Subaqueous Yangtze Delta Research Articles

Impacts of human activity on the late-Holocene development of the subaqueous Yangtze delta, China, as shown by magnetic properties and sediment accumulation rates

Development of the Yangtze delta during the late Holocene, and its relationship to human activities in the drainage basin, was analyzed using data from 16 cores collected from distributaries to the prodelta. We used AMS 14C dating and digital elevation model (DEM) data from marine charts from 1864 through 2005 to determine ages and estimate sediment accumulation rates. The results demonstrate that the latest major subaqueous delta front formed within the past c. 0.8 cal. ka and features remarkably high accumulation rates (1—4 cm/yr) in comparison with those of previous delta fronts. We also examined the temporal distribution of grain size and magnetic susceptibility in all 16 cores. Results show soil-derived superparamagnetic (SP) minerals generally occur, and even dominate, in the recent ( c. 1.7 cal. ka) Yangtze delta fine-grained sediment, as shown by high values of frequency-dependent magnetic susceptibility (both χFD and χFD%). Rock-derived magnetite dominates generally in the river channel and delta front sand bodies as a result of hydrodynamic sorting, but is also enriched in both fine and coarse-grained sediment formed more recently ( c. 0.8 cal. ka), as evidenced by rising values of mass specific magnetic susceptibility (χLF). SP grains were deposited as early as the late Neolithic, possibly indicating local deforestation associated with the use of fire at that time. We suggest major deforestation in the drainage basin started c. 1.7 cal. ka BP, and intensified after c. 0.8 cal. ka BP when both χLF and χFD show the highest values. We therefore conclude that upland deforestation and cultivation as a result of the migration of human populations from northern China since c. 1.7 cal. ka BP resulted in increased sediment discharge of the Yangtze and played an important role in recent delta construction.

50,000 dams later: Erosion of the Yangtze River and its delta

Using 50 years of hydrologic and bathymetric data, we show that construction of ~ 50,000 dams throughout the Yangtze River watershed, particularly the 2003 closing of the Three Gorges Dam (TGD), has resulted in downstream channel erosion and coarsening of bottom sediment, and erosion of the Yangtze's subaqueous delta. The downstream channel from TGD reverted from an accretion rate of ~ 90 Mt (1Mt = 1000 000 t)/yr between the mid-1950 s and mid-1980 s to an erosion rate of ~ 60 Mt/yr after closing of the TGD. The delta front has devolved from ~ 125 Mm 3 (1 Mm 3 = 1000 000 m 3)/yr of sediment accumulation in the 1960 s and 1970 s, when river sediment load exceeded 450 Mt/yr, to perhaps 100 Mm 3/yr of erosion in recent years. As of 2007 erosion seemed to have been primarily centered at 5–8 m water depths; shallower areas remained relatively stable, perhaps in part due to sediment input from eroding deltaic islands. In the coming decades the Yangtze's sediment load will probably continue to decrease, and its middle-lower river channel and delta will continue to erode as new dams are built, and the South-to-North Water Diversion is begun.

Sedimentary record of environmental evolution off the Yangtze River estuary, East China Sea, during the last ∼13,000 years, with special reference to the influence of the Yellow River on the Yangtze River delta during the last 600 years

A 35.60-m-long core (ECS-0702) recovered from a water depth of 22 m in the muddy area off the Yangtze River estuary was analyzed for sedimentary characteristics, clay and detrital mineral components, and element geochemistry as well as by AMS 14C dating to document sediment provenance changes and environmental evolution during the postglacial period in the study area. On the basis of the lithology, the benthic foraminiferal and ostracod assemblages, and the AMS 14C ages, we divided the core into four depositional units (DU1–DU4 in descending order) representing the postglacial sedimentary sequence during the last ∼13,000 years. DU 4 was deposited in a littoral to tidal-flat environment during ∼13,000–11,500 cal yr BP; DU 3 in a nearshore, subtidal environment upwards to the nearshore shelf with tidal influence in response to the postglacial sea-level rise during ∼11 500–7400 cal yr BP; DU 2 in a tide-affected, nearshore shallow-sea environment during ∼7400–540 cal yr BP, on the delta-front of the Yangtze River when the delta was actively prograding; and DU 1 in a nearshore shallow-sea environment during the last ∼540 years, on the delta-front under modern marine conditions. In DU 1, the depositional rate was much higher than in the underlying units, and the sediment composition reflects more influence from the Yellow River; this unit can therefore be logically linked to the period when the Yellow River last discharged into the South Yellow Sea, from AD 1128 to 1855. According to historical records, this period can be divided into an early stage (AD 1128–1494), when the shoreline near the Yellow River mouth was advancing slowly eastward, and a late stage (1495–1855), when the shoreline was advancing rapidly. The initiation of DU 1 deposition corresponds to the latter part of the early stage, suggesting that since that time, a relatively large amount of sediment from the Old Yellow River mouth area has been transported to the offshore area of the Yangtze River mouth. Our study has identified a high-resolution sedimentary signature associated with the last course shift of the Yellow River in the Yangtze River delta-front, thus highlighting the intricate relationship between sediment sources and sinks in coastal areas. A significant contribution of the Old Yellow River sediments to the Yangtze subaqueous delta during the last ∼600 years must be considered when the evolutional history of the Yangtze delta is examined and especially when the influence of the Three Gorge Dam construction on the future changes of the Yangtze Estuary and ecological environments in the East China Sea is estimated.

