Grain size records reveal variability of the East Asian Winter Monsoon since the Middle Holocene in the Central Yellow Sea mud area,China

Three cores(ZY-1,ZY-2 and ZY-3) retrieved from the Central Yellow Sea mud(CYSM) were analyzed in sensitive grain size and AMS 14 C dating to reconstruct the history of the East Asian Winter Monsoon(EAWM) since the Middle Holocene in the study area.The results show that these data provide a continuous history of the EAWM over the past 7.2 ka and that the EAWM can be divided into three periods:strong and highly fluctuating during 7.2-4.2 ka BP;moderate and relatively stable during 4.2-1.8 ka BP;and weakened during 1.8-0 ka BP.Compared with the East Asian Summer Monsoon(EASM) recorded in the previous studies,the evolutionary history of the EAWM broadly follows the orbital-derived winter insolation with a similar long-term step-decreased trend as the EASM.At the centennial scale,however,the EAWM intensified events correlate well with the EASM weakened events and the North Atlantic climatic variations(Bond events 0 to 5) within the dating error,most likely forced by the reduction of solar irradiance through changes in the oceanic-atmospheric circulation patterns.     

Clay mineral records of East Asian monsoon evolution during late Quaternary in the southern South China Sea

High-resolution clay mineral records combined with oxygen isotopic stratigraphy over the past 190 ka during late Quaternary from core MD01-2393 off the Mekong River in the southern South China Sea are reported to reconstruct a history of East Asian monsoon evolution. The dominating clay mineral components indicate a strong glacial-interglacial cyclicity, with high glacial illite, chlorite, and kaolinite contents and high interglacial smectites content. The provenance analysis indicates the direct input of clay minerals via the Mekong River drainage basin. Illite and chlorite derived mainly from the upper reach of the Mekong River, where physical erosion of meta-sedimentary rocks is dominant. Kaolinite derived mainly from active erosion of inhered clays from reworked sediments in the middle reaches. Smectites originated mainly through bisiallitic soils in the middle to lower reaches of the Mekong River. The smectites/(illite+chlorite) and smectites/kaolinite ratios are determined as mineralogical indicato     

Late Quaternary clay minerals off Middle Vietnam in the western South China Sea: Implications for source analysis and East Asian monsoon evolution

High-resolution clay mineral records combined with oxygen isotopic stratigraphy over the past 450 ka during late Quaternary from Core MD05-2901 off Middle Vietnam in the western South China Sea are re-ported to reconstruct a history of East Asian monsoon evolution. Variations in Illite, chlorite, and kaolinite contents indicate a strong glacial-interglacial cyclicity, while changes in smectite content present a higher frequency cyclicity. The provenance analysis indicates a mixture of individual clay minerals from various sources surrounding the South China Sea. Smectite derived mainly from the Sunda shelf and its major source area of the Indonesian islands. Illite and chlorite originated mainly from the Mekong and Red rivers. Kaolinite was provided mainly by the Pearl River. Spectral analysis of the kaolin-ite/(illite chlorite) ratio displays a strong eccentricity period of 100 ka, implying the ice sheet-forced win-ter monsoon evolution; whereas higher frequency changes in the smectite content show an ice sheet-forced obliquity period of 41 ka, and precession periods of 23 and 19 ka and a semi-precession period of 13 ka as well, implying the tropical-forced summer monsoon evolution. The winter monsoon evolution is generally in coherence with the glacial-interglacial cyclicity, with intensified winter monsoon winds during glacials and weakened winter monsoon winds during interglacials; whereas the summer monsoon evolution provides an almost linear response to the summer insolation of low latitude in the Northern Hemisphere, with strengthened summer monsoon during higher insolation and weakened summer monsoon during lower insolation. The result suggests that the high-latitude ice sheet and low-latitude tropical factor could drive the late Quaternary evolution of East Asian winter and summer monsoons, respectively, implying their diplex and self-contained forcing mechanism.     

