Abstract
The sediment sources of the Yangtze Shoal were traced by analysing surface and core sediment particle size, detrital and clay minerals, carbon and nitrogen isotopes, and radioisotope dating. In the estuary, the sediments are dominated by silty clay, high stable mineral, and extremely high illite/chlorite. Stable organic carbon isotopes (δ13C-TOC) indicated a marine-dominated mixture. On the shoal, the sediments are mainly composed of fine sand, high unstable mineral. The δ13C-TOC indicated predominantly marine sedimentation. The average TOC of core sediments was ~0.26%, and the average TN was ~0.05%. The TOC/TN was 5.4–7.8, the δ13C-TOC was −19.8 to −22.1‰, and the age of the sediments spanned the last ~10.8 ka (Holocene). The sediments and provenance of the Yangtze Shoal have been controlled by the East Asian monsoon, sea level change, riverine sediment flux and ocean circulation. The intervals 8.3–6.3 ka and 3.8–1.5 ka, are characterized by Yangtze River sources, whereas 6.3–3.8 ka and 1.5–0.8 ka, are characterized by a source mixture with Yellow River input. Tracing the multiple sources effectively confirms the hypothesis that the southern Yangtze Shoal was a delta formed by combined sedimentation from the Yangtze River and Yellow River during times of low sea level.
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Introduction
The Yangtze Shoal, the largest in the Yangtze Estuary with an area of 3 × 104 km2, is located on the estuary’s eastern side69.
Compared with mineral contents of Core ZC1 (b) and Core ZC2 (a) and records of floods and sediment input of the Yangtze River and the Yellow River over the Holocene (Compared with Zhou and Yu, 2013). The ancient flood events were mainly concentrated in the Yangtze River basin over three periods: 7.5–6.5 ka, 5.0–3.5 ka and 1.0–0.5 ka. In the Yellow River basin, the flood events were mainly concentrated in 4.2–2.8 ka and 1.5–0.8 ka. This is consistent with the provenance of the detrital and clay minerals. Data of sediment discharge of Yangtze River and the Yellow River cites http://www.mwr.gov.cn/sj/. Paleo flood records refer to Zhou68.
Conclusions
The sediment types found in the area around the Yangtze Shoal showed zonal distribution. The area around the Yangtze Estuary is dominated by silty clay (6–8 Φ), with mainly fine sand (3–6 Φ) on the shoal. The grain size of the sediment in the grooves was small, and relatively large in the sand ridges or beach surface, consistent with the groove distribution trend. The core sediments changed in grain size from the top to the bottom of the cores with alternations of fine-coarse-thin-coarse layers. For surface sediments on the Yangtze Shoal, the illite content was high (average 62%), followed by smectite (17%), this is a typical characteristic of the Yangtze River terrigenous supply. Terrigenous material from the Yellow River was mainly distributed on the northern and western part of the shoal.
The sedimentary sequence of the beach and trenches on the southern Yangtze Shoal showed rhythmic thickness changes, indicating a change in multi-period hydrodynamic conditions as a result of the balance between transgression and regression and river recharge. The results of 14C dating showed that the base of Core ZC2 (trough) was ~10.8 ka (Holocene). The age of the base of Core ZC1 (ridge) was 3.36 ka, corresponding to the mid-late Holocene, indicating that the sedimentation rate was higher at ZC1. The slower sedimentation rate in the trough also indicates that the beach ridge in the study area is dominated by erosion–deposition processes (material re-transport), and the trench is dominated by slow and stable sedimentation. The clay and detrital mineral analysis showed different values and zoned results of smectite to illite and chlorite (S/(I + C)) that indicate sediments from different intervals mainly originated from the suspended material of the Yellow or Yangtze rivers. The mineral composition of amphibole, emerald, limonite and magnetite also indicates the same source; the clay minerals were in agreement with grain size changes. This comprehensive comparison of sediment grain size, clay and detrital minerals, and 14C dating in core samples indicated the source of sediment and changes in hydrodynamic conditions.
During the Holocene, the material of the southern Yangtze Shoal mainly originated from the Yangtze and Yellow rivers. Under the influence of sea level changes and the amount of riverine sediment supply, the deposition characteristics of the Yangtze Shoal consists of a mixed source from the Yangtze River, which is predominant; the contribution from the Yellow River increased during specific periods. During the periods 6.0–4.0 ka and 1.5–0.8 ka, sea level rose and the Yellow River was diverted southwards because of flood events. Material from the Yellow River was supplied to the Yangtze Shoal through coastal currents, resulting in increased contribution from the Yellow River on the shoal. During times of lower sea level, parts of the Yangtze Shoal were exposed and received recharge from the paleo-Yellow and Yangtze rivers. While the material input from the Yangtze River drastically reduced at 8.0 ka, the material from the Yellow River increased. During all other studied intervals, material from the Yangtze River dominated.
