Impact of depositional and biogeochemical processes on small scale variations in nodule abundance in the Clarion‐Clipperton Fracture Zone

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Highlights

  • We evaluated the pore-water geochemistry with respect to nodule coverage in the CCFZ.

  • We found that hydrogenetic and oxic-diagenetic processes control the present-day nodule growth.

  • Surface sediments covered with big or medium-sized nodules are enriched in mobilizable Mn.

Abstract

Manganese nodules of the Clarion–Clipperton Fracture Zone (CCFZ) in the NE Pacific Ocean are highly enriched in Ni, Cu, Co, Mo and rare-earth elements, and thus may be the subject of future mining operations. Elucidating the depositional and biogeochemical processes that contribute to nodule formation, as well as the respective redox environment, in both water column and sediment, supports our ability to locate future nodule deposits and to evaluate the potential ecological and environmental effects of future deep-sea mining. For these purposes we studied the local hydrodynamics and pore-water geochemistry with respect to the nodule coverage at four sites in the eastern CCFZ. Furthermore, we carried out selective leaching experiments at these sites in order to assess the potential mobility of Mn in the solid phase, and compared them with the spatial variations in sedimentation rates. We found that the oxygen penetration depth is 180–300 cm at all four sites, while reduction of Mn and NO3 is only significant below the oxygen penetration depth at sites with small or no nodules on the sediment surface. At the site without nodules, potential microbial respiration rates, determined by incubation experiments using 14C-labeled acetate, are slightly higher than at sites with nodules. Leaching experiments showed that surface sediments covered with big or medium-sized nodules are enriched in mobilizable Mn. Our deep oxygen measurements and pore-water data suggest that hydrogenetic and oxic-diagenetic processes control the present-day nodule growth at these sites, since free manganese from deeper sediments is unable to reach the sediment surface. We propose that the observed strong lateral contrasts in nodule size and abundance are sensitive to sedimentation rates, which in turn, are controlled by small-scale variations in seafloor topography and bottom-water current intensity.

Introduction

Manganese nodules are polymetallic concretions consisting mainly of manganese (~26–30%) and iron (~6–7%), but also including economically significant amounts (~3% combined) of nickel, copper, cobalt, lithium, molybdenum, zirconium, and rare earth elements (Cronan, 1980, Halbach et al., 1988, Rühlemann et al., 2011, Hein et al., 2010, Hein et al., 2013). They form and grow over long timespans (106 yr) on the surface of marine sediments by the sorption of manganese and iron around a nucleus. Due to their long formation times, manganese nodules represent important paleoceanographic archives (Ito et al., 1998, Frank et al., 2002, Han et al., 2003, Ito and Komuro, 2006), and due to their composition, may be considered economical for metal mining (Mero, 1965, Clark and Neutra, 1983, Koschinsky et al., 2003, Morgan, 2012, Hein and Koschinsky, 2013, Hein et al., 2013). They are typically 1–5 cm wide, but can reach sizes of up to 12 cm in diameter.

Manganese nodules occur throughout the global ocean. The most extensive deposits have been found in the Clarion–Clipperton Fracture Zone (CCFZ), the Peru Basin (PB) and the Penrhyn Basin in the Pacific Ocean, as well as in the Central Indian Ocean Basin (CIOB) (e.g., Glasby et al., 1982, Pattan and Mudholkar, 1990, von Stackelberg, 1997, Koschinsky et al., 2001, Glasby, 2006). Nevertheless, nodules have been found in diverse locations as the Argentine Basin, the Cape Basin, the south western Atlantic and the Circum-Antarctic Ocean (e.g., Dietrich, 1988, Albarede et al., 1997, Kasten et al., 1998). The existence of manganese nodules has been known for over 150 yr, but detailed investigations of their growth processes and spatial abundance did not take place until the 1980s with missions such as the manganese nodule project (MANOP) in the Eastern Pacific (e.g., Emerson et al., 1980, Klinkhammer, 1980, Emerson et al., 1982, Lyle et al., 1984). During the project disturbance and recolonization experiment (DISCOL) a disturbance of a 10.8 km2 area in the southeastern Pacific and the Hawaii-Tahiti Transect in the South Pacific was conducted in early 1989 (Andrews et al., 1983). This program had the objective of simulating the potential disturbance effects of manganese nodule mining and the influence of the dispersing sediment cloud during the dredging process on the bottom-dwelling biota. Seven years later the former “DISCOL area” was examined again in the framework of the ATESEPP project (effects of technical interventions into the ecosystem of the deep sea in the Southeast Pacific Ocean). The aim was to investigate how benthic macrofaunal communities developed after the disturbance experiment (e.g., Borowski and Thiel, 1998, Becker et al., 2001, Borowski, 2001). Between 1996 and 1998, sedimentological, geochemical, hydrographic and ecological studies, relevant for environmental impact assessment studies of polymetallic nodule mining, were undertaken during the project TUSCH (e.g., Haeckel et al., 2001, Oebius et al., 2001, Thiel and Tiefsee-Umweltschutz, 2001). Furthermore, in the Penrhyn Basin, Cronan et al. (2010) investigated manganese nodule substrates in comparison with those of the PB and CCFZ, partly to provide a background to future manganese nodule mining activities in that area.