Yangtze- and Taiwan-derived sediments on the inner shelf of East China Sea

X-ray diffraction (XRD) mineralogical and grain-size analyses indicate that inner continental shelf sediments in the East China Sea (ECS) represent a unique mixing of clays derived from the Yangtze River and silts/sands from small western Taiwanese rivers. Taiwanese (e.g., Choshui) clays (<2 μm) display no smectite but the best illite crystallinity and are only distributed along southeastern Taiwan Strait. Both Yangtze and Taiwanese river clays are illite-dominated, but the poor illite crystallinity and the presence of smectite and kaolinite indicate that Taiwan Strait clays are mainly Yangtze-dominated. In contrast, medium silts (20–35 μm) and very fine sands (63–90 μm) in the Taiwan Strait are characterized by low feldspar/quartz, low K-feldspar/plagioclase and high kaolinite/quartz, indicating their provenance from Taiwanese rivers. Taiwanese silts and sands are introduced primarily by the way of typhoon-derived floods and transported northward by the Taiwan Warm Current during summer–fall months. Yangtze clays, in contrast, are widely dispersed southward about 1000 km to the western Taiwan Strait, transported by the China Coastal Current during winter–spring months. Since most Taiwan Strait samples were collected in May 2006, clay results in this paper might only represent the winter–spring pattern of the dispersal of Yangtze sediments.     

Distribution,formation and evolution of sand ridges on the East China Sea shelf

Based on the integrated results of multiple data types including MBES (Multi-Beam Echo Sounding) and historical topography maps,the LSR (Linear Sand Ridges) on the ECS (East China Sea) shelf are identified,divided into subareas,and classified.The distribution of sand ridge crests is also established.The strikes of the LSR on the ECS shelf fall in a normal distribution with the center point being 155° azimuth with additional peak points at 125°,130°,140°,and 180° azimuth.The distribution of the ECS shelf san...     

Quaternary clay mineralogy in the northern South China Sea (ODP Site 1146) —— Implications for oceanic current transport and East Asian monsoon evolution

The East Asian monsoon system is a thermodynamic atmospheric circulation induced by the different potential heating between the 揥estern Pacific Warm Pool?(WPWP) and the Asian continent. The circulation patterns dominate seasonal patterns of winds, preci…     

Ancient Changjiang channel system in the East China Sea continental shelf during the last glaciation

The global sea level had started to rapidly fall about at 25 cal. ka before present. The last glaciation maxi-mum (LGM) is from 23 to 19 cal. ka[1,2]. The sea level at that time is under 130 m depth lower than the mod-ern sea level[3,4]. The continental shelf plain is gradu-ally submerged with rapid sea level rise in the last de-glaciation cycle. Whether rivers occurred in the sere continental shelf plain during the last glaciation cycle has been the hotspot in academia at all times. There a…     

Grain-size records at ODP Site 1146 from the northern South China Sea: Implications on the East Asian monsoon evolution since 20 Ma

273 samples from Ocean Drilling Program (ODP) Site 1146 in the northern South China Sea (SCS) were analyzed for grain-size distributions using grain-size class vs. standard deviation method and end-member modeling algorithm (EMMA) in order to investigate the evolution of the East Asian monsoon since about 20 Ma. 10–19 μm/1.3–2.4 μm, the ratio of two grain-size populations with the highest variability through time was used to indicate East Asian winter monsoon intensity relative to summer monsoon. The mass accumulation rate of the coarsest end member EM1 (eolian), resulting from EMMA, can be used as a proxy of winter monsoon strength and Asian inland aridity, and the ratio of EM1/(EM2 EM3) as a proxy of winter monsoon intensity relative to summer monsoon. The combined proxies show that a profound enhancement of East Asian winter monsoon strength and winter monsoon intensity relative to summer monsoon occurred at about 8 Ma, and it is possible that the summer monsoon simultaneously intensified with winter monsoon at 3 Ma. Our results are well consistent with the previous studies in loess, eolian deposion in the Pacifc, radiolarians and planktonic foraminifera in the SCS. The phased uplift of the Himalaya-Tibetan Plateau may have played a significant role in strengthening the Asian monsoon at 8 Ma and 3 Ma.     

