Differences in Precipitation and Related Wind Dynamics and Moisture and Heat Features in Separate Areas of the South China Sea before and after Summer Monsoon Onset

Using surface and balloon-sounding measurements, satellite retrievals, and ERA5 reanalysis during 2011–20, this study compares the precipitation and related wind dynamics, moisture and heat features in different areas of the South China Sea(SCS) before and after SCS summer monsoon onset(SCSSMO). The rainy sea around Dongsha(hereafter simply referred to as Dongsha) near the north coast, and the rainless sea around Xisha(hereafter simply referred to as Xisha) in the western SCS, are selected as two typical research subregions. It is found that Dongsha, rather than Xisha, has an earlier and greater increase in precipitation after SCSSMO under the combined effect of strong low-level southwesterly winds, coastal terrain blocking and lifting, and northern cold air. When the 950-h Pa southwesterly winds enhance and advance northward, accompanied by strengthened moisture flux, there is a strong convergence of wind and moisture in Dongsha due to a sudden deceleration and rear-end collision of wind by coastal terrain blocking. Moist and warm advection over Dongsha enhances early and deepens up to 200 h Pa in association with the strengthened upward motion after SCSSMO, thereby providing ample moisture and heat to form strong precipitation. However, when the 950-h Pa southwesterly winds weaken and retreat southward, Xisha is located in a wind-break area where strong convergence and upward motion centers move in. The vertical moistening and heating by advection in Xisha enhance later and appear far weaker compared to that in Dongsha, consistent with later and weaker precipitation.

Morphological and Sulfur-Isotopic Characteristics of Pyrites in the Deep Sediments from Xisha Trough,South China Sea

Pyrite is one of the common authigenic minerals in marine sediments.Previous studies have shown that the morphological and isotopic characteristics of pyrite are closely related to the geochemical environment where it is formed.To better understand the for-mation mechanism of authigenic pyrite,we analyzed the isotopic composition,morphology,and distribution of pyrite in the sediment at 500m below the seafloor from Xisha Trough,South China Sea.Mineral morphologies were observed by scanning electron micros-copy and Raman spectrography.X-Ray computed tomography was applied to measure the particle size of pyrite.The size of pyrite crystals in the matrix sediment mainly ranged between 25 and 65µm(av.ca.40µm),although crystals were larger(av.ca.50μm)in the veins.The pyrites had a fine-grained truncated octahedral shape with occasionally well-developed growth steps,which implies the low growth rate and weak anaerobic oxidation of methane-sulfate reduction when pyrite was formed.Theδ^(34)S values of pyrites ranged from+20.8‰Vienna-defined Canyon Diablo Troilite(V-CDT)to+33.2‰V-CDT and from+44.8‰V-CDT to+48.9‰,which suggest two growth stages.In the first stage,with the continuous low methane flux,the pyrite possibly formed in an environment with good access to seawater.In the second stage,the pyrites mainly developed in sediment fractures and appeared in veins,probably due to the limited availability of sulfate.The less exposure of pyrite to the environment in the second stage was probably caused by sediment accumulation or perturbation.In this study,an episodic pyritization process was identified,and the paleoenvironment was reconstructed for the sediment investigated.

U-Pb zircon geochronology of basaltic pyroclastic rocks from the basement beneath the Xisha Islands in the northwestern South China Sea and its geological significance

As one of the micro-blocks dispersed in the South China Sea(SCS), the basement of the Xisha Islands has rarely been drilled because of the thick overlying Cenozoic sediments, which has led to a confused understanding of the pre-Cenozoic basement of the Xisha Islands. Well CK-1, a kilometer-scale major scientific drill in the Xisha Islands in the northwestern SCS, penetrated thick reefal limestone(0–888.4 m) and the underlying basement rocks(888.4–901.4 m). In this study, we present the zircon U-Pb ages of basement basaltic pyroclastic rocks from Well CK-1 in the Xisha Islands of the northwestern SCS to investigate the basement nature of the Xisha microblock. The basement of Well CK-1 consists of basaltic pyroclastic rocks on the seamount. The zircon grains yielded apparent ages ranging from ca. 2 138.9 Ma to ca. 36 Ma. The old group of zircon grains from Well CK-1 was considered to be inherited zircons. Two Cenozoic zircons gave a weighted mean 206Pb/238U age of(36.3 ± 1.1) Ma,Mean Squared Weighted Deviations(MSWD) = 1.2, which may represent the maximum age of the volcano eruption. The Yanshanian inherited zircons(116.9–105.7 Ma and 146.1–130.2 Ma) from Well CK-1 are consistent with the zircons from Well XK-1, indicating that the basement of Chenhang Island may be similar to that of Well XK-1. We propose that the Xisha micro-block may have developed on a uniform Late Jurassic metamorphic crystalline basement, intruded by Cretaceous granitic magma.

