查看更多>>摘要：Composite analyses were performed in this study to reveal the difference in spring precipitation over southern China during multiyear La Niña events during 1901 to 2015.It was found that there is significantly below-normal precipitation during the first boreal spring,but above-normal precipitation during the second year.The difference in spring precipitation over southern China is correlative to the variation in western North Pacific anomalous cyclone(WNPC),which can in turn be attributed to the different sea surface temperature anomaly(SSTA)over the Tropical Pacific.The remote forcing of negative SSTA in the equatorial central and eastern Pacific and the local air-sea interaction in the western North Pacific are the usual causes of WNPC formation and maintenance.SSTA in the first spring is stronger than those in the second spring.As a result,the intensity of WNPC in the first year is stronger,which is more likely to reduce the moisture in southern China by changing the moisture transport,leading to prolonged precipitation deficits over southern China.However,the tropical SSTA signals in the second year are too weak to induce the formation and maintenance of WNPC and the below-normal precipitation over southern China.Thus,the variation in tropical SSTA signals between two consecutive springs during multiyear La Niña events leads to obvious differences in the spatial pattern of precipitation anomaly in southern China by causing the different WNPC response.
查看更多>>摘要：The wave-induced liquefaction of seabed is responsible for causing damage to marine structures.Particle composition and consolidation degree are the key factors affecting the pore water pressure response and liquefaction behavior of the seabed under wave action.The present study conducted wave flume experiments on silt and silty fine sand beds with varying particle compositions.Furthermore,a comprehensive analysis of the differences and underlying reasons for liquefaction behavior in two different types of soil was conducted from both macroscopic and microscopic perspectives.The experimental results indicate that the silt bed necessitates a lower wave load intensity to attain the liquefaction state in comparison to the silty fine sand bed.Additionally,the duration and development depth of liquefaction are greater in the silt bed.The dissimilarity in liquefaction behavior between the two types of soil can be attributed to the variation in their permeability and plastic deformation capacity.The permeability coefficient and compression modulus of silt are lower than those of silty fine sand.Consequently,silt is more prone to the accumulation of pore pressure and subsequent liquefaction under external loading.Prior research has demonstrated that silt beds with varying consolidation degrees exhibit distinct initial failure modes.Specifically,a dense bed undergoes shear failure,whereas a loose bed experiences initial liquefaction failure.This study utilized discrete element simulation to examine the microscopic mechanisms that underlie this phenomenon.
查看更多>>摘要：At present,the main controlling factors of helium accumulation is one of the key scientific problems restricting the exploration and development of helium reservoir.In this paper,based on the calculation results of He generation rate and the geochemical characteristics of the produced gas,both the similarities and differences between natural gas and He resources in the Bohai Bay,Ordos and the surrounding Songliao Basin are compared and analyzed,discussing the main controlling factors of helium resources in the three main petroliferous basins of the North China Craton.It is found that the three basins of Bohai Bay,Ordos and Songliao have similar characteristics of source rocks,reservoirs and cap rocks,that's why their methane resource characteristics are essentially the same.The calculated 4He generation per cubic metamorphic crystalline basement in the three basins is roughly equivalent,which is consistent with the measured He resources,and it is believed that the 4He of radiogenic from the crust is the main factor controlling the overall He accumulation in the three basins;there is almost no contribution of the mantle-derived CH4,which suggests that the transport and uplift of mantle-derived 3He carried by the present-day magmatic activities along the deep-large faults is not the main reason for the mantle-derived 3He mixing in the basins.Combined with the results of regional volcanic and geophysical studies,it is concluded that under the background of the destruction of North China Craton,magma intrusion carried a large amount of mantle-derived material and formed basic volcanic rocks in the Bohai Bay Basin and Songliao Basin,which replenished mantle-derived 3He for the interior of the basins,and that strong seismic activities in and around the basins also promoted the upward migration of mantle source 3He.This study suggests that the tectonic zone with dense volcanic rocks in the Cenozoic era and a high incidence of historical strong earthquakes history may be a potential area for helium resource exploration.
