JOURNAL OF RARE EARTHS, Vol. 30, No. 11, Nov. 2012, P. 1184 Sediment provenance discrimination in northern Okinawa Trough during the last 24 ka and paleoenvironmental implication: rare earth elements evidence XU Zhaokai (徐兆凯)1, LI Tiegang (李铁刚)1, CHANG Fengming (常凤鸣)1, CHOI Jinyong2, LIM Dhongil3, 4 XU Fangjian (徐方建) (1. Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China; 2. Department of Oceanography, Kunsan National University, Kunsan 573-701, Korea; 3. South Sea Institute, Korea Ocean Research and Development Institute, Geoje 656-830, Korea; 4. School of Geo- sciences, China University of Petroleum, Qingdao 266555, China) Received 28 February 2012; revised 28 September 2012 Abstract: Rare earth elements (REE) compositions and discriminant function were successfully used to examine high resolution sediment source changes in the northern Okinawa Trough over the last 24.1 ka, especially for the influence from the Yellow River and the Tsushima Warm Current (TWC) that has not been well solved. Variations of these parameters were clearly divided into three distinct depositional units. During Interval 1 (24.1–16.0 ka BP), the paleo-Yellow River and the paleo-Yangtze River mouths were situated near the studied area and could have played major roles in the sedimentation therein. In Interval 2 (16.0–7.3 ka BP), these river mouths gradually retreated with global sea-level rise, leading to less fluvial inputs from them to the northern Okinawa Trough. Meanwhile, formation of the TWC could carry some sediment loads of Taiwan to the studied core, especially during its late phase (8.0–7.3 ka BP). Modern oceanographic conditions, with a predominance of the TWC, were finally established since the beginning of Interval 3, causing more terrigenous contribution from Taiwan to the studied area. Sub- sequently, modern depositional environments mainly influenced by the Yellow River, the Yangtze River, and the TWC were finally formed. Keywords: sediment provenance; discriminant function; rare earth elements; Tsushima Warm Current; northern Okinawa Trough The northern Okinawa Trough is influenced not only by the discriminant function (DF) based on REE pairs has been saline water masses derived from the Tsushima Warm Cur- successfully used for sediment source research in the north- rent (TWC) but also mainland China freshwater in the late western Pacific marginal seas[23], given that the ratios of Quaternary (Fig. 1)[1–3]. Therefore, its sediment provenance these elements could better offset the grain size and carbon- study is of great significance for the reconstruction of ate dilution effects than the absolute contents of them. source-to-sink history of materials from surrounding land- However, a comprehensive provenance discrimination based mass and current as well as of land-ocean interaction during on REE indices with high time resolution has not been pre- the late Quaternary[4–10]. Previous researches deduced that viously conducted for core sediments in the northern Oki- sediment provenances of the Okinawa Trough mainly in- nawa Trough. Based on abundant AMS 14C, grain size, and clude terrigenous sediments carried by the Yangtze River, REE data of core PC-1, detailed sediment provenance the Yellow River, and the TWC[1,4,5,9,11,12]. Besides, materials change, paleoenvironmental evolution, and the mechanism eroded from the outer shelf of the East China Sea and the were discussed in this paper. episodic volcanic eruptions possibly influenced the Okinawa [4,5,13] Trough sedimentation, too . But detailed contribution 1 Materials and methods changes of them over time have remained unresolved, espe- cially in the northern Okinawa Trough. For example, Core PC-1, 812 cm long, was collected by the Institute of whether and how far the materials carried by the Yellow Oceanology, Chinese Academy of Sciences (IOCAS) in 1993 River and the TWC can arrive at there? from a water depth of 590 m in the northern Okinawa Trough Rare earth elements (REE) are well accepted as reliable (Fig. 1). It is characterized by continuous deposition of silty provenance tracers[4,14–21]. Whereas biologic materials (e.g., clay ooze (0–165 cm), clayey silt (165–410 cm), silty clay carbonate) only have minor contributions to the bulk REE (410–610 cm), and fine silt (610–812 cm). Two layers of vol- compositions in the Okinawa Trough sediments[20]. In fact, canic matter are confined to 123–130 and 315–330 cm, re- there is only limited change in carbonate content in the spectively[24]. northern Okinawa Trough over the last 25 ka[22]. In particular, 395 samples generally at 2 cm intervals were powdered. Foundation item: Project supported by the