Types,Characteristics and Significances of Migrating Pathways of Gas-bearing Fluids in the Shenhu Area,Northern Continental Slope of the South China Sea

The first marine gas hydrate expedition in China has been conducted by Guangzhou Marine Geological Survey in the Shenhu Area, northern continental slope of the South China Sea. Previous study has analyzed the P-T conditions, geophysical anomalies and saturation calculations of these gas hydrates, but has not documented in detail the migration of gas-bearing fluids in the study area. Based on the interpretations of 2D/3D seismic data, this work identified two types of migration pathways for gas-bearing fluids in the Shenhu area, i.e., vertical and lateral pathways. The vertical pathways（largescale faults, gas chimneys and mud diapirs） presented as steep seismic reflection anomalies, which could be traced downward to the Eocene source rocks and may penetrate into the Late Miocene strata. The deeper gases/fluids might be allowed migrating into the shallower strata through these vertical conduits. However, the distributions showed distinct differences between these pathways. Large-scale faults developed only in the north and northeast of the Shenhu area, while in the drilling area gas chimneys were the sole vertical migration pathways. Since the Pliocene, normal faults, detachment faults and favorable sediments have constituted the lateral pathways in the Shenhu gas hydrate drilling area. Although these lateral pathways were connected with gas chimneys, they exerted different effects on hydrate formation and accumulation. Gas-bearing fluids migrated upward along gas chimneys might further migrate laterally because of the normal faults, thereby enlarging the range of the chimneys. Linking gas chimneys with the seafloor, the detachment faults might act as conduits for escaping gases/fluids. Re-deposited sediments developed at the early stage of the Quaternary were located within the gas hydrate stability zone, so hydrates would be enriched in these favorable sediments. Compared with the migration pathways（large-scale faults and mud diapirs） in the LW3-1 deep-sea oil/gas field, the migration efficiency of the vertical pathways（composed of gas chimneys） in the gas hydrate drilling area might be relatively low. Description and qualitative discrimination of migration pathways in the Shenhu gas hydrate drilling area are helpful to further understand the relationship between good-quality deep source rocks and shallow, mainly biogenicallyproduced, hydrates. As the main source rocks of the Baiyun sag, lacustrine mudstones in the Wenchang and Enping Formations may provide thermogenic methane. Gas chimneys with relatively low migration efficiency created the vertical pathways. Caused by the Dongsha tectonic movement, the release of overpressured fluids might reduce the vertical migration rates of the thermogenic methane. The thick bathyal/abyssal fine-grained sediments since the Late Miocene provided migration media with low permeability. These preconditions may cause carbon isotopic fractionation ofthermogenic methane during long-distance vertical migrations. Therefore, although geochemical analyses indicate that the methane forming gas hydrate in the Shenhu area was mainly produced biogenically, or was mixed methane primarily of microbial origin, thermogenic methane still contribute significantly.

