0―210 m (0―0.21 Ma) with ex-tremely high sedimentation rates of about 100 cm/ka; (2) 210―405 m (0.21―0.9 Ma) with sedimentation rates of only 28.3 cm/ka; and (3) 405―519 m (0.9―1.05 Ma)

with increasing sedimentation rates to about 76 cm/ka[3]. The average contents of the terrigenous material varied accordingly, indicating that the input of the terrigenous material played a significant role in controlling the sedimentation rates. Therefore, whether the sedimentary source areas in the northern SCS had also changed with the changing sedimentation rates in different periods remains an important question to be answered.

2.1 Distribution characteristics of REE

It has been proved that the contents of rare earth ele-ments (REE) and their distribution patterns are prove-nance related. Sediments derived from different source rocks have quite distinct REE contents and distribution patterns[14―16]. Although sedimentation rates varied with time at Site 1144, the REE values and distribution pat-terns from different core levels remain almost un-changed. The REE distribution patterns show a clear LREE enrichment, unchanged HREE and a negative Eu-anomaly. Compared with PAAS distribution of the upper continental crust, the REE content of terrigenous sediments from Site ODP1144 was on the low side of the absolute value, but the LaN/LuN ratio of PAAS (9.455) differs considerably from its counterpart at Site 1144 (7.563), illustrating higher HREE values at Site 1144 than PAAS (Figure 2(a)). In fact, the high content of volcanic component in the source rock always shows low LREE but comparatively high HREE. The presence of Eu-depletion follows the general pattern of Eu-deple- tion in the upper continental crust and Eu-enrichment in the lower crust based on element fractionation[15―17]. According to Figure 2(b), the REE patterns of terri-genous sediments around the northern SCS are similar, and REE values are close to or less than PAAS, but the values of HREE are slightly higher. Comparatively, the REE (HREE) values from Xijiang River sediments were slightly higher (lower) than PAAS, while the ratio of LaN/LuN in river sediments reaches 11.735, obviously higher than the PAAS value (Figure 2(b)).

2.2 Nd isotope distribution characteristics

After weathering, transportation, and diagenesis, only stable minerals might be preserved as sediments, leading to different chemical compositions from different sedi-mentary rock types. It has been demonstrated that the chemical composition of source rocks is the major factor determining the chemistry of sediments, whereas sig-nificant chemical changes may take place during deposi-tion[17]. Of great importance is the observation that some

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Figure 2 Chondrite-normalised REE distribution for samples from ODP Site 1144 (a) and samples from other northern SCS and surrounding localities (b), showing a parallel trend in REE distribution for all analyzed samples with those of PAAS, but with slightly higher HREE and relatively constant REE values at times.

critical elemental ratios (e.g. Sc/Ti, etc.) are not seri-ously affected by the processes of deposition. These two elements have almost the identical nature, and their contents varied enormously in different rocks. They both are high in basite and low in acidite, but their contents in sedimentary mudstone-sandstone-carbonate rock types are often reduced significantly, compared to mother rock types[18―21]. As they are almost insoluble in water, they are the best indices for provenance analysis. Similar to Nd isotopes, isotopes and ratios in some other elements also show little effects by sedimentation processes, so they can also help to trace the evolutionary history of sedimentary provenance[20―22]. Figure 3 indicates that the distribution of Nd isotopes shows distinct provin-cialism in the northern SCS, with a maximum initial ratio in the coastal area of South China, εNd(0) ranging from −8.576 to −10.186, followed by samples from Taiwan Island from −11.078 to −11.501, and the Ling-dingyang Sea at about −11.815. The εNd(0) values in sediments from the Xijiang River change little, from

−13.135 to −13.533, while those from the Pearl River Delta gradually increase from shallow water to deep ocean, i.e., from −12.323 on the continental shelf, to −12.128 on continental slop and −12.074 in the deep sea (Figure 3). In a whole, the northeast area to a proposed separation line along the Lingdingyang Sea-Dongsha Islands has a relatively high ratio of Nd isotopes, while in the southwest area the ratio is low, indicating the dis-tribution characteristics of sediments from different ori-gins in the northern SCS.

2.3 Provenance analysis Recent terrigenous sediments in the northern SCS are mainly transported by the Pearl River, Hanhe River, riv-ers in Taiwan Island, and ocean currents from the Tai- wan Strait and Bashi Channel. Sediments from the Red River hardly reach east Hainan due to the separation by Hainan Island and the Coriolis force. Although the Pearl River is the largest river entering the northern SCS, it transports only about 82.78×106 tons of sediments per year into the region. The Hanhe River is similar to the

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Figure 3 εNd(0) distribution in sediments from the northern SCS and its surrounding areas.

Pearl River in terms of the concentration of sediment load. Its water volume is only one tenth of the Pearl River, and the amount of its transported sediments is just about 7×106 t/a. However, the total drainage area of the Kaoping River and Tsengwen rivulets in southwestern Taiwan Island is less than one hundredth of the Pearl River, but their transported sediments amount to a total of 67×106 t/a, in a similar range as those of the Pearl River[23,24]. In addition, several other rivulets along the continental side of the Taiwan Strait also flow into the strait. Therefore, the northeast coast including Taiwan Island is an essential sedimentary source in the northern SCS. Several cores have been drilled at three Sites 1146, 1147 and 1148 in the southwest of the Dongsha Islands during the ODP Leg 184 cruise. As these sites are closer to the delta of the Pearl River than Site 1144[3], they were expected to have received more deposits from the river. However, the sedimentation rate at Site 1144 is much higher than that at Sites 1146, 1147 and 1148, as evidenced by the average Quaternary sedimentation rates: 97 cm/ka at Site 1144, 12 cm/ka at Site 1146, 4.1 cm/ka at Site 1147, and only 2 cm/ka at Site 1148. Therefore, the sediment drift in the southeast of the Dongsha Islands would have received sediments from sources other than the Pearl River.