查看更多>>摘要：To better understand the community patterns mediated by connectivity in artificial reefs of coastal areas,it is necessary to understand the distribution and coexistence of organisms with artificial reefs area and adjacent waters.This study was conducted to examine main catches assemblages collected by trawls in Haizhou Bay,which included five habitats:the artificial reef area(AR),aquaculture area(AA),natural area(NA),estuary area(EA)and comprehensive effect area(CEA).The result shows that the total abundances of species in the five habitats were highly different(univariate PERMANOVA:P = 0.001,n = 24),but some species were also unique in their habitat(e.g.Scapharca subcrenata and Glossaulax didyma in AA).The body size distribution of specific species between habitats are different.For Collichthys lucidus,their body size in AR(14.63 cm±1.64 cm)and EA(14.3 cm±0.85 cm)is higher than that in NA(10.65 cm±1.64 cm),CEA(11.28 cm±1.85 cm)and AA(12.1 cm±0.43 cm),which indicates the potential connection from AR to EA mediated by their adult population.We concluded that artificial reefs in AR can be considered key components that have the ability to support species assemblages in adjacent habitats.This study has implications for the conservation and monitoring of species assemblages in coastal areas in terms of that artificial reefs can be applied in different stages of habitat protection implementation and in different combinations of scenarios.
查看更多>>摘要：High-resolution sea-level data and high-precision dating of corals in the northern South China Sea(SCS)during the Holocene provide a reference and historical background for current and future sea-level changes and a basis for scientific assessment of the evolutionary trend of coral reefs in the SCS.Although sporadic studies have been performed around Hainan Island in the northern SCS,the reconstructed sea level presents different values or is controversial because the indicative meaning of the sea-level indicators were neither quantified nor uniform criteria.Here,we determined the quantitative relationship between modern living coral and sea level by measuring the top surfaces of 27 live Porites corals from the inner reef flat along the east coast of Hainan Island and assessed the accuracy of results obtained using coral as sea-level indicators.Additionally,three in situ fossil Porites corals were analyzed based on elevation measurements,digital X-ray radiography,and U-Th dating.The survey results showed that the indicative meanings for the modern live Porites corals is(146.09±8.35)cm below the mean tide level(MTL).It suggested that their upward growth limit is constrained by the sea level,and the lowest low water is the highest level of survival for the modern live Porites corals.Based on the newly defined indicative meanings,6 new sea-level index points(SLIPs)were obtained and 19 published SLIPs were recalculated.Those SLIPs indicated a relative sea level fluctuation between(227.7±9.8)cm to(154.88±9.8)cm MTL between(5 393±25)cal a BP and(3 390±12)cal a BP,providing evidences of the Mid-Holocene sea-level highstand in the northern SCS.Besides that,our analysis demonstrated that different sea-level histories may be produced based on different indicative meanings or criteria.The dataset of 276 coral U-Th ages indicates that coral reef development in the northern SCS comprised the initial development,boom growth,decline,and flourishing development again.A comparison with regional records indicated that synergistic effects of climatic and environmental factors were involved in the development of coral reefs in the northern SCS.Thus,the cessation of coral reef development during the Holocene in the northern SCS was probably associated with the dry and cold climate in South China,as reflected in the synchronous weakening of the ENSO and East Asian summer monsoon induced by the reduction of the 65°N summer insolation,which forced the migration of the Intertropical Convergence Zone.
查看更多>>摘要：Based on high-tide shoreline data extracted from 87 Landsat satellite images from 1986 to 2019 as well as using the linear regression rate and performing a Mann-Kendall(M-K)trend test,this study analyzes the linear characteri-stics and nonlinear behavior of the medium-to long-term shoreline evolution of Jinghai Bay,eastern Guangdong Province.In particular,shoreline rotation caused by a shore-normal coastal structure is emphasized.The results show that the overall shoreline evolution over the past 30 years is characterized by erosion on the southwest beach,with an average erosion rate of 3.1 m/a,and significant accretion on the northeast beach,with an average accretion rate of 5.6 m/a.Results of the M-K trend test indicate that significant shoreline changes occurred in early 2006,which can be attributed to shore-normal engineering.Prior to that engineering construction,the shorelines are slightly eroded,where the average erosion rate is 0.7 m/a.However,after shore-normal engineer-ing is performed,the shoreline is characterized by significant erosion(3.2 m/a)on the southwest beach and significant accretion(8.5 m/a)on the northeast beach,thus indicating that the shore-normal engineering at the updrift headland contributes to clockwise shoreline rotation.Further analysis shows that the clockwise shoreline rotation is promoted not only by longshore sediment transport processes from southwest to northeast,but also by cross-shore sediment transport processes.These findings are crucial for beach erosion risk management,coastal disaster zoning,regional sediment budget assessments,and further observations and predictions of beach morphodynamics.
