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Ferrimagnetic greigite (Fe3S4) is widespread in the sedimentary environment. Despite abundant reports of greigite occurrence in marine and lacustrine deposits, its formation mechanisms in deltaic deposits remain poorly studied. Here we investigate greigite in Holocene Yangtze River Delta deposits using granulometric, magnetic, and geochemical methods. The studied cores consist of tidal river, estuary, shallow marine, and delta facies in ascending order. The greigite‐bearing layers are found predominantly in the accreting tidal flat facies during the transgression stage and secondarily in the shallow marine facies during the regression stage of the delta's Holocene development. These sedimentary intervals have a higher total sulfur (TS) content and TS to total organic carbon ratios (TS/TOC) suggesting the accumulation of iron sulfides, including greigite, under reducing estuarine and shallow marine conditions. The greigite‐bearing layers in the tidal flat facies have lower Sr/Ba ratios, in comparison to the shallow marine facies, indicating a lower salinity environment. Supported by the dating results, it is suggested that the higher sedimentation rate of the tidal flat facies, caused by rapid sea‐level rise during the early Holocene, favors the formation and preservation of greigite. Our results indicate that the magnetic detection of greigite provides a simple and useful tool for inferring salinity and sedimentation rate changes, and hence better an understanding of the heterogeneity of depositional processes in Holocene delta environments. Plain Language Summary: Greigite (Fe3S4), an early or late diagenetic magnetic mineral, is an iron sulfide commonly produced during the carbon‐iron‐sulfur (C‐Fe‐S) cycle in aquatic environments. Its occurrence is generally found to be linked to water salinity change, which can be used to infer changes in past environmental conditions such as drought and flood events. So far, studies on greigite have focused on marine and lacustrine deposits. Its formation mechanisms in delta environments remain poorly known, where fresh and marine water mixing results in intensive spatial and temporal salinity change. In this study, environmental magnetic measurement and salinity indictors for example, the ratio of strontium to barium (Sr/Ba), the ratio of total sulfur to total organic carbon (TS/TOC) and TS concentration are combined to reveal greigite occurrence and its formation conditions. Our results show that higher sedimentation rates and marked salinity change associated with rapid sea level rise in the early Holocene is favorable for greigite generation and preservation. We find that magnetic detection of greigite can provide valuable information regarding spatial/temporal variabilities in fresh‐marine water mixing and depositional processes in deltaic environment. Key Points: We identify greigite at the boundary of sedimentary facies with marked salinity change in the Yangtze River DeltaGreigite‐bearing layers occurs predominantly in accreting tidal flat facies with a higher sedimentation rateMagnetic identification of gregite can assist stratigraphic differentiation in delta deposits [ABSTRACT FROM AUTHOR]

Abstract The optically stimulated luminescence (OSL) sensitivity and OSL signal components of quartz grains were used to investigate provenance changes of Holocene sediments from the Yangtze River delta. The variation of luminescence sensitivity was observed in multiple grain aliquots and single grains of quartz from different sedimentary units of the Yangtze River delta. Laboratory experiments suggest that repeated dosing/bleaching cycles increase the luminescence sensitivity of quartz from the studied sediments. High variable thermal activation curves were observed even for samples from the same sedimentary unit, implying highly diverse sources for the delta deposits of the Yangtze River. Different sedimentary units show quartz with similar OSL component contributions, and repeated dosing/bleaching cycles and heating treatment are unable to affect the relative contributions of the fast and medium components to the bulk OSL signal. The samples from unit 1 (U1, tidal river, 15–11 ka), unit 2 (U2, estuary, 11–9 ka) and unit 6 (U6, delta plain, ca. 1 ka to the present) show relatively higher luminescence sensitivity in comparison to unit 3 (U3, tidal sand ridge, 9–4 ka), unit 4 (U4, prodelta, 4–2.5 ka) and unit 5 (U5, delta front, 2.5–1 ka), implying changing sediment sources over time. Such a temporal variation of sediment source can be explained by the transgressive/regressive history of the Yangtze River delta as well as by Asian monsoon variability since the last deglaciation. It demonstrates that luminescence sensitivity of quartz has great potential for tracing sediment sources in the Yangtze River delta, but more work is needed to characterize specific sources to establish a source-to-sink linkage. Highlights • Provenance changes of Holocene Yangtze River delta deposits were studied with luminescence properties. • The luminescence sensitivity and thermal activation curve vary with stratigraphic units. • Luminescence sensitivities reflect sediment source variation of the Yangtze River delta. • Temporal variation of sediment source corresponds to the transgressive/regressive history and monsoon variation. [ABSTRACT FROM AUTHOR]

