Vertical Distribution of Particulate Matter in the Clarion Clipperton Zone (German Sector)—Potential Impacts From Deep-Sea Mining Discharge in the Water Column

Benjamin Gillard,Rob P Harbour,Laurenz Thomsen,Nicolas Nowald,Morten H Iversen

doi:10.3389/fmars.2022.820947

Abstract

Most studies on the potential impacts of deep-sea mining in the Clarion Clipperton Zone (CCZ) have largely focused on benthic ecosystems but ignore the pelagic environment. To model full-scale impacts, it is important to understand how sediment discharge might affect the pelagic zone as well. This study combines in situ optics, hydrography, and remote sensing to describe particle abundance and size distribution through the entire water column in the CCZ (German sector). CCZ surface waters were characterized as productive over the year. During the winter, we observed the formation of a sharp transition zone in Chla concentration, identifying the area as a productive transitional zone toward a more depleted ocean gyre. In the German sector, median particle size was small (± 77 μm), and large particles (&gt;300 μm) were rare. By assessing particle flux attenuation, we could show that the presence of a thick oxygen minimum zone (OMZ) plays an essential role in export and transformation of settling aggregates, with strong diel variations. We suggest that the combination of small aggregate size, bottom currents and slow seafloor consolidation may explain the extremely low sedimentation rate in the CCZ. We conclude that sediment incorporations and ballasting effect on settling particulate matter represent the most significant hazard on midwater and benthic ecosystems.

Highlights

A growing demand for metals and rare earth elements for use in frontier technologies is driving accelerated research and development into viable deep-sea mining solutions (Miller et al, 2018)
Polymetallic nodules may form on Clarion Clipperton Zone (CCZ) Particulate Matter Vertical Distribution the abyssal seafloor at depths between 4,000 and 6,000 m when the water is well oxygenated, and the sedimentation rate is less than 10 mm·kyr−1 (Gollner et al, 2017)
This study aims to: (i) provide an overview of the oceanographic background of the area, (ii) describe how particulate matter is distributed in the water column, (iii) associate fundamental transport or alteration processes based solely on particle size-distribution and abundance variations through the water column and (iv) reappraise the risk of mining waste released into the pelagic environment

Summary

Introduction

A growing demand for metals and rare earth elements for use in frontier technologies is driving accelerated research and development into viable deep-sea mining solutions (Miller et al, 2018). The International Seabed Authority has awarded 16 licenses for mining exploration in the CCZ, covering a 1 million km area of seabed (ISA, 2019). The collection of nodules will be carried out by autonomous mining crawlers (Figure 2), resulting in sediment plumes released from the seafloor by the crawler’s movement and harvesting activities (Gillard et al, 2019). Based on an estimation of 260 days per year of operation, a mining vessel could release up to 1.2 × 108 kg of sediment in the water column (Muñoz-Royo et al, 2021). After release and dilution of mining plumes, increased inorganic nutrients and trace metal concentrations in the surface water might result in a shift in the planktonic community (e.g., diatoms) and affect the entire food web structure (Hyun et al, 1998)

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