Methods
A total 261 surface samples were used in this study with 111 samples collected during two cruises by the Science III (Open Cruise of National Science Foundation of China) on the southern part of the Yangtze Shoal in June 2013 (n = 79) and July 2015 (n = 32, Fig. 2). The remaining surface samples were collected by a fishing vessel (NO. 34002) in June 2013 (n = 15) and August 2014 (n = 12). Other surface samples were collected by “908” sedimentary survey27. Additionally, two sediment cores ZC1 and ZC2 were retrieved by another fishing vessel (NO. 79423) in August 2014. The cored sediments are dominated by continuous homogenous grey clay or silt with or without visible bioturbation. Samples were taken at 2 cm or 4 cm intervals from ZC1 and ZC2, respectively. Both surface and core sediments were used for multiple analyses in this study.
Particle size
Grain size distribution of terrigenous particles was measured on bulk sediments with a Malvern Mastersizer 2000 Particle Size Analyzer at the Third Institute of Oceanography after removing carbonate and organic matter. Bulk sediments were treated successively with 10% H2O2 and 0.1 N HCl to remove organic material and carbonate30, respectively. A Shepard method was applied to calculate mean grain size (Mz), standard deviation (σi), skewness (SKi) and kurtosis (Kg), following the formulas utilized at the Third Institute of Oceanography.
Detrital minerals
Bulk sediments were sieved and heavy minerals separated from the 63–125 μm fraction using bromoform (CHBr3) with a density of 2.89 g•cm−3. Both light and heavy minerals were analysed at Test central of minerals and rocks. Light minerals were mixed with epoxy (Epoxy-TEOA, 6:1), mounted and dried at 70 °C for 48 h, they were identified and counted using an optical microscope. For the heavy mineral analysis, a total of 300–500 particles were identified, and the sum and percentage of each species counted70.
Clay minerals
Clay mineralogy was analysed by X-ray diffraction (XRD) using a PANalytical X’Pert PRO diffractometer at the State Key Laboratory of Marine Geology (Tongji University) on oriented mounts of carbonate-free clay-sized particles (<2 μm)32,33. All the samples were measured 3 times under air-dry conditions after ethylene glycol solvation for 24 h and heating at 490 °C for 2 h. Identification and interpretation of clay mineral species were made according to the (001) basal reflections on three XRD diagrams. The proportions of clay minerals in the assemblage were calculated semi-quantitatively using the MacDiff software (Petschick, 2000), based on peak areas of basal smectite reflections, including mixed layers (15–17 Å), illite (10 Å) and kaolinite/chlorite (7 Å), of the glycolated curve. The relative proportions of kaolinite and chlorite were calculated according to the ratio of the 3.57/3.54 Å peak areas. Replicate analysis of a few selected samples gave a precision of ±2% (2σ). The semi-quantitative evaluation based upon the XRD method had an accuracy of 5% for each clay mineral species.
TOC and TN content and isotopic values
A Thermo NE1112 CN Elemental Analyzer connected to a Delta Plus AD Isotope Mass Spectrometer via Conflo III was used for sample analysis online. The elemental analysis furnace was set to 1020 °C, the reduction furnace to 650 °C and the column temperature to 40 °C31. Laboratory cylinders of CO2 and N2 were calibrated with the standards USGS-24, GBW4408 and IAEA-N1. Carbon and nitrogen isotopes are referenced to Pee Dee Belemnite (PDB) and atmospheric nitrogen, respectively. Laboratory determination accuracy was ±0.2‰. The measurements were conducted at the Third Institute of Oceanography (State Oceanic Administration).
AMS 14C dating
Approximately 15 g of sample were processed for micropaleontological identification. Each sample was fully soaked, washed over standard 250 mesh (0.063 mm) sieve and dried. The retained material was then floated with carbon tetrachloride to concentrate any microfossils, isolated specimens were identified under a binocular microscope. In general, a minimum of 300 foraminifera were counted for quantitative analyses22 and selected foraminifera were sent to the United States Beta laboratory for 14C analyses. The age model of core was established using planktonic foraminifera (G. ruber) combined with eight AMS 14C data (Table 2). These AMS 14C dates were analyzed at the Beta Analytic Laboratory (USA) and were converted to calendar years by using the CALIB 7.0 program, with a 400-year reservoir correction and intercept of radiocarbon age with calibration curve. The result (S1) shows an age of ~10806a BP at the bottom (1.56 m) of the core.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant Nos 41276058 and 41406059), the Province Natural Science Foundation of Fujian (Grant No. 2016J01190).
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Chao Cao designed the study, wrote the main manuscript and prepared all figures. Feng Cai contributed to the improvement of the manuscript design and guaranteeing the quality of the manuscript. Yongling Z and **g**g Bao processed the data. Chengqiang Wu, Huiquan Lu. And Quan Sun collected the data. All authors reviewed the manuscript.
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Cao, C., Cai, F., Zheng, Y. et al. Temporal and spatial characteristics of sediment sources on the southern Yangtze Shoal over the Holocene. Sci Rep 8, 15577 (2018). https://doi.org/10.1038/s41598-018-33757-5
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DOI: https://doi.org/10.1038/s41598-018-33757-5
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