Establishing a reliable chronology is essential for understanding delta evolution, which is normally performed using radiocarbon ( 14 C) dating and the recently emerging technique of optically stimulated luminescence (OSL). The application of the latter one to the Holocene Yangtze River (Changjiang) Delta deposit is still quite limited. In this study, two 60.9-m-long cores were collected from Taizhou (TZ) and Nantong (NT) within the paleo-incised valley of the Yangtze River, and a total of seven and nine OSL samples were collected from the TZ and NT cores, respectively. In addition, ten accelerator mass spectrometry (AMS) 14 C ages of the TZ core were presented with eight AMS 14 C ages that were previously obtained from the NT core. The single-aliquot regenerative-dose (SAR) protocol was applied to coarse silt-sized (45–63 μm) and fine sand-sized (90–125 μm) quartz. The results showed that the grains in the 45–63 μm size fractions appeared to be better bleached than those in the 90–125 μm size fractions, and the detection of insufficiently bleached sediments is required in order to obtain accurate age estimates. The ages adopted for the samples range from 3 ka to 9 ka for the TZ core, and from 1 ka to 14 ka for the NT core, which were in general internally coherent within the limits of experimental errors and with respect to their stratigraphic order. The AMS 14 C age of the TZ core were significantly older than their OSL ages, while those from the NT core generally showed good agreement with their OSL ages. One should be cautious when using AMS 14 C to date deltatic deposits. Based on the OSL ages, two periods of rapid accumulation rates can be found in both cores, which are linked to the rapid sea level rise in early Holocene and migration of delta front facies in late Holocene. These investigations indicate that OSL technique is an effective method with which to date Holocene deltaic deposits, especially in coarse sedimentary layers where organic carbon material is sparse. [ABSTRACT FROM AUTHOR]

Sulfur isotopic signatures of pyrite (δ34S py) sensitively react to local environmental change. Here we elucidate mechanism of δ34S py variation in the Holocene sedimentary strata of the Yangtze River Delta, through the analyses of multiple geochemical and physical indicators including total organic carbon, total nitrogen, stable isotopes of organic carbon, different iron solid phases, Sr/Ba ratios, and changes in sedimentation rates. Two heavy δ34S py (∼30‰) layers that formed under contrasting redox conditions are preserved in the early Holocene transgressive tidal flat and late Holocene regressive mouth bar facies, respectively. Two hypotheses are proposed to explain these heavy δ34S py layers: 1) in the transgression process, the heavy δ34S py layers formed under stronger reducing conditions due to methane leakage; 2) in the regression process, the heavy δ34S py layers formed under relatively weaker reducing conditions due to rapid deposition and low salinity. This study emphasizes the importance of physicochemical condition characterization in sediments, including redox conditions, in distinguishing the mechanisms that act to produce heavy δ34S py signals. • Iron phase analysis reveals heavy δ34S py layers formed in distinct sedimentary facies and redox conditions in the Holocene deposits. • Heavy δ34S py layers in accreting tidal flat facies is formed in stronger reducing conditions due to methane leakage. • Heavy δ34S py layer in mouth-bar facies is formed in weaker reducing conditions due to rapid deposition and low salinity. [ABSTRACT FROM AUTHOR]

In fluvial and deltaic environments, different grain-size fractions experience varying degrees of bleaching of OSL signals due to variations in hydrodynamics and depositional processes. This has led to ambiguity for selection of grain-size fraction suitable for OSL dating. Comparison of OSL results from multiple grain-size fractions is one approach to ensure the reliability of OSL ages. We determined the quartz OSL ages of four grain-size fractions from fine silt to sand, together with the single grain OSL sensitivity in a Holocene sediment core from the Yangtze delta. For samples from the lower part of the sediment succession (including tidal river, tidal trough fill, estuarine front, tidal sand and delta front facies), OSL ages determined for different grain-size fractions were generally mutually consistent within the age range of ~2–9 ka, implying minor portion of incompletely bleached grains in the sediment at the time of burial. In contrast, significant partial bleaching was observed for three samples from younger delta front facies deposited during 0.4–0.6 ka. In these three samples, fine-silt quartz OSL ages were overestimated by ~0.5–2.0 ka compared with ages of coarser fractions. Based on the investigation of quartz luminescence sensitivity, we suggest that the southward shift of the Yellow River and increasing erosion in the Yangtze River catchment have led to enhanced water turbidity, resulting in partial bleaching of the sediment and substantial overestimation of the quartz OSL ages of fine silt during 0.4–0.6 ka. Alternatively, the erosion of older deposits and their redeposition elsewhere could retain large residual OSL ages. Notably, fine-silt quartz provides reliable ages for the period of 9–2 ka, suggesting that there was a lower degree of water turbidity compared with that since 2 ka. Therefore, the degree of OSL age overestimation may provide an indication of changes in water turbidity and sedimentary environment. Overall, our findings highlight the complexity of OSL dating in delta environments, and that an understanding of sediment transport and depositional processes is critical for reliable OSL dating. • OSL ages consistency and discrepancy among four quartz fractions were found in Holocene sediment of the Yangtze Delta. • The adopted OSL ages obtained using four different fractions were mutually consistent within the age range of ~20.5–2.0 9 ka. • Fine-silt quartz OSL ages were overestimated by ~0.5–2.0 ka for delta facies deposited during 0.4–0.6 ka. • Enhanced water turbidity due to increased sediment flux or relocation of eroded old deposit causes partial bleaching. [ABSTRACT FROM AUTHOR]

