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[SCG55-01] Estimation of REE discharge fluxes from pelagic sediments to the ocean via pore water on the Shatsky Rise, western North Pacific Ocean
Keywords:Pelagic sediments, Pore water, Rare-earth elements, Shatsky Rise
With the growing global trend to achieve carbon neutrality, deep-sea sediments that have abundant rare-earth elements (REEs), which are essential for low-carbon technologies, are attracting attention (Kato et al., 2011). In order to narrow down promising areas for development of the REE-rich mud, which is widely distributed on the seafloor, it is important to understand the genesis of REE-rich muds. To this end, various approaches have been taken so far.
Yasukawa et al. (2016) showed that REEs in REE-rich muds are derived from seawater, and that the bulk sedimentation rate must be sufficiently small for the formation of REE-rich muds by calculating the flux of REEs deposited from seawater to the seafloor. Matsunami et al. (2024) constructed an ocean-sediment Nd mass balance model and conducted a sensitivity analysis of environmental factors that could be related to the grade of REE-rich muds. The results show that, in addition to sedimentation rate, Nd flux from continental margin to the ocean have a significant effect on the grade of REE-rich muds.
However, how much REEs are discharged from sediments to the ocean in the pelagic region remains poorly constrained based on observation. In this study, we analyzed the chemical composition of sediments and pore water of three multiple core samples (MC01-03) collected from the southwest slope of the Tamu Massif, Shatsky Rise, in the western North Pacific Ocean during the KH-24-1 cruise (April 2-19, 2024, Chief Researcher: Erika Tanaka). Major lithology of each core was nannofossil ooze in MC01 (3,890 m water depth) and clay in MC02 (4,352 m water depth) and MC03 (5,673 m water depth). Pore water was collected onboard using a Rhizon sampler. Sediments were sampled onboard every 1-2 cm after pore water sampling. XRF and ICP-MS were used to analyze bulk chemical composition of the sediments, and ICP-MS was used for major and trace element concentrations of the pore water.
The REE composition of the pore water in each core normalized by average shale (PAAS) showed positive anomalies of La and Y and negative anomalies of Ce, and was enriched in HREE compared to LREE, all of which indicate that the overall characteristics of the pore water were similar to those of oxic seawater. The Ce negative anomalies in the pore water decreased with increasing depth from the seafloor surface. In MC03, that was collected from the deepest site, the REE concentration in the porewater tended to be higher in the deeper part of the core than in the other two cores.
Using the REE concentrations in the pore water obtained in this study, REE discharge fluxes from the pelagic sediments to the ocean were estimated based on Fick's first law of diffusion. The REE discharge fluxes from the sediment of Shatsky Rise were 2-3 orders of magnitude smaller than that fluxes from the continental margin of the Pacific coast of North America reported by Abbott et al. (2015). The order of REE fluxes estimated in this study were MC01 < MC02 < MC03, suggesting that REE fluxes from sediments to the ocean tend to be larger for pelagic clays than for calcareous ooze. Our results also support the suggestion of a previous study (Matsunami et al., 2024) that REE fluxes from the sediments of continental margin contribute significantly to the ocean-sediment REE budget including the formation of REE-rich muds. In order to investigate the cause of REE-rich muds in more detail, including ocean-sediment interactions, it is necessary to accumulate more observation data.
References:
Kato et al. (2011) Nature Geoscience 4, 535-539.
Yasukawa et al. (2016) Scientific Reports 6, 29603.
Matsunami et al. (2024) Ore Geology Reviews 175, 106338.
Abbott et al. (2015) Geochimica et Cosmochimica Acta 154, 186-200.
Yasukawa et al. (2016) showed that REEs in REE-rich muds are derived from seawater, and that the bulk sedimentation rate must be sufficiently small for the formation of REE-rich muds by calculating the flux of REEs deposited from seawater to the seafloor. Matsunami et al. (2024) constructed an ocean-sediment Nd mass balance model and conducted a sensitivity analysis of environmental factors that could be related to the grade of REE-rich muds. The results show that, in addition to sedimentation rate, Nd flux from continental margin to the ocean have a significant effect on the grade of REE-rich muds.
However, how much REEs are discharged from sediments to the ocean in the pelagic region remains poorly constrained based on observation. In this study, we analyzed the chemical composition of sediments and pore water of three multiple core samples (MC01-03) collected from the southwest slope of the Tamu Massif, Shatsky Rise, in the western North Pacific Ocean during the KH-24-1 cruise (April 2-19, 2024, Chief Researcher: Erika Tanaka). Major lithology of each core was nannofossil ooze in MC01 (3,890 m water depth) and clay in MC02 (4,352 m water depth) and MC03 (5,673 m water depth). Pore water was collected onboard using a Rhizon sampler. Sediments were sampled onboard every 1-2 cm after pore water sampling. XRF and ICP-MS were used to analyze bulk chemical composition of the sediments, and ICP-MS was used for major and trace element concentrations of the pore water.
The REE composition of the pore water in each core normalized by average shale (PAAS) showed positive anomalies of La and Y and negative anomalies of Ce, and was enriched in HREE compared to LREE, all of which indicate that the overall characteristics of the pore water were similar to those of oxic seawater. The Ce negative anomalies in the pore water decreased with increasing depth from the seafloor surface. In MC03, that was collected from the deepest site, the REE concentration in the porewater tended to be higher in the deeper part of the core than in the other two cores.
Using the REE concentrations in the pore water obtained in this study, REE discharge fluxes from the pelagic sediments to the ocean were estimated based on Fick's first law of diffusion. The REE discharge fluxes from the sediment of Shatsky Rise were 2-3 orders of magnitude smaller than that fluxes from the continental margin of the Pacific coast of North America reported by Abbott et al. (2015). The order of REE fluxes estimated in this study were MC01 < MC02 < MC03, suggesting that REE fluxes from sediments to the ocean tend to be larger for pelagic clays than for calcareous ooze. Our results also support the suggestion of a previous study (Matsunami et al., 2024) that REE fluxes from the sediments of continental margin contribute significantly to the ocean-sediment REE budget including the formation of REE-rich muds. In order to investigate the cause of REE-rich muds in more detail, including ocean-sediment interactions, it is necessary to accumulate more observation data.
References:
Kato et al. (2011) Nature Geoscience 4, 535-539.
Yasukawa et al. (2016) Scientific Reports 6, 29603.
Matsunami et al. (2024) Ore Geology Reviews 175, 106338.
Abbott et al. (2015) Geochimica et Cosmochimica Acta 154, 186-200.