Modern bioturbation structures and their mixing effects in the Yangtze River Estuary and its adjacent sea areas

Bioturbation structures are not only critical indicators to the state of the seafloor environment but also serve as important proxies for reconstructing paleo-environments. Furthermore, bioturbation processes will cause the redistribution of materials and early diagenesis in sediments, which makes it a challenging component to explore elements cycles or interpret sedimentary records over centennial time scale. This study, based on sediment cores from 31 stations in the Yangtze River Estuary and its adjacent areas, utilized high-resolution X-ray computed tomography (CT) scanning technology and MATLAB data processing method, along with measurements of sediment grain size and ²¹⁰Pb activity, to qualitatively and quantitatively characterize the bioturbation structures in the area and estimate mixing intensity. The results indicate that bioturbation structures can be functionally categorized into four types: biodiffusor structure, conveyors structure, regenerator structure and gallery-diffusor structure, with conveyors structure and gallery-diffusor structure being the most common. The vertical distribution patterns of bioturbation structures within the cores exhibit variations, including exponential decay pattern, fluctuating decay pattern, pulse distribution pattern, and uniform distribution pattern. Based on bioturbation intensity and their combinations, three bioturbation structural zones were identified: the Yangtze River Estuary bioturbation zone, the coastal sand ridge and northern Zhejiang-Fujian bioturbation zone, and the transitional phase and northern Zhejiang-Fujian muddy bioturbation zone. The mixing coefficients in this area ranges from 0.02 to 884.24 cm²/yr, exhibiting significant spatial variation. Strong mixing is observed in the pro-delta and delta front-shelf transition zones, while weaker mixing occurs in the northern Zhejiang-Fujian muddy area and the southern coastal region of northern Jiangsu coastal. The mixing effects are influenced not only by bioturbation but also by non-biological factors such as physical mixing.