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The Arakawa incised-valley fill beneath the Tokyo area, Japan, has formed by delta progradation during Holocene transgression and highstand. This paper presents the integrated stratigraphy of this valley fill as deduced from three continuous sediment cores with rich radiocarbon age control and abundant data from the surrounding, and compares the filling history with the adjacent Nakagawa valley. Six depositional units make up the valley fill, distinguished based on sedimentary facies. In a sequence-stratigraphic framework, these units span from lowstand fluvial deposits (latest Pleistocene) through retrogradational to transgressive marine deposits (early and middle Holocene), to highstand fluvio-deltaic deposits (middle and late Holocene). The isochron and stacking pattern of sedimentary facies indicate that around 8 cal ka BP the shoreline trajectory flipped from landwards to seawards and maximum flooding occurred in the Arakawa valley. This is considerably earlier than in the neighboring Nakagawa valley, where this happened between 7 and 6 cal ka BP. Differences in the sediment supply at the time of transgression explain the diachronicity. The findings identify the Tone River, which was at that time feeding the Arakawa valley, as a system of very large sediment supply, compared to average rivers worldwide. [ABSTRACT FROM AUTHOR]

The fluvial to marine transition zone (FMTZ) is the area of coastal rivers in which sedimentation is controlled by the interaction of fluvial and marine processes. This study examines the FMTZ of the Mekong River delta, along a total channel length of ~660 km. Methods consist of collection and analysis of channel bed sediment samples, measurements of channel morphological parameters, and recognition of mangrove, molluscan, and diatom species. The process regime, salinity, morphological, and sedimentary trends recognized were used to define two main tracts for this FMTZ: an upstream, fluvial-dominated tract and a downstream, tide-dominated tract. In more detail, they allow the identification of four subzones, from upstream to downstream: 1) fluvial-dominated, tide-affected; 2) fluvial-dominated, tide-influenced; 3) tide-dominated, fluvial-influenced; and 4) tide-dominated, fluvial-affected. Tide-induced water-level changes affect the entire study area and extend into Cambodia. Measured salinity intrusion extends ~15 km upstream of the river mouth during wet season, and ~50 km during dry season. Brackish water species of mangroves, mollusks, and diatoms, however, occur landward of these limits, suggesting that highly diluted brackish water may reach ~160 km upstream of the river mouth during the dry season. In the fluvial-dominated tract, channels are sinuous and show a seaward-deepening trend, whereas width is relatively constant. In the tide-dominated tract, channels are straight, and show seaward-widening and seaward-shallowing trends. Natural levees are present in the fluvial-dominated, tide-affected subzone, but are replaced by mangroves elsewhere along the FMTZ. In the fluvial-dominated tract, mud content is low, sand grain size fines seaward, and gravelly sand and sand are the dominant facies. In the tide-dominated tract, mud content is high, sand grain size is constant, recycled sand is common, and tidal rhythmites are the dominant facies. Mud pebbles are common in sediments throughout a large part of the FMTZ. These trends characterizing the FMTZ of the Mekong River delta seem to be present in other systems and likely represent a general FMTZ pattern. Nonetheless, minor differences may be observed between different types of systems, or because of differences in local conditions. The comprehensive description of trends and their mutual relationships along the FMTZ presented herein provides critical information that can form the basis of a general conceptual model and can help to better understand these complex zones. [ABSTRACT FROM AUTHOR]

To detect long-term change in the reservoir effect of the eastern coast of Korea, we measured the radiocarbon ages of terrestrial plant and marine shell pairs from the same horizons from approximately 12 m of Holocene sediments under reclaimed land around the Mae-ho Lagoon. The lagoon sediment was examined for lithology, and sedimentary structures, as well as mollusk and diatom assemblages. From bottom to top the sediment comprised three units: transgressive lagoon, restricted lagoon, and delta. The offset in the radiocarbon age of nine pairs were also divided into three stages: 300 ± 90 to 410 ± 120 years (transgressive lagoon sediment in 7350–5800 cal BP), 640 ± 150 to 800 ± 150 years (restricted lagoon sediment in 4320–1600 cal BP), and 120 ± 140 to 330 ± 100 years (deltaic sediment in 1200–390 cal BP). The reservoir ages (R) in the sediments of transgressive lagoon and the restricted lagoon include the variation associated with dietary of shells, herbivorous and deposit-feeding, respectively. The R values in the deltaic lagoon sediment were the most representative of those in the Mae-ho Lagoon because they were obtained from the shells of a filter-feeding taxon, Potamocorbula amurensis , in the most recent sediment. These values from the deltaic lagoon sediment are also consistent with previously reported modern reservoir ages from the Korean Peninsula and Peter the Great Gulf around Vladivostok. [ABSTRACT FROM AUTHOR]

