An in situ Investigation of Sediment Hydrodynamics for Advancing Seabed Mining Plume Monitoring and Modeling

Abstract

The potential of seabed mining for mineral resources, vital in transitioning to a metals-based economy, is rapidly gaining momentum and is poised to materialize within the next decade. A pivotal step in this process requires establishing seabed exploitation regulations that call for a proactive scientific approach to assess potential disturbances to the benthic environment. A critical environmental concern, in particular, revolves around sediment plumes formed during the extraction of deposits from the top sediment layer of the ocean floor. This thesis seeks to provide timely insights into the behavior of these plumes through an in-depth in situ investigation into the properties of deep-sea sediment.

To achieve this goal, a cutting-edge instrument has been developed to provide real-time measurements of size and settling velocity distributions in the abyssal deep-ocean environment. This instrument was deployed during two pilot deep-sea mining trials at a depth of 4500 meters in the Clarion Clipperton Fracture Zone in the Pacific. Throughout these trials, unprecedented sediment data was collected, offering a unique insight into suspended sediment generated from deep-sea nodule collector vehicle operations.

The suspended sediment grain size and other key particle shape characteristics are observed to be influenced by the nature of the hydrodynamic processes that generate the sediment in suspension. The data collected in situ not only revealed marked differences from previous laboratory-based measurements but also challenged the significance of particle size in dictating sediment settling rates. This implies that ex-situ size measurements and single parametrization-based settling velocity estimates are inadequate for evaluating sinking sediment flux, which is crucial in determining the extent of seabed mining plumes.

The broader implications of these findings extend to monitoring seabed mining sediment plumes, emphasizing the imperative for well-informed in situ measurements rooted in an understanding of the complex hydrodynamic processes at play for a nuanced parameterization of sediment plume transport models.