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Studies of the evolution of coastal lowlands since the Last Glacial Maximum (LGM) typically ignore radiocarbon data from sediment samples that have undergone reworking. However, these samples contain information on their sediment sources and the timing of their redeposition. We analyzed 738 radiocarbon dates obtained from shell and plant material in samples of post-LGM coastal sediment from north of Tokyo Bay, Japan. Of these samples, 245 (33%) were reworked. Furthermore, the percentage of reworked samples and their average age offsets increased with the depth of the water environment (terrestrial, 15% and 360 ± 250 years, respectively; intertidal, 26% and 470 ± 620 years; subtidal, 39% and 550 ± 630 years). Taking these radiocarbon samples as a proxy for clastic material, our results imply that channel erosion accounted for relatively little clastic removal in the terrestrial and intertidal environments over short timescales, whereas ~ 40% of clastics were removed by storm winnowing and transported in stepwise fashion to deeper water over longer timescales and ~ 60% in the subtidal environment were transported by floods directly from river mouths. These findings imply that radiocarbon ages from reworked samples can be used to quantify clastic recycling processes and their history in coastal areas. [ABSTRACT FROM AUTHOR]

The Ba Lai distributary channel of the Mekong River Delta was abandoned and infilled with sediment during the Late Holocene, providing a unique opportunity to investigate the sediment fill, timing and mechanisms of channel abandonment in tide-dominated deltaic systems. Based on analysis and age dating of four sediment cores, we show that the channel was active since 2.6 ka and was abandoned at 0.7 ka as marked by the abrupt disappearance of the sand fraction and increase in organic matter and sediment accumulation rate. We estimate that the channel might have been filled in a time range of 45–263 years after detachment from the deltaic network, with sediment accumulation rates of centimetres to decimetres per year, rapidly storing approximately 600 Mt of organic-rich mud. We suggest that the channel was abandoned due to a sediment buildup favoured by an increase in regional sediment supply to the delta. This study highlights that mechanisms for abandonment and infilling of tide-dominated deltaic channels do not entirely fit widely used models developed for fluvial-dominated environments. Their abandonment might be driven by autogenic factors related to the river-tidal and deltaic dynamics and favoured by allogenic factors (e.g., human impact and/or climate change). [ABSTRACT FROM AUTHOR]

The Holocene sea-level history of the east coast of India is relatively unexplored. We analysed a 17.37-m-long sediment core from Kolleru Lake, a fresh waterbody located in a deltaic setting along the east coast of India, to reconstruct the climate, environmental, and sea-level history of the region. Sedimentary facies and pollen assemblages, with nine accelerator mass spectrometry 14C dates (two mollusk shells and seven plant samples) from the core revealed Holocene relative sea-level changes, and provide the first insights into the climate of the region from the Last Glacial Maximum (LGM) to present. A layer of anhydrous calcium sulfate at the core bottom, along with a 14C age of 18.4 cal ka BP, combined with the absence of pollen in mottled yellowish clay, indicated a desiccated lake surface, reflecting a dry LGM climate. Palynomorphs in the overlying calcareous-concretion-bearing light brown silty clay showed a change from arid terrestrial herbaceous plants to freshwater taxa, indicating a change from dry to wet climate after the LGM and before 8.0 cal ka BP. Further upward in the core, black, sticky silty clay with abundant mangrove pollen and mollusk shells indicated a marine environment in Kolleru Lake and aggradational sediment stacking related to the middle Holocene sea-level rise from 8.0 to 4.9 cal ka BP. The uppermost sandy/silty clay, with terrestrial/aquatic pollen and a 14C age of 3.7 cal ka BP indicated a freshwater environment during the late Holocene. Our results show that Kolleru Lake, on the east coast of India, changed from a desiccated state during the dry LGM to a brackish lagoon during the middle Holocene, and then to a freshwater lake by the late Holocene, under the influence of climate and relative sea-level changes. [ABSTRACT FROM AUTHOR]

