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Suspended sediment floc dynamics, particularly in the bottom boundary layer (BBL), has long been recognized as a complex and crucial factor influencing flow hydrodynamic and bed morphodynamics in tidal estuarine regions. Periodic salt-water intrusion in micro-tidal estuaries plays a crucial role due to its direct impacts on ecosystem, biological habitats, and human activities. To gain deeper understanding of the interactions between sediment flocculation and salt-water dynamics in estuaries, a continuous 115-h in-situ field survey onboard was conducted in a micro-tidal estuary, specifically the Huangmaohai estuary in China. This survey involved the measurements of suspended sediment concentrations (SSCs), floc properties, tidal current, turbulence and salinity in the BBL, utilizing state-of-art instrumentation such as the LISST-200X and ADV. Principal component analysis (PCA) was employed to examine the characteristics of sediment flocculation and analyze the influences of dynamic and structural factors on flocculation. The results reveal that, with the movement of the salt-wedge, two distinct stages of sediment flocculation process can be observed. Stage 1 occurs when the water column is vertically stratified, and a salt-wedge forms. Despite a reduction in SSC, this stage is characterized by a flocculation-dominated regime, in which large-sized, low density and low fractal dimension flocs settle to the bed. The presence of a stable salt-wedge enhances the flocculation by facilitating the formation of large-sized flocs with low density, low fractal dimension and high settling velocity while minimizing the disruptive effects of turbulence in the BBL. Statistical analysis reveals that saline water stratification contributes 19.76% to flocculation efficiency during this stage. Stage 2 occurs when the water column undergoes vertical mixing, causing the salt-wedge to dissipate. The regime transitions to a de-flocculation-dominated phase, leading to the resuspension of fine particles from the bed surface due to intense turbulence shear in the BBL. Consequently, flow shear becomes the key factor controlling factor for flocculation in the BBL with a contributing rate of 18.82%, statistically. A comprehensive conceptual model is presented to elucidate the intricate interactions between salt-wedge intrusion-retreat cycles and sediment flocculation within the BBL of a micro-tidal estuary. • Flocculation-dominated when the BBL within the salt-wedge. • De-flocculation-dominated when the BBL in the front of the salt-wedge. • Water stratification controls the flocculation in flocculation-dominated regime. • Flow shear controls the flocculation in de-flocculation-dominated regime. [ABSTRACT FROM AUTHOR]

Abstract: The formation of a broad tidal flat along the coast of the northern Jiangsu Province of China depends largely on the sediment supply from the Yellow and Yangtze Rivers although the relative contributions from each of these two large rivers remain uncertain. Knowledge of sediment sources to the tidal flat is critical for understanding the evolution of this muddy coast impacted by the large rivers. The present study focuses on tracking the sediment source of the tidal flat deposits based on sediment magnetic properties and geochemical analyses as well as statistical analysis. The study shows that sediments to the north of Yangkougang (site 9) have lower values of saturation isothermal remanence magnetization (SIRM), magnetic susceptibility (χ), demagnetization parameter S−100 ratios, lower Fe/Al values and higher Ca concentrations, while the opposite is true for samples south to Haozhigang (site 18). In addition, SIRM values of the <16μm and >63μm fractions generally display increasing trend from north to south. These results suggest that sediment sources rather than particle size variation are the dominant factors influencing the bulk magnetic properties. In light of the sediment composition comparison between the Yellow River and Yangtze River, factor analysis is used to identify sediment source of the tidal flat sediments. It is indicated that the Yellow River in its former course is the dominant supplier for sites to the north of Yangkougang (site 9), while the Yangtze River is the dominant supplier for sites to the south of Haozhigang (site 18). The coast between Yangkougang (site 9) and Haozhigang (site 18) is a transition zone influenced by both rivers. The inferred provenance contrasts are consistent with the pattern of coastal geomorphological evolution. Our data also suggest that sediments eroded from the former Yellow River delta have contributed to the evolution of the Yangtze Estuary in historical time. This study demonstrates that a combined magnetic and geochemical fingerprinting techniques in couple with statistical analysis is valuable for identifying sediment sources of tidal flats and deltas influenced by large rivers in the world. [Copyright &y& Elsevier]

Abstract: Loess deposits are widely distributed over the islands in the Bohai Gulf and neighboring regions of northern China. The coastal loess, located to the east of the Chinese Loess Plateau (CLP), is believed to have a local source from exposed marine sediments and a distant source from inner arid regions in Asia. In this study, magnetic, geochemical and diffuse reflectance spectroscopic analyses were conducted on the loess to explore its relationship with the loess on the CLP and its paleoclimatic implications. We find that the coastal loess shows similar geochemical composition, background susceptibility and magnetic enhancement to loess on the CLP. Magnetic minerals characteristic of pedogenesis are more important in the paleosol layer, with fine ferrimagnetic minerals abundant in paleosol layer SB1 and hematite in paleosol layer SB2. Geochemical evidence suggests that weathering intensity in paleosol SB2 is similar to or even stronger than in paleosol SB1. Pedogenic enhancement of magnetic susceptibility in the study area is not so strong, which may be due to enhanced hematite formation as opposed to ferrimagnetic mineral production and preservation. [Copyright &y& Elsevier]