Quanhong Zhao

Sedimentary facies of the tide-dominated paleo-Changjiang (Yangtze) estuary during the last transgression

Abstract A large estuary was formed by marine inundation of the paleo-Changjiang (Yangtze) incised valley during the transgression after the Last Glacial Maximum. This paper presents the sedimentary facies and architecture of the estuary fill, based on the analysis of three sediment cores (CM97, JS98, and HQ98) obtained from the present Changjiang delta plain. Estuary fill deposits showing an upward-fining succession were grouped into five depositional facies: tidal river, distributary channel, muddy intertidal to subtidal flats, transgressive lag, and estuary front, based on sedimentary textures, lithology, and physical sedimentary structures. Sand–mud couplets are common in these deposits, indicating that tides played a significant role in producing these sedimentary structures and that the estuary type was tide-dominated. Some of the successive sand–mud couplets probably recorded neap-spring cycles as well as semidiurnal tidal cycles. The nature of the estuary was very different from other representative tide-dominated estuaries in sediment facies, its distribution, and sediment source for estuarine fill. Unlike the other estuaries that receive sediments mainly from the sea, the paleo-Changjiang estuarine fill deposits were supplied largely from the river. This difference would also have a great influence on the sedimentological and morphological component in the estuary. The sediment distribution of the estuary showed fining-seaward and estuary-mouth sand bodies fed by marine-source sand were absent. The architecture model of tide-dominated estuaries should be divided into two types by the degree of fluvial sediment supply. The paleo-Changjiang estuary shows a good example for an estuary of large rivers.

Architecture and evolution of the tide-dominated Changjiang (Yangtze) River delta, China

Abstract The Changjiang (Yangtze) River, China, has formed a large tide-dominated delta on the western coast of the East China Sea. This paper presents the architecture and Holocene evolution of the delta, based on analyses of three borehole samples taken from the present delta plain. The deltaic sediments are 25–30 m thick and composed largely of subaqueous deposits, which can be divided into three facies: prodelta, delta front and delta plain, in ascending order. Tide-influenced sedimentary structures such as sand–mud couplets and bi-directional crosslaminations characterize the delta plain and delta front deposits. An upward-coarsening facies succession from prodelta to delta front is overlain by an upward-fining facies succession from the uppermost delta front to the delta plain. The deposits at the boundary between delta plain and delta front are composed of well-sorted, coarse deposits because it is in the zone of highest energy. The depth from the top of the intertidal facies of the delta plain to this boundary is almost the same, 9–11 m, among the three cores. Sediment accumulation rates are highest in the delta front facies (ca. 10 m/kyr maximum) and decrease toward both delta plain and prodelta facies (ca. 1 m/kyr). The evolution of the Changjiang delta, which began in a bay approximately 8 kyr bp , can be divided into two stages by its stacking pattern: aggradational and progradational. The aggradational system developed during the slow rise of sea level from 8 to 6 kyr bp , and the progradational system developed after sea level reached or approached its present position. The evolution of distributary channels and related river-mouth sand bars was an important process of delta migration or progradation.

Sedimentary facies and Holocene progradation rates of the Changjiang (Yangtze) delta, China

Abstract The Changjiang (Yangtze) River, one of the largest rivers in the world, has formed a broad tide-dominated delta at its mouth during the Holocene sea-level highstand. Three boreholes (CM97, JS98, and HQ98) were obtained from the Changjiang delta plain in 1997–1998 to clarify the characteristics of tide-dominated delta sediments and architecture. Based on sediment composition and texture, and faunal content, core sediments were divided into six depositional units. In ascending order, they were interpreted as tidal sand ridge, prodelta, delta-front, subtidal to lower intertidal flat, upper intertidal flat, and surface soil deposits. The deltaic sequence from the prodelta deposits to the delta front deposits showed an upward-coarsening succession, overlain by an upward-fining succession from the uppermost part of the delta front deposits to the surface soil. Thinly interlaminated to thinly interbedded sand and mud (sand–mud couplets), and bidirectional cross laminations in these deposits show that tide is the key factor affecting the formation of Changjiang deltaic facies. Sediment facies and their succession combined with AMS 14C dating revealed that isochron lines cross unit boundaries clearly, and delta progradation has occurred since about 6000 to 7000 years BP, when the rising sea level neared or reached its present position. The average progradation rate of the delta front was approximately 50 km/kyear over the last 5000 years. The progradation rate, however, increased abruptly ca. 2000 years BP, going from 38 to 80 km/kyear. The possible causes for this active progradation could have been an increase in sediment production in the drainage basin due to widespread human interference and/or decrease in deposition in the middle reaches related to the channel stability caused by human activity and climatic cooling after the mid-Holocene.