Figure 4 clearly shows that the modern sediment in the northern SCS is mainly controlled by two prove-nances: (1) Pearl River, including the Pearl River Delta

and Site 1148, with elements characterized by wide-spread Palaeozoic carbonates; and (2) Taiwan Island and coastal areas of South China with numerous Mesozoic- Cenozoic petrosilex in addition to Mesozoic-Palaeozoic sedimentary rock, resulting in a maximum ratio of Nd isotopes. While the sedimentary rock types in Taiwan Island are similar to those in the coastal area of South China, Mesozoic-Palaeozoic petrosilex was under de-veloped, but the presence of some volcanic rocks gave rise to a lower Nd initial ratio than those contemporary strata from the coastal area. The Lingdingyang Sea is the estuary of the Dongjiang and Beijiang rivers, two branches of the Pearl River, and has a drainage area in the coastal South China. Apart from these two rivers, the sea also receives sediments transported southwestward by alongshore current from the source region of coastal area of South China, thus the εNd(0) is −11.815 in sedi-ments from this region. If the samples from the Pearl River Delta and southwestern shelf of Taiwan Island are taken as two end members (Figure 4), it is clear that over 80% sediments at Site 1144 come from Taiwan Island and coastal areas of South China and only less 20% from the Pearl River source, in good accordance with the results of other studies[8,11].

Terrigenous sediments from the northeast are trans-ported by water currents into the SCS. Seismic profiles provide evidence that there are strong deep water current transport and deposition in the northern SCS[11,25], car-

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Figure 4 The εNd(0) and Sc/Ti ratios in sediments from the north-ern SCS and surrounding areas, showing two distinctive source rock provenances.

rying sediments from northeast to southwest[8,11]. The results of pollen and foraminifer δ 13C analyses indicate strong water currents flowing via the Bashi Channel into the SCS[9,10]. Therefore, it is most likely that sediments from the coastal northeast including Taiwan Island were transported through the Penghu Channel into the SCS, and affected by water currents from the Bashi Channel, forming a contourite deposition. Under the effect of Coriolis force, sediments from the Pearl River are mainly carried to the southwest after they enter the SCS. While a great amount of them is unloaded on the shelf to form the Pearl River Delta, a smaller amount can be transported farther into the deep sea. Therefore, the dis-tribution of sediments from the Pearl River is limited to the southwest of the Dongsha Islands.

Due to the alongshore current, sedimentation between the Pearl River Delta and the Leizhou Peninsula is con-trolled by the source of the Pearl River Delta[26], al-though the influence by the Hainan Island on the re-gional sedimentation is not yet clear.

3 Conclusions

The REE patterns in Quaternary and modern terrigenous sediments from around the northern SCS were similar, with enriched LREE, constant HREE and a negative Eu-anomaly. Compared to the standard values from the upper continental crust (PAAS), the REE values in sediments from around the northern SCS are similar or slightly less, while the HREE values are slightly higher

with a significantly lower LaN/LuN ratio. In Xijiang River sediments, the REE values were slightly higher and the HREE values were slightly lower than PAAS, while the LaN/LuN ratio was noticeably higher than PAAS. It is apparent, therefore, that REE differences exist between the Pearl River source region and the source region along the coastal South China and Taiwan Island. Although the sedimentation rate of the sediment drift on the northern slope of the South China Sea varied with time, the REE values and their distribution pattern are almost the same, indicating the same source area over time.

The Nd isotopes and the Sc/Ti ratio show strong pro-vincialism in the northern SCS, with a maximum εNd(0) in the coastal area of South China, a minimum εNd(0) in Xijiang River sediments, and intermediate values in samples from Taiwan Island and neighboring areas. If linking between the Lingdingyang Sea and Dongsha Islands as a proposed separation line, the northeast SCS area has a relatively high ratio of Nd isotopes, while the southwest has a lower ratio. In the Pearl River Delta, the εNd(0) increases gradually offshore, from −12.323 on continental shelf, −12.128 on continental slop, then to −12.074 in the deep sea, indicating the distribution char-acteristics of sediments from different origins in the northern SCS.

The recent terrigenous sediments in the northern SCS are mainly contributed by the Pearl River, the coastal area of South China and Taiwan Island. To the southwest of the line from the Lingdingyang Sea to the Dongsha Islands, the Pearl River source region exerts a major impact, which was controlled by the Xijiang River, the main branch of the Pearl River, with elements charac-terized by Palaeozoic carbonates. To the northeast of the line, the source region of the coastal area of South China and Taiwan Island is most important. Numerous Meso-zoic-Cenozoic petrosilex bodies occur in the coastal area of South China in addition to Mesozoic-Palaeozoic sedimentary rocks, which differ from Taiwan Island dramatically. However, the difference in elemental components between these two regions and the Pearl River source region is much bigger, as characterized by higher HREE values and εNd(0) and a lower Sc/Ti ratio than the Pearl River source. The Lingdingyang Sea represents the estuary of Dongjiang and Beijiang rivers, two branches of the Pearl River. In addition to sediments from these two rivers, it also receives sediments trans-ported by the alongshore current from coastal areas of

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South China. Over 80% sediments of the high sedimen-tation rate drift on the northern slope of the SCS come from Taiwan Island and coastal areas of South China, and only less than 20% from the Pearl River source.

We thank Prof. Li Xinhua, Dr. Liang Xirong and Ms. Liu Yin (Guangzhou Institute of Geochemistry, CAS) for help with chemical analyses. Prof. Wang Pinxian provided critical support in writing up this paper. Prof. Zhou Di and Prof. Qiu Yan reviewed the manuscript and offered valuable comments.

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