Abstract
The manganese nodule belt within the Clarion and Clipperton Fracture Zones (CCZ) in the abyssal NE Pacific Ocean is characterized by numerous seamounts, low organic matter (OM) depositional fluxes and meter-scale oxygen penetration depths (OPD) into the sediment. The region hosts contract areas for the exploration of polymetallic nodules and Areas of Particular Environmental Interest (APEI) as protected areas. In order to assess the impact of potential mining on these deep-sea sediments and ecosystems, a thorough determination of the natural spatial variability of depositional and geochemical conditions as well as biogeochemical processes and element fluxes in the different exploration areas is required. Here, we present a comparative study on (1) sedimentation rates and bioturbation depths, (2) redox zonation of the sediments and element fluxes as well as (3) rates and pathways of biogeochemical reactions at six sites in the eastern CCZ. The sites are located in four European contract areas and in the APEI3. Our results demonstrate that the natural spatial variability of depositional and (bio)geochemical conditions in this deep-sea sedimentary environment is much larger than previously thought. We found that the OPD varies between 1 and 4.5 m, while the sediments at two sites are oxic throughout the sampled interval (7.5 m depth). Below the OPD, manganese and nitrate reduction occur concurrently in the suboxic zone with pore-water Mn2+ concentrations of up to 25 µM. The thickness of the suboxic zone extends over depth intervals of less than 3 m to more than 8 m. Our data and the applied transport-reaction model suggest that the extension of the oxic and suboxic zones is ultimately determined by the (1) low flux of particulate organic carbon (POC) of 1–2 mg Corg m−2 d−1 to the seafloor, (2) low sedimentation rates between 0.2 and 1.15 cm kyr−1 and (3) oxidation of pore-water Mn2+ at depth. The diagenetic model reveals that aerobic respiration is the main biogeochemical process driving OM degradation. Due to very low POC fluxes of 1 mg Corg m−2 d−1 to the seafloor at the site investigated in the protected APEI3 area, respiration rates are twofold lower than at the other study sites. Thus, the APEI3 site does not represent the (bio)geochemical conditions that prevail in the other investigated sites located in the European contract areas. Lateral variations in surface water productivity are generally reflected in the POC fluxes to the seafloor across the various areas but deviate from this trend at two of the study sites. We suggest that the observed spatial variations in depositional and (bio)geochemical conditions result from differences in the degree of degradation of OM in the water column and heterogeneous sedimentation patterns caused by the interaction of bottom water currents with seafloor topography.
Original language | English |
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Pages (from-to) | 159-172 |
Number of pages | 14 |
Journal | Deep-Sea Research Part I: Oceanographic Research Papers |
Volume | 140 |
DOIs | |
Publication status | Published - 1 Oct 2018 |
Funding
We thank captain Lutz Mallon, the crew and the scientific party of RV SONNE cruise SO239 for the technical and scientific support. Thanks to Jennifer Ciomber, Benjamin Löffler and Vincent Ozegowski for their participation in onboard sampling and analysis. For analytical support in the home laboratory and during data evaluation we are grateful to Ingrid Stimac, Olaf Kreft, Dennis Köhler, Ingrid Dohrmann and Dr. Gerhard Kuhn (all at AWI). For providing the bathymetric grids we thank Prof. Dr. Jens Greinert (GEOMAR). Special thanks to Dr. Matthias Haeckel (GEOMAR), Prof. Dr. Gerhard Bohrmann (MARUM, University of Bremen), Dr. Timothy G. Ferdelman (MPI Bremen) and Dr. Gerard Versteegh for much appreciated discussions. We thank the four reviewers for their helpful comments that have improved this manuscript. This study is funded by the German Federal Ministry of Education and Research (BMBF) in the framework of the JPI Oceans project MiningImpact (grant no. 03F0707G ) as part of the JPI-Oceans pilot action EcoMining-DEU – Ecological Aspects of Deep-Sea Mining. Dr. José M. Mogollón was partially funded by the Netherlands Earth System Science Centre (NESSC). We acknowledge further financial support from the Helmholtz Association (Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research). The data are available via the data management portal OSIS-Kiel and the geological data network PANGAEA. Appendix A
Keywords
- Biogeochemical processes
- CCZ
- Deep-sea sediments
- Eastern equatorial Pacific
- Manganese nodule belt
- Oxygen penetration depth
- Redox zonation