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Distribution, residence time, autotrophic production, and heterotrophic removal of DOP in the Mirs Bay, northern South China Sea
Huaming Shi, Yu Ma, Xulu Li, Xiaoyong Shi, Junxiao Zhang
doi: 10.1007/s13131-019-1384-8
The importance of dissolved organic phosphorus (DOP) as a potential nutrient source for primary producers in marine systems has been recognized for up to eight decades, but currently, the understanding of the biogeochemistry of DOP is in its infancy. In the present study, monthly data between 2000 and 2014 were used to analyze the temporal and spatial distributions of DOP in the Mir Bay, the northern South China Sea. The DOP residence time (TDOP) was also investigated using a simple regression analysis in combination with chlorophyll a (Chl a) measurements while excess DOP (ΔDOP), produced by the biogeochemical processes of autotrophic production and heterotrophic removal, was determined using a two-component mixing mass-balance model in combination with salinity measurements. The results showed that the DOP concentration was (0.017±0.010) mg/L higher in the surface-water compared with the bottom-water and higher in the inner Tolo Harbour and waters adjacent to Shatoujiao compared with the main zone of the bay. Although seasonal changes and annual variability in the DOP were small, the surface DOP concentration was higher in the wet season (April–September) than in the dry season (October–March) due to the impacts of seaward discharges and atmospheric deposition into the bay. Measurement and regression results showed that the DOP release rate from phytoplankton production was about 1.83 (g P)/(g Chl a) and the TDOP was about 7 d, which implied that the DOP cycle in the bay was rapid. The ΔDOP was calculated from the model to be about 0.000 mg/L in the main zone of the bay and about 0.002 mg/L in the inner Tolo Harbour and waters adjacent to Shaotoujiao, suggesting that the autotrophic production of DOP was almost balanced by the heterotrophic removal in the main zone of the bay and dominated in the inner Tolo Harbour and waters adjacent to Shaotoujiao. In conclusion, the Mirs Bay is very productive and fairly heterotrophic.
key words: dissolved organic phosphorus, temporal and spatial distributions, residence time, autotrophic production, heterotrophic removal, Mirs Bay, northern South China Sea
Miocene paleoenvironmental evolution based on benthic foraminiferal assemblages in the Lufeng Sag, northern South China Sea
Liyuan XUE, Xuan DING, Renjie PEI, Xiaoqiao WAN
doi: 10.1007/s13131-019-1405-7
The Miocene epoch marks the most crucial period during the Cenozoic cooling trend, characterized by the Middle Miocene Climatic Optimum (MMCO) and a series of short–lived cooling events (Miocene isotope events). To understand the paleoenvironmental evolution along the shallow water shelf in the South China Sea during the Miocene, the benthic foraminiferal assemblage and total organic carbon content (TOC) were analyzed at Hole LF 14 located in the Lufeng Sag, northern South China Sea. Three benthic foraminiferal assemblages (e.g., the Uvigerina spp. assemblage, the Cibicides spp. assemblage, and the Cibicidoides spp. assemblage), corresponding to different watermass conditions, were recognized based on Q–mode factor analysis. Early studies suggested that Hole LF14 was deposited under semienclosed bay, middle to outer shelf or even upper bathyal environment during ~18.7–4.53 Ma. The dominant Uvigerina spp. assemblage was characterized by low diversity and shallow infaunal to infaunal species, indicating a warm, low–oxygenation and eutrophic conditions since the Early Miocene to MMCO (~18.7–14.24 Ma). An abrupt sea level drop and significant faunal changes were recorded during 14.24–13.41 Ma, suggesting development of the East Antarctic Ice Sheets, which resulted in a drop of sea level and change in benthic foraminiferal assemblages along the shallow water shelf. Beyond the Uvigerina spp. assemblage, the Cibicides spp. assemblage became important during the middle–late Middle Miocene (14.24–11.54 Ma). This assemblage was dominated by epifaunal species with relative high diversity, suggesting high–energy, high–oxygenation and oligotrophic conditions with episodic supply of organic food. The dominant Cibicidoides spp. assemblage with high diversity, indicates a mesotrophic conditions with relative high–oxygen content during the Late Miocene to Pliocene (11.54–4.53 Ma). The appearance and continuous occurrence of Ammonia spp. and Pseudorotalia spp. since 10.02 Ma, may reflect the influence of the Kuroshio Current.