Since the 1990s the Mekong River delta has suffered a large decline in sediment supply causing coastal erosion, following catchment disturbance through hydropower dam construction and sand extraction. However, our new geological reconstruction of 2500-years of delta shoreline changes show that serious coastal erosion actually started much earlier. Data shows the sandy coast bounding river mouths accreted consistently at a rate of +2 to +4 km2/year. In contrast, we identified a variable accretion rate of the muddy deltaic protrusion at Camau; it was < +1 km2/year before 1400 years ago but increased drastically around 600 years ago, forming the entire Camau Peninsula. This high level of mud supply had sharply declined by the early 20th century after a vast canal network was built on the delta. Since then the Peninsula has been eroding, promoted by the conjunction of mud sequestration in the delta plain driven by expansion of rice cultivation, and hysteresis of long-term muddy sedimentation that left the protrusion exposed to wave erosion. Natural mitigation would require substantial increases in sediment supply well above the pre-1990s levels. [ABSTRACT FROM AUTHOR]

We investigated carbon sources and cycling in a canal connected to Lake Kasumigaura (Japan) using stable carbon isotope values (δC) in dissolved inorganic carbon (DIC) of ambient water and in shell carbonate of the commercially cultured freshwater pearl mussel ( Hyriopsis sp., Unionidae). The δC showed correlations with DIC concentrations, pH, monthly precipitation and pCO. Thus, the carbon isotopic composition can be used as a proxy for environmental variables. The pCO ranged from 3,046 to 9,535 μatm, with a mean value of 6,035 μatm, and remained well above the atmospheric value throughout the years of study. Changes in CO solubility, outgassing and biological processes in the river had only minor influence on the carbon isotope composition. DIC concentration and δC were controlled primarily by external inputs of soil and weathering-derived DIC associated with precipitation. The δC values in different growth zones of the Hyriopsis shells were similar, suggesting that given a constant δC source throughout shell formation, the influence of size-related metabolic variations on shell δC was negligible. Comparison of δC values in DIC and Hyriopsis shell indicated that mussel ontogeny did not influence carbon isotope fractionation, but inter-specimen differences suggested that physiological factors exert systematic control on δC values in Hyriopsis. Results also suggest that if the annual mean δC value is to be inferred for a fluvial system from shell δC values, multiple specimens should be sampled, and the annual mean value yields a temporally integrated measure of the stable isotopic composition of the river system. Given the carbon isotope fractionation between ambient DIC and shell is +2.0 ± 0.7 ‰ (2 se), we estimated that pCO averaged ~6,850 μatm during shell growth periods from 2002 to 2007. The pCO concentration was an order of magnitude greater than the atmospheric equilibrium value in all years, suggesting that CO was lost from surface water to the atmosphere or downstream to the lake. These losses must be accounted for when considering the regional redistribution of CO. Our results extend the utility of δC measures in freshwater bivalves for inferring the δC history of ambient waters. This tool provides information that is complementary to that gained from direct δC measurement and should enable inference of long-term changes in critical variables such as pCO. [ABSTRACT FROM AUTHOR]

We present oxygen and carbon isotope ratios and the morphological structure of the cultured freshwater pearl mussel ( Hyriopsis sp., Unionidae) shell and pearl. The number of first-order fluctuations of δ18O of the outer shell layer along the maximum growth axis was consistent with the number of cultured years. The dominant factor controlling annual δ18O fluctuations was water temperature with a minor contribution from the variation in δ18O of ambient water, especially during the rainy season. The δ13C values were approximately constant throughout the life of the mussel, suggesting that the contributions of body size to δ13C of the shell were minor. We observed nine distinct disturbance rings on the outer surface of the shell. Five rings coincided with the five winter peaks of the δ18O profile, indicating winter growth cessation below approximately 10°C, probably because of either inactive growth at low water temperatures or reproduction. Summer disturbance rings were not observed in all years. Moreover, some summer rings showed discontinuity in the inner structure. These findings suggest that summer growth cessation may be caused by occasional events such as heavy rains, as the decrease of dissolved oxygen concentration. The δ18O profile and shell structures indicated that shell aragonite was precipitated at close to equilibrium conditions with respect to the oxygen isotope composition of the ambient water. Hyriopsis sp. shells can potentially be used for reconstruction of past hydrologic conditions. The δ18O of a pearl indicated that calcification occurred over a temperature range of at least 13–23°C. The optimal temperature for pearl calcification in this species is lower than that for marine pearl calcification. [ABSTRACT FROM AUTHOR]