East Asian paleoclimate change in the Weihe Basin(central China)since the middle Eocene revealed by clay mineral analysis

Obtaining a continuous sedimentary record of Cenozoic East Asian paleoclimate change is key to understand the origin, evolution and driving mechanism of the East Asian monsoon and climate change. Based on the continuous Cenozoic fluvial and lacustrine sedimentary sequence in the Weihe Basin, central China, we carried out research on the content and crystal parameters of clay minerals. The paleoclimate change since the middle Eocene was reconstructed accordingly. The results show that smectite and illite are the dominant clay minerals in fluvial-lacustrine sediments of Weihe Basin. The crystallinity of illite,the chemical index of illite, the ratio of smectite to illite and chlorite decrease gradually, which indicates that chemical weathering in the Weihe River watershed stepwise weakened since the middle Eocene, under the background of a semi-arid and semi-humid climate. The formation of palygorskite may be affected by a weak diagenesis. In this case, the content of smectite and the illite crystallinity caused by the weathering in the corresponding period may be actually higher than that of the current,which indicates that the climate in the Weihe Basin region was relatively warm and humid from the middle Eocene to Pliocene.With the decrease of Cenozoic global temperature and the continuous accumulation of sediments, the intensity of chemical weathering in the Weihe Basin gradually weakened, and the East Asian monsoon climate gradually evolved until becoming dry in the Quaternary. Evidence from clay minerals also indicates that the aridification in the Weihe Basin from the late Eocene to the Oligocene may be a response to the global cooling event at the Eocene-Oligocene transition.     

Biomarker evidence for changes in terrestrial organic matter input into the Yellow Sea mud area during the Holocene

We present lipid biomarker records of two cores(ZY1 and ZY3) from the central South Yellow Sea mud area to investigate the changes in sources and transport processes of the sedimentary organic matter(OM) throughout the Holocene. Based on the analysis of marine biomarker content(SPB(Phytoplankton Biomarker, total content of brassicasterol, dinosterol and C_(37)-alkenones) and crenarchaeol), and terrestrial biomarkers(Sn-alkanols and br GDGTs) as well as TMBR′ and BIT index values, the marine organic matter(MOM) and terrestrial organic matter(TOM) deposition history was reconstructed. Changes in TOM and MOM were related to variations in land vegetation density and marine productivity, as well as transport processes dominated by the oceanic circulation system. The marine biomarker contents from the South Yellow Sea have generally increased throughout the Holocene, indicating that the increased MOM contents were mainly controlled by the strengthening of the circulation system. The terrestrial biomarkers, on the other hand, were more variable, indicating more complex influence of TOM burial in the Yellow Sea. During the Early Holocene(7200–6000 cal yr BP), the moderate TOM input revealed by the terrestrial proxy records may result from abundant land source supply by strong river transport despite the lack of transport via circulation system. The Mid-Holocene(6000–3000 cal yr BP) was characterized by decreased terrestrial biomarker contents. The balance between the decrease in land source supply and increase of transportation by the current system of the TOM resulted in the lower but stable contents of TOM. During the Late Holocene(3000 cal yr BP to present), the TOM deposition in the South Yellow Sea increased as the current system was further enhanced and thus transported more TOM to the central South Yellow Sea, although the land supply of TOM was further reduced.     

末次冰盛期以来东亚季风变化历史——中国北方的地质记录

全球变暖对中国北方气候的影响已经引发了学术界的广泛关注,地质增温期东亚季风变迁历史可为理解未来气候变化提供重要参考.文章主要基于中国北方夏季风边缘带的湖泊、黄土等记录的古植被信息,探讨了末次冰盛期至全新世全球增温过程中东亚季风的变化历史.地质记录显示,末次冰盛期东亚冬季风增强,夏季风减弱,中国北方气候干冷,大部分地区呈现荒漠草原或干草原景观,贺兰山以东沙地的东南边界和现代沙地边界接近或略向东南方向小幅扩张.在由冷转暖的末次冰消期,东亚冬季风逐渐减弱,夏季风逐渐增强,但在快速变冷的Heinrich 1(H1)和Younger Dryas(YD)事件期间,中国北方气候变干.全新世冬季风减弱,夏季风显著增强,季风雨带向西北推进至少300km,中国中东部干旱区范围大幅度缩小,北方喜暖喜湿植物显著增加.从夏季风边缘带的记录看,中全新世夏季风最为强盛.显然,古增温有利于东亚夏季风的增强,从而极大改善中国北方的生态环境.如果全球变暖持续下去,中国北方将变得湿润.与轨道尺度记录相比,高分辨率的古植被记录较为缺乏,全新世气候突变事件以及百年-十年尺度气候旋回尚需深入研究,应作为今后研究的重点.     