Intensive studies in the CCFZ, concerning nodule composition and sediment geochemistry, were performed and presented by von Stackelberg and Beiersdorf (1982) and Halbach et al. (1988). These and other investigations have revealed that the metal source for the manganese nodules may be either seawater (hydrogenetic growth) and/or surface sediments as well as the underlying porewater (oxic and suboxic diagenetic growth). Although most manganese nodules throughout the world oceans form by a combination of these processes, certain locations are currently characterized specifically by hydrogenetic and oxic-diagenetic (e.g., the CCFZ) or by suboxic diagenetic (e.g., the PB) growth (e.g., Halbach et al., 1988, Müller et al., 1988, Koschinsky, 2001, Stummeyer and Marchig, 2001, Glasby, 2006).

Studies, relating the sediment biogeochemistry to nodule growth and abundance, have been mostly carried out in environments dominated by suboxic diagenetic growth, for instance the PB (Haeckel et al., 2001). In the PB, intensive investigations of the redox zonation (e.g., oxygen penetration depth), the sediment geochemistry, and the impact of mining on the sedimentary environment were performed (e.g., Becker et al., 2001; Borowski, 2001; Haeckel et al., 2001; Koschinsky et al., 2001; Koschinsky et al., 2003). However, in contrast to the investigations and experiments performed in the PB, the geochemical zonation in the CCFZ has only been the subject of a few studies (e.g., Halbach et al., 1988, Müller et al., 1988). Although in the western CCFZ seafloor disturbance experiments in areas with hydrogenetic and suboxic diagenetic nodules have been performed during the French Nodinaut cruise in 2004 (Khripounoff et al., 2006), geochemical analyses in the CCFZ are so far insufficient to assess the impact of future mining operations on the sedimentary geochemical zonation. In light of increasing commodity prices and the contracting of exploration license areas in the CCFZ, a renewed push towards understanding the possible effects of mining/extraction on the sedimentary geochemical zonation and on deep-sea habitats is of imminent importance.

In this study, we examined two areas (one close to and one more distant to a seamount) within the eastern CCFZ (Fig. 1). In each area two long sediment cores were recovered from neighboring sites, characterized by contrasting nodule size and abundance. The sediment geochemistry (including high resolution deep oxygen measurements, pore-water and solid-phase analyses) at these four sites is evaluated and compared, in order to assess the local sedimentary geochemical zonation that governs the various nodule populations. To our knowledge, this is the first extensive study performed in the CCFZ which relates the seafloor nodule size and abundance to the biogeochemistry of the upper 10 m of sediment. To point out the effects of sediment accumulation and local variations in bottom water current velocities on nodule coverage, we determined the content of mobilizable manganese, and performed sediment grain-size analyses.

Section snippets

Geological and oceanographic setting

The Manganese Nodule Belt of the CCFZ is located in the north-eastern equatorial Pacific Ocean between the Clarion (north) and Clipperton (south) fractures, which emanate from the East Pacific Rise (EPR) in the east, and drift with a velocity of 16 cm yr-−1 north-westwards (Wessel et al., 2006). The area of approximately 4.5×106 km2 stretches ~5000 km between 116 °W and 155 °W and ~1000 km between 5 °N and 15 °N. The upper water column of the CCFZ is characterized by a broad and very pronounced oxygen

Material and methods

The sediment cores were retrieved during RV SONNE cruise SO-205 in April/May 2010 in the German license area “East”, situated in the equatorial northeastern Pacific Ocean (Rühlemann et al., 2010). Four sites were investigated in two working areas (Fig. 1), which differ significantly with respect to nodule abundance and nodule size. The sampling area in the western part of license area “East” is characterized by a topographical depression situated immediately east of a 1500 m high seamount and is

Results

Nodule abundance is highest at site A5-1-BN (29.9 kg m−2) and lowest at site A1-2-NN (0.16 kg m−2). At site A5-2-SN (18.8 kg m−2) nodule abundance is slightly higher than at site A1-1-MN (12.8 kg m−2) (Fig. 4). Oxygen profiles for all sites are shown in Fig. 5, depth profiles for pore-water constituents and respiration rates in Fig. 6. Solid-phase constituents are displayed in Fig. 7.

Geochemical zonation and biogeochemical processes

In the upper 50 cm the ex situ oxygen concentration profiles obtained in this study agree with oxygen measurements performed in the central equatorial Pacific Ocean by Murray and Grundmanis (1980), Hammond et al. (1996) and a site in the CCFZ (Khripounoff et al., 2006). Whereas Hammond et al. (1996) and Khripounoff et al. (2006) performed in situ O2 measurements by use of a benthic chamber, Murray and Grundmanis (1980) measured O2 on pore-water samples retrieved using an in situ pore-water

Conclusion

Four sites were investigated in two working areas within the Clarion–Clipperton Fracture Zone, one area with and one without a seamount nearby. All four sites differed significantly with respect to nodule abundance and nodule size. Results of the sediment and pore-water analyses indicate that the sites without or with small nodules are characterized by higher sedimentation rates, lower contents of leachable manganese and higher rates of nitrate and manganese oxide reduction below the oxygen

Acknowledgment

We thank the captain and crew of RV SONNE during cruise SO-205. We are indebted to Prof. Dr. Wiebke Ziebis (University of Southern California, Los Angeles) for the supply of and support with the equipment for oxygen measurements. We would like to thank Ingrid Stimac, Ludmila Baumann, Dr. Sven Kretschmer, Olaf Kreft and Jennifer Ciomber (all AWI Bremerhaven), as well as Dr. Reiner Dohrmann and Irene Bitz (BGR) for their great support in the laboratory, and Jan Hansen for his support onboard RV

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    1

    Present address: Department of Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA-02138, USA.

    2

    Present address: Department of Geosciences – Geochemistry, Utrecht University, P.O. Box 80.021, 3508TA Utrecht, Netherlands.

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