key words: South China Sea, Lufeng Sag, Miocene, benthic foraminiferal assemblage, total organic carbon, shelf paleoenvironment
High-resolution sequence architecture and depositional evolution of the Quaternary in the northeastern shelf margin of the South China Sea
Hanyao LIU, Changsong LIN, Zhongtao ZHANG, Bo ZHANG, Jing JIANG, Hongxun TIAN, Huan LIU
doi: 10.1007/s13131-019-1442-2
The northeastern shelf margin of the South China Sea (SCS) is characterized by the development of large scale foresets complexes since Quaternary. Based on integral analysis of the seismic, well logging and paleontological data, successions since ~3.0 Ma can be defined as one composite sequence, consist of a set of regional transgressive to regressive sequences. They can be further divided into six 3rd order sequences (SQ0–SQ5) based on the Exxon sequence stratigraphic model. Since ~1.6 Ma, five sets of deltaic systems characterized by development of wedge-shaped foresets complexes or clinoforms had been identified. High-resolution seismic data and the thick foresets allowed further divided of sub-depositional sequences (4th order) of regression to transgression, which is basically consistent with published stacked benthic foram O-isotope records. Depositional systems identified in the study area include deltaic deposits (inner-shelf deltas and shelf-edge deltas), incised valleys, and slope slumping massive deposits. Since ~1.6 Ma, clinoforms prograded from the southern Panyu Lower Uplift toward the northern Baiyun Depression, shelf slope break migrated seaward, whereas the shelf edge of SQ0 migrated landward. The development of incised valleys in the continental shelf increased upward, especially intensive on the SB3 and SB2. The slumping massive deposits increased abruptly since SB2, which corresponds to the development of incised valleys. The evolution of depositional systems of continental slope mainly controlled by the combined influence of sea level changes, tectonic movements, sediment supply and climate changes. Since ~3.0 Ma, relative sea level of the northern SCS had been experienced transgression (~3.0 Ma BP) to regression (~1.6 Ma BP). The regional regression and maximum transgressions of the composite sequences were apparently enhanced by uplift or subsidence related to tectono-thermal events. In addition, climatic variations including monsoon intensification and the mid-Pleistocene transition may have enhanced sediment supply by increasing erosion rate and have an indispensable influence on the development of the incised valleys and 5 sets of deltaic systems since ~1.6 Ma.
key words: sequence architecture, depositional systems, continental slope, Quaternary, Zhujiang (Pearl) River Mouth Basin
The Cenozoic activities of Yangjiang-Yitongdong Fault: insights from analysis of the tectonic characteristics and evolution processes in western Zhujiang (Pearl) River Mouth Basin
Yuhan LI, Rongwei ZHU, Hailing LIU, Xuelin QIU, Haibo HUANG
doi: 10.1007/s13131-019-1477-x
The Yangjiang-Yitongdong Fault (YJF) is an important NW-trending regional fault, which divides the Zhujiang (Pearl) River Mouth Basin (ZRMB) into western and eastern segments. In Cenozoic, the northern continental margin of the South China Sea (SCS) underwent continental rifting, breakup, seafloor spreading and thermal subsidence processes, and the Cenozoic activities of YJF is one part of this series of complex processes. Two long NW-trending multichannel seismic profiles located on both sides of the YJF extending from the continental shelf to Continent-Ocean Boundary (COB) were used to study the tectonic and sedimentary characteristics of western ZRMB. Using the 2D-Move software and back-stripping method, we constructed the balance cross-section model and calculated the fault activity rate. Through the comprehensive consideration of tectonic position, tectonic evolution history, featured structure, and stress analysis, we deduced the activity history of the YJF in Cenozoic. The results showed that the YJF can be divided into two segments by the central uplift belt. From 65 Ma to 32 Ma, the YJF was in sinistral motion as a whole, inherited the preexisting sinistral motion of Mesozoic YJF, in which, the southern part of YJF was mainly in extension activity, controlling the formation and evolution of Yunkai Low Uplift, coupled with slight sinistral motion. From 32 Ma to 23.8 Ma, the sinistral motion in northern part of YJF continued, while the sinistral motion in southern part began to stop or shifted to a slightly dextral motion. After 23.8 Ma, the dextral motion in southern part of YJF continued, while the sinistral motion in northern part of YJF gradually stopped, or shifted to the slightly dextral motion. The shift of the YJF strike-slip direction may be related to the magmatic underplating in continent-ocean transition, southeastern ZRMB. According to the analysis of tectonic activity intensity and rift sedimentary structure, the activities of YJF in Cenozoic played a regulating role in the rift extension process of ZRMB.