The East Asian Monsoon since the Last Glacial Maximum: Evidence from geological records in northern China

The impact of global warming on the climate of northern China has been investigated intensively, and the behavior of the East Asian monsoon during previous intervals of climatic warming may provide insight into future changes. In this study, we use paleovegetation records from loess and lake sediments in the marginal zone of the East Asian summer monsoon(EASM) to reconstruct the EASM during the interval of warming from the Last Glacial Maximum(LGM) to the Holocene. The results show that during the LGM, desert steppe or dry steppe dominated much of northern China; in addition, the southeastern margin of the deserts east of the Helan Mountains had a distribution similar to that of the present-day, or was located slightly further south, due to the cold and dry climate caused by a strengthened East Asian winter monsoon(EAWM) and weakened EASM. During the last deglaciation, with the strengthening of the EASM and concomitant weakening of the EAWM, northern China gradually became humid. However, this trend was interrupted by abrupt cooling during the Heinrich 1(H1) and Younger Dryas(YD) events. The EASM intensified substantially during the Holocene, and the monsoon rain belt migrated at least 300 km northwestwards, which led to the substantial shrinking of the desert area in the central and eastern part of northern China, and to the large expansion of plants favored by warm and humid conditions. Paleoclimatic records from the marginal zone of the EASM all show that the EASM reached its peak in the mid-Holocene, and past global climatic warming significantly strengthened the EASM, thereby greatly improving the ecological environment in northern China. Thus, northern China is expected to become wetter as global warming continues. Finally, high resolution Holocene vegetation records are sparse compared with the numerous records on the orbital timescale, and there is a need for more studies of Holocene climatic variability on the centennial-to-decadal scale.     

The origin of strata within the inner accretionary prism of Nankai Trough: Evidence from clay mineral assemblages along the NanTroSEIZE transect

One of the more prominent architectural elements of the Nankai subduction margin, offshore southwest Japan, is an out‐of‐sequence thrust fault (megasplay) that separates the inner accretionary prism from the outer prism. The inner prism (hanging wall of the megasplay) is dominated by mudstone, which is enigmatic when the sedimentary facies is compared to coeval deposits in the Shikoku Basin (i.e. inputs from the subducting Philippine Sea plate) and to coarser‐grained turbidite sequences from the Quaternary trench wedge. Clay mineral assemblages amplify the mismatches of sedimentary facies. Mudstones from the inner prism are uniformly depleted in smectite, with average bulk values of 23–24 wt%, whereas the Shikoku Basin deposits show progressive decreases in proportions of smectite over time, from averages of 46–48 wt% at 10 Ma to 17–21 wt% at 1 Ma. Plate‐boundary reconstructions for the Philippine Sea region provide one solution to the conundrum. Between 15 Ma and 10 Ma, the Pacific plate subducted near the NanTroSEIZE transect, and a trench‐trench‐trench triple junction migrated to the northeast. Accretion during that period involved sediments that had been deposited on the Pacific plate. Motion of the Philippine Sea plate changed from 10 Ma to 6 Ma, resulting in sinistral slip along the proto‐Nankai Trough. Sediments accreted during that period probably had been deposited near the triple junction, with a hybrid detrital provenance. Renewed subduction of the Philippine Sea plate at 6 Ma led to reorganization of watersheds near the Izu–Honshu collision zone and gradual incision of large submarine canyons on both sides of the colliding Izu arc. Accreted Pliocene mudstones share more of an affinity to the triple junction paleoenvironment than they do to Shikoku Basin. These differences between subducting Shikoku Basin strata and accreted Pacific plate sediments have important implications for interpretations of frictional properties, structural architecture, and diagenetic fluid production.     

