Present Sedimentary Environments And Environment Evolvement Since Late Pleistocene For The Pearl River Delta And Intracoastal Estuary And Sea Area

With active interactions between ocean and land, the near sea and estuary areas around deltas response sensitively to natural processes and human activities. So, studies of modern and paleo-environments around those areas are valuable for science and social development.Through the analysis of grain size, minerals and trace metal elements of 78 surficial sediments from the Pearl River estuary and near sea area, the impact of sediment sources, transport processes and human being activities on the sedimentary environments were studied. Further more, the signification of source-sink was discussed via simple model. Data of grain sizes, detrital minerals, trace elements, microfossils, sporopollen and ¹⁴C dating of 3 sediment cores were used in the interpretation of the evolvement of depositional environment from late Pleistocene epoc and the impaction of sea level fluctuation, paleo-climate changes and the instability of regional tetonics on the sedimentary environments.Mainly composed of quartzs and feldspars with less than 10% heavy minerals, which characterised with mineral assemblages of granitic and metamorphic rocks, detrital minerals in surficial sediments indicated the contributions of terrigenous deposition via runoff, land and islands direct inputs. Other sources of the sediments include authigenic biogenic deposition and volcanignic via tide and seawater mass from the slope. The content and distribution of several minerals such as olivine and limonite revealed the obvious influence from public works such ad sea-filling project.Spatial distributions of trace metal elements showed following features: Distributions of Co, Cu, Ni and Ba showed their connections with the compositions of source rocks, indicating that, they were dominated by hydrodynamics, coupled with mild anthropogenic influence. Sr were mainly controlled by sea environment factors especially aquiculture. Pb and Zn showed unnatural characteristics, and enrichment factor analysis confirmed significant anthropogenic influence.Modal calculation showed that, the amount of annual deposition and exportation of metal elements in the studied area was huge. As an ideal place of sedimentary environment and transition place between the sea and land, the Pearl River estuary showed important source-sink functions, providing sediments to South China Sea and around the estuary.Studies of 3 sediment cores revealed the following pictures. Before Late Pleistocene epoc, Pearl River estuary and near sea area were main land, with the east islands and inlets as corroded and denudated foothills. In the time in LGH, the modern Pearl river delta and Pearl river estuary was unified to be a big estuary, with the east edge as islands in the east side. In LGL, most of the area was bared in air, leaving some low area in the west influenced by the sea, the hypsography of foothills in the east becoming low and aggregated fluvial sediments for long time denudation and corrosion. In the post-glacial period, with the invadation of the sea, the area became gradually to be shallow slope area. Around 5000 a B.P., the sea level didn't fluctuated as before, so the modern Pearl river delta intruded to the sea and the coast line intrude too, the slope sea water with high salinity fell back, therefore, study area became to be near land shallow water area just like today. All these indecated that, Pearl river was a young delta, generated from mid-Holocene Epoch, as the outcome of the interactions between the sea and the land.The movement of geologic structure around the Pearl River delta also had an influence on the sedimentary environment. In the early stage of the last glacial age, the subsidence of the west part of Xijiang-Modaomen Fault resulted in the transgression of the sea, received the earliest marine sedimentation. In the Holocene epoch, the tectonic sinking of the depressed area between the east Faults resulted in huge sedimentary layers as witnessed by the core NNZ1.Sedimentary core studies also revealed that, besides two big transgressions since, namely, Lile and Guizhou transgression, the Pearl river estuary region also suffered a short time and small scale sea level fluctuation between them.The pollen analysis reflected the tropical-subtropical climate characteristics since Late Pleistocene epoch and several climate changes in the period. Especially, Pollen and factor line marked Young Dryas event in this area. The well coupling of paleoclimate and sea level fluctuation reflected that the paleoclimate controlled the environmental evolvement ultimately.