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Controls on the chemical composition of ferromanganese nodules in the Clarion-Clipperton Fracture Zone, eastern equatorial Pacific

Controls on the chemical composition of ferromanganese nodules in the Clarion-Clipperton Fracture Zone, eastern equatorial Pacific
Controls on the chemical composition of ferromanganese nodules in the Clarion-Clipperton Fracture Zone, eastern equatorial Pacific

Ferromanganese nodules have long been considered an attractive source for Mn, Co, Ni and Cu and, potentially, for the rare earth elements and yttrium (REY). Sixteen exploration contracts have recently been granted for the Clarion-Clipperton Fracture Zone (CCFZ) in the eastern equatorial Pacific and, because of concern about the environmental impacts of seafloor mining, some parts of the CCFZ have been designated ‘Areas of Particular Environmental Interest’ (APEI) from which mining will be excluded. As the APEIs were selected based on surface ocean characteristics and seafloor topography, estimated from satellite altimetry, the abundance and composition of nodules in these areas are unknown, and as a result their relevance as an intended baseline for mining disturbance has not been demonstrated. To fill this gap, we have undertaken a detailed study of the chemical composition of nodules and seafloor sediments in the UK Contract area in the CCFZ and its closest APEI, APEI-6. There are distinct differences between the two sites: nodules from the UK Contract area are larger (10–15 cm in diameter) compared to those from APEI-6 (2–4 cm diameter), and they have faster growth rates (~11 vs ~3 mm/Ma). Nodules from APEI-6 have, on average, higher contents of Fe (7.2 vs. 5.7 wt%), Co (0.28 vs. 0.13 wt%) and total REY (1120 vs. 715 ppm) relative to those from the UK Contract area. Analyses of individual growth layers reveal that nodules from the UK Contract area contain a greater proportion of Mn-rich phases that are also enriched in Ni and Cu. Although pore waters in the uppermost sediments are oxygen replete today, these layers likely precipitated from suboxic sediment pore waters indicating that the supply of organic material to the sediments in this area has been higher in the past. Sediments from the UK Contract area, in turn, have higher Mn/Al ratios compared to sediments from APEI-6 and have a higher proportion of Mn in easily-reducible phases. Nodules from APEI-6 have a greater proportion of Fe-rich layers that are also enriched in Co and the REY, and likely precipitated from seawater. These differences in the chemical composition of nodules from the UK Contract area and APEI-6, as well as differences in the oxygen penetration depth in the sediments, mean that APEI-6 does not provide a relevant geochemical baseline for mining disturbance in the UK Contract area, or for many other contract areas within the CCFZ.

APEI-6, Equatorial Pacific, Manganese nodules, Suboxic diagenesis, UK Contract area
0025-3227
1-14
Menendez, Amaya
378704ee-93ff-478f-ba13-83177846dd9c
James, Rachael H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Lichtschlag, Anna
be1568d9-cc63-4f85-bd38-a93dfd7e245f
Connelly, Douglas
d49131bb-af38-4768-9953-7ae0b43e33c8
Peel, Kate
77f319ec-6c69-4f96-b184-ed67bf8a1204
Menendez, Amaya
378704ee-93ff-478f-ba13-83177846dd9c
James, Rachael H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Lichtschlag, Anna
be1568d9-cc63-4f85-bd38-a93dfd7e245f
Connelly, Douglas
d49131bb-af38-4768-9953-7ae0b43e33c8
Peel, Kate
77f319ec-6c69-4f96-b184-ed67bf8a1204

Menendez, Amaya, James, Rachael H., Lichtschlag, Anna, Connelly, Douglas and Peel, Kate (2019) Controls on the chemical composition of ferromanganese nodules in the Clarion-Clipperton Fracture Zone, eastern equatorial Pacific. Marine Geology, 409, 1-14. (doi:10.1016/j.margeo.2018.12.004).

Record type: Article

Abstract

Ferromanganese nodules have long been considered an attractive source for Mn, Co, Ni and Cu and, potentially, for the rare earth elements and yttrium (REY). Sixteen exploration contracts have recently been granted for the Clarion-Clipperton Fracture Zone (CCFZ) in the eastern equatorial Pacific and, because of concern about the environmental impacts of seafloor mining, some parts of the CCFZ have been designated ‘Areas of Particular Environmental Interest’ (APEI) from which mining will be excluded. As the APEIs were selected based on surface ocean characteristics and seafloor topography, estimated from satellite altimetry, the abundance and composition of nodules in these areas are unknown, and as a result their relevance as an intended baseline for mining disturbance has not been demonstrated. To fill this gap, we have undertaken a detailed study of the chemical composition of nodules and seafloor sediments in the UK Contract area in the CCFZ and its closest APEI, APEI-6. There are distinct differences between the two sites: nodules from the UK Contract area are larger (10–15 cm in diameter) compared to those from APEI-6 (2–4 cm diameter), and they have faster growth rates (~11 vs ~3 mm/Ma). Nodules from APEI-6 have, on average, higher contents of Fe (7.2 vs. 5.7 wt%), Co (0.28 vs. 0.13 wt%) and total REY (1120 vs. 715 ppm) relative to those from the UK Contract area. Analyses of individual growth layers reveal that nodules from the UK Contract area contain a greater proportion of Mn-rich phases that are also enriched in Ni and Cu. Although pore waters in the uppermost sediments are oxygen replete today, these layers likely precipitated from suboxic sediment pore waters indicating that the supply of organic material to the sediments in this area has been higher in the past. Sediments from the UK Contract area, in turn, have higher Mn/Al ratios compared to sediments from APEI-6 and have a higher proportion of Mn in easily-reducible phases. Nodules from APEI-6 have a greater proportion of Fe-rich layers that are also enriched in Co and the REY, and likely precipitated from seawater. These differences in the chemical composition of nodules from the UK Contract area and APEI-6, as well as differences in the oxygen penetration depth in the sediments, mean that APEI-6 does not provide a relevant geochemical baseline for mining disturbance in the UK Contract area, or for many other contract areas within the CCFZ.

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More information

Accepted/In Press date: 10 December 2018
e-pub ahead of print date: 15 December 2018
Published date: 1 March 2019
Keywords: APEI-6, Equatorial Pacific, Manganese nodules, Suboxic diagenesis, UK Contract area

Identifiers

Local EPrints ID: 428972
URI: https://eprints.soton.ac.uk/id/eprint/428972
ISSN: 0025-3227
PURE UUID: c48adb40-b7cd-4e59-99e6-4fcaad17e2d9

Catalogue record

Date deposited: 15 Mar 2019 17:30
Last modified: 15 Mar 2019 17:30

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Contributors

Author: Amaya Menendez
Author: Anna Lichtschlag
Author: Douglas Connelly
Author: Kate Peel

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