Humans have been sailing across seas and oceans for thousands of years. However, the story of large ships capable of affecting coastal ecology and shelf sedimentary processes is only about 100 years old. Modern large seagoing vessels with a draft of 10–20 m can cause resuspension of seabed sediment, erosion of the channel slope and shoal, enhancement of seafloor sediment activity and thickening of the active layer, thereby having a significant impact on seabed topography and sedimentation processes. However, little is known about the effects of this anthropogenic agent on shelf sedimentation due to limited observational data. Here, two sediment cores were collected from a shipping lane used by vessels of 5,000- to 50,000-ton off the coast of China to analyze their sedimentary properties, with focus on both the grain size and elements. It was found that ship disturbance selectively modified the sedimentary record, with the fine-grained sediment becoming increasingly unstable. In addition, there was a reduction in grain size of sediment finer than 6.25 Φ, which decreased by 11% after the disturbance by ship. Biogenic elements that were closely related to the ecological environment were significantly altered, with Br/Cl, Si/Ti, and Ca/Ti ratios all becoming significantly smaller. This indicated that frequent disturbance caused by ships had reduced the productivity in the waters near the shipping lane. In terms of sensitivity to the effects of ship navigation, the sedimentation response was relatively rapid and began to emerge from the commencement of ship navigation, whereas the ecological response became evident later than the sedimentation response and only appeared after a significant growth in the maritime transportation of China. Following the comparison of the two sediment cores, we propose that the constant rate of supply (CRS- with ship disturbance)–constant initial concentration (CIC- without ship disturbance) dual dating model be used to establish a dating framework in waters frequently disturbed by ship. This type of anthropogenic sedimentary dynamic process and its sedimentary–ecological effects deserve attention on this era where there is a surge in shipping globally. Shipping lanes present an excellent area for quantitative studies on the impacts of human activity and defining the Anthropocene in the context of shipping.
Delta evolution in the context of no sediment discharge has become a global concern, and an accretion-to-erosion conversion is occurring in the Yangtze estuary. This conversion could threaten Changjiang subaqueous delta development. Sediment erodibility is an important indicator of subaqueous delta vulnerability. However, the present and future erodibility of the Changjiang subaqueous delta remains unclear. In this study, 37 short cores were collected from the Changjiang subaqueous delta, and the critical shear stress of the sediment was measured using a cohesive strength meter (CSM) and compared with estimates based on an empirical Shields diagram. The sediment erodibility was analyzed by comparing the sediment critical shear stress with the bed shear stress simulated using a numerical model (i.e., FVCOM), and sediment activity was introduced to discuss the geomorphological change in the subaqueous delta. The CSM-derived critical shear stress is significantly higher than that derived from the empirical Shields formula, but it better shows the erodibility of the sediment. The annual surface sediment activity ranges from 5% to 30% based on the CSM, indicating low surface erodibility. Moreover, the critical shear stress in this region increases as water depth increases, but the bed shear stress shows the opposite trend. Therefore, the erodibility of the Changjiang subaqueous delta is lower than that of the shallow area, indicating no accretion-erosion conversion or continued vertical erosion under sediment starvation in the coming decades. These findings can provide suggestions for erosion assessment and management in large river deltas under decreasing sediment discharge.
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