key words: Yangjiang-Yitongdong Fault, Zhujiang (Pearl) River Mouth Basin, tectonic evolution, strike slip movement, Cenozoic
Tectono-thermal evolution of the Liwan Sag, deepwater area in the Zhujiang River Mouth Basin, northern South China Sea
Xiaoyin TANG, Shaopeng HUANG, Shuchun YANG, Guangzheng JIANG, Mo JI, Shengbiao HU
doi: 10.1007/s13131-017-1125-9
The Liwan Sag, with an area of 4 000 km2, is one of the deepwater sags in the Zhujiang River (Pearl River) Mouth Basin, northern South China Sea. Inspired by the exploration success in oil and gas resources in the deepwater sags worldwide, we conducted the thermal modeling to investigate the tectono-thermal history of the Liwan Sag, which has been widely thought to be important to understand tectonic activities as well as hydrocarbon potential of a basin. Using the multi-stage finite stretching model, the tectonic subsidence history and the thermal history have been obtained for 12 artificial wells, which were constructed on basis of one seismic profile newly acquired in the study area. Two stages of rifting during the time periods of 49–33.9 Ma and 33.9–23 Ma can be recognized from the tectonic subsidence pattern, and there are two phases of heating processes corresponding to the rifting. The reconstructed average basal paleo-heat flow values at the end of the rifting events are ~70.5 and ~94.2 mW/m2 respectively. Following the heating periods, the study area has undergone a persistent thermal attenuation phase since 23 Ma and the basal heat flow cooled down to ~71.8–82.5 mW/m2 at present.
key words: tectono-thermal evolution, thermal history, heat flow, deepwater area, Liwan Sag
Different vertical distribution of zooplankton community between North Pacific Subtropical Gyre and Western Pacific Warm Pool: its implication to carbon flux
Dong SUN, Dongsheng ZHANG, Ruiyan ZHANG, Chunsheng WANG
doi: 10.1007/s13131-018-1237-x
The mesozooplankton in both epipelagic and mesopelagic zones is essentially important for the study of ecosystem and biological carbon pump. Previous studies showed that the diel vertical migration (DVM) pattern of mesozooplankton varied among ecosystems. However, that pattern was largely unknown in the Western Pacific Warm Pool (WPWP). The vertical distribution, DVM and community structure of mesozooplankton from the surface to 1 000 m were compared at Stas JL7K (WPWP) and MA (North Pacific Subtropical Gyre, NPSG). Two sites showed similarly low biomass in both epipelagic and mesopelagic zones, which were in accordance with oligotrophic conditions of these two ecosystems. Stronger DVM (night/day ratio) was found at JL7K (1.31) than that at MA (1.09) on surface 0–100 m, and an obvious night increase of mesopelagic biomass was observed at JL7K, which was probably due to migrators from bathypelagic zone. Active carbon flux by DVM of zooplankton was estimated to be 0.23 mmol/(m2·d) at JL7K and 0.16 mmol/(m2·d) at MA. The community structure analysis showed that calanoid copepods, cnidarians and appendicularians were the main contributors to DVM of mesozooplankton at both sites. We also compared the present result with previous studies of the two ecosystems, and suggested that the DVM of mesozooplankton was more homogeneous within the WPWP and more variable within the NPSG, though both ecosystems showed typically extremely oligotrophic conditions. The different diel vertical migration strength of mesozooplankton between NPSG and WPWP implied different efficiency of carbon pump in these two ecosystems.