Magnetic properties of East China Sea shelf sediments off the Yangtze Estuary: Influence of provenance and particle size

The East China Sea continental shelf offshore from the Yangtze River estuary is a sedimentary system characterized by complicated hydrodynamic conditions and large variability in sediment texture. Granulometric, geochemical and environmental magnetic analyses were carried out on surface sediments from this area to investigate the spatial variations of sediment composition and their underlying mechanisms. Magnetic properties of three particle-size fractions (< 16 µm, 16–63 µm, > 63 µm) were further analyzed to assist data interpretation. Our results reveal four distinct units: subaqueous Yangtze delta and inner-shelf mud deposits (Unit A), relict sands on the middle shelf (Unit B), the transitional environment between Unit A and B (Unit C), and the area northern to Unit A (Unit D). The distributions of organic carbon, iron, manganese and their ratios (Mn/Fe) suggest minor effects of reductive diagenesis on the surface sediment magnetic properties. Because of differences in sediment source and hydrodynamic conditions, sediments in Unit A have the highest ferrimagnetic and antiferromagnetic mineral concentrations, as well as the finest ferrimagnetic grain size. The significant variations of magnetic properties within Unit A indicate the effect of particle size sorting during sediment transportation and deposition. Sediments in Unit B have the coarsest ferrimagnetic grain size and the highest proportion of ferrimagnetic versus antiferromagentic minerals. Unit C, which has a bimodal size distribution, can be subdivided into a southern and northern part magnetically, reflecting the influence of provenance and particle size. The distinct magnetic characteristics of Unit D are probably due to the strong influence of materials from the Yellow River. Our results provide a new method for characterizing East China Sea shelf sediments offshore of the Yangtze River mouth, against which the extent of the influence of the Yangtze River on the East China Sea can be examined. Furthermore, it may provide a baseline for future long-term monitoring in this sensitive continental-margin environment that is influenced by the Yangtze River.

Lithological and palynological evidence of late Quaternary depositional environments in the subaqueous Yangtze delta, China

AMS 14C ages of post-glacial core sediments from the subaqueous Yangtze delta, along with sedimentary structures and distributions of grain size, pollen spores, and dinoflagellate cysts, show an estuarine depositional system from 13 to 8.4 cal ka BP and a deltaic system from 5.9 cal ka BP to the present. The estuarine system consists of intertidal to subtidal flat, estuarine, and estuarine-front facies, characterized by sand–mud couplets and a high sedimentation rate. The deltaic system includes nearshore shelf and prodelta mud featured by lower sedimentation rate, markedly fewer coastal wetland herbaceous pollens, and more dinoflagellate cysts. We explain the extremely high sedimentation rate during 9.2–8.4 cal ka BP at the study site as a result of rapid sea-level rise, high sediment load due to the unstable monsoonal climate, and subaqueous decrease of elevation from inner to outer estuary. A depositional hiatus occurred during 8.2–5.9 cal ka BP, the transition from estuarine to deltaic system, caused possibly by a shortage of sediment supply resulting from delta initiation in paleo-incised Yangtze valley and strong tidal or storm-related reworking in offshore areas. The subsequent development of deltaic system at the study site indicates accelerated progradation of Yangtze delta post-5.9 cal ka BP.

Sedimentary features of the Yangtze River-derived along-shelf clinoform deposit in the East China Sea

A predominant sigmoidal clinoform deposit extends from the Yangtze River mouth southwards 800 km along the Chinese coast. This clinoform is thickest (∼40 m) between the 20 and 30 m isobaths and progressively thins offshore, reaching water depths of 60 and 90 m and distances up to 100 km offshore. Clay mineral, heavy metal, geochemical and grain-size analyses indicate that the Yangtze River is the primary source for this longshore-transported clinoform deposit. 210Pb chronologies show the highest accumulation rates (>3 cm/yr) occur immediately adjacent to the Yangtze subaqueous delta (north of 30 °N), decreasing southward alongshore and eastward offshore. The interaction of strong tides, waves, the China Coastal Current, winter storms, and offshore upwelling appear to have played important roles in trapping most Yangtze-derived sediment on the inner shelf and transporting it to the south.

