Daidu Fan

Stratigraphy and paleoenvironmental changes in the Yangtze Delta during the Late Quaternary

The Yangtze Delta area may be subdivided into paleo-valley and paleointerfluves of the last glaciation. The postglacial transgressive sedimentary cycle (PTSC) on the front zones of the paleointerfluves is composed of marsh-nearshore and barrier-lagoon, shallow marine and nearshore-tidal flat units, with a basement in stiff clay. The PTSC in back zones of the paleointerfluves consists of lacustrine-marsh deposits. The PTSC in the incised valley contains river channel, floodplain-estuary, estuarine-shallow marine and deltaic units, with an erosional surface at its bottom. The stiff clay and the erosional surface constitute the PTSC lower boundary. The stiff clay, consisting of a paleosol, experienced deposition alternating with pedogenesis during the falling of sea-level during δ18O stage 3; ongoing pedogenesis in the sea-level lowstand of δ18O stage 2; and early diagenesis after paleointerfluve inundation by the sea-level rise of δ18O stage 1. The climate during the paleosol formation was temperate with more than 500-mm a−1 rainfall and frequently fluctuating groundwater. The Yangtze River incised its course during falling sea level of stage 3, and a huge incised valley was formed during the sea-level lowstand (stage 2). The filling of the incised valley took place during the postglacial sea-level rise, and delta formation occurred at a late stage of PTSC development.

Late Quaternary incised-valley fill of the Yangtze delta (China): its stratigraphic framework and evolution

A Late Quaternary stratigraphic framework of the Yangtze delta, China, has been established based on the analysis and correlation of over 600 cores. Thirty cores were drilled and analyzed during this study, and the rest were collected from numerous publications. The Late Quaternary stratigraphic framework of the Yangtze delta consists of incised-valley fill and two interfluvial sequences. The incised valley roughly coincides with the modern Yangtze delta. From bottom to top, the incisedvalley fill sequence is typically composed of channel, floodplain–estuary, estuary–shallow marine, and modern deltaic facies. The interfluvial sequences occur along the two flanks of the incised-valley fill. On the southern flank, seaward of the maximumtransgression line, the interfluvial sequence is typically composed of marsh–nearshore, shallow marine, and tidal flat facies with predominantly muddy deposits. On the northern flank, the interfluvial sequence is typically composed of barrier–lagoon, tidal sand ridge, and littoral–tidal flat facies with predominantly sandy deposits. The interfluvial sequence landward of the maximum-transgression line on both flanks is composed mainly of lacustrine muddy deposits. The post-glacial fining-upward incised-valley fill sequence was formed by retrogressive aggradation in the lower reach of the Yangtze River, induced by sealevel rise. The upstream extent of the retrogressive aggradation exceeds the reach of flood tidal currents, which explains the lack of marine fossil and tidal sedimentary structures in the channel sand deposits. A huge estuary was formed in the present Yangtze delta area when the post-glacial transgression reached maximum at about 7500 years BP. The shallow marine mud was deposited broadly in the estuary and the maximum flooding surface, which separates the underlying transgressive sequence from overlying regressive sequence, located within the muddy stratum. An erosional surface between the channel sand unit and deltaic sand unit exists at the apical area of the Yangtze delta. The modern Yangtze delta developed after the post-glacial transgression maximum was reached. The delta development was dominated by the abundant fluvial sediment supply. Six subdeltas with a southward migrating trend have developed. D 2002 Elsevier Science B.V. All rights reserved.

Fluorescence characteristics of chromophoric dissolved organic matter in shallow water along the Zhejiang coasts, southeast China.

