Geological settings of sediment source areas
The lands around the northern South China Sea include South China, Taiwan, Luzon, and the northern Indochina Peninsula. In the north, South China has been tectonically stable since Mesozoic. Its stratigraphy is mainly characterized by Mesozoic-Cenozoic granitic rocks and Paleozoic sedimentary rocks (Fig. 1.2). In particular, Permian-Triassic limestone dominates the upstream drainage area of Xijiang River, the main stream of the Pearl River, while Mesozoic-Cenozoic granitic rocks and Paleozoic limestones and sandstones dominate the downstream drainage area of the Pearl River. Taiwan is a tectonically active accretionary prism. It was developed by the eastward subduction of the South China Sea oceanic crust beneath the Philippine Sea Plate (Huang et al., 1997). The stratigraphy is mainly composed of Tertiary sedimentary rocks with associated metamorphic rocks in the Central Range and of volcanic arc complex in the Coastal Range (Fig. 1.2).
Climatic settings of sediment source areas
The East Asian monsoon prevails in the lands adjacent to the northern South China Sea, controlling the seasonal precipitation and runoff regimes by the alternation of summer and winter monsoon wind directions. From November to April, the continent cools and the high pressure cell over northern Asia develops, inducing the northeast winter monsoon across the South China Sea; on the contrary, the southwesterly summer monsoon prevails from May to October, accompanied by continental heating and low pressure over central China (Wang and Li, 2009). Driven by the seasonality of monsoon winds, the inter-tropical convergence zone (ITCZ) shifts meridionally and the precipitation rate changes seasonally (Wang et al., 2003).
In South China, the mean temperature varies between 10-30 °C, and the annual precipitation reaches 1700-2000 mm with ~80% of the rainfall occurring in summer (Fig. 1.3). Taiwan is situated at the same latitude as South China. The temperature is also moderate and the precipitation also arrives mainly in summer (June-September) (Fig. 1.3), with heavier annual rainfall than in South China (over 2500 mm) because Taiwan is located in the “typhoon corridor”. There were 255 typhoons that hit Taiwan between 1949 and 2009 (Liu et al., 2013b). The frequent typhoon events and tropical storms largely increase the precipitation in summer. In Luzon located south of Taiwan, the temperature is high and stable (27 °C ± 5 °C), while the precipitation pattern is more complex (Fig. 1.3). In northern and central Luzon, the rainy season with 80% of the total rainfall occurs in summer (June-October), while the dry season dominates the rest of the year. Such a shift in precipitation is not observed in southern Luzon where 45% of the total rainfall occurs in October to December, while 55% of the rest is distributed during the other seasons (Schopka et al., 2011).
Magnetic susceptibility and remanent magnetizations
The continuous measurements of low-field susceptibility and remanent magnetizations were performed on the u-channels of core MD12-3432 following the standard procedure followed at LSCE (Kissel et al., 1998; 2010; 2013).
The magnetic susceptibility was measured with a Bartington MS2C 45-mm diameter susceptibility bridge at 2 cm intervals with a resolution of about 4 cm. The susceptibility (κ) was normalized to volume.
The Natural Remanent Magnetization (NRM) was measured every 2 cm using a 2G-755R cryogenic magnetometer equipped with high-resolution pick-up coils (~ 4 cm resolution) and placed in the μ-metal shielded room at LSCE. It was stepwise demagnetized using alternating fields (AF) along the three perpendicular axes using AF coils installed in-line with the magnetometer. The successive peak fields used for demagnetization are 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60 and 80 mT. The u-channels were translated at a 4 cm/s speed through the coils. The stepwise demagnetizations allow to remove the contribution of low-stability secondary magnetization that formed after deposition and thus to define the stable component of NRM called Characteristic Remanent Magnetization (ChRM). The direction (declination and inclination) of the ChRM at each horizon has been determined using a principle component analysis and a least-square fit (Kirschvink, 1980; Mazaud, 2005). The accuracy of the directions was given by the maximum angular deviation (MAD). The obtained declinations were used to orient the core in geographic frame assuming that on the long-term, declinations should average around 0°.
The Anhysteretic Remanent Magnetization (ARM) was acquired in-line along the vertical axis in a 100 mT alternating field superimposed to a steady 50 μT bias field parallel to the AF. The samples were translated through the coil at a speed of about 1 cm/s during the acquisition (Brachfeld et al., 2009). The ARM was then progressively demagnetized with 11 steps at 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 80 mT and measured after each demagnetization step using the same instrument and same setting as for NRM.
