Characterization and mechanism of burst resuspension events at the toe of Shenhu canyon

Understanding sediment resuspension processes in deep-sea environments is crucial for informing environmental impact assessments and making efficient planning of engineering operations. In this study, we deployed a lander at the toe of the Shenhu Canyon on the northern slope of South China Sea at a water depth of 1450 m to investigate deep-sea sediment resuspension processes. The lander was equipped with different sensors to measure near-bottom current velocity, temperature, turbidity, and digital videos. Several burst resuspension events were recorded. The suspended particular matter (SPM) concentration was calculated by analyzing the digital videos. The deep-sea digital video analysis showed that sometimes smaller particles were numerically dominant, but they do not contribute as significantly to the total particle volume as larger particles. Different resuspension mechanisms are identified by analyzing the characteristics of near-bottom current velocity and temperature variation. Two short-term burst resuspension events were attributed to up-slope propagating fronts arising from the nonlinear effect of internal tides. Another short-term burst resuspension event was triggered by a down-slope propagating front, likely generated by the oblique propagation of internal tides and the flow over a nearby promontory. The intensity of the short-term burst resuspension event triggered by the down-slope propagating front was greater than others triggered by up-slope propagating fronts. Other long-term burst resuspension events were induced by internal tides. The wavelet analysis of temperature data indicated that a distinct sub-inertial frequency during the resuspension period, which was consistent with the presence of a dynamical regime known as the sub-inertial event and internal gravity wave (SIE-IGW) regime, was associated with the burst resuspension events. This research contributes to the knowledge of burst resuspension processes in deep-sea environments, enhancing our understanding of the complex interplay between internal tides, seafloor topography, and the distribution of suspended particles.

厘清深海环境中的沉积物再悬浮（sediment resuspension）过程，可为环境影响评价及工程作业高效规划提供关键科学依据。本研究于南海北部陆坡神狐峡谷坡脚处部署水深1450米的深海着陆器（lander），以探究深海沉积物再悬浮过程。该着陆器搭载多类传感器，用于观测近底流速（near-bottom current velocity）、温度（temperature）、浊度（turbidity）及采集数字视频（digital videos）。研究记录到多起突发性再悬浮事件，通过分析数字视频计算得到悬浮颗粒物（SPM）浓度。深海数字视频分析结果显示，尽管小粒径颗粒物在数量上占据优势，但其对总颗粒体积的贡献远不及大粒径颗粒物。通过分析近底流速与温度变化特征，本研究识别出不同的再悬浮机制：两起短期突发性再悬浮事件由内潮（internal tides）非线性效应产生的上坡传播锋面引发；另一起短期突发性再悬浮事件由下坡传播锋面触发，该锋面可能由内潮斜向传播及流经附近海岬（promontory）的水流共同形成，且此类下坡锋面触发的突发性再悬浮事件强度高于上坡锋面引发的事件。其余长期突发性再悬浮事件均由内潮引发。对温度数据的小波分析（wavelet analysis）表明，再悬浮时段存在显著的次惯性频率（sub-inertial frequency），这与被称为次惯性事件-内重力波（SIE-IGW）模态的动力系统特征一致，且该特征与突发性再悬浮事件密切相关。本研究深化了对深海突发性再悬浮过程的认知，进一步阐明了内潮、海底地形（seafloor topography）与悬浮颗粒物分布之间的复杂相互作用。