Abstract:

Beibu Gulf is located in the summer monsoonal zone along the southeastern margin of Euro-Asia continent as a coastal basin of Titetan Plateau, Hengduan Mountains and their sketch, which is also a broadest shelf along northern South China Sea(SCS). It constitutes an integrity with Sunda shelf called Pan-Sunda in the aspects of geomorphoric and geological environment showing an ecological connection and similarity on a larger scale. The Plateau-Mountain-Basin complex is characteristized by a semi-closed, long, and multiple-fractal-dimensional coast line but receiving less attention on palaeo-peat development in Beidu Gulf. We collected a complete Quaternary sediment core ZK9 with a length of 80.05 m. Subsequently, we performed lab analysis, dating, and identification of micro-paleontological, granularity, geochemical micro-elements, and detrital minerals for 92 samples. With combination of the data analysis and shallow seismic profiles collection, the results show that the ZK9 bottom age was 171.0 ka. There was tropical and subtropical monsoonal vegetation of main Castanopsis sp., Quercus sp., Microlepia sp., Polypodiaceae and Cibotium barometzetc etc appeared from ~165 ka in a variation style and primarily fell into three evolution stage 165-110 ka,110-10.5 ka, and Holocene relating to PGM, last inter-glacial, glacial and deglacial, and basically corresponded to change stages of granularity, geochemical-micro elements, and detrital minerals below. Sparse diatom Cyclotella striata and C.stylorum displayed from 56.0 ka and diatom species became abundant during Holocene, and warm shallow-water type of foraminifera appeared during the SCS transgression from middle-Holocene(6.0 ka). Main composition is comprised of more sand, but the gray and silt both became abundant in some depth or ages with maximum 65.78%, 59.71%, respectively, and generally bad sorting. There were relatively richer detrital minerals with dominant quartz of an average 88.57%, and others such as feldspar, and weathering minerals including mica and hematite. Geochemical micro-elements concentration ranged from 2.6×10^-6 to 347×10^-6, with an average organic matter of 1.07%. A series of P1-P5 peat cycles showed millennium scales driven by strong warming oscillation during glacial/interglacial periods, one of which was polar-coupled oscillation D-O/AIM a11 revealing a highly unstable southward shift of westlies, and it developed different sedimentary environments under fluvial/riverine and brackish facies but also shared some common characteristics in micro-element behaviour, organic matter growth, fine sediments and reduction pyrite and so on. There was also highly carbonizated wood in upper core and black-charcoaled sub-strata in neighboring core during some peat cycles exhibiting a carbon reservoir feature like Sunda shelf. Peat cycles indicated that the neo-tectonic uplift movement caused a significant vertical terrain differences and patterns in the Plateau-Maintain-Basin complex during mid-late Pleistocene.Therefore, it acted as an intensive water production, utilization, maintaining and gradient transport area under warm and humid Indian and East-Asian summer monsoonal field through the climatic-tectonic coupled effect. Beibu Gulf was ever a closed basin before opening to the sea thereafter it became a coastal plain in Late Pleistocene, and the reconstruction for its lowland surface distributed woods and grass and widely distributed palaeo-river system when being exposed during glacial stages. In summary, Beibu Gulf was sensitive to global and regional climatic changes, and this exposed coastal basin might have also played a role in southward transfer and compensation for high-middle latitude shrink of palaeo-productivity and-ecological function. The river systems incised to the continent slope to transport biogenic matters into sea basin could be an important mechanism to enhance marine CO₂ biological pump especially from the viewpoint of whole low-latitude monsoonal Pan-Sunda during glacial stages except the way of aerial dust input at high latitude, meaning Pan-Sunda fedbacked regional even global changes.

Key words: Beibu Gulf, variation of sedimentary element, peat cycle, driving mechanism, carbon reservoir