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Modern source-to-sink processes in the southern South China Sea (SCS) remain poorly understood. In this study, clay minerals and magnetic properties are used to identify the sources and transport pathways of surface sediments in the southern SCS. The distributions of four clay mineral species in the deep southern SCS are mainly controlled by sediment sources, while clay minerals on the outer Sunda Shelf mostly reflect reworking/recycling of older weathered sediments. Clay mineral analyses indicate that the Mekong River is the predominant contributor of clay minerals to the southern SCS, with minor contributions from Luzon and/or Palawan and northern Borneo. The contribution from the Luzon and/or Palawan arc to the southern SCS is semi-quantitatively evaluated as 20% and is mainly smectite. Most sediments in the southern SCS are magnetically dominated by mixtures of magnetite, hematite and goethite in variable proportions. Concentration-dependent magnetic parameters decrease gradually seaward and then sharply towards the outer deep sea. The grain size of magnetite particles also shows a significant fining trend with distance from the southwestern sources. In addition, the magnetic minerals change from hematite- and goethite-dominated to magnetite-dominated type as going further offshore. These magnetic characteristics suggest that the magnetic minerals in the southern SCS are mainly sourced from the southwest through summer monsoon-driven surface currents. Finally, combining the provenance signature indicated by clay minerals and the transport information revealed by magnetic properties, this study highlights that the Mekong River plays a major role in modern source-to-sink processes in the southern SCS. In brief, our study sets a good example for tracing sediment provenance and transport in other marginal seas worldwide. • Clay minerals and magnetic properties are used to identify the sources and transport pathways. • The Mekong River plays a predominant role in the source-to-sink processes of the SW SCS. • Magnetic minerals are sourced from the southwest through summer monsoon-driven surface currents. • Coupled analyses of clays and magnetic minerals can yield valuable and complementary information. [ABSTRACT FROM AUTHOR]

The South Yellow Sea (SYS) Basin, which is part of the West Pacific Continental Margin, began to form in the late Mesozoic to early Cenozoic as a result of regional tectonic activity in Asia. The pre-Middle Pleistocene sedimentary history of the area remains poorly understood or controversial, mainly due to a lack of long borehole cores penetrating pre-Quaternary strata. Borehole core CSDP-1 (300.10 m long), the longest sediment core recovered thus far from the SYS, was drilled in 2013 in the western SYS. Recently, a reliable timescale for the core for the last ~ 3.50 Myr was obtained by magnetostratigraphy (Liu et al., 2016a). Here we report the results of integrated analyses of the core, including sedimentary characteristics, downcore changes in environmental proxies (benthic foraminifera and ostracods), and accelerator mass spectrometry 14 C and optically stimulated luminescence dates. In addition, we analyze 546 km of single-channel seismic profiles and ~ 50 km of a boomer seismic profile around the core site, and correlate the regional stratigraphy. The core sedimentary succession contains 17 depositional units (lithofacies associations) corresponding to three major seismic units, reflecting a variety of depositional environments during the last ~ 3.50 Myr. Our results show that fluvial environments prevailed in the western SYS from ~ 3.50 to 1.66 Ma, followed by alternations of tidal-flat to shallow-subtidal environments with fluvial environments from ~ 1.66 to 0.83 Ma, and alternations of modern-like marine environments and fluvial environments from ~ 0.83 Ma to the present-day in response to glacio-eustatic sea-level cycles. The depositional succession during the last ~ 3.50 Myr was dominantly controlled by tectonism and glacio-eustatic sea-level cycles. The presumed first marine transgression since the Neogene in the western SYS at ~ 1.66 Ma is interpreted to suggest that sea water could partially flood the Zhe-Min Uplift (ZMU) into the SYS due to tectonic subsidence, and further subsidence of the ZMU and the SYS shelf is inferred to have resulted in establishment of normal marine environments in the SYS during the highstands of glacio-eustatic sea-level