Vegetation and climate changes in the Changjiang (Yangtze River) Delta, China, during the past 13,000 years inferred from pollen records

Abstract A high-resolution pollen record from 2 boreholes from the Changjiang (Yangtze River) delta shows a series of well-defined changes in vegetation and climate over the last 13,000 yr . In latest Pleistocene to early Holocene time (12,900– 10,300 cal yr BP), pollen dominated by the xerophytic herb Artemisia and common conifers indicate coniferous forest and grasslands and a cool, dry climates, possibly represented by the Younger Dryas. From 10,300 to 9000 cal yr BP, a warm, wet climate fostered the development of mixed broadleaved evergreen–deciduous forests to grow on the grasslands and surrounding hills and uplands. From 9000 to 7600 cal yr BP, the dominance of conifers and the reduction in broadleaved evergreen trees pollen suggest cool, dry conditions. The mid-Holocene “hypsithermal” (7600– 4800 cal yr BP) is represented by a large expansion of subtropical evergreen–deciduous broadleaved forests. Between 4800 and 1300 cal yr BP, which corresponds to a Neoglacial period elsewhere, conifers are dominant, along with common herbs, whereas the area with broadleaved evergreen trees shrink under the cool, dry conditions. The first occurrence of Fagopyrum pollen at 4500 cal yr BP suggests human influence. The significant change in arboreal and herbaceous pollen after 1300 cal yr BP reflects widespread intensive human influence.

Evolution of the Coastal Depositional Systems of the Changjiang (Yangtze) River in Response to Late Pleistocene-Holocene Sea-Level Changes

ABSTRACT The paleo-Changjiang (Yangtze) incised-valley fills, approximately 80-90 m thick, provide an opportunity to document the evolution of coastal depositional systems with large sediment supply in response to late Pleistocene-Holocene sea-level fluctuations on time scales of 103 to 104 years. The sedimentary facies of the incised-valley fills record three main depositional systems: fluvial, tide-dominated estuary, and tide-dominated delta. Radiocarbon ages for the incised-valley fills suggest that these depositional systems developed before about 11 ka, between 11 and 8 ka, and after approximately 8 ka, respectively. By applying sequence-stratigraphic concepts, the evolution of the depositional systems can be divided into three systems tracts--a lowstand systems tract (LST), a transgressive systems tract (TST), and a highstand systems tract (HST). Sea-level changes on a 104-year time scale controlled the basic architecture of the sequence of the incised-valley fill. On the other hand, sea-level changes on a millennial time scale affected the stacking pattern of the systems tracts. In particular, the continuous sea-level rise with episodic rapid rises during the last deglaciation affected the stacking pattern of the TST, which is characterized by a combination of aggradation and backstepping. The aggradation of fluvial and estuarine systems was dominant and the shoreline migrated only gradually landward under the relatively slow rise in sea level, and a very rapid sea-level rise around 12 and 10 ka caused the system to migrate abruptly landward. Unlike the transgressive estuarine phase, the stacking pattern of the regressive tide-dominated delta (HST), which developed within the almost filled incised valley and on the surrounding interfluve zones, was characterized by seaward progradation with clinoform architecture. It was initiated with aggradational and progradational stacking about 8 ka during the last phase of decelerated sea-level rise, and was followed by a progradational phase after the highest sea level about 6 ka.

Foraminiferal responses to major Pleistocene paleoceanographic changes in the southern South China Sea

A detailed age model for core 17957-2 of the southern South China Sea was developed based on delta(18)O, coarse fraction, magnetostratigraphy, and biostratigraphy for the last 1500 kyr. The delta(18)O record has clear similar to 100-kyr cycles after the Mid-Pleistocene Revolution (MPR) at the entrance of marine isotopic stage (MIS) 22. Planktonic foraminifera responded to the MPR immediately, showing the increased sea surface temperature (SST) and dissolution after the MPR. Benthic foraminifera did not respond to it until the Brunhes/Matuyama boundary. Since the MPR, the depth of thermocline gradually became shallower until MISs 6-5. This major change within MISs 6-5 was also reflected in the decreased SSTs and increased productivity and Deep Water Mass. Thus two major Pleistocene paleoceanographic changes were found: One was around the MPR; the other occurred within MISs 6-5, which speculatively might be ascribed to the reorganization of surface and deep circulation, possibly induced by tectonic forces.

