Distribution Of TS/TOC And Iron-sulfides In The Holocene Sediments From The Taihu Lake Plain And Its Implications For The Sedimentary Environment

We obtained a Holocene sediment core SL67at Xiaokunshan from the Taihu Lake plain. The sedimentary environmental evolution was reconstructed by AMS14C dating, stratigraphic and sedimentological analyses. On this basis, we employed element geochemistry and environmental magnetism in order to reveal the distribution pattern of the organic geochemical elements and authigenic greigite in the sedimentary micro-facies of tidal flat in the Holocene sediment core. Furthermore, we evaluated the application and limitation of sulfur and iron-sulfides to distinguish sedimentary micro-facies of the tidal flat in Holocene sediment core.After measuring the AMS14C ages, we decided the Holocene sedimentary environmental evolution according to the lithology and sediment grain size at the core site. There is layer of a stiff mud at the bottom of core, formed during the Last Glacial Maximum (LGM). A transgressive succession, i.e. supratidal-saltmarsh-upper tidal flat-mid-tidal flat-upper tidal flat-mid-tidal flat was formed during the early to mid-Holocene when the study area was influenced by rapid sea-level rise. As the sea-level rise decelerated from the mid-Holocene, a large amount of sediment input was derived from the Yangtze River. This subsequently formed a regressive succession of lower tidal flat-mid-tidal flat-upper tidal flat-supratidal-lacustrine plain.Results demonstrate some certain rules in the distribution of geochemical element in different sedimentary micro-facies of the tidal flat. Firstly, the content of TOC, TN and TS in the regressive succession was much lower than that in the transgressive one. In the transgressive succession, content of TOC, TN and TS is the highest in the saltmarsh sediments, followed by that in the upper tidal flat and mid-tidal flat. In the regressive succession, content of of TOC, TN and TS is the highest in the mid-tidal flat sediments.The composition of organic matters varies in different sedimentary facies. In the transgressive succession, the ratios of TOC/TN exceed10in the supratidal and saltmarsh, reflecting their major origin of terrestrial vegetation. TOC/TN declines from supratidal to mid-tidal flat, and its value is consistently lower than10, indicating the increased contribution from marine algae. In the regressive succession, the ratio of TOC/TN is low in each sedimentary facies except the lacustrine plain, implying major organic contribution from algae.Besides, ratios of TS/TOC in sediments of SL67are also related to the sedimentary micro-facies of the tidal flat. In the transgressive succession, TS content is extremely high in the saltmarsh and upper tidal flat sediments. The ratio of TS/TOC is the minimum in the supratidal flat and saltmarsh sediments while it is the highest in the upper tidal flat sediments. The regression line slope of TS-TOC plot has the similar change trend as TS/TOC. In the regressive succession, TS content and TS/TOC are generally in high values but with large fluctuations in the mid-and lower tidal flat sediments. By contrast, the values in the supratidal and upper tidal flat are much lower. TS and TOC shows a poor correlation in the sediments of regressive succession.Early diagenesis prevails in core SL67. Pyrite was found all over the tidal flat sediments in core SL67while greigite just markedly showed up in the basal saltmarsh and the mid-tidal flat sediments of regressive succession. In addition, pyrrhotite also occurred in the basal saltmarsh sediments. In the upper and mid-tidal flat of transgressive succession and the lower tidal flat of the regressive succession, there also appeared weak signals of greigite.TS/TOC is much lower in the basal saltmarsh than that in the upper tidal flat of transgressive succession, suggesting the sulfate limitation in the saltmarsh. This explains the much stronger signals of greigite and pyrrhotite in the basal saltmarsh than in upper tidal flat sediments. We also suggest that the eluviation of the supratidal-upper tidal flat sediments and the early diagenesis of the underlying sediments result in the high values of TS/TOC and occurrence of greigite in mid-and lower tidal flat of regressive succession. Seepage of methane and H2S produced in the underlying sediments made storage and further early diagenesis in the coarser-grained sediments of lower and mid-tidal flat sediments of regressive succession.Present study also shows that the application of TS/TOC can improve the differentiation of sedimentary facies.