Abstract: The Godavari delta in India is a major wave dominated delta of a tropical monsoon-fed river with one of the largest sediment deliveries in the world. While several earlier studies revealed the nature of landforms and progradation style of the delta plain during the Holocene, the present study attempts to reconstruct the depositional environment of the Godavari delta through the analysis of core sediment and accelerator mass spectrometry (AMS) 14C dating from three locations. The sediment core obtained from a 27.06m deep borehole at Vilasavilli (VV) supported by 13 14C dates revealed the complete succession of the Holocene deposits unconformably overlying Pleistocene sediments. Textural analysis indicates the lower upward-fining and upper upward-coarsening units in the Holocene succession. Total organic carbon (TOC)/total nitrogen (TN) ratios of >20; high content of TOC around 1.5–2.5%; and black to very dark colored sediment throughout the muddy part of the Holocene succession indicated a predominance of terrigenous material. The VV core and two other cores (DRP and SDG) with 14 14C dates indicated the thickness of the Holocene sediments in the Godavari delta plain is in the order of ~20–50m, unconformably resting on a seaward sloping Pleistocene basement. Sediment facies and sediment accumulation of the three cores show the evolution of the Godavari delta. A transgressive phase is recognized as an upward-fining succession in the VV core, 8.4–8.0cal ky BP, followed by a low accumulation period, 8.0–6.3cal ky BP including the Holocene maximum transgression in the Godavari delta. After 6.3cal ky BP, areas of high accumulation rates have changed laterally between the central part (VV site) and southwestern part (DRP and SDG sites), may be controlled by the location of river-mouths. Further analysis coupled with 11 more 14C dates compiled from earlier works indicated that the strandplain of the Godavari delta prograded seaward in three stages, and that the rate of progradation accelerated during the past ~3 ky, particularly in the last millennium. However, pronounced shoreline erosion led to a net negative growth of the delta during the recent decades due to sediment retention by upstream dams. Although this study provided insights into the sedimentation patterns and rate of seaward progradation of the Godavari delta, further studies on three-dimensional volume analysis of the deltaic sediments are necessary to estimate the overall rates of past sediment discharge and the Holocene growth of the Godavari delta. [Copyright &y& Elsevier]

Abstract: We investigated oxygen isotope and Sr/Ca, Ba/Ca and Mg/Ca ratios in shells of a commercially cultured freshwater pearl mussel (Hyriopsis sp., Unionidae) and ambient water. The first-order fluctuations of oxygen isotope ratios of the outer shell layer along the maximum growth axis recorded the mussels'' annual growth from 2002 to 2007. Clear annual Sr/Ca cycles suggest that Sr was incorporated into the shells at a high rate during warm seasons. All three examined shells showed a clear decrease in annual average Sr/Ca with age. Inter- and intra-annual trends suggest that Sr incorporation was higher during periods of faster growth. The Ba concentration and the Ba/Ca ratio of ambient water showed neither an annual periodicity nor a seasonal trend. In contrast to the findings of an inorganic aragonite precipitation experiment, Ba/Ca was positively correlated with ambient temperature. In addition, Ba/Ca and Sr/Ca ratios were positively correlated with each other. These results suggest that not thermodynamic equilibrium partitioning but the shell growth rate controls Ba incorporation into Hyriopsis shells. The Mg/Ca profiles of the three shells suggest that complex factors control the incorporation of Mg into shell aragonite. Water temperature and annual growth rates did not significantly influence Mg incorporation, and substantial interspecimen differences suggested that physicochemical factors do not exert any systematic control on Mg/Ca ratios in Hyriopsis. Instead, the Mg content of shell aragonite might reflect biological contamination with organic material or adsorption phenomena during crystal precipitation rather than regulated substitution into the aragonite crystal lattice. Our results suggest that trace element profiles in aragonitic bivalve shells could be interpreted as the result of strong biological controls over-riding environmental influences on trace element incorporation. [Copyright &y& Elsevier]

Abstract: A 35.60-m-long core (ECS-0702) recovered from a water depth of 22m 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 ∼13,000–11,500calyrBP; DU 3 in a nearshore, subtidal environment upwards to the nearshore shelf with tidal influence in response to the postglacial sea-level rise during ∼11 500–7400calyrBP; DU 2 in a tide-affected, nearshore shallow-sea environment during ∼7400–540calyrBP, 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. [Copyright &y& Elsevier]