Dynamics of picoplankton in the Nansha Islands area of the South China Sea

Dynamics of major picoplankton groups, Synechococcus (Syn), Prochlorococcus (Pro), picoeukaryotes (Euk) and heterotrophic bacteria (Bact) was investigated by flow cytometry for the first time in the Nansha Islands area in the South China Sea. Averaged over the whole investigation area, depth-weighted integrated cell abundance (DWA) of Syn, Pro, Euk and Bact was 1.6 (0.4-5.7)×103, 5.4 (0.1-7.3)×104, 0.7 (0.2-2.2)×103, and 2.3 (1.4-3.2)×105 cells/mL respectively. Picoautotrophic cell abundance was low in the northwest part of the Nansha Islands where surface water temperature was low and the upper mixed layer was shallow. Concurrently, a surface maximum vertical distribution pattern was observed in this area. While in the southeast and east zones where temperatures were relatively higher and nitraclines were deeper, picoplankton is abundant and a subsurface maximum around 50-75 m is observed. Coupling of horizontal and vertical distribution patterns of picoplankton abundance and hydrological status was found, suggesting a strong influence of currents and water column structure on picoplankton distribution in the investigation area. Contrary to that in the shelf water in the East China Sea, the relationship between Pro and Bact in the Nansha Islands area in the South China Sea was not significantly negative but weakly positive. Moreover, a similar distribution pattern of Syn and Pro was observed. Possible reasons for these differences in the two marine regimes were discussed.

The Relationship between Tectonic Subsidence and BSR of Upper Neogene in the Deep-Water Area of the Northern Continental Slope, South China Sea

BSR （Bottom Simulating Reflector） occurs widely in the strata since the late Miocene in the deep-water area of the northern continental slope of South China Sea （SCS）. It is an important seismic reference mark which identifies the gas hydrate and its distribution influenced by the tectonic movements. Single-point basin modeling was conducted using 473 points in the study area. To discuss the relationships between the tectonic subsidence and BSR, the volume and rate of tectonic subsidence in each geological time have been simulated. The results show that there are three tectonic accelerate subsidence processes in the study area since the late Miocene, especially since 1.8Ma the tectonic subsidence accelerates more apparently. Since the Late Miocene to Pleistocene, the rate of tectonic subsidence in deep-water underwent a transformation from weak to strong. The ratio of tectonic subsidence to the total subsidence was relatively high （65-70%）. Through the superposition of the BSR developed areas and the contours of tectonic subsidence in this area, it was discovered that more than 80% of BSR tend to be distributed at the slope break or depression-uplift structural transfer zone and the average tectonic subsidence rate ranges from 70 m/Ma to 125 m/Ma.

Coexistence of natural gas hydrate,free gas and water in the gas hydrate system in the Shenhu Area,South China Sea

Shenhu Area is located in the Baiyun Sag of Pearl River Mouth Basin,which is on the northern continental slope of the South China Sea.Gas hydrates in this area have been intensively investigated,achieving a wide coverage of the three-dimensional seismic survey,a large number of boreholes,and detailed data of the seismic survey,logging,and core analysis.In the beginning of 2020,China has successfully conducted the second offshore production test of gas hydrates in this area.In this paper,studies were made on the structure of the hydrate system for the production test,based on detailed logging data and core analysis of this area.As to the results of nuclear magnetic resonance(NMR)logging and sonic logging of Well GMGS6-SH02 drilled during the GMGS6 Expedition,the hydrate system on which the production well located can be divided into three layers:(1)207.8–253.4 mbsf,45.6 m thick,gas hydrate layer,with gas hydrate saturation of 0–54.5%(31%av.);(2)253.4–278 mbsf,24.6 m thick,mixing layer consisting of gas hydrates,free gas,and water,with gas hydrate saturation of 0–22%(10%av.)and free gas saturation of 0–32%(13%av.);(3)278–297 mbsf,19 m thick,with free gas saturation of less than 7%.Moreover,the pore water freshening identified in the sediment cores,taken from the depth below the theoretically calculated base of methane hydrate stability zone,indicates the occurrence of gas hydrate.All these data reveal that gas hydrates,free gas,and water coexist in the mixing layer from different aspects.

Geoacoustic Inversion for Bottom Parameters in the Deep-Water Area of the South China Sea

Bottom acoustic parameters play an important role in sound field prediction. Acoustic parameters in deep water are not well understood. Bottom acoustic parameters are sensitive to the transmission-loss （TL） data in the shadow zone of deep water. We propose a multiple-step fill inversion method to invert sound speed, density and attenuation in deep water. Based on a uniform liquid hMf-space bottom model, sound speed of the bottom is inverted by using the long range TL at low frequency obtained in an acoustic propagation experiment conducted in the South China Sea （SCS） in summer 2014. Meanwhile, bottom density is estimated combining with the Hamilton sediment empirical relationship. Attenuation coefficients at different frequencies are then estimated from the TL data in the shadow zones by using the known sound speed and density as a constraint condition. The nonlinear relationship between attenuation coefficient and frequency is given in the end. Tile inverted bottom parameters can be used to forecast the transmission loss in the deep water area of SCS very we//.