Public Science and Technology Research Funds Projects of Ocean, State Oceanic Administration of the People's Repub- lic of China (201005003), the National Natural Science Foundation of China (41230959, 41106043, 41076030), and the National Basic Research Program of China (2007CB815903) Corresponding author: LI Tiegang (E-mail: [email protected]; Tel.: +86-532-82898522) DOI: 10.1016/S1002-0721(12)60202-6 XU Zhaokai et al., Sediment provenance discrimination in northern Okinawa Trough during the last 24 ka and … 1185 nese Academy of Geological Sciences. Differences between the determined and certified values of national geostandards GSD9, GSS22, and GSS23 were mostly less than 5%. Be- sides, grain size measurement was performed on bulk sam- ple by a Cilas Particle Size Analyzer (Cilas 940L) in IOCAS. Cilas 940L accounts for grains from 0.5 to 2000 µm with analytical precision better than 2%. AMS 14C ages of planktonic foraminifer and mollusk shells from ten layers were measured by the National Ocean Sciences AMS Facility, Woods Hole Oceanographic Institu- tion and calibrated using Calib6.0 (http://calib.qub.ac.uk/ calib/calib.html). All ages here are reported as calibrated calendar 14C ages before AD 1950. The average higher linear sedimentation rate (LSR) of core PC-1 (33.7 cm/ka) than those of nearby cores in the northern Okinawa Trough[3,7], together with the present high average sampling resolution of 60.8 years, provides good chance for detailed sediment provenance study. DF was used to estimate the proximity of the studied sam- ple to possible sediment source. The absolute DF value be- Fig. 1 Schematic map showing the locations of core PC-1 and other low 0.5 is generally accepted as indicating a close relation- typical reference cores in the Okinawa Trough and adjacent ship in material origin between them. The smaller DF value areas (KCC=Korea Coastal Current, YSCC=Yellow Sea indicates the closer relationship. The formula used for DF Coastal Current, TWC=Tsushima Warm Current, YDW= calculation is cited from Xu et al.[23]: Yangtze River Diluter Water, ZFCC=Zhejiang-Fujian Coastal DF=|(C1M/C2M)/(C1P/C2P)–1| (1) Current, TSF=Taiwan Strait Flow, PYV1=paleo-Yellow where (C1M/C2M) is the ratio of concentrations of element 1 River valley, PYV2=paleo-Yangtze River valley) to element 2 in the studied sediment, (C1P/C2P) is the ratio of concentrations of element 1 to element 2 in the possible In each case, powdered sample was weighed and put into a provenance. Teflon pot, and admixed with nitric acid and hydrofluoric acid. These mixtures were put into a sealed container and 2 Results heated on a hot plate at 150 ºC for 24 h. Then, the hot plate was cooled to 120 ºC and perchloric acid was added, and A summary of grain size and REE compositions of core samples were dried with containers open. The resultant cake PC-1 sediments and of the reference materials is presented in was cooled and dissolved in cold nitric acid. Each solution Table 1. The anterior 6 elements of REE (La–Eu) are called was analyzed for contents of REE and selected elements LREE, expressed by ΣLREE, and the remaining 9 elements (Mn, Ti, Nb, and Th) using ICP-MS and ICP-OES in the In- (Gd–Lu and Y) have a name of HREE, represented by stitute of Geophysical and Geochemical Exploration, Chi- ΣHREE. The total REE amount is denoted as ΣREE. Table 1 A summary of grain size and REE compositions of core PC-1 sediments and potential provenances Samples Mz/ ΣREE/ (La/Yb)UCC (La/Sm)UCC (Gd/Yb)UCC References Φ (µg/g) PC-1 Interval 3 7.5 130.8 0.89 0.91 0.98 This study Interval 2 7.3 144.3 0.97 0.96 1.03 Interval 1 7.2 162.3 1.08 0.99 1.09 Whole 7.3 150.1 1.01 0.96 1.05 Yangtze River Surface sediments 6.3 186.6 1.17 0.93 1.40 Yang et al.[15,21] – 213.8 1.06 0.88 1.08 Xu et al.[17] Yellow River Surface sediments 5.1 148.0 1.05 0.93 1.32 Yang et al.[15,21] 5.6 178.8 0.95 0.86 1.02 Xu et al.[17] Taiwan Main source – 156.4 0.93 0.86 1.14 Chen et al.[26] end-member Okinawa Volcanic – 93.7 0.29 0.39 0.92 Liu and Meng[20] Trough end-member [27] Volcanic rocks – 146.5 0.37 0.62 0.74 Shinjo and Kato a Mz=mean grain size, UCC refers to upper continental crust normalization, – means data unavailable 1186 JOURNAL OF RARE EARTHS, Vol. 30, No. 11, Nov. 2012 Overall, these indices show regular fluctuations (Fig. 2). to the underlying Intervals 1 and 2 sediments. Characteristically, the core contains two distinct layers at 123–130 cm and 315–330 cm with remarkably different 3 Discussion components, corresponding to the K-Ah tephra of 7.3 ka BP in age and possible to the U-Oki tephra at 9.3–12.8 ka 3.1 Provenance discrimination of the core sediments BP[4,13,24,25].