Will Seabed Mining Increase Climate Change?

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Gerald McCormack, Natural Heritage Trust
First published CI News 26 Sept. 2019, modified 10 Feb. 2021.

South Penrhyn Basin nodules above the sediment

In July 2019, the Greenpeace report “In Deep Water” warned: “By impacting on natural processes that store carbon, deep sea mining could even make climate change worse by releasing carbon stored in deep sea sediments or disrupting the processes which ……. deliver it to those sediments. Deep sea sediments are known to be an important long-term store of ‘blue carbon’, the carbon that is naturally absorbed by marine life, a proportion of which is carried down to the sea floor as those creatures die.”

On the 19th September 2019, an article in the Cook Islands News echoed the same warning: “carbon stored in deep sea sediment, once disrupted by deep sea mining equipment, can be released and exacerbate the negative impacts of climate change.” And in the context, there was a strong implication that this would be the case in the Cook Islands.

In 2016, I wrote a booklet for the Natural Heritage Trust: “Cook Islands Seabed Minerals – a precautionary approach to mining”. The idea was to see if more locally focused scientific information would raise the level of debate about nodule mining above “it could” versus “it won’t”. The release of carbon was not included in the booklet, and I would like to correct that oversight.

It is important to understand I am not concerned with what might happen in other countries or areas. And, within the Cook Islands, I am focused on the central South Penrhyn Basin (SPB), which is the area with our most significant concentration of polymetallic nodules.

Inorganic carbon deposition

Cross-section of the Manihiki Plateau and the South Penrhyn Basin showing the key features and also a graph of water density.

The South Penrhyn Basin (SPB) is immediately east of the immense and high Manihiki Plateau.

The plateau formed about 120Ma (million years ago). Since then it has slowly subsided to be about 2,500mbsl (metres below sea level). Since its formation the calcareous skeletons of dead plants and animals have been raining down upon it. Today those skeletons have consolidated into a calcareous cap, up to one kilometre thick. The plateau is a significant depository for inorganic carbon.

The SPB is about 110Ma. The nodule-covered seafloor is about 5,200mbsl, which is below the Carbonate Compensation Depth, causing all the sinking calcareous skeletons to dissolve before they reach the seafloor. Without any calcareous sediment, the SPB seafloor is a reddish volcanic sediment, upon which polymetallic nodules have been growing over the last 20Ma.

The SPB deep-seabed is not a significant depository for inorganic carbon.

Organic carbon deposition

Microscopic plant plankton in the top 200m of water capture carbon dioxide to grow, and they are eaten by animal plankton which in turn are eaten by larger animals. When the plant plankton and the pelagic animals die, they can sink to the seafloor and this organic carbon enables the growth the seafloor animals and bacteria. Eventually some of this carbon can be locked into the sediment and removed from circulation.

In our particular case, the very low nutrient levels in the SPB means it supports a very low quantity of microscopic plant plankton, leading to low levels of animal plankton and other pelagic animals. Thus, there is a minimal amount of organic remains, known as Particulate Organic Carbon (POC), to sink down to feed seafloor animals or to be stored in the sediment.

Okamoto (2003)((OKAMOTO (2003) Summary Report on the Japan/SOPAC Deep-sea Cooperative Deep-sea Mineral Resources Study Programme, R/V Hakurei-Maru No.2 Cruise for Polymetalic Manganese Nodules, the EEZ of the Cook Islands, SOPAC Technical Report 359.)) reported the Japanese research on the sediment in the main nodule area of the SPB. They found a very low quantity of animals, as we would expect from the low level of POC flux, and 80% of the tiny animals lived in the top 1cm (one centimetre) of sediment.

In the sediment they found very low concentrations of organic carbon at 0.4% or less, which was slightly less than the level in the CCZ (Clarion-Clipperton Zone) in the tropical East Pacific, which has a higher POC flux.

In the CCZ, Sweetman et al. (2019)((Sweetman et al. (2019) Key role of bacteria in the short-term cycling of carbon at the abyssal seafloor in a low particulate carbon flux region of the eastern Pacific Ocean. Limnol. Oceanogr. 64:694-713.)) showed that bacteria are much more important in assimilating carbon from POC flux than the tiny animals and, furthermore, bacteria can slightly increase organic carbon by using some dissolved inorganic carbon (DIC) from the water. In the SPB we would also expect bacteria to be dominant in processing the small amount of POC flux on the seafloor, and that they would convert some DIC into organic carbon.

Although there is a very small amount of organic carbon in the sediment, we would expect some to be mobilised in any sediment plume and should consider its fate. Experiments and models of such plumes show that almost all the sediment resettles within a few kilometres. Furthermore, because of the high density of the bottom current, the Antarctic Bottom Water (AABW), the plume would not rise more than a few hundred metres above the seafloor. This seabed mobilised organic carbon will not reach the surface waters.

In contrast to the seabed plume, the release of any uplifted sediment at the surface would pose a number of problems. In the 2016 booklet I strongly recommended that all uplifted sediment and nutrient rich abyssal water be returned to the seafloor or, at least, to a great depth. The main concern here was the effects of nutrients and sediments on water-column biodiversity, rather than the release of carbon dioxide, which would be negligible.

The SPB is not a significant depository for organic carbon.

Dissolved inorganic carbon

The SPB has dissolved inorganic carbon in the immense seabed current, the Antarctic Bottom Water (AABW).

Globally the oceans absorb about 30% of the CO2 generated by human activity (“anthropogenic CO2”), and the Southern Ocean is particularly important, absorbing about 40% of this oceanic uptake (Murata et al. 2019)((Murata et al. (2019) Decadal-Scale Increases of Anthropogenic CO2 in Antarctic Bottom Water in the Indian and Western Pacific Sectors of the Southern Ocean. Geophysical Research Letters, 46, 833-841)). The CO2 absorbed near Antarctica is carried northward by seafloor currents, especially the AABW into the Pacific. Much of the AABW flows on the seabed through the SPB and its high oxygen content is a key factor in the unusually high abundance of nodules.

The process of harvesting nodules on the seabed will not release dissolved carbon, but any abyssal water brought to the surface would release some of its dissolved inorganic carbon as CO2. Although the released CO2 would be very small and have no significant effect on the climate, it is important to return abyssal water to the depths because of its high nutrient load.


The present scenarios for seabed mining in the SPB would not release significant amounts of deposited inorganic carbon, deposited organic carbon or dissolved carbon, and would not have a significant effect on the climate crisis. This is mainly because of the nature of the SPB environment, rather than anything special about the mining scenarios.

Despite the predicted trivial effect on climate change, the precautionary principle applies. Mining proposals should report on how their activities might alter the distribution of carbon, so this can be considered in the EIA process before mining is approved.


McCormack, Gerald. (2021). Will Seabed Mining Increase Climate Change? Cook Islands Natural Heritage Trust. https://cinature.org/2021/02/10/will-seabed-mining-increase-climate-change/


Gerald McCormack

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Gerald has worked on Cook Islands marine and terrestrial biodiversity since 1980. He was the foundation Director of the Natural Heritage Trust from 1990 to 2020.

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