Abstract
We measured sedimentation fluxes around JA06 Seamount (Xufu Guyot; 19°30′N, 158°00′E) as part of an environmental baseline survey in Japan’s exploration area for cobalt-rich crusts in the subtropical Northwest Pacific. Sinking particles were collected at the flat top (sediment trap depths: 900 and 1000 m) and northeastern base (sediment trap depths: 1000 and 4720 m) of the seamount from June 2016 to April 2017. Total mass fluxes were very low, with average values of 4.3–4.9 mg m–2 d–1 and 9.3 mg m–2 d–1 in the shallow traps at the flat top and base, respectively, which is consistent with an oligotrophic system. The lower fluxes at the flat top probably reflect lower productivity of siliceous microplankton, such as diatoms. However, we were unable to substantiate any potential mechanisms for this difference in productivity and cannot evaluate whether this is representative of typical conditions. When combined with previous observations at two adjacent seamounts, our results indicate widespread seasonality in sediment fluxes with a peak in late summer (August–September). However, satellite data indicate that summer is the season with the lowest primary production. This discrepancy could be explained either by phytoplankton blooms fueled by symbiotic nitrogen fixation that only cause minor increases in surface-ocean chlorophyll or short-lived blooms induced by passing typhoons under thick cloud cover. At the base site, we also analyzed material and element transport rates from shallow to deep waters. Half of the organic matter and >80% of the carbonate in sinking particles was not degraded in the water column, suggesting that most of the regeneration of these materials occurs near or on the sediment surface. Furthermore, four major processes appeared to control elemental fluxes in the area: lithogenic (Al, Ti, Fe), carbonate (Mg, Ca, Sr), biogenic (+scavenging) (Ni, Zn, Cd, Pb), and scavenging (V, Mn, Co, Cu, rare earth elements) processes. The estimation of excess flux based on the composition of upper continental crust demonstrated that >85% of total Mn, Co, Ni, Cu, Zn, Cd, and Pb fluxes were attributable to scavenging (+biogenic uptake). Scavenging-dominant metal fluxes are likely ubiquitous in the oligotrophic open ocean.
Highlights
Deep-sea mining has attracted increased interest in recent years due to rising demand for metallic elements that are crucial for cutting-edge and green technologies
According to CTD measurements taken around the seamount in May 2016, surface water temperature and salinity are 27.7°C and 34.8, respectively; there were no marked differences between water column profiles taken at the flat top and base (Yamaoka et al, submitted)
The main findings obtained in this study are as follows: (1) Total mass fluxes on the seamount flat top were half of those on the base throughout the year
Summary
Deep-sea mining has attracted increased interest in recent years due to rising demand for metallic elements that are crucial for cutting-edge and green technologies. One potential target for deep-sea mining is cobalt-rich ferromanganese crusts, which are marine polymetallic mineral deposits composed of hydrogenetic manganese oxide layers that slowly precipitate on seamount basement rocks. The subtropical Northwest Pacific is considered a prime area for exploration because of the presence of thick crusts on old seamount groups. On the high seas of the northwest Pacific, several countries including Japan, China, Korea, and Russia have already signed exploration contracts for cobalt-rich crusts with the International Seabed Authority (ISA). To minimize environmental impacts from any future deep-sea mining, careful environmental surveys, and assessments are essential. The ISA environmental guidelines (ISBA/25/LTC/6) mandate the study of six environmental baseline data groups: 1) physical oceanography, 2) chemical oceanography, 3) geological properties, 4) biological communities, 5) bioturbation, and 6) fluxes to the sediment
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