Sediment source identification is critical for understanding delta evolution processes and for managing delta sustainability, particularly for deltas experiencing significant recent fluvial sediment discharge. Sediment sources for the Yangtze River Delta (YRD) are commonly assumed to be derived from the Yangtze River, despite the fact that the YRD is a tide-dominated delta that can receive marine-sourced sediments in addition to fluvial inputs. In particular, potential contributions from the neighbouring sediment-laden Yellow River, when it discharged into the Yellow Sea during 1128–1855 CE, remains unclear. Here we present provenance analysis of three cores from the northern YRD using size-specific magnetic characterizations. We find that magnetic properties of sediments younger than ~400 years have large differences among the studied cores. Comparison of magnetic properties to potential sources, including the major Yangtze River tributaries (i.e., Jinsha River, Jialing River, and Han River) and Old Yellow River (OYR), indicates that the northern core received enhanced Yellow River sediment loads over the last 400 years, while the southern core had a dominant Yangtze influence, which is most pronounced in size fractions less than 16 μm. This interpretation is supported by geochemical results. The documented spatial sediment source heterogeneity is caused by differences in tidal-fluvial interaction among delta distributary channels. Our results imply that the neighbouring OYR delta to the north exerts a remote influence on the YRD through longshore transport. This coastal connectivity between deltas should be assessed when forecasting future tide-dominated delta changes in the context of global change. • Size-specific magnetic characterizations are studied to trace the sediment source of late Holocene Yangtze River Delta. • Yellow River contributes sediment to the Yangtze River Delta over the last 400 years. • Sediment source in the Yangtze River Delta is spatially heterogeneous in response to hydrodynamics. [ABSTRACT FROM AUTHOR]

Abstract The Yangtze (Changjiang) mega-delta, China, has a high risk of coastal erosion owing to the recent high rate of relative sea-level rise and reduced sediment supply. The study of the Holocene evolution of the delta can provide information about its response to rapid sea-level rise and changes in sediment supply caused by climate or human activity, although this has yet to be fully explored because of the lack of integrated studies using age-constrained sedimentary records. Here we document stratigraphic architecture and morphological changes over the last 11,700 years and estimate the amount of sediment trapped in the delta region on a millennial scale using a dataset of 344 sediment cores, 658 radiocarbon and 28 optically stimulated luminescence (OSL) ages (of which we obtained 64 cores, 345 radiocarbon and 28 OSL ages, and the others we sourced from the literature). Using this dataset we present the temporal and spatial morphodynamic evolution of the entire Yangtze River mouth from its early Holocene transgressive estuary to a mid- to late-Holocene regressive delta, making it possible to produce a quantitative and sequential analysis of sediment deposition. A destructive phase of the river mouth region was identified at 10 to 8 cal. kyr BP, including significant coastal erosion of tidal flats and troughs within the estuary and of tidal ridge-and-trough topography offshore; these resulted from the reshaping of the river mouth morphology caused by rapid sea-level rise at that time. As a result, the rate of sediment trapping at the river mouth declined from an average of 224 Mt yr−1 at 11.7–10 cal. kyr BP to 137 Mt yr−1 between 10 and 8 cal. kyr BP. Since delta initiation 8000 years ago, a retreat of the subaqueous delta occurred and the sediment trapping rate declined from 151 Mt yr−1 at 8–6 cal. kyr BP to 99–113 Mt yr−1 between 6 and 2 cal. kyr BP, caused by the reduction in sediment supply linked to summer monsoon weakening ~6000 years ago. In the last 2000 years the sediment trapping rate has increased to 162 Mt yr−1 due to intensified human activity. The present-day level of sedimentation in delta (49 Mt yr−1 in 2003–2011), after the completion of the Three Gorges Dam, is far lower than the 'natural' range in the Holocene. We thus infer a potential for system regime shift in terms of coastal erosion and a transition to a new equilibrium in delta morphology in the near future. [ABSTRACT FROM AUTHOR]