Yangtze offshore, China: highly laminated sediments from the transition zone between subaqueous delta and the continental shelf

This study examines the deposition of highly laminated muddy sediment in vibrocores recovered from a depth of 40–55 m of water and located 120 km east of the Yangtze River mouth. X-radiographs show numerous sharp-based sedimentary rhythmics with nearly parallel and undulated laminations, interbedded with silty lenses and interbeds. The laminated sediment varies from clayey silt to silty clay. AMS radiocarbon dates on well-preserved bivalves are primarily younger than 5500 y BP, indicating the formation of the laminated sediment under the present sea-level conditions. Macro- and microfossils in the sediments are consistent with the modern offshore sedimentary setting. The laminated sediment originated from the Yangtze estuary, but the sedimentation processes were not deltaic. The highly laminated sediment that lies below the normal wave base of 10 m was deposited on the seaward margin of the Yangtze subaqueous delta, closely associated with submarine tidal and storm-generated currents.

Delta response to decline in sediment supply from the Yangtze River: evidence of the recent four decades and expectations for the next half-century

In order to examine the delta response to decline in sediment supply from the Yangtze River during the recent decades, bathymetric maps of the Yangtze subaqueous delta surveyed from 1958 to 1997 were analyzed together with the river sediment dataset of the same period. The net accretion rate over the study area decreased from 38 mm/yr in 1958–1978 to 8 mm/yr in 1978–1997, while the river sediment discharge decreased from 466×10 6 to 394×10 6 t/yr and the suspended sediment concentration decreased from 0.543 to 0.448 kg/m 3 during the same periods, respectively. The outer subaqueous delta is more sensitive to decline in river sediment load in contrast with the inner subaqueous delta. From 1958–1978 to 1978–1997, the net accretion rate decreased from 51 to 2 mm/yr in the outer subaqueous delta and from 25 to 14 mm/yr in the inner subaqueous delta. The accretion rate decreased more rapidly than the river sediment discharge because the amount of sediment transported out of the delta has not decreased. A concept of the critical threshold of river sediment discharge that separates delta progradation from recession is proposed. Although this critical value was found to change temporally and spatially, it was instrumental in predicting the future delta response. Under the impacts of human activities, particularly, the Three Gorges Dam Project, the river sediment load would be reduced below the critical value during the next five decades. This would result in a general delta recession replacing delta progradation; the recession would occur first in the outer subaqueous delta.

Late Quaternary evolution of the sub-aqueous Yangtze Delta, China: sedimentation, stratigraphy, palynology, and deformation

Four sediment belts (A–D) of the Yangtze sub-aqueous delta are defined from the sea-bottom sediment distribution. Belt A, delta front fine sand and silt, with trough cross stratification; Belt B, prodelta silty clay and clayey silt, with abundant burrowing and rich in organic matter; Belt C, prodelta to continental shelf sand–silt–clay, with thin layers of sand and clay interbedded; and Belt D, late Pleistocene relict sands (medium to fine grained), mixed with shell fragments. The late Quaternary stratigraphy of the delta consists from bottom upward of: late Pleistocene terrigenous (fluvial and lacustrine) sand and mud; Holocene transgressive silt, prodelta clay and delta front fine sand and silt. In addition, four pollen-spore zones (a–d) derived from these sediment sequences reveal a general trend of climate warming from early to late Holocene. The Zone c, distributed primarily within the prodelta muds, provides evidence for a temperature maximum starting at ca. 7000 years B.P. (Atlantic period). This indicates that the delta initiation was essentially coeval with the climate optimum in early Holocene. Sub-bottom seismic surveys reveal three (I–III) acoustic facies. Facies I and II are characterized by seaward remobilization of water-saturated Holocene prodelta muds; facies III is composed of highly-contorted late Pleistocene strata and mud diapirs that erupted from deep-seated late Pleistocene sediments in front of the estuarine depocenter. Gravity subsidence due to Holocene sediment loading within the depocenter is the major factor in causing the deformation of underlying unconsolidated strata. Furthermore, the sub-aqueous Yangtze Delta proper is now obviously diverting to southeast owing to the long-term, tectonically induced southward shift of the river mouth, and to the marine currents associated with the Coriolis effect. We thus predict that the large Chongming island in river mouth area will have been coalesced with the northern Yangtze coast in the recent future.