Twenty-eight surface water samples from rivers, muddy intertidal flats, sand shores, and bedrock coasts were collected along the Zhejiang coastline in southeast China. In addition, three samples from the Changjiang (Yangtze River) were collected for comparison. CDOM (chromophoric dissolved organic matter) absorption and fluorescence excitation-emission matrix (EEM) spectroscopy, as well as nutrients and DOC were measured in these samples. According to salinity, nutrient, and DOC constituents, the 28 Zhejiang samples were categorized into four groups, i.e. highly-polluted, river derived, muddy-flat derived, and saltwater dominated ones. Among the six parameters (two humic-like and two protein-like peak intensities in fluorescence EEM contours, absorption at 300 nm, and DOC concentration) for the Zhejiang samples, any two of them were positively correlated. The submarine groundwater discharge, rather than local rivers, might have provided most of the freshwater that interacted with the saltwater during the mixing process. The high protein-like EEM peaks in samples from muddy salt marshes and rivers were probably caused by terrestrial inputs, land-based pollution, and local biological activities in combination.

Characteristic and paleoenvironmental evolution of subaerial tidal sand body in Subei coastal plain

The subaerial tidal sand area in the northern Jiangsu Province (Subei), stretching from Dongtai towards east with a fan shape, is an early developing stage of radial sand ridges distributed in the South Yellow Sea. Since 5000–6000 a BP, after the Holocene transgression maximum in the northern Jiangsu Province, subaqueous tidal sand bodies were exposed and changed into land gradually. The environmental magnetism analysis shows that subaerial tidal sand strata are formed by the convergent-divergent paleo-tidal current field. The sediment source of tidal sand strata came early from the Changjiang River and late from the Yellow River. Sea floor erosion by tidal currents also served as an important sand source. Drilling cores and ground-penetrating profile show that there exists no probability of sand supplying directly by a large river through the apical area of tidal sand ridges either on land or in the sea. Fluvial deposits supplied the tidal sand bodies by alongshore transportation, which corresponds to the conclusions obtained by the analyses of provenance and paleocurrent field.

Open-Coast Tidal Flats

Recent research advances highlight the importance of open-coast tidal-flat depositional system in both modern and ancient coastal environments. The system is unique in its wave- and tide-dominated physical setting, notably distinct from the tide-dominated barred tidal flats and the wave-dominated shorefaces. Interactions of waves and tides over different time scales produce not only cyclic morphologic variations in terms of erosion and deposition, but also rhythmic depositional units consisting of storm-generated sand-dominated layers (SDLs) and post-storm mud-dominated layers (MDLs). Ancient deposits of the open-coast tidal flats can be distinguished by abundance of storm-generated structures and scarcity of tidal-channel deposits from those of barred tidal flats, and by abundance of the structures created by combined flows or the interactions of waves and tides from those of (tidal) shorefaces. Difference is also remarkable between muddy and sandy open-coast tidal flats. Muddy open-coast tidal flats tend to develop along mega-river deltas and the adjacent chenier plains, have a general accretional convex-up profile with clear zonation, and produce aggradational fining-upward intertidal successions. Sandy open-coast tidal flats are common in the open-mouth estuaries of small rivers and the adjacent strand plains, usually develop an erosional concave-up profile with common presence of inner swash bars having the coarsest sediment near the high water, and produce coarsening-upward retrogradational successions. The vertical successions of sandy open-coast tidal flats generally contain more storm-generated beds volumetrically than those of muddy open-coast tidal flats. Notably, there are some accretional sandy open-coast tidal flats, lying in between the above two settings. A new spectrum of coastal morphodynamic settings is therefore proposed to change from the tide-dominated barred tidal flats, the wave-influenced and tide-dominated muddy open-coast tidal flats, the wave- and tide-dominated accretional sandy open-coast tidal flats, the wave-dominated erosional sandy open-coast tidal flats, to the wave-dominated tidal shorefaces.