key words: mesopelagic zone, diel vertical migration, biological pump, vertical distribution, active carbon flux
Basin modeling in the initial stage of exploration: a case study from the North Subbasin of the South Yellow Sea Basin
Yumao PANG, Xunhua ZHANG, Xingwei GUO, Guolin XIAO, Zuozhen HAN
doi: 10.1007/s13131-017-1112-1
Basin modeling has become an important tool for analyzing sedimentary basins. The North Subbasin of the South Yellow Sea Basin is filled with thick Meso-Cenozoic terrigenous deposits during the rift evolution stage. The accumulation of data and achievements of geological investigations in recent years have provided the preconditions for basin modeling. The necessary parameters and geological elements for simulations are collated and summarized. Modeling of tectono-thermal evolution is performed and the related trend in heat flow is reconstructed and calibrated. The heat flow value commences from an average level of 61 mW/m2 during Middle-Late Jurassic, rises to about 80 mW/m2 from circa 145 Ma to circa 74 Ma, and then undergoes a gradual decline to 65 mW/m2 until the end of Oligocene.Three evolutionary phases, namely, the initial rifting phase, syn-rifting phase, and post-rifting phase, have been identified. The modeling results show that the North Subbasin generally enters into a stage of strong rifting during Cretaceous and undergoes rapid subsidence until the Late Cretaceous, then follows by a stage of moderate rifting during the Paleogene. The input and general workflow involved in 3-D modeling are introduced. Reconstruction of the petroleum system in the North Subbasin reveals that the threshold depth of hydrocarbon generation is located near the top of the Paleogene Funing formation, and the underlying Jurassic and Cretaceous source rocks have reached or exceeded peak oil generation and have almost completed the generation and expulsion of hydrocarbons. The main generation and expulsion in the Jurassic source rocks take place during the syn-rifting and post-rifting phases, whereas the peak generation and expulsion in the Cretaceous and Paleogene source rocks take place during the post-rifting phase. Although the study area is still a relatively less explored sedimentary basin, the results of modeling can provide valuable information for exploration. A preliminary discussion of the main uncertainty factors is also presented.
key words: basin modeling, tectono-thermal evolution history, petroleum system, South Yellow Sea Basin
Geochemical characteristics of Oligocene-Miocene sediments from the deepwater area of the northern South China Sea and their provenance implications
Shuhui CHEN, Peijun QIAO, Houhe ZHANG, Xiaojun XIE, Yuchi CUI, Lei SHAO
doi: 10.1007/s13131-017-1127-7
Geochemical and detrital zircon U-Pb dating data for drilled sediments from the Baiyun deepwater area of the northern South China Sea demonstrate a change of sedimentary sources from the Oligocene to the Miocene. Zircon ages of the pre-rift Eocene sequences are dominated by Yanshanian ages with various peak values (110–115 Ma for U1435 and L21; 150 Ma for H1), indicating local sediment supply from the pre-existing Mesozoic magmatic belt. For the Oligocene sediments in the northern part of the basin, the rare earth elements show different distribution characteristics, indicating sediment supply from the paleo-Zhujiang River (Pearl River), as also confirmed by the multimodal zircon age spectra of the Lower Oligocene strata in Well X28. By contrast, a positive Eu anomaly characterizes sediments from the western and southern parts of the basin, indicating potential provenances from intermediate to basic volcanic rock materials. The Baiyun Movement at the end of the Oligocene contributed to a large-scale subsidence in the deepwater area and also a northward retreat of continental shelf break, leading to deepening depositional environment in the basin. As a result, all the detrital zircon ages of the Upper Oligocene strata from Wells X28, L13, and L21 share a similar distribution, implying the possible control of a common source like the Zhujiang River. During the Miocene, whereas sediments in the northern area were mainly sourced from the Zhujiang River Delta, and those in the southern deepwater area continued to be affected by basic volcanic activities, the Dongsha Uplift could have contributed as the main source to the eastern area.