cycles since ~ 0.83 Ma. The average subsidence rates in the western SYS were ~ 120–165 m/Myr during the last ~ 1.66 Myr. The sedimentary succession comprises fifth- and fourth-order sequences, which are interpreted to have been driven by the 20-kyr or 41-kyr and by the 100-kyr glacio-eustatic sea-level cycles, respectively, since the late Pliocene. The sequences deposited since the Late Pleistocene are relatively complete, recording deposits from MIS 5 to MIS 1. However, the pre-Upper Pleistocene sequences show significant stratigraphic hiatuses, presumably caused by fluvial erosion during the glacial sea-level lowstands and subsequent transgressive erosion. This type of significant discontinuity in the sedimentary record can be considered to be diagnostic of shelf sequences controlled by glacio-eustatic sea-level cycles. [ABSTRACT FROM AUTHOR]

This study investigates Holocene sedimentary evolution and hypoxia development using borehole cores CJK06 and CJK09, in combination with other published core data. Based on lithology and microfossil (benthic foraminifera) characteristics, seven types of sedimentary facies were identified from the base upward: river, tidal flat, tide-influenced river, transgressive lag, estuary, inner-shelf, and prodelta. Isochronous correlation among the cores was established relying on accelerator mass spectrometry 14C dates. Facies succession indicates that a weak progradation occurred in coastal environments (12–10 ka BP) due to the Younger Dryas-induced deceleration of sea-level rise; rapid deposition locally occurred in the southern marginal area of tide-dominated estuary environments (10–8 ka BP), likely due to the junction of the Yangtze and Qiantangjiang River currents; and marine current-induced fluid mudflows prevailed in the shallow marine environments (8–1 ka BP), with the cooling climates of 5–1 ka BP. Furthermore, prodelta and shallow marine environments co-occurred with an uneven progradation of the delta within the last 1 ka, while deposition occurred just inside the more southern, delta-influenced area. The occurrence of euryhaline benthic foraminifera suggests that an enhanced freshwater discharge of the mid-Holocene (8–5 ka BP) was followed by a sharp decline in the late Holocene (5–1 ka BP) with climate change. The occurrence of cold-water benthic foraminifera indicates a severe cold-water condition during the mid-Holocene due to the intrusion of upwelling currents triggered by the propelling force of warm currents and summer monsoon winds. In addition, the occurrence of low-oxygen foraminiferal assemblages reveals that hypoxia has developed since 10 ka BP in stages consistent with the sedimentary evolution: localized hypoxia formation coincident with the southern depocenter during the early Holocene (10–8 ka BP); severe hypoxia with enhanced freshwater discharge and upwelling current intrusion during the mid-Holocene; and weakened hypoxia of the late Holocene, mainly due to the sharp decline in freshwater discharge. Within 1 ka BP, freshwater discharge from the approaching river mouth and increased nutrient inputs from enhanced human activities on land have contributed to the prevalence of hypoxia, especially in the southern deltaic area. Overall, it was revealed that the freshwater discharge modulated with climate variations and delta progradation plays a primary role in controlling the sedimentary evolution and hypoxia development during the mid-late Holocene. • Gravity flows (fluid mudflow) prevailed during the late Holocene (5–1 ka BP). • Holocene freshwater discharge was enhanced (8–5 ka BP) and then sharply declined. • Severe upwelling current intrusion occurred during the mid-Holocene (8–5 ka BP). • Hypoxia boomed during the mid-Holocene (8–5 ka BP). [ABSTRACT FROM AUTHOR]

The East China Sea (ECS) is a vast river-dominated depositional basin because of its broad shelf and extensive river sediment supply. This study investigated the mid–late Quaternary stratigraphic framework and evolution of the outer ECS shelf based on a borehole core (DH02) and seismic profiles. Seven sedimentary facies were recognized in the core sediments according to lithology, grain-size composition, sediment structure, and microfossil assemblages, while eight unconformity-bounded major stratigraphic units were identified in the seismic profiles. Based on seismic–core correlations, dating data (radiocarbon and optically stimulated luminescence), and comparison between sedimentary cycles and sea-level