Paleoenvironmental And Paleoclimatic Records From The South China Sea Since The Late Last Glaical Period

The South China Sea (SCS) is the largest marginal sea of the Western Pacific. It is surrounded by the Eurasia continent to the north and west, and Philippine islands and Borneo to the east and south. Due to the high sedimentation rate and carbonate preserving efficiency, the SCS is an ideal location for the paleoceanography studies. During the late last glacial period, especially for the Last Glacial Maximum (LGM), there were numerous climatic changes. The studies of the paleoenvironmental variation have attacted the scholars' interests. Determining the provenance of marine sediments is essential to understand the environmental and climatic conditions of its source areas. The SCS receives～260 Mt of fluvial sediments annually from the north direction, the majority of which is delivered by the Pearl River and small mountainous rivers in Taiwan. However, due to the similarity lying in the geological setting of the Pearl and Taiwanese rivers (e.g. characters of source rocks and Nd isotope value), the source of the sediments accumulated in the SCS has not been well established during the last glacial period. Clay mineral assemblages in marine sediments in general represent a well-suited tracer concerning their sources and the weathering condition of the surrounding continents. Although previous clay mineral studies in the SCS addressed the influence of chemical weathering and physical erosion on the surrounding continents in the past 20 Ma, few of them focuses on the entire period since the late last glacial period. Meanwhile, we also know little about the delivering process and distribution of mud from the Pearl River, the second largest river that drains into the SCS. Based on the review of the clay mineral research in the northern SCS (NSCS), I find that there is limited compilation for the source of the fine-grained sediment in the whole SCS. Therefore, a coupled approach based on clay mineral assemblages, planktonic foraminiferal oxygen and carbon isotopes, element analysis, organic carbon, and calcium carbonate content from the AMS 14C-dated borehole is used to trace the sources of the fine-grained sediments and to investigate the paleoenviornmental evolution in this area. I combine the research which conducted in the NSCS with the published data of the clay minerals in surface sediments of the SCS to trace the source of the clay mineral in the whole SCS. I also present the Chirp sonar high resolution sub-bottom profiles, which were obtained from the NSCS geophysical investigation in 2007, and adjacent sediment cores, which were published by previous scholars, to help better understand the Holocene delivering process and distribution of the Pearl River-derived sediment. Detailed conclusions are shown below:1.I use both planktonic foraminifera G. ruber AMS 14C dating and age correlations between cores ZHS-176 and SONNE 17940 for an age control of the past 22 cal. ka. The switch point from the Marine Isotope Stage (MIS) 2 to MIS 1 occurs at 345 cm depth. Below the 345 cm is MIS 2; above the 345 cm is MIS 1. The dating data show that the bottom of the borehole is not the start of the MIS 2.2. The clay mineral assemblages of core ZHS-176 are mainly composed of four species, which are dominated by illite (～39%) and chlorite (～27%), with associated with smectite (～21%) and kaolinite (～13%). The clay mineral assemblage analysis suggests multiple sediment sources. The denudation rates in Taiwan are almost the highest in the world, with providing numerous illite and cholorite to the NSCS. The East China Sea (ECS)-derived clay mineral assemblage is similar with Taiwan's. Kaolinite comes mainly from the Pearl River, and Luzon Island is the main contributor for smectite.3. Due to the closure of the Taiwan Strait, and lower sedimentation rates, sediment from the ECS is hard to enter the NSCS during the late last glacial period. The influence from the Pearl River increases during the cold interval due to a fallen sea level and the seaward extended paleo-Pearl River estuary. During the Holocene, the coastline generally remains the same position as it is today, the ECS-derived sediment reenter the SCS.4. The planktonic foraminiferal oxygen isotopic oscillations in core ZHS-176 during the late last glacial period reveal the paleoclimatic variations, including the LGM, Heinrich event 1, Bφlling-Allerφd (B/A), and Younger Dryas. During the Holocene, I also find three periods of strong precipitation stages (S1～S3) and three periods of weak precipitation stages (W1～W3). The oxygen isotopic record exhibits correlation with climate records from distant regions, including the high-latitude area of North Atlantic, providing evidence for global tele-connection among regional climates.5. Among the climatic oscillations which are found in core ZHS-176, the Holocene event 3 attacts huge interests. The weak solar activity is the