Rhythmic deposition on mudflats in the mesotidal Changjiang Estuary, China

ABSTRACT The mesotidal Changjiang Estuary is characterized by high suspended sediment concentration, broad mudflats, no barrier islands, and a semidiurnal tidal regime controlled by the tropical period. Study mudflats are located at the south flank of the Changjiang Delta, facing the East China Sea. Thickness and number of tidal couplets deposited on the mudflats were measured daily and fortnightly over three neap-spring cycles. Changes in mudflat elevation were monitored from May (pre-typhoon season) to August (mid-typhoon season). Daily observation of sedimentary features shows that short-term deposition on the mudflats during calm weather is controlled mainly by tides. Neap-spring cyclicity can be extracted from the variation of sand-mud couplet thickness. High suspended sediment concentration allows deposition of two couplets per semidiurnal tide. However, number of couplets preserved per tidal cycle is reduced because of reworking and erosion by tidal currents and waves. Over a fortnightly period, tidal cyclicities are obscured, with ca. 80% of the couplets reworked. On a seasonal scale, sand-dominated layers reflect deposition during storms and mud-dominated layers reflect deposition from tides during calm weather. Two orders of sand-mud lamina alternations were examined in the tidal-flat succession. The first-order alternations are in the range of centimeters to decimeters (consisting of 16-26 couplets) and related to storm-calm weather variation; the second-order, millimeter-thick alternations are related to tidal rhythm. Deposition rates decrease exponentially as time scales increase. The open-coast tidal-flat succession is truncated with numerous depositional hiatuses.

Influences of Storm Erosion and Deposition on Rhythmites of the Upper Wenchang Formation (Upper Ordovician) Around Tonglu, Zhejiang Province, China

ABSTRACT Tonglu rhythmites (Upper Ordovician) in Zhejiang Province, east-central China, display three orders of cyclicity in sandstone and mudstone layer thickness. Millimeter-thick alternations of sandstone and mudstone laminae are ascribed to single tidal cycles. Centimeter-thick alternations of sand-dominated layers (SDLs) and mud-dominated layers (MDLs) are interpreted to be related to alternation of storm and calm weather conditions with a periodicity longer than that forced by neap-spring tidal cyclicity. The SDLs are interpreted as storm deposits on the basis of presence of scour structures, abundant intraformational mud pebbles, oscillation ripples, and thinning-upward trends in the sandstone laminae. A third, meter-thick cycle of variations in sandstone-lamina thickness is interpreted as a reflection of cross-shore changes in coastal dynamics and water depth in the subtidal-intertidal environment. Storm waves, usually considered to be random destructive factors to normal cyclic deposits, are here highlighted as effective agents of sediment transport and deposition of the sand-dominated layers. This study aims at improving our facies-level understanding of the genesis and preservation of storm-related tidal-flat rhythmites on open coasts, and highlights the fact that storm related facies can mimic the cyclicity that is commonly ascribed to neap-spring tidal variation.

Temporal Distribution of Diastems in Deposits of an Open-Coast Tidal-Flat With High Suspended Sediment Concentrations

Abstract Based on field observations of the preservation potential and genesis of couplets and small sequences, diastems occur within tidal flat deposits at scales of semi-tidal, daily, monthly, and yearly intervals. Thus, tidal flat deposition is riddled with more diastems of different scales than previous assumptions. Units that are apparently complete when examined at longer time scales actually contain many diastems when examined at a shorter time scale. Diastems within tidal flat deposits are generated not only by high-magnitude storm waves, which occur during typhoon seasons, but also by daily erosion produced by tides and weak waves during calm-weather seasons. The missing intervals in diastems generated by storm waves can be several days to several years. Preservation potential of storm-generated small sequences is of the order of one-third. Diastems generated by erosion, by tides, and weak waves can have spans from a semidiurnal cycle to several days. They are easily overlooked because of the low-magnitude destruction and lack of distinctive sedimentary features. Net erosion by tides and weak waves is unexpectedly increased by day-to-day processes. Alternations of sandy and muddy laminae are related to tidal rhythm, but individual laminae can be separated by a significant diastem. Documentation of erosion by tides and weak waves under normal (non-typhoon) conditions is the principal focus of this report.

