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.

深入理解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（Shenhu Canyon）趾部水深1450米处布设着陆器（lander），以开展深海沉积物再悬浮过程的相关研究。该着陆器搭载多类传感器，可测量近底流速、温度、浊度并采集数字视频。研究期间记录到多起突发性再悬浮（burst resuspension）事件。通过对数字视频的分析，可计算得到悬浮颗粒物（suspended particular matter, SPM）浓度。深海数字视频分析结果显示，部分时段细颗粒在数量上占优，但相较于粗颗粒，其对总颗粒体积的贡献并不显著。通过分析近底流速与温度变化的特征，可识别出不同的再悬浮机制。两起短期突发性再悬浮事件，归因于内潮（internal tides）非线性效应所引发的上坡传播锋面。另有一起短期突发性再悬浮事件由下坡传播锋面触发，该锋面大概率由内潮斜向传播及流经附近海岬的海流共同作用形成。该下坡传播锋面触发的短期突发性再悬浮事件，其强度高于上坡传播锋面引发的同类事件。其余长期突发性再悬浮事件则由内潮诱发。对温度数据的小波分析结果显示，再悬浮时段存在显著的亚惯性频率特征，这与被称为亚惯性事件-内重力波（sub-inertial event and internal gravity wave, SIE-IGW）模态的动力状态相吻合，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环境中突发性再悬浮过程的认知，进一步增进了我们对内潮、海底地形（seafloor topography）与悬浮颗粒物分布之间复杂相互作用的理解。