main mechanism of this cold and dry event in most part of the low and middle latitudes in North Hemisphere. It causes the sea surface temperature, thermohaline circulation and monsoon decreasing, and Intertropical Convergence Zone southward moving. It appears in different proxies. This cold/dry or cold/wet climate fluctuation maybe the main reason of the collapse of the Old Kingdom in Nile River, Akkadian empire in Mesopotamia, urban Harappan civilization in Indus valley, and Neolithic cultures in China.6. There is a brief negative planktonic foraminifera carbon isotopic shift in core ZHS-176 during B/A. Due to the massive input of freshwater from Antarctica, the density of the Antarctic Intermediate Water decreases and the North Atlantic thermohaline circulation intensifies, which leads to the NSCS intermediate water warming at the Bφlling oscillation. During the relative low sea level stage (-70～-80 m), rapid increase of temperature in NSCS will dissociate the gas hydrate at depth and expel massive methane into the overlying sediments and ocean water. Releasing of gas hydrate markedly affects the planktonic foraminifera as shown by the carbon isotopic variation in core ZHS-176. Similar carbon isotope excursions also happened in other areas, such as the Santa Barbara Basin, Guaymas Basin, Lake Baikal, Greenland Sea, Peru, East Greenland continental shelf, Papua New Guinea, and southern SCS.7. The calcium carbonate content variation history of core ZHS-176 reveals that there is a low calcium carbonate event during the early Holocene. Compared with that of other five cores, I find this event occurs widely in the whole NSCS. There are two main reasons for this excursion: (1) abundant precipitation, cold events and centralized volcanic eruptions reinforce the dilution of terrigenous sediment, (2) strong carbonate dissolution, and (3) low primary productivity reduces the carbonate content. The terrigenous sediment dilution dominates the calcium carbonate content in the NSCS, the carbonate dissolution and sea surface primary productivity are the secondary mechanisms.8. The major and trace element variations of core ZHS-176 show that the amplitude of chemical weathering is low, and the terrigenious input is high during the late last glacial period, while the amplitude of chemical weathering is high, and the terrigenious input is low during the Holocene. The oscillation of the elements is related to the planktonic foraminerfera oxygen isotope variation. The organic carbon is mainly biogenic origin. The terrigenious organic carbon relative content increases with the strengthen summer monsoon, while it decreases around 3 ka BP.9. The clay mineral assemblages in surface sediments of the SCS consist of illite, chlorite, kaolinite, and smectite. The relative abundance of these clay mineral components varies in different regions and also with different water depth. It is mainly controlled by the provenance. I combine the research which conducted in the NSCS with the peers' published data of the clay minerals in surface sediments of the SCS, and divide the SCS into four parts to trace the sources. The Taiwan Island and Luzon Island are the main provenances for the eastern SCS. The Mekong River, Borneo, Sunda shelf and Indonesia arc are the contributors for surface sediments in the southern SCS. The clay minerals in surface sediments of the western SCS are mainly from the Mekong, Red, and Pearl Rivers, Sunda shelf, Indonesia arc and Borneo. The Pearl, and Yangtze Rivers, Taiwan and Luzon Island are the sources for surface sediments in the NSCS.10. High resolution Chirp profiling and coring reveals an elongated (～350 m) Holocene Pearl River-derived mud area (thickness<30 m) extending from the Pearl River delta plain southwestward off the Guangdong coast to the Leizhou Peninsula. Most of the terrigenous sediments are transported and trapped on the NCSC continental shelf, because of the convergence of the southward-flowing China coastal current and northward-flowing SCS warm current. On the continental shelf off the west Guangdong Province, the mud is deposited in water shallower than 50 m; while to the southeast of the Pearl River estuary, the mud area can extend to the-120 m isobath. Through analyzing the terrigenous sedimentary sequences from other continental shelves of the Western Pacific marginal seas, we find the formation of the mud area can be further divided into two stages:before the mid-Holocene sea-level highstand (MHSH) (～7.0 cal. ka BP), the distal mud was deposited between 11.2 and 9.8 cal. ka BP, when the sea-level rose slowly after the meltwater pulse (MWP)-1B. The proximal mud was deposited after 9.0 cal. ka BP, when the sea-level rose slowly after MWP-1C; after the MHSH, clinoform developed on the continental shelf off the west Guangdong Province, extending～150 km from the Pearl River estuary. This clinoform thins offshore, from～10 m thickness around 5-10 m water depth to< 1～2 m around 20-30 m water depth.