Search

Abstract This study investigated the late Quaternary sedimentary evolution of the outer shelf of the East China Sea (ECS) using borehole core data (DH02). Three unconformity-bounded sedimentary facies, namely, prodelta facies of marine isotopic stage (MIS) 3, intertidal to subtidal flat facies of the deglacial period, and offshore tidal sand ridge facies of the Holocene, were identified from the base upward according to combined evidence of lithology, microfossil assemblages, and geochronology. Through comparative analysis with other neighboring borehole cores, these three facies, along with the MIS 3 nearshore facies and the last glacial maximum (LGM) river facies, were found to constitute the late Quaternary sediment strata. Sedimentary processes of these five facies were interpreted based on a synthetic consideration of sea-level change, climate variation, paleotopographic setting, and paleoriver-sea interactions. In particular, the development of the delta was attributable to integration of the slow sea-level decline during the later stage of the MIS 3 and abundant sediment supply resulting from the warm-wet climate. The gentle shelf topography suggests the presence of shallow and wide rivers in the delta, and the lateral shift of river channels might have induced the local formation of nearshore facies at the abandoned river mouth. The LGM river facies, which was rarely observed (presumably due to marine erosion with deglacial transgression), was inferred to have been deposited during the latter half of the LGM in response to the rise in river base level. Occurrences of tidal flat facies imply the formation of a tide-dominated estuary with the deglacial transgression. Furthermore, tidal flat sediments were reworked by enhanced tidal currents in the estuary front environment, forming mud ridges during subsequent deglacial transgression. Formation of tidal sand ridges in offshore environments of the Holocene was attributable to the uneven base caused by the underlying estuarine erosion mud ridges, and this promoted the occurrence of tidal depositional processes. The sedimentary evolution in association with sea-level change was constructed using the above-mentioned results. Overall, this study systematically elucidates the late Quaternary sedimentary processes and evolution of the outer ECS shelf, and provides an insight into the sedimentary development of analogous shelves around the world. [ABSTRACT FROM AUTHOR]

The East China Sea Shelf Basin (ECSSB) is an important tectonic unit of the western Pacific active continental margin. It is divided further into a series of sags and basement ridges. The Xihu Sag on the east margin of the ECSSB is an important hydrocarbon exploration area and contains significant petroleum resources. The Huagang Formation is the major reservoir in Xihu Sag, and its provenance has been the focus of debate at all times. The major and trace elements (including rare earth elements) compositions were analyzed for the Oligocene Huagang Formation sandstone and mudstone samples in the Xihu Sag. The provenance and tectonic setting of these clastic rocks are discussed. The Chemical Index of Alteration value and A–CN–K diagrams suggest low to moderate weathering in the source areas of the Huagang Formation. The mudstones have lower index of compositional variation values (<1), indicating that the mudstone samples are compositionally mature, whereas the index of compositional variation values for the sandstone samples (0.59 to 1.87) indicate that the sandstones are compositionally immature and mature. The major and trace element composition and ratios of the clastic rock samples indicate that mudstones are mainly from Late Mesozoic igneous rocks in SE China, which is dominated by the acidic volcanic rock. The provenance of sandstones is from two different directions, the Central Uplift Group of ECSSB and the Taiwan–Sinzi Fold Belt with more mafic volcanic rock. Based on the geochemical analysis of the clastic rock samples, the tectonic setting of Xihu Sag was Japanese‐type continental margin during the Huagang Formation deposition time (Oligocene). [ABSTRACT FROM AUTHOR]

A 35.30 m-long core (MZ02) that was recovered from a water depth of 32.4 m from the inner shelf mud deposit of the East China Sea is analyzed to determine its sedimentary characteristics, color reflectance, clay mineral content, element geochemistry components and AMS 14 C dating to investigate the sedimentation rate, sediment provenance and paleoclimate evolution during the fast transgression period. Rare earth element and clay mineral proxies indicate that mixed-provenance sediment accumulated in the foreshore–nearshore region at the beginning of the fast transgression period, with a higher sedimentation rate of 5.58 m/ka. From the early Holocene (9.8–9.5 ka B.P.), the sedimentation rate declined by approximately 1.73 m/ka, and the sediment provenance obviously changed. The silt fraction resembled that of Taiwan's mountainous rivers and the clay fraction was primarily transported from the Yangtze River. A multiple proxy system including sediment redness (a*), chemical index of alteration (CIA), clay mineral ratio (smectite/kaolinite), and major and trace element ratios (CaO/MgO, Ba/Sr), also provides a good paleoclimate record during the fast transgression period, which can be divided into three units. All the proxies barely changed during Unit I (12.88–10.85 ka B.P.), revealing that the climate remained relatively stable. Obvious fluctuations occurred during Unit II (10.85–10.05 ka B.P.), and the temperature kept decreasing for more than 1 ka until the Younger Dryas event, showing a notable regional response to global climate changes. A continuous warming trend resumed again during Unit III (10.05–8.15 ka B.P.), which may mark the threshold of the Holocene warm period. In addition, we find significant 80-yr, 89-yr and 100-yr cycles in the CIA, CaO/MgO and Ba/Sr records, which imply the possible influence of solar activity on regional climate change during the fast transgression period in the Asian margin region. [ABSTRACT FROM AUTHOR]

AMS14C dating and analysis of grain size, major elements and clay minerals were applied to Core MZ01 from the mud area on the inner shelf of the East China Sea. Based on the environmentally sensitive grain size, clay mineral and major element assemblages, the history of the East Asia winter monsoon since the mid-Holocene could be reconstructed. These three proxies, mean grain size (>9.71 µm), chemical index of alteration (CIA) and ratio of smectite to kaolinite in particular, show similar fluctuation patterns. Furthermore, 10 extreme values corresponding to the contemporary cooling events could be recognized since the mid-Holocene; these extreme values are likely to have been caused by the strengthening of the East Asia winter monsoon. The cooling events correlated well with the results of the δ 18 O curves of the Dunde ice core and GISP2, which therefore revealed a regional response to global climate change. Four stages of the East Asia winter monsoon were identified, i.e. 8300-6300 a BP, strong and unstable; 6300-3800 a BP, strong but stable; 3800-1400 a BP, weak and unstable; after 1400 a BP, weak but stable. [ABSTRACT FROM AUTHOR]