查看更多>>摘要：The Oligocene Zhuhai sandstones are significant reservoirs for hydrocarbons in the Baiyun Sag,South China Sea.For effective appraisal,exploration and exploitation of such a deep-water hydrocarbon sandstone,samples of five wells from depths of 850 m to 3 000 m were studied.A series of comprehensive petrographic and geochemical analyses were performed to unravel the diagenetic features and their impact on the reservoir quality.Petrographically,the sandstones are dominated by feldspathic litharenites and lithic arenites with fine to medium grain sizes and moderate to good sorting.The reservoir quality varies greatly with a range of porosity from 0.2%to 36.1%and permeability from 0.016×10-3 μm2 to 4 301×10-3 μm2,which is attributed to complex diagenetic evolution related to sedimentary facies;these include compaction,cementation of calcite,dolomite,siderite and framboidal pyrite in eogenetic stage;further compaction,feldspar dissolution,precipitation of ferrocalcite and ankerite,quartz cements,formation of kaolinite and its illitization,precipitation of albite and nodular pyrite,as well as hydrocarbon charge in mesogenetic stage.The dissolution of feldspar and illitization of kaolinite provide internal sources for the precipitation of quartz cement,while carbonate cements are derived from external sources related to interbedded mudstones and deep fluid.Compaction is the predominant factor in reducing the total porosity,followed by carbonate cementation that leads to strong heterogeneity.Feldspar dissolution and concomitant quartz and clay cementation barely changes the porosity but significantly reduces the permeability.The high-quality reservoirs can be concluded as medium-grained sandstones lying in the central parts of thick underwater distributary channel sandbodies(>2 m)with a high content of detrital quartz but low cement.
查看更多>>摘要：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 micro-block.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.
查看更多>>摘要：The southwest Lahad Datu felsic rocks were previously thought to have formed in the late Triassic as part of the microcontinental crystalline basement.Based on U-Pb ages,geochemistry,and the Hf isotopes of zircon from the southeastern Sabah gabbro and granite,in this study,the tectonic properties of the Sabah area during the Triassic were investigated.The weighted average U-Pb zircon ages of the gabbro and granite samples were determined to be(230.9±2.5)Ma and(207.1±3.3)Ma,respectively.The granite had SiO2 contents of 66.54%-79.47%,low TiO2 contents of 0.08%-0.3%,Al2O3 contents of 10.97%-16.22%,Na2O contents of 5.91%-6.39%,and low K2O contents of 0.15%-0.65%.The chondrite-normalized rare earth element(REE)patterns exhibit light REE enrichment,with right-sloping curves.The primitive mantle-normalized trace element spider diagrams exhibit Th,U,La,Sr,and Zr enrichment and Nb,Ta,P and Ti depletions,i.e.,the geochemical characteristics of typical island arc igneous rocks.The tectonic discriminant diagram indicates that the granite is a volcanic arc granite.The Hf isotopic analysis of gabbro zircon revealed that the zircons have εHf(t)values of 12.08-16.24(mean of 14.32)and two-stage model ages(tDM2)of 223-491 Ma(mean of 347 Ma).This indicates that the diagenetic magma of the gabbro was mainly derived from melting of newly formed crustal materials.The ophiolite in southeast Sabah has existed since the early Late Triassic.The crystalline basement granite in southeastern Sabah was emplaced lasted from late Triassic to early Cretaceous.Based on previous studies and global plate reconstruction models,it is speculated that the southeastern Sabah granite may have been formed in an island arc setting,i.e.,where the oceanic crust of the Paleo-Tethys Ocean collided with the oceanic crust of the Panthalassa Ocean.
查看更多>>摘要：The tephra layers in multiple sediment cores from the offshore region of the Mahanadi basin in the northern Bay of Bengal were investigated for possible volcanic sources.The glass shards from those tephra layers were studied for size distribution,texture,and elemental geochemistry to establish chronostratigraphic markers for regional and global Quaternary correlation.The textural features of fine-grained(silty)volcanic glasses suggest the distal source of these tephra deposits.Major element composition with elevated SiO2 contents ranging between 75%-76%and dominance of K2O(>4.5%)over CaO(<0.9%)suggest ashes have originated from siliceous rhyolitic melts,similar to the petrographic composition of tephra from the Toba volcano.The bulk trace element compositions of the same glass shards were comparable with those reported in the youngest Toba tephra reported elsewhere.Likewise,the LREE-dominated chondrite normalized REE profiles of tephra from the Mahanadi basin closely resemble the characteristic REE patterns in Toba ash from other parts of the Indian Ocean and thus confirmed the contribution of the youngest Toba super-eruption for this ash layers.
万方数据
维普
NETL
NSTL