A theoretical model for the intrusion of the Kuroshio across the continental shelf of East China Sea

Here we use a two-layer model to study the dynamics of the intrusion of the Kuroshio onto the continental shelf. Results show that the interaction of the Kuroshio water and shelf water produces a stable upwelling zone above 100 m depth northeast of Taiwan, which provides a dynamical explanation to the presence of the cold core previously observed by satellite. The affected shelf water from the interaction has an onshore portion, which turns right and becomes a northward alongshore flow when it moves closer to shore. This implies that the Kuroshio water cannot penetrate deep onto the inner part of the continental shelf, but it generates a strong northward jet that is formed mainly by the shelf water.     

Reevaluation of mixing among multiple water masses in the shelf: An example from the East China Sea

East China Sea (ECS) is bounded by the continent where the fourth largest river of Changjiang discharges large amounts of freshwater to the west and by the Kuroshio in the East and connected to the South China Sea via Taiwan Strait, therefore water characteristics are very complex and undergo great seasonal changes. The dominant source waters in the ECS are found to be Kuroshio Surface Water (KSW), Kuroshio Sub-surface Water (KSSW), Changjiang Diluted Water (CDW), and Taiwan Strait Warm Water (TSWW). Optimum multiparameter analysis (OMP) using temperature, salinity and ²²⁶Ra were applied to quantify the contribution of individual source water to the surface water of the ECS in summer. The successful application of radium isotope in OMP analysis demonstrates the usefulness of ²²⁶Ra in the discrimination of mixing among multiple water sources. In 1987, one interesting phenomenon was that the KSSW entered the surface with the upwelling at the margin of continental shelf, and affected the coastal water obviously. In 1999, the TSWW extended northward continuously up to the Changjiang Estuary.     

Detailed structure of the Kuroshio frontal eddy along the shelf edge of the East China Sea

The detailed three-dimensional structure of the Kuroshio frontal eddy along the shelf edge of the East China Sea is revealed by the CTD, ADCP, and satellite-tracked drifter observations. The length and width of the cold core of the Kuroshio frontal eddy are about 60 and 40 km, respectively, and its phase speed is about 30 cm s^-1. The calculated buoy tracks with the use of the observed ADCP data well reproduce the observed tracks of satellite-tracked drifters around the frontal eddy. The observed maximum horizontal velocity around this frontal eddy are 40 cm s^-1 and the center of this eddy shifts offshore in the deep layer. Nutrient is advected onshore across the shelf edge by passing of this frontal eddy while it is advected offshore without the frontal eddy at the shelf edge.     

Numerical study of seasonal circulation and variability over the inner shelf of the northern South China Sea

This study examines seasonal circulation, hydrography, and associated spatial variability over the inner shelf of the northern South China Sea (NSCS) using a nested-grid coastal ocean circulation model. The model external forcing consists of tides, atmospheric forcing, and open boundary conditions based on the global ocean circulation and hydrography reanalysis produced by the Hybrid Coordinate Ocean model. Five numerical experiments are conducted with different combinations of external forcing functions to examine main physical processes affecting the seasonal circulation in the study region. Model results demonstrate that the monthly mean circulation in the study region features the Guangdong Coastal Current (GCC) over coastal waters and the South China Sea Warm Current (SCSWC) in the offshore deep waters. The GCC produced by the model flows nearly southwestward in winter months and northwestward in summer months, which agrees with previous studies. The SCSWC flows roughly northeastward and is well defined in summer months. In winter months, by comparison, the SCSWC is superseded by the southwestward strong wind-driven currents. Analysis of model results in five different experiments demonstrates that the monthly mean circulation over coastal and inner shelf waters of the NSCS can be approximated by barotropic currents forced by the southwestward monsoon winds in winter months. In summer months, by comparison, the monthly mean circulation in the study region is affected significantly by baroclinic dynamics associated with freshwater runoff from the Pearl River and advection of warm and saline waters carried by the SCSWC over the NSCS.     

