Increasing hypoxia in the Changjiang Estuary during the last three decades deciphered from sedimentary redox-sensitive elements

Abstract(#br)Ranked as one of the world’s largest seasonal hypoxic water bodies (Dissolved oxygen, DO≤2 mg l − 1 ), the Changjiang hypoxia has been reported to grow rapidly worse in recent decades according to cruise observations, but it has been seldom studied by sedimentary records. In this paper, four gravity cores (E1-E4), retrieved from the Changjiang Estuary, were dated by excess 210 Pb and analyzed with grain-size compositions, total organic carbon (TOC) contents, and RSEs (redox sensitive elements) compositions. We aim to decipher RSEs enrichment characteristics and controlling mechanisms in the Changjiang Estuary. The results show that Mo and V enrichment is highly promoted by particle absorption and FeMn redox cycling at the shallow estuarine environment (E1 and E2) with occasional hypoxic disturbances. Scavenging of Mo and V by organic complexation becomes significant at the hypoxic center (E3), together with great influence by FeMn redox cycling, but they do not work effectively for U enrichment because of its easy remobilization and abundant riverine input. Moreover, upcore increasing trends of Mo, V, and U in E3 match well with a general lowering trend of bottom water DO minima since the mid-1980s. There are two progressive hypoxic development stages intercalated with a less DO-depleted period 1991–1997 as shown by both cruise observations and RSEs/Al records. These findings are vital to better understanding coastal hypoxic development and RSEs enrichment mechanisms in the seasonal hypoxic settings, because hypoxia is predicted to increase in the near future due to intensifying human disturbances

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

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

Sedimentation Rate and Sedimentary Break in Tidal Deposits

Based on the study of tidal laminae of non tidal inlet in the Wenchang Formation of northern Zhejiang Province and at the muddy coast facing open sea in the Yangtze Delta region,the sedimentation rates have been calculated,which are close to that of similar study abroad.Field observation and research of coast pragradation rate and tidal flat sedimentation show that the calculated sedimentation rates are unreasonable.The investigation of present tidal flat in the Yangtze Delta reveals that tidal laminae are formed and reworked during a simidiurnal tidal cycle,and laminae preserved in the layer of sixteen days observation cover only 10% of that could be formed,other 90% have been eroded.The small sequence composed of sandy and muddy layers is a storm sequence,which is formed in the condition of alternation of storm and calm weather.Preservation of the small sequence covers about 10% of that could be formed,and other 90% are reworked.The laminae in the small sequence disappeared when the sandy and muddy layers eroded,so that the laminae preserved in tidal flat sequence are only 0 2% of that could be created.Reworked laminae and layer only have erosion surfaces,i.e.diastems.The attention should be paid to the sedimentary breaks,when we are doing high resolution study of the tidal deposits

Cross-Shore Variations in Morphodynamic Processes of an Open-Coast Mudflat in the Changjiang Delta, China: With an Emphasis on Storm Impacts

On the open coast of the Changjiang Delta, waves temporally play their dominant roles in shaping the tidal-flat profile, especially during typhoons. Detailed analyses are presented of the variations in grain sizes of surface sediments and bed level, measured in the summer of 1999 at Nanhui mudflats, south flank of the Changjiang Delta, China. Cross-shore variations in bed level are distinctly site-specific in response to waves. The site-specific erosion rates are related to local water depth, sediment properties, vegetation, and exposure time per semidiurnal tidal cycle. A great difference exists between the higher and lower intertidal mudflats bordered at the mean sea level (MSL): the higher section is dominated by continuous accretion, while the lower section is characteristic of dynamic changes in erosion and accretion phases. Swells play their more important roles in shaping the profile than local wind-driven waves at the study mudflat, where swells propagate onshore without great barriers’ damping and local winds are not highly strengthened by distant typhoons. Storm processes are greatly modulated by tides. The magnitude of erosion is greater by a weak storm during spring tides than a strong storm during neap tides. Significant changes in entrainment capacity of tidal currents from neap to spring tides account for the different erosion and accretion models of the intertidal mudflat

Bruun Rule and Its Application to the Coastal Zone of China

Brunhes The (Bruun) rule is the earliest method for predicting coastal erosion caused by sea level rise, and the simplest method. According to the situation of China\u27s sandy and silty coast, the law of grace can qualitatively explain the relationship between sea level rise and coastal erosion. It is possible to use it to predict coastal erosion and developmental periods that meet its requirements. However, if the coastal environment and conditions are not strictly examined, it is expected to be more common as a general pattern for predicting coastal erosion in sea level rise. Research proves

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

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