Diversity of symbiotic algae of the genus Symbiodinium in scleractinian corals of the Xisha Islands in the South China Sea

Symbiotic algae （Symbiodinium sp.） in scleractinian corals are important in understanding how coral reefs will respond to global climate change. The present paper reports on the diversity of Symbiodinium sp. in 48 scleractinian coral species from 25 genera and 10 families sampled from the Xisha Islands in the South China Sea, which were identified with the use of restriction fragment length polymorphism （RFLP） of the nuclear ribosomal DNA large subunit gene （rDNA）. The results showed that： （i） Symbiodinium Clade C was the dominant zooxanthellae in scleractinian corals in the Xisha Islands; （ii） Symbiodinium Clade D was found in the corals Montipora aequituberculata, Galaxea fascicularis, and Plerogyra sinuosa; and （iii） both Symbiodinium Clades C and D were found simultaneously in Montipora digitata, Psammocora contigua, and Galaxeafascicularis. A poor capacity for symbiosis polymorphism, as uncovered by RFLP, in the Xisha Islands indicates that the scleractinian corals have low adaptability to environmental changes. Further studies are needed to investigate zooxanthellae diversity using other molecular markers.

Regional hard coral distribution within geomorphic and reef flat ecological zones determined by satellite imagery of the Xisha Islands,South China Sea

Coral reefs in the Xisha Islands （also known as the Paracel Islands in English）, South China Sea, have experienced dramatic declines in coral cover. However, the current regional scale hard coral distribution of geomorphic and ecological zones, essential for reefs management in the context of global warming and ocean acidification, is not well documented. We analyzed data from field surveys, Landsat-8 and GF-1 images to map the distribution of hard coral within geomorphic zones and reef fiat ecological zones. In situ surveys conducted in June 2014 on nine reefs provided a complete picture of reef status with regard to live coral diversity, evenness of coral cover and reef health （live versus dead cover） for the Xisha Islands. Mean coral cover was 12.5% in 2014 and damaged reefs seemed to show signs of recovery. Coral cover in sheltered habitats such as lagoon patch reefs and biotic dense zones of reef flats was higher, but there were large regional differences and low diversity. In contrast, the more exposed reef slopes had high coral diversity, along with high and more equal distributions of coral cover. Mean hard coral cover of other zones was 〈10%. The total Xisha reef system was estimated to cover 1 060 km2, and the emergent reefs covered -787 km2. Hard corals of emergent reefs were considered to cover 97 km2. The biotic dense zone of the reef flat was a very common zone on all simple atolls, especially the broader northern reef flats. The total cover of live and dead coral can reach above 70% in this zone, showing an equilibrium between live and dead coral as opposed to coral and algae. This information regarding the spatial distribution of hard coral can support and inform the management of Xisha reef ecosystems.

Simulation of the Accumulation Process of Biogenic Gas Hydrates in the Shenhu Area of Northern South China Sea

Previous work has largely discussed the relations between sediment structures and accumulation of gas hydrates in the Shenhu area of South China Sea, but has not documented why the gas hydrates occurred at the seafloor topographic highs. Many gas hydrate exploration examples abroad also indicate that the saturation of gas hydrates was higher at seafloor topographic highs. This work aims to understand why gas hydrates accumulated at topographic highs and why their saturation is higher.

Geochemical characteristics of Oligocene-Miocene sediments from the deepwater area of the northern South China Sea and their provenance implications

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.

Study of radioactive status in the Zhujiang Estuary and adjacent sea area in South China Sea (SCS)

Through the investigation on radioactive activities of water, sediment and some marine organismsin the Zhujiang Estuary, adjacent sea area and the distributary mouths of the Zhujiang River, activities of total α. radioactivity, total β radioactivity, artificial radioactive 90Srand 157Cs, and factors inflencing the distribution and the content of U in seawater are studied.The mainly radioactive pollution substances and their sources in the sea area are studied by γ spectra obtained from sediment in the sea area. The results show that the main radioactivity substances are natural radioactivity U,Th series and 40K. which were produced by the modern industry and transported into the sea through the main current of the Zhujiang River.

