Palaeomagnetic And Palaeoenvironmental Studies On Recent Sediments From The Bohai Sea, China

In July 1991, scientists from both of the Chinese side and the British side had completed the Cruise JX91 in eleven days. In the Bohai Sea area, using the British Mackereth type corer and the mini corer, together with the Chinese piston gravity corer, we have taken JX91-2A and JX91-2B from the Central Basin , and JX91-3m, JX91-3A, JX91-3B and JX91-4G in the western Laizhou Bay outside the Huanghe River Delta. This thesis is accomplished on the multi-discipline studies on the above six cores during my study in the Department of Geology and Geophysics, University of Edinburgh under a joint project.The experimental measurements are mainly of two folds: palaeomagnetism and geochemistry.1. Palaeomagnetism. Measurements of NRM (Natural Remnant Magnetisation), AF (Alternating Field) demagnetisation, susceptibility and AMS (Anisotropy of Magnetic Susceptibility) have been carried out on 451 samples from cores JX91-2A (168), JX91-2B (80), JX91-3A (66) and JX91-3B (137). Rock magnetism has been applied on JX91-2A, JX91-2B and JX91-3A, including ARM (Anhysteretic Remnant Magnetisation), 1RM (Isothermal Remnant Magnetisation), SIRM (Saturated IRM) and its decay curve of a few pilots. MDF (Medium Destruction Field) has been determined for JX91-3B.2. Geochemistry. XRF (X-Ray Fluorescence) analysis, which can yield concentrations of 10 major elements and 21 trace elements, and XRD (X-Ray Diffraction) analysis have been carried out on 223 samples from JX91-2A (79), JX91-3B (67), JX91-3m (50) and JX91-4G (27). Analysis of organic carbon, paleontology, stable isotopes of oxygen and carbon and wet sediment density have been conducted on JX91-2A, and measurement of ²¹⁰Pb on JX91-3B. Furthermore, water content and particle size distributions of all the four cores have also been carried out.All of the experimental data are processed and analysed with computer, and comprehensive multi-discipline correlations have been made. Having referred fully to the previous work of others, this thesis provides some new insights into the four fields below.1. The Bohai Sea turbidity current system and the shallow water turbidites outside the Huanghe river mouth. There are quite a few evidences to show the existence of this turbidity current system. 1) Calculated from the present day suspension concentration of the Huanghe River, the turbid Huanghe river water flowing into the Bohai Sea is heavier than the Bohai seawater diluted byfresh river water, which can definitely enable the turbidity current. 2) Practical investigations have revealed a widespread sheet of turbid water near the Bohai Sea seafloor with typical appearance of turbidity current, which shows its maximum suspension concentration along the direction of the Huanghe River channel and becomes smaller away from it. 3) In Cruise JX91, a strong bottom current with extraordinary strength has been observed flowing seaward from the Huanghe river mouth. Particle size analysis on the Huanghe deltaic sediments have confirmed they are shallow water turbidites, and have revealed two basically different patterns of particle size distribution and two types of turbidites. The Bohai Sea shallow water turbidites are quite homogeneous inside and the deposition phase dominates throughout, though classic turbidites can also be found, which were formed under special conditions.2. High resolution chronology of the Bohai Sea sediments. Curves of sea-level changes and palaeointensity of geomagnetic field since the Holocene are used to date the sediment from the Central Basin in the Bohai Sea, and the bottom of JX91-2A is dated as 8,500 years BP. The chronology of the Huanghe deltaic sediment cores, JX91-3B and JX91-4G, are first controlled loosely with sedimentation rates calculated from theoretical models and 2iopb measurements, and then are determined on a year-layer basis according to the precise records about the Huanghe River channel switchings. The establishment of high resolution chronology of the Bohai Sea sediment has provided a firm background for various correlations with other cores using all these corresponding data.3. Method of correction of the palaeomagnetic records and the establishment of the type curve of palaeomagnetic SV (Secular Variations) in the Bohai Sea area. A great deal of rock magnetic work and alike prove that the main magnetic minerals in the Bohai Sea sediments are magnetite and low-Ti titanomagnetite. Therefore, the magnetic fabric can reflect the general micro structure of the sediments. AMS analysis point out that the corer was tilted during coring, after having confirmed the compaction effects, and accordingly tilt corrections have been made on the Kirschvink directions derived from AF demagnetisation. The geographic direction of the maximum axes of the susceptibility ellipsoid is determined by the consistent directions of tidal current, slope and sediment source, and hence used to reset the declination of the palaeomagnetic SV records. The high resolution palaeomagnetic SV type curves since 8,500 years BP are therefore established for the Bohai Sea area. As for the cores taken from outside the Huanghe River Delta, the magnetic signals recorded are more likely to indicate the current change than the palaeomagnetic SV, as the hydraulic energy may be much higher.4. Comprehensive analysis of the palaeoenvironment in the Bohai Sea and the Huanghe River channel switchings since the Holocene. The palaeoenvironment in the Bohai Sea is mainly influenced by the sea-level changes and the Huanghe River and its sediment supply. Geochemistry etc. reveals that almost all the sediment in the Central Basin and the Laizhou Bay in the Bohai Sea is from the Huanghe River. However, there are many evidences showing that the Huanghe River was flowing into the South Huanghai Sea instead of the Bohai Sea in the early Holocene, and only in 2278 BC did it flow back to the Bohai Sea. These evidences include historical and archaeological records, the buried Huanghe River palaeo-channels in the South Huanghai Sea formed in the early Holocene, stable foraminiferal content and combination indicating normal seawater, the formation of yellow brown soft mud and the abrupt change in foraminiferal content and combination after the Huanghe River flew back to the Bohai Sea.The sea-level was rising at an average speed of 1 cm/yr from 8,500 to 6,000 years BP, when the transgression reached its maximum. The high sea-level had remained for quite a while, when the climate was warm and humid, and the sedimentation rate in the central Bohai Sea had dropped below 10 cm/ka. About 2,200 years ago, the Huanghe sediment supply began to rise sharply, and the sedimentation rate in the Central Basin jumped to 290 cm/ka in the Song Dynasty and thus helped develop the turbidity current system. This soon resulted in the Huanghe River lower-reach channel switching in 1128 AC. As soon as the Huanghe River flew back from the South Huanghai Sea, it started to build the huge modern Huanghe River Delta and switch its channel frequently on the Delta. This history indicates the strong influences of the human activities on the palaeoenvironmental changes. First, the large scale development of agriculture started over two thousand years ago in the Huanghe drainage area had destroyed the vegetation and ecology significantly, which leads to worsening of the climate and severe land erosion. Secondly, the industrialisation in the last two hundred years had brought in global wanning, which enhanced the dry warm climate on the Loess Plateau and made the severe land erosion even worse.