海啸在冲绳海槽和东海浅水大陆架地形上产生和传播的数值模拟

运用数值模拟的方法对在冲绳海槽产生9.0级地震,并引发海啸的过程和海啸波在东海浅水大陆架地形上的传播过程进行研究.模拟的结果表明,数值模拟产生的波浪符合海啸波的特点,东海浅水大陆架适合海啸波的传播.     

Effect of the mesoscale hydrographic features on larval fish distribution across the shelf break of East China Sea

Surveys of fish larvae and oceanographic conditions were conducted along transects across the shelf break region in the East China Sea (ECS) in May 2001. The objective of this study was to investigate the distribution and assemblages of fish larvae across the shelf break region and their relationships to mesoscale hydrographic features. There was a warm surface streamer from the Kuroshio which extended toward the shelf region of the ECS; concurrently, we observed the intrusion of the less saline shelf water into the subsurface layer towards the offshore. In all 66 taxa (65 families and 1 order) of larvae were collected by oblique net tows using a bongo net, the larval fish density and number of families sampled were lower in the low-temperature area. Based on cluster analysis and environmental factor, two larval fish assemblages were identified: off-shelf/Kuroshio and shelf break. In shelf break assemblage, Auxis spp. and Diaphus spp. were abundant in the warm surface streamer. In contrast, Maurolicus japonicus, Synagrops spp., Bregmaceros sp. and Champsodon spp. were found in the off-shelf region where the offshore intrusion of less-saline water occurred. This different pattern of the horizontal distribution would reflect vertical distribution of the larvae in the water column. Moreover, copepod nauplii density was higher in the upper layer of the shelf region. Thus, dynamic interactions between the Kuroshio and shelf waters at the frontal boundary are concluded to potentially affect larval fish transport and prey availability.     

Terrestrial and marine biomarker estimates of organic matter sources and distributions in surface sediments from the East China Sea shelf

Revealing of the sources and distributions of sedimentary organic matter in the East China Sea (ECS) is important for understanding its carbon cycle, which has significant temporal and spatial variability due to the influences of recent climate changes and anthropogenic activities. In this study, we report the contents of both terrestrial and marine biomarkers including ∑C₂₇+C₂₉+C₃₁n-alkanes (38.6-580 ng/g), C₃₇ alkenones (5.6-124.6 ng/g), brassicasterol (98-913 ng/g) and dinosterol (125-1521 ng/g) from the surface sediments in the Changjiang River Estuary (CRE) and shelf areas of the ECS. Several indices based on biomarker contents and ratios are calculated to assess the spatial distributions of both terrestrial and marine organic matter in the ECS surface sediments, and these results are compared with organic matter distribution patterns revealed by the δ¹³C (−20.1‰ to −22.7‰) and C/N ratio (5-7.5) of total organic matter. The contents of terrestrial biomarkers in the ECS surface sediments decrease seaward, controlled mostly by Changjiang River (CR) inputs and surface currents; while higher contents of the two marine biomarkers (brassicasterol and dinosterol) occur in upwelling areas outside the CRE and in the Zhejiang-Fujian coastal zone, controlled mostly by marine productivity. Four proxies, f_Terr(δ¹³C) (the fraction of terrestrial organic matter in TOC estimated by TOC δ¹³C), odd-alkanes (∑C₂₇+C₂₉+C₃₁n-alkanes), 1/P_mar-aq ((C₂₃+C₂₅+C₂₉+C₃₁)/(C₂₃+C₂₅) n-alkanes) and TMBR (terrestrial and marine biomarker ratio) (C₂₇+C₂₉₊C₃₁n-alkanes)/((C₂₇+C₂₉+C₃₁) n-alkanes+(brassicasterol+dinosterol+alkenones)), reveal a consistent pattern showing the relative contribution of terrestrial organic matter (TOM) is higher in the CRE and along the Zhejiang-Fujian coastline, controlled mostly by CR inputs and currents, but the TOM contribution decreases seaward, as the influences of the CR discharge decrease.