Pliocene–Pleistocene stable isotope and paleoceanographic changes in the northern South China Sea

Abstract Based on the stable isotopic analysis of planktonic and benthic foraminifers from Ocean Drilling Program Core 1148 of the northern South China Sea (SCS), Pliocene–Pleistocene isotope stratigraphy and events have been reconstructed. The benthic foraminiferal δ18O record shows that the Pacific intermediate water had a greater influence upon the SCS or the Pacific deep water above ∼2600 m was warmer before ∼3.2 Ma than at present. After that, the benthic δ18O conspicuously increased during the ∼3.2–2.5 Ma period, in correspondence to the formation of the Northern Hemisphere ice sheet, whereas the planktonic δ18O signal suggests a stepwise overall decrease of sea surface temperature during the ∼2.2–0.9 Ma period. Compared to the equatorial Pacific records, the decrease in planktonic (Globigerinoides ruber) δ13C during the ∼3.2–2.2 Ma period is particularly striking, suggesting that fertility of surface water increased noticeably. According to the modern δ13C distribution of G. ruber in the northern SCS, it is inferred that the East Asian winter monsoon strengthened during this interval. Afterwards, there were several conspicuous decreases of G. ruber δ13C at ∼1.7, 1.3, 0.9, 0.45 and 0.15 Ma BP, that is, about every 0.4 Ma, suggesting that the East Asian winter monsoon became episodically stronger. This is confirmed by changes in relative abundance of planktonic foraminifer species Neogloboquadrina dutertrei, a typical East Asian winter monsoon proxy. The deep-water δ13C of the SCS is close to that of the Pacific, but lighter than that of the Atlantic, implying that the pattern of deep water originating mainly from the Atlantic and through the Pacific entering the SCS existed at least since the early Pliocene. After 1.4 Ma, the benthic δ13C signal decreased conspicuously but with a periodicity of ∼100 ka, suggesting that the deep-water ventilation of the SCS was reduced, probably corresponding to a decrease of the North Atlantic Deep Water and/or further isolation of the SCS deep basin from the Pacific during glaciations.

Thirty million year deep-sea records in the South China Sea

In the spring of 1999 the Ocean Drilling Program (ODP) Leg 184 Shipboard Party cored 17 holes at 6 deep water sites in the northern and southern parts of the South China Sea (SCS). Chinese scientists actively participated in the entire process of this first deep-sea drilling leg off China, from proposal to post-cruise studies. More than 30 categories of analyses have been conducted post-cruise in various Chinese laboratories on a large number of core samples, and the total number of analyses exceeded 60 thousand. The major scientific achievements of the Leg 184 studies are briefly reported in three successive papers, with the first one presented here dealing with deep-sea stratigraphy and evolution of climate cycles. This ODP leg has established the best deep-sea stratigraphic sequences in the Western Pacific: the 23-Ma isotope sequence from the Dong-Sha area is unique worldwide because of its continuity; the last 5-Ma sequence from the Nansha area represents one of the best 4 ODP sites worldwide with the highest time-resolution for that time interval, and the sequences of physical properties enable a decadal-scale time resolution. All these together have provided for the first time high-quality marine records for paleoenvironmental studies in the Asian-Pacific region. This new set of stratigraphic records has revealed changes in climate cyclicity over the last 20 Ma with the fluctuating power of the 100 ka, 400 ka, 2000 ka eccentricity cycles, indicating the evolving response of the climate system to orbital forcing along with the growth of the Antarctic and Northern Hemisphere ice sheets.

Distribution of the ostracod genera Krithe and Parakrithe in bottom sediments of the East China and Yellow seas

Abstract A total of 520 grab and core-top samples collected from the Yellow and East China seas was quantitatively examined to determine the distribution of Krithe and Parakrithe and their relationship to various environmental parameters. The numerical data (incidence, relative abundance and simple species diversity) and species composition of Krithe and Parakrithe are closely linked to water masses (the Cold Water Mass of the Yellow Sea, the Surface, Lower, Intermediate and Deep Water Masses of the East China Sea). Samples barren of the two genera occur in the marginal area of the Yellow Sea and on the inner and middle shelf of the East China Sea. Their absence is attributed to the combined effect of higher temperature, lower salinity and coarser sediments in those localities. It is confirmed by the co-occurrence of krithinid ostracods with different vestibular sizes in the oxygen minimum zone (1.2-2.0 ml/l) that there is no correlation between the size of anterior vestibulum and the level of dissolved oxygen in seawater.

Microtektites in the Middle Pleistocene deep-sea sediments of the South China Sea

This is a preliminary study on the microtektites that were found in large numbers from the interval between 7.80 and 8.10 m depth of core S095-17957-2 (10°53.9’N, 115°18.3’E, water depth 2 195 m), northern Nansha area of the South China Sea. The microtektites vary in shape, with spherules predominating, and are commonly less than 1 mm in diameter, transparent or semitransparent, brownish in color, with bubbles inside. Based on coarse fraction stratigraphy and foraminifera/nanofossil biostratigraphical events the microtektite layer was assigned to nearly the Bm-hes/Matuyama magnetic reversal boundary (some 0.78 MaBP). Obviously, the present microtektites, and those found from the middle Pleistocene of the Indian Ocean, Australia and loess of northern China, were products of the same impact event and therefore, are useful as a reliable mark in Quaternary stratigraphy, as well as in paleoclimatic studies.