Distribution Characteristics Of Particle Size And TS/TOC In Sediments From The Modern Tidal Flat Of Yangtze Estuary And Its Indication To Distinguish Sedimentary Micro-facies

Basal peat and microfossil-based transfer functions are popular methods to reconstruct the high-resolution Holocene sea level curves for coastal areas in the world. These methods, however, are limited due to the unique sedimentary environment of Yangtze delta. According to the principles of sulfur enrichment in the seawater and its close relationship with the organic carbon and nitrogen during the early diagenesis, here we investigated distributions of sediment grain size and TS/TOC in the modern tidal flat of Yangtze River mouth. This study tends to develop a new technique for distinguishing sedimentary micro-facies of the tidal flat in the Holocene sediment cores for the study area.We obtained11short cores from FX, TJS, BBG and BQY located at the erosional and aggradational tidal flats of the Hangzhou Bay and Yangtze delta, respectively. We examined firstly the sediment grain size in detail to distinguish micro-facies of tidal flat in each core with help of the morphologic location and tidal level at the core site. Results show typical frequency curves of sediment grain size for different micro-facies of tidal flats. The upper tidal flat is characterized by frequency curves of wide plateau with major peak value at～6μm or20μm. Asymmetrical curve with single peak at the coarser side dominates in mid-tidal and lower tidal flats. Curves with plateau also appear in the mid-tidal flat sediments. Mode of lower tidal flat sediments is generally at35μm, coarser than that of mid-tidal flat.Measurement also reveals that TOC, TS and TN change synchronously in sediments, and their concentration decreases from upper tidal flat to mid-tidal flat and to lower tidal flat gradually.In terms of TOC/TN, the ratio in upper tidal flat and mid-tidal flat of FX cores are6.650and6.658respectively. TOC/TN in mid-tidal flat of TJS cores is8.037, which is slightly higher than that of upper tidal flat (7.374) and lower tidal flat (6.723). In the northern Chongming Island TOC/TN is9.295in upper tidal flat and is7.327in mid-tidal flat. These similar vales of TOC/TN between upper and mid-tidal flats suggest that organic matter in the sediments originated from suspended matter of the Yangtze River. TOC/TN in mid-tidal flat of BBG cores is a distinctly low value at5.6, which indicates origin of marine algae. By contrast, organic matter in upper tidal flat and lower tidal flat sediments of BBG is from fluvial suspended matter as their TOC/TN are8.26and7.952respectively.As far as the regression line slopes of TS-TOC plots are concerned, upper tidal flat and mid-tidal flat of FX cores have similar slopes (0.11and0.103). The main reason is that both have same organic matter source. The same conclusion could be obtained in BQY cores, which the slope of upper tidal flat is0.068and the slope of mid-tidal flat is0.07. The regression line slope of TS-TOC plots of mid-tidal flat is as high as0.129in BBG cores, implying that marine algae is more in favor of the enrichment of sulfur. The slopes, however, of lower tidal flat is0.048in BBG cores, suggesting the limitation of low content of TOC. The scatters in TJS cores are not linear, close to the ones of freshwater environment, reflecting the dominant Yangtze freshwater at the tidal flat.