| Ever since the Industrial Revolution, global economy has grown rapidly, and environmental change has become significant. The sea level rise, which was caused by global climate warming, has impacted the entire human beings, especially those who live in coastal lowlands and delta areas. To assess and predict future sea level change, we need summarize the nature of sea level fluctuations from the geological evolution history. Thus, it is critical to reconstruct the regional high-resolution sea level rising pattern of the Holocene epoch.The Yangtze River delta, which is a vast and densely populated area, has attracted extensive attention of coastal scientists, due to its sensitive depositional environmental response to the Holocene sea level fluctuations. Many sea level curves have been established in this area, using sedimentary stratigraphic approach, the elevation of Neolithic sites, and basal peats. However, the precision of these curves are relatively low. Due to its special depositional environment, such as high sedimentation rate and abundant sediment input into the river mouth, some new techniques of reconstructing high-resolution sea level curve are not feasible in the Yangtze River delta. Therefore, it is necessary to develop a new proxy, basing on the identification of the sedimentary micro-facies in the saltmarsh/tidal flat, to help reconstruct high-resolution sea level curve of Holocene in the Yangtze River delta.We took sediment cores from different parts of tidal flat along the edge of the Yangtze River delta, including Fengxian at northern Hangzhou Bay, and Tuanjiesha, Bei Qiyao and Beibaogang at Chongming Island. In addition, we took surficial sediments from the subaqueous Yangtze delta as well. We measured the magnetic properties of these sediment samples, and revealed the distribution pattern of authigenic greigite, which is the typical magnetic mineral in the tidal flat environment. Content of geochemical elements and sulfates are also measured to be compared with the magnetic properties. By comparing the phenomenon of early diagenesis, and the result of geochemical measurement, we discussed the mechanism of the formation of authigenic greigite, and evaluate its application for identifing the sedimentary micro-facies in the saltmarsh/tidal flat. This could also provide a practicable tool for the reconstruction of high-resolution sea level rising pattern.Here are our research findings:1. Ferromagnetic minerals dominate in the core sediments from the modern tidal flat of the Yangtze River mouth, and especially the authigenic greigite are recognized in tidal flat. In the sediment cores TJS and FX from erosional tidal flat, greigite occurs in the upper tidal flat sediments. In the sediment cores BQY and BBG from aggradational tidal flat, greigite occurs in sediments frorm the high-tidal flat and mid-tidal flat, except in BQY01and BBG01which are also taken from the bare part of tidal flat. Overall, greigite is found in the high-and mid-tidal flat which are covered by plant, but not in the bare part of tidal flat or in the sediment samples from the subaqueous Yangtze delta.2. Magnetic parameters, such as χARM, χARM/χ and χARM/SIRM, can imply the magnetic domains. The magnetic domains of in sediments from TJS tidal flat are bigger significantly than BQY, BBG, FX tidal flat sediments, and the magnetic grain in sediments from the Yangtze river distributary channel also are greater significantly than the sediment samples from three other area of the subaqueous Yangtze delta. we assume it is caused by the strong influence of Yangtze river runoff, which leaded to the lack of authigenic minerals in sediments from the TJS column and Yangtze river distributary channel. As in different parts of a single tidal flat, magnetic mineral domains are also different, being finest in the upper tidal flat,coarsest in the lower tidal flat, while the domains in the mid-tidal flat are of medium size. We suggest this phenomenon is mainly caused by the difference in the tide current and wave strength, and the post-depositional early diagenesis that is featured by the dissolution of fine grains of magnetite.3. Greigite enriched in the upper tidal flat sediment of cores TJS, FX, BQY02, BQY03and BBG03. The upper tidal flat sediments are rich in organic matters and fine grain sediments, which are more conductive to the formation of greigite comparing to mid-and low-tidal flat. The formation of pyrite is a slow process, and the sedimentation rate of tidal flat is often higher than the rate of pyrite formation, causing the greigite deposited in the sediment before being transformed into pyrite. We imply either the coarser sediments in the sample of TJS01have cause lack of greigite in the sediment, or the large amount of freshwater discharge from the Yangtze River, which have diluted the content of sulphate (SO42-), is responsible for the deficiency of greigite. The absence of greigite in samples BQY01and BBG01is believed to be the result of weak early diagenesis, which is just in the stage of dissolving iron oxides. The low content of organic matters is the main factor that leads to the lack of greigite in the sediment samples taken from subaqueous delta. Therefore, organic matters are critical to the geochemical cycling of element S and the distribution characteristics of greigite in tidal flat could provides a new technique for the reconstruction of the Holocene sea level in the study area. |
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