Zircon SHRIMP U-Pb dating of the Neogene coral reefs,Xisha Islands,South China Sea:implications for tectonic evolution

The Xisha Block is a minor one in the South China Sea and an important tectonic unit in the northwestern part of the region.Zircon SHRIMP U-Pb ages for three volcanic intrusive core samples from Xike-1,an exploratory well penetrating the bioherms of the Xisha Islands.The core samples are from the Miocene reef carbonate bedrock and are recognized as dark-gray biotite-hornblende gabbro,gray fne-grained biotite diorite,and gray fine-grained granite,respectively.Zircon cathodoluminescence (CL)images and trace Th,U and Pb compositions of the zircons show that these rocks are of volcanic intrusive origin.Zircon SHRIMP U-Pb dating yielded six groups of ages,ranging from 2451-1 857 Ma to early Cretaceous, which indicate that the formation and evolution of the Xisha Block was affected by the evolution and closure of Neotethys Ocean,probably within its eastern extension into South China Sea.Both old,deepsourced material,including fragments from Rodina supercontinent,and recent mantle-derived magma products contributed to the emergence and formation of the Xisha block.The SHRIMP U-Pb results also proved that this process differed from that of the Kontum massif,the Hainan Block,and the South China Block,but is similar to that of the Nansha and Zhongsha blocks.The process was associated with the effects of Yanshanian magmatism induced by subduction mechanisms of the Paleo-Pacific Plate or the reworking of the multiple magmatisms since the Early to mid-Yanshanian,possibly jointly experienced by the Xisha-Zhongsha-Nansha Block.

Assessment of natural gas hydrate reservoirs at Site GMGS3-W19 in the Shenhu area,South China Sea based on various well logs

To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19,extensive geophysical logging data and cores were analyzed to assess the reservoir properties.Sediment porosities were estimated from density,neutron,and nuclear magnetic resonance(NMR)logs.Both the resistivity and NMR logs were used to calculate gas hydrate saturations,the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data.The density porosity was closely in agreement with the core-derived porosity,and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates.The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity.For the gas hydrate reservoirs at GMGS3-W19,the porosity,gas hydrate saturation and free gas saturation was 52.7%,42.7%and 10%,on average,respectively.The various logs provide different methods for the comprehensive evaluation of hydrate reservoir,which supports the selection of candidate site for gas hydrate production testing.

Petroleum geology controlled by extensive detachment thinning of continental margin crust: A case study of Baiyun sag in the deep-water area of northern South China Sea

The relationships between crustal stretching and thinning,basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic,gravity,magnetic,ocean bottom seismic(OBS),deep-water exploration wells and integrated ocean drilling program(IODP).During the early syn-rifting period,deep-water area was a half-graben controlled by high angle faults influenced by the brittle extension of upper crust.In the mid syn-rifting period,this area was a broad-deep fault depression controlled by detachment faults undergone brittle-ductile deformation and differentiated extension in the crust.In the late syn-rifting period,this area experienced fault-sag transition due to saucer-shaped rheology change dominated by crustal ductile deformation.A broad-deep fault depression controlled by the large detachment faults penetrating through the crust is an important feature of deep-water basin.The study suggests that the broad-deep Baiyun sag provides great accommodation space for the development of massive deltaic-lacustrine deposition system and hydrocarbon source rocks.The differentiated lithospheric thinning also resulted in the different thermal subsidence during post-rifting period,and then controlled the development of continental shelf break and deep-water reservoir sedimentary environment.The high heat flow background caused by the strong thinning of lithosphere and the rise of mantle source resulted in particularities in the reservoir diagenesis,hydrocarbon generation process and accumulation of deep-water area in northern South China Sea.