The Saturation Isothermal Remanent Magnetizaiton (SIRM) was acquired within 6 steps (0.05, 0.1, 0.2, 0.3, 0.5, 1 T) also along the vertical (z) axis using a 2G 1.6 m long pulsed solenoid. The IRM was also measured every 2 cm after each step in order to construct the acquisition curves giving the access to the magnetic hardness of the sediment at each measured horizon. The SIRM was then stepwise demagnetized using the same steps as for ARM. During the demagnetization and measurement, the samples moved at a speed of about 4 cm/s through the coils. The median destructive field of IRM (MDFIRM) as well as the percentage of magnetization remaining after demagnetization at 80 mT with respect to SIRM (IRMAF@80mT% = 100% × IRMAF@80mT/IRM1T) could be calculated. A backfield of -0.3 T was applied to the SIRM1T to calculate the S-ratio (S0.3T = -IRM-0.3T/IRM1T) (King and Channell, 1991), HIRM (= 0.5×(SIRM+IRM-0.3T)) (Robinson, 1986).
The hysteresis loops were obtained between +1 and -1 T using an alternating gradient magnetometer (AGM2900). Saturation magnetization (Ms), remanent saturation magnetization (Mrs), and coercive force (Hc) were determined after the correction for high field slope representing the para- and diamagnetic components. The remanent coercive force (Hcr) was measured separately after applying stepwise increasing back-fields to the Mrs. Replicate measurements were performed on randomly selected samples to check the precision and sample representativeness. High-resolution IRM acquisition curves (150-200 steps between 0 and 1 T) were then obtained from 32 samples. They were decomposed (Kruiver et al., 2001; Robertson and France, 1994) into different coercivity components defined by their half IRM acquisition field (B1/2) and their relative contribution to SIRM.
The thermal evolution of the different coercivity components has been evaluated by high-temperature demagnetization of three-axis IRM. Fields of 1, 0.3, and 0.1 T were successively applied to the three axes of dried cubic samples using the Applied Physics pulse magnetizer 2G660. The samples were then demagnetized by stepwise heating from room temperature to 700°C using a zero-field PYROX furnace, in which the temperature gradient over the samples are lower than 3-4 °C at high temperature. The IRM remaining on each axis after each step was measured using a 2G-755R cryogenic magnetometer equipped with high-homogeneity pick-up coils also placed in the μ-metal shielded room at LSCE together with the furnace.
Low-resolution quantitative analysis
To quantitatively determine the major element composition, wavelength dispersive X-ray fluorescence analyses (hereafter called “WD-XRF”) were conducted on 102 discrete bulk sediment samples taken every 50 cm from the center of the working sections with a cubic container (2×2×2 cm). These discrete samples were firstly freeze-dried and manually ground in order to eliminate the potential influences of grain size and water content. Then, 0.70000±0.00008 g of sediment was melted with 7.0000±0.0008 g of a fusion agent (mixture of lithium tetraborate and lithium metaborate) into a bead (3.7 cm diameter, 0.7 cm thick). The prepared sample beads were measured with a PANalytical AXIOSmAX wavelength dispersive XRF spectrometer. Replicated analyses were performed on the Chinese rock and sediment standards (i.e. GSR6 and GSD15) every day at the beginning of the measurement runs to control the analytical precision and accuracy. These analyses indicate a relative standard deviation lower than 1% and a relative accuracy lower than 6%.
The inductive coupled plasma atomic emission spectrometer (ICP-AES) analyses were performed on 22 additional samples to confirm the reliability of WD-XRF major element concentrations. Dried and ground bulk sediments were heated at 600°C for 4 hours to remove organic matter. Their weights were measured before and after heating to calculate the loss on ignition (LOI). The sediments were digested in HF+HNO3 mixed acid. Major elements were then measured on an IRIS Advantage ICP-AES. A series of Chinese rock and sediment standards (i.e. GSR5, GSR6 and GSD9) were measured together with the sediments. The mean relative accuracy estimated from these standard samples is better than 4%.
Table of contents :
Chapter 1 Geological and Environmental settings
1.1 Physiography of the South China Sea
1.2 Geological settings of sediment source areas
1.3 Climatic settings of sediment source areas
1.4 Oceanic circulation
1.4.1 Surface current
1.4.2 Deepwater current
1.5 Fluvial inputs and surface sediments distribution
Chapter 2 Material and Methods
2.2 Clay minerology
2.3 Magnetic properties
2.3.1 Magnetic susceptibility and remanent magnetizations
2.3.2 Hysteresis parameters
2.3.3 Thermal demagnetization
2.4 Major element compositions
2.4.1 Low-resolution quantitative analysis
2.4.2 High-resolution semi-quantitative analysis
2.5 Granulometry (sortable silt)
Chapter 3 Marine sediment XRF scanning calibration
Chapter 4 Proxies for the East Asian summer monsoon
Chapter 5 Land-sea interactions in the northern South China Sea
Chapter 6 Conclusions and Perspectives
Appendix A Supporting information for Chapter 3
Text S1. Accuracy evaluation of WD-XRF element concentrations
Text S2. Description of existing XRF calibration methods
S2.1 Correction for water absorption effect
S2.2 Normalized median-scaled calibration (NMS)
S2.3 Multivariate log-ratio calibration (MLC)
S2.4 Comparison between NMS and W-NMS results and between MLC and W-MLC results
Text S3. Description of proposed improved calibration methods
S3.1 Normalized polynomial-scaled calibration (NPS)