key words: geochemistry, zircon U-Pb age, provenance, South China Sea, Baiyun deepwater area
Chemical kinetics evaluation and its application of natural gas generation derived from the Yacheng Formation in the deep-water area of the Qiongdongnan Basin, China
Long SU, Dongwei ZHANG, Haizhang YANG, Ying CHEN, Guojun CHEN, Jianjing ZHENG, Yongchang XU
doi: 10.1007/s13131-018-1158-8
The natural gas generation process is simulated by heating source rocks of the Yacheng Formation, including the onshore-offshore mudstone and coal with kerogens of Type II2-III in the Qiongdongnan Basin. The aim is to quantify the natural gas generation from the Yacheng Formation and to evaluate the geological prediction and kinetic parameters using an optimization procedure based on the basin modeling of the shallow-water area. For this, the hydrocarbons produced have been grouped into four classes (C1, C2, C3 and C4-6). The results show that the onset temperature of methane generation is predicted to occur at 110°C during the thermal history of sediments since 5.3 Ma by using data extrapolation. The hydrocarbon potential for ethane, propane and heavy gaseous hydrocarbons (C4-6) is found to be almost exhausted at geological temperature of 200°C when the transformation ratio (TR) is over 0.8, but for which methane is determined to be about 0.5 in the shallow-water area. In contrast, the end temperature of the methane generation in the deep-water area was over 300°C with a TR over 0.8. It plays an important role in the natural gas exploration of the deep-water basin and other basins in the broad ocean areas of China. Therefore, the natural gas exploration for the deep-water area in the Qiongdongnan Basin shall first aim at the structural traps in the Ledong, Lingshui and Beijiao sags, and in the forward direction of the structure around the sags, and then gradually develop toward the non-structural trap in the deep-water area basin of the broad ocean areas of China.
key words: deep-water area, geological prediction, natural gas, Yacheng Formation, evaluation, Qiongdongnan Basin
Geochemistry and age of seamounts in the West Pacific: mantle processes and petrogenetic implications
Limei TANG, Yanhui DONG, Fengyou CHU, Ling CHEN, Weilin MA, Yonggang LIU
doi: 10.1007/s13131-019-1371-0
Research on seamounts provides some of the best constraints for understanding intraplate volcanism, and samples from seamounts reveal crucial evidence about the geochemical makeup of the oceanic mantle. There are still many seamounts in the West Pacific Seamount Province (WPSP) that have not been studied, meaning their ages and geochemistry remain unknown. A better understanding of these seamount trails and their evolutionary history, investigated with age and geochemistry data, will enable better understanding of the geological processes operating underneath the Pacific Ocean Plate. Here, new 40Ar/39Ar ages and trace element and Sr-Nd-Pb isotopic data for seven basalt rocks from four seamounts in the WPSP are provided. Chemically, these rocks are all Oceanic Island Alkali basalt (OIA type); analysis of olivine phenocrysts shows that the magmas experienced strong olivine fractionation and changed from olivine + plagioclase to olivine + plagioclase + clinopyroxene cotectic during their evolution. Rare earth element (REE) patterns and a spider diagram of the samples in this study show OIB (Ocean Island Basalt) like behavior. The range of 87Sr/86Sr values is from 0.704 60 to 0.706 24, the range of 206Pb/204Pb values is from 18.241 to 18.599, and the range of 143Nd/144Nd values is from 0.512 646 to 0.512 826; together, these values indicate magma sources ranging from EMI to EMII. Finally, new 40Ar/39Ar age data show that these seamounts formed at ~97 and ~106 Ma, indicating that some may have undergone the same formation processes as seamounts in the eastern part of the Magellan Seamount Trail, but other seamounts likely have different origins.
key words: 40Ar/39Ar ages, geochemistry, magmatic evolution, basalts, West Pacific

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