Oceanic bottom mixed layer in the Clarion-Clipperton Zone: potential influence on deep-seabed mining plume dispersal

Abstract

The oceanic bottom mixed layer (BML) is a well mixed, weakly stratified, turbulent boundary layer. Adjacent to the seabed, the BML is of intrinsic importance for studying ocean mixing, energy dissipation, particle cycling and sediment-water interactions. While deep-seabed mining of polymetallic nodules is anticipated to commence in the Clarion-Clipperton Zone (CCZ) of the northeastern tropical Pacific Ocean, knowledge gaps regarding the form of the BML and its potentially key influence on the dispersal of sediment plumes generated by deep-seabed mining activities are yet to be addressed. Here, we report recent field observations from the German mining licence area in the CCZ that characterise the structure and variability of the BML locally. Quasi-uniform profiles of potential temperature extending from the seafloor reveal the presence of a spatially and temporally variable BML with an average local thickness of approximately 250 m. Deep horizontal currents in the region have a mean speed of 3.5 cm s$$^{-1}$$ - 1 and a maximum speed of 12 cm s$$^{-1}$$ - 1 at 18.63 ms above bottom over an 11 month record. The near-bottom currents initially have a net southeastward flow, followed by westward and southward flows with the development of complex, anticyclonic flow patterns. Theoretical predictions and historical data show broad consistency with mean BML thickness but cannot explain the observed heterogeneity of local BML thickness. We postulate that deep pressure anomalies induced by passing surface mesoscale eddies and abyssal thermal fronts could affect BML thickness, in addition to local topographic effects. A simplified transport model is then used to study the influence of the BML on the interplay between turbulent diffusion and sediment settling in the transport of deep-seabed mining induced sediment plumes. Over a range of realistic parameter values, the effects of BML on plume evolution can vary significantly, highlighting that resolving the BML will be a crucial step for accurate numerical modelling of plume dispersal.

Turbulent diapycnal fluxes as a pilot Essential Ocean Variable

“…generate benthic plumes of sediment, and other biogeochemical factors (e.g., dissolved heavy metals), that have the potential to negatively impact the ocean environment (Peacock and Ouillon, 2023). Recent modeling studies identified turbulent vertical mixing as a critical parameter that influences model predictions of plume extent, which will be the basis of decision-making by regulators (Chen et al, 2023).…”

What Causes Excess Deepening of the Sediment Mixed Layer in the Deep Ocean?

“…However, there is no evidence showing that benthic storms mix sediment to depths greater than 20 cm (DeMaster et al., 1985; Volz et al., 2020). Observational records indicate that benthic storms and sediment resuspension events, due to abyssal benthic flows, are relatively weak in the manganese nodule zone compared with other places around the world ocean (Chen et al., 2023). Bottom current speeds measured over nodule fields are too low (<5 cm s −1 ) to erode sediments (Dutkiewicz et al., 2020), indicating that the enhanced sediment mixing, observed in the CCZ during the post exploration period, was not caused by natural ocean dynamic processes, further supporting evidence for anthropogenic disturb.…”

Intensified Currents Associated With Benthic Storms Underneath an Eddying Jet

“…With rising interests in deep‐sea mining and marine carbon dioxide removal (S.‐Y. S. Chen et al., 2023; Miller et al., 2018; Peacock & Ouillon, 2023; Ryabinin et al., 2019), there is also a pressing need to understand transient motions in the deep ocean.…”