curves, the 100 m thick strata below the seafloor were divided into three sequences corresponding to glacial–interglacial sea-level cycles of marine isotope stage (MIS) 1–2, MIS 3–6, and MIS 7–8 (Sq1–Sq3 from top to bottom). In particular, Sq1 is developing in the present sea-level highstand and consists mainly of deposits of transgressive systems tract (TST), including tide-influenced river, tidal flat, and tide-dominated estuarine facies. Sq2 and Sq3 consist mainly of prodelta and related inner-shelf facies of forced regressive systems tract (FRST) and transgressive deposits similar to those of Sq1. For all three sequences, a thin-sand sheet layer of open-shelf facies represents deposits of highstand systems tract (HST). Deposits of lowstand systems tract (LST) were typically represented by river facies. In addition, comprehensive analyses of seabed sand ridge distribution, core sediment characteristics, and ridge deposition mechanism suggest that the occurrence of a paleoestuary with transgression determined the development of a tidal sand ridge field. Based on all these results, a conceptual stratigraphic sequence model in association with the deposition of LST, TST, HST, and FRST was constructed. Overall, our stratigraphic framework, along with a comparison of the sequence stratigraphy to that of other shelves, suggested that glacial–interglacial sea-level fluctuations, huge amounts of river sediment supply, active subsidence, gentle shelf topography, and marine erosion processes (transgression and regression) controlled the mid–late Quaternary stratigraphic development of the ECS shelf. • A mid-late Quaternary stratigraphic framework (since MIS8) was established. • Three sequences were geochronologically corresponded to the recent three glacial/interglacial sea-level cycles. • The paleoestuary retreated with post-LGM transgression, yielding modern seabed tidal sand ridge field. • Comparison of the sequence stratigraphy to other shelves suggested controlling factors of stratigraphic development. [ABSTRACT FROM AUTHOR]

The internal fabric of mud caps, which are on top of sediment gravity flow deposits, remains unexplored so far. We identified four hybrid event beds on one of the widest and gentlest shelf (East China Sea Shelf). Three types of lithofacies, including massive sand, chaotic mud and homogeneous mud, were distinguished. Homogeneous mud represents mud caps of these hybrid event beds. Scanning electron microscopy performed in both homogeneous mud and chaotic mud has revealed the presence of mud aggregates (mud clasts) conforming sand-size clasts. Chemical analyses of the aggregates show the presence of abundant gypsum/anhydrite and clay mineral components as well as relicts of quartz, muscovite and feldspar, suggesting an origin related to erosional events and an intraclast origin. The vertical shortening, deformation together with the pore spaces of mud clasts indicate a shallow burial depth of erosion, where mud sediments were still unconsolidated and water-saturated. No background sedimentation is present in the mud caps, because we did not observe any bioturbated interval in core images and X-radiographs. The lack of laminated muds and synsedimentary trace fossils in the homogeneous muds suggest an origin related to fluid mud flows. As proved on previous experiments, the mud caps could be related to head erosion and shearing of the muddy debris flow/mud flow's upper surface which generated a slower moving fluid mud flow. Our study further confirms that no laminated sand/silt (H4 division) would be present in hybrid event beds with mud-dominated H3 division. Head erosion of muddy debris flow/mud flow would not allow laminated H4 division to form, because the fluid mud flow derived from the parent mud-dominated debris flow was mainly composed of grains smaller than 30 μm which can effectively damp turbulence effectively. Grain size analysis performed on samples from fluid mud flow deposit fall into pelagites/hemipelagites area (T area), suggesting that fluid mud flow deposits are also a candidate for T area, and the area does not necessarily reflect a deposition under un-stressed environment. • 1 Mud caps of hybrid event beds from ECSS are interpreted as fluid mud flow deposits. • 2 SEM and chemical analyses of mud caps suggest a shallow burial depth. • 3 CM pattern of fluid mud flow deposit fall into pelagite/hemipelagite area (T area). • 4 muddy debris flows generate a trailing fluid mud flow. [ABSTRACT FROM AUTHOR]