Variation of Air-Sea Heat Fluxes over the Western Pacific Warm Pool Area and Its Relationship with the South China Sea Summer Monsoon Onset

Based on oceanic and atmospheric parameters retrieved by satellite remote sensing using a neural network method, air-sea heat fluxes over the western Pacific warm pool area were calculated with the advanced the advanced Coupled Ocean-Atmosphere Response Experiment 3.0 (COARE3.0) bulk algorithm method. Then, the average annual and interannual characteristics of these fluxes were analyzed. The rela- tionship between the fluxes and the South China Sea (SCS) summer monsoon onset is highlighted. The results indicate that these fluxes have clear temporal and spatial characteristics. The sensible heat flux is at its maximum in the Kuroshio area, while the latent heat flux is at its maximum in the North Equatorial Current and Kuroshio area. The distribution of average annual air-sea heat fluxes shows that both sensible and latent heat fluxes are maximized in winter and minimized in summer. The air-sea heat fluxes have obvious interannual variations. Correlation analysis indicates a close lag-correlation between air-sea heat fluxes in the western Pacific warm pool area and at the SCS summer monsoon onset. The lagcorrelation can therefore predict the SCS summer monsoon onset, providing a reference for the study of precipitation related to the monsoon.

A 3D basin modeling study of the factors controlling gas hydrate accumulation in the Shenhu Area of the South China Sea

Great advancement has been made on natural gas hydrates exploration and test production in the northern South China Sea.However,there remains a lot of key questions yet to be resolved,particularly about the mechanisms and the controls of gas hydrates enrichment.Numerical simulaution would play signficant role in addressing these questions.This study focused on the gas hydrate exploration in the Shenhu Area,Northern South China Sea.Based on the newly obtained borehole and multichannel reflection seismic data,the authors conducted an integrated 3D basin modeling study on gas hydrate.The results indicate that the Shenhu Area has favorable conditions for gas hydrate accumulation,such as temperature,pressure,hydrocarbon source,and tectonic setting.Gas hydrates are most concentrated in the Late Miocene strata,particularly in the structual highs between the Baiyun Sag and the Liwan Sag,and area to the south of it.It also proved the existence of overpressure in the main sag of source rocks,which was subject to compaction disequilibrium and hydrocarbon generation.It also shown that the regional fault activity is not conducive to gas hydrate accumulation due to excess gas seepage.The authors conjecture that fault activity may slightly weaken overpressure for the positive effect of hydrocarbon expulsion and areas lacking regional fault activity have better potential.

Distribution of Organic Matter,Iron,Mangenese in Surface Sediments in the Nansha Islands Sea Area,South China Sea

Concentrations of organic matter, iron and manganese in the deep sea surface sediments in the Nansha Islands sea area, South China Sea are measured. Horizontal and vertical distributions of iron and manganese are discussed. The vertical distribution of iron and manganese in the sediments results from reduction, diffusion, and redeposition of manganese (or iron) oxide and hydroxide in the sediment. There are the maxima of iron and manganese in solid phase in the top of the sediment, which is caused by the penetration of O2 and the upward flux of Mn2+ ( or Fe2+ ). Manganese bacteria play a very important role in the cycle of solid-phase iron and manganese in the ocean environment. Manganese bacteria oxidize Mn2+ ( or Fe2+ ) in dissolved state to Mn4+ ( or Fe3+ ) in oxidized state under the aerobic condition, whereas they reduce iron and manganese in anaerobic conditions.

Abnormality of the monsoon wind in the sea area along the southeasterncoast of China and the response of the northern areaof the South China Sea during the ENSO events

-Variations of monsoon wind field in the sea area along the southeastern coast of China during the ENSO events and its influence on the sea level and sea surface temperature (SST) are explored mainly on the basis of the data of monthly mean wind at 850 hPa and five coastal stations during 1973-1987. The results from the analyses of the data and theoretical estimation show that the southwest wind anomalies appeared in the study area during the events, and northeast wind anomalies occurred in general before the events. With the coastline of the area being parallel basically to the direction of the wind, an Ekman transport will result in an accumulation of the water near the coast or a departure of the water from the coast. As a result , the sea level and SST there will be affected markedly. During the events, southwest wind will intensify in the summer, and northeast wind will weaken in the winter. Their total effect is that a large negative anomaly of the sea level and SST will occur. The estimations indicate that the monsoon wind is stronger in the summer and weaker in the winter than the normal by 1-1. 5 m/s during the events, and this anomaly will cause a decrease of the sea level by 7-11 cm . Changes of the wind field, therefore, is mainly responsible for a large negative anomalies of the sea level and SST there during the ENSO events.