Abstract. The thriving interest in harvesting deep-sea mineral resources, such as
polymetallic nodules, calls for environmental impact studies and,
ultimately, for regulations for environmental protection. Industrial-scale
deep-sea mining of polymetallic nodules most likely has severe consequences
for the natural environment. However, the effects of mining activities on
deep-sea ecosystems, sediment geochemistry and element fluxes are still
poorly understood. Predicting the environmental impact is challenging due to
the scarcity of environmental baseline studies as well as the lack of mining
trials with industrial mining equipment in the deep sea. Thus, currently we
have to rely on small-scale disturbances simulating deep-sea mining
activities as a first-order approximation to study the expected impacts on
the abyssal environment. Here, we investigate surface sediments in disturbance tracks of seven
small-scale benthic impact experiments, which have been performed in four
European contract areas for the exploration of polymetallic nodules in the
Clarion–Clipperton Zone (CCZ) in the NE Pacific. These small-scale disturbance experiments
were performed 1 d to 37 years prior to our sampling program in the
German, Polish, Belgian and French contract areas using different
disturbance devices. We show that the depth distribution of solid-phase Mn
in the upper 20 cm of the sediments in the CCZ provides a reliable tool for
the determination of the disturbance depth, which has been proposed in a
previous study from the SE Pacific (Paul et al., 2018). We found that the upper 5–15 cm of the
sediments was removed during various small-scale disturbance experiments in
the different exploration contract areas. Transient transport-reaction
modeling for the Polish and German contract areas reveals that the removal
of the surface sediments is associated with the loss of the reactive labile total organic carbon (TOC) fraction. As a result, oxygen consumption rates decrease significantly
after the removal of the surface sediments, and, consequently, oxygen
penetrates up to 10-fold deeper into the sediments, inhibiting
denitrification and Mn(IV) reduction. Our model results show that the return
to steady-state geochemical conditions after the disturbance is controlled
by diffusion until the reactive labile TOC fraction in the surface sediments
is partly re-established and the biogeochemical processes commence. While
the re-establishment of bioturbation is essential, steady-state geochemical
conditions are ultimately controlled by the delivery rate of organic matter
to the seafloor. Hence, under current depositional conditions, new steady-state geochemical conditions in the sediments of the CCZ are reached only on
a millennium scale even for these small-scale disturbances simulating
deep-sea mining activities.
Deep Sea Mining of Submarine Hydrothermal Deposits and its Possible Environmental Impact in Manus Basin, Papua New Guinea