5:15 PM - 6:30 PM
[MZZ46-P02] Fluxes of sinking particles in the exploration area for cobalt-rich ferromanganese crusts in the Northwest Pacific
Keywords:seamount, sinking particle, ferromanganese crust
To assess the environmental impact of deep-sea mining, it is important to understand natural spatial/temporal variations and their mechanisms. As part of the environmental baseline survey in the exploration area for cobalt-rich ferromanganese crusts in the Northwest Pacific, we have conducted sediment trap observations around five seamounts (JA02, JA03, JA04, JA06, JA17). The sinking particles mainly consisted of biogenic carbonate, and total mass fluxes were low (~5 mg m–2 d–1), which are consistent with the characteristics of the oligotrophic open ocean. The JA17 Seamount, which is located in the northernmost portion of the area (~24Nº), showed relatively elevated total mass fluxes and a seasonal peak corresponding to the spring bloom. The sinking particles also contained higher biogenic opal, suggesting contribution of diatoms. On the other hand, the total mass fluxes at other seamounts showed seasonality with a sharp peak in late summer (August–September), although satellite data indicated the lowest primary production in summer. This discrepancy could be explained by short-lived or subsurface blooms induced by passing typhoons (Yamaoka et al., 2020).
We also measured elemental composition of the sinking particles collected around JA06 Seamount located in the south of the area. The results demonstrated that elemental fluxes in the area are controlled by four major processes: lithogenic (Al, Ti, Fe), carbonate (Mg, Ca, Sr), biogenic (+scavenging) (Ni, Zn, Cd, Pb), and scavenging (V, Mn, Co, Cu, REE). 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. Interestingly, the excess fluxes of Mn (1.3–7.0 µg m–2 d–1) and Co (0.04–0.1 µg m–2 d–1) for sinking particles in this study roughly corresponded to the metal fluxes into ferromanganese crusts (1.9–3.0 µg m–2 d–1 for Mn and 0.03–0.11 µg m–2 d–1 for Co) estimated by Sato and Usui (2018). Recently, Usui et al. (2020) presented evidence of ongoing precipitation of Fe-Mn oxide particles (a few micrometers in diameter) from normal seawater at 900–4500 m depth based on in situ 15-year exposure tests. Such particulate precipitates could be a common source of Mn for both cobalt-rich crusts and sinking particles.
We also measured elemental composition of the sinking particles collected around JA06 Seamount located in the south of the area. The results demonstrated that elemental fluxes in the area are controlled by four major processes: lithogenic (Al, Ti, Fe), carbonate (Mg, Ca, Sr), biogenic (+scavenging) (Ni, Zn, Cd, Pb), and scavenging (V, Mn, Co, Cu, REE). 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. Interestingly, the excess fluxes of Mn (1.3–7.0 µg m–2 d–1) and Co (0.04–0.1 µg m–2 d–1) for sinking particles in this study roughly corresponded to the metal fluxes into ferromanganese crusts (1.9–3.0 µg m–2 d–1 for Mn and 0.03–0.11 µg m–2 d–1 for Co) estimated by Sato and Usui (2018). Recently, Usui et al. (2020) presented evidence of ongoing precipitation of Fe-Mn oxide particles (a few micrometers in diameter) from normal seawater at 900–4500 m depth based on in situ 15-year exposure tests. Such particulate precipitates could be a common source of Mn for both cobalt-rich crusts and sinking particles.