Distinction and grain-size characteristics of intertidal heterolithic deposits in the middle Qiantang Estuary (East China Sea)

The routine sampling procedure for grain-size analysis of intertidal heterolithic deposits runs a high risk of inadvertent mixing of two or more different sedimentation units, which would consequently complicate data interpretation. Traditionally, sedimentologists pay less attention to muddy layers due to a lack of internal structures, although the grain-size populations of such layers should encode more information on fine-mud flocculation processes than sandy layers. In this paper, individual muddy and sandy layers of nine short cores from the Da-Jian-Shan tidal flats of the middle Qiantang Estuary in the East China Sea, which experiences tidal bores, were sampled separately for grain-size analysis. A core taken at Huang-Jia-Yan from the lower estuary, not affected by tidal bores, served for comparison. A curve-fitting method was employed to decompose each grain-size distribution into two Gaussian populations. Cumulative plots indicate that intertidal sediments are mostly dispersed as intermittent and uniform suspension loads, traction loads being absent or very subordinate. This is conceivably linked to flows agitated by tidal bores, and to the highly dynamic nature of fine sand and coarse silt particles. Selective transport and deposition have produced three distinct sedimentation units, namely, tidal-bore deposits, tidal sandy deposits, and tidal muddy deposits. These can also be discriminated on bivariate plots of any two textural parameters. Shoreward attenuation of tidal flows is reflected in the gradual fining and thinning of sandy layers from lower-flat massive sands, through middle-flat hybrid deposits (alternations of massive sands and tidal rhythmites), to upper-flat tidal rhythmites. This gradient is also well represented in slightly decreasing (increasing) sorting and decreasing (increasing) proportions of the coarser (finer) hydraulic populations in the muddy layers. Although no corresponding trends are discernible in the hydraulic populations of the sandy layers, these can be distinguished on the basis of characteristic sedimentary structures. The floc limit and floc volume fraction, estimated from the modes and proportions of the finer hydraulic populations, are 8~10 μm (16 μm) and on average 41.73% (26.41%) for muddy (sandy) layers, respectively. The most plausible explanation is that the floc limit sensitively responds to subtle changes in the suspended sediment composition and the ambient hydraulic and hydrochemical settings. In comparison, the Huang-Jia-Yan core features blurred bedding and higher contents of fine mud and flocs, these being consistent with the weaker energy on the upper tidal flat of the lower estuary where neither tidal bores nor bigger waves occur.

Tracing the quarter‐diurnal signatures of nutrients and dissolved organic matter to evaluate their nonconservative behaviors in coastal seawaters

[1] In June 2009, six consecutive observations with duration times of 13 h or 26 h were performed at the sea surface of outer Hangzhou Bay in the East China Sea. Physical (current direction and velocity), hydrological (salinity, suspended particulate matter (SPM) turbidity, total volume and mean size of suspended flocs), and chemical tracer (nutrients, chromophoric dissolved organic matter, and dissolved organic nitrogen) data were collected. Harmonic analysis that included diurnal, semidiurnal, and quarter‐diurnal constituents was conducted on time series data for all of the parameters measured. Spectral analysis was performed to determine whether the three harmonics could characterize the variations observed in the data. The amplitude ratio of quarter‐diurnal harmonic to the sum of diurnal and semidiurnal harmonics was expected to quantify the nonconservative extent of the chemical tracers. This ratio generally was higher in turbid seawater, whereas the lowest ratio occurred at a station where the seawater displayed zero turbidity throughout the experiment. The quarter‐diurnal phase lags between SPM and chemical tracers were believed to provide information about how SPM and chemicals interact. How field processes such as resuspension and flocculation/ disaggregation affected the kinetics of the chemical tracers also was explored by plotting the quarter‐diurnal phase spectra along the 32 size classes obtained from Laser In Situ Scattering and Transmissometry measurements. This study provided an interesting and potentially useful technique to estimate the nonconservative behaviors of biogenic elements and to study the relationships between the fields of physical oceanography and biogeochemistry in estuarine and coastal seawaters. Citation: Gao, L., D. Fan, Y. Zhang, D. Li, and J. Cai (2011), Tracing the quarter‐diurnal signatures of nutrients and dissolved organic matter to evaluate their nonconservative behaviors in coastal seawaters, J. Geophys. Res., 116, G03015,