[S-CG55] Ocean Floor Geoscience

[SCG55-03] Constraints on REE-rich mud formation based on an ocean-sediment Nd mass balance model: Implications from REE and P fluxes in the ocean

*Ryosuke Matsunami¹, Kazutaka Yasukawa¹, Kentaro Nakamura^1,2, Yasuhiro Kato^1,2 (1.The University of Tokyo, 2.Ocean Resources Research Center for Next Generation, Chiba Institute of Technology.)

Keywords:REE-rich mud, simulation, seafloor sediment, seafloor mineral resource, mass balance model

Deep-sea sediments highly enriched in rare-earth elements (REEs), termed “REE-rich mud,” have recently attracted attention as a potential mineral resource for industrially critical metals [1]. Previous studies of the geochemistry and mineralogy of REE-rich mud have suggested that the sedimentation rate is key for the formation of REE-rich mud, and, therefore, that Earth system dynamics affecting pelagic sedimentation rates control the formation, ore grade, and distribution of REE-rich mud on a geological time scale [2]. However, the environmental factors controlling the formation of REE-rich mud and their secular variations have never been studied systematically nor quantitatively. In this study, to investigate the dominant factors promoting the formation of REE-rich mud in the Pacific Ocean, we constructed a new multi-box neodymium (Nd) mass balance model that considers interactions between the ocean and seafloor sediments [3]. Sensitivity analysis results showed that dilution by eolian dust, which is not enriched in REEs, results in decreased REE contents in REE-rich mud, whereas the discharge of REEs from the continental margin to the ocean can have a strong positive impact on the REE content of REE-rich mud. Long-term simulations through the Cenozoic demonstrated that dust fluxes dominantly control the secular trend of REE contents in REE-rich mud. The calculated REE content range is consistent with the range observed in data previously reported for North and South Pacific REE-rich mud. However, our model could not reproduce the highly REE-rich mud with >3,000 ppm of total REE that is observed in some areas. This result implies that such REE peaks reflect regional or local processes, as previously proposed [4]. Numerical experiments considering the effect of biogenic calcium phosphates (BCP) showed that the addition of REE to the sediment is necessary to generate the highly REE-rich mud. Furthermore, a preliminary calculation of phosphorus fluxes in the ocean, which includes BCP, suggested that the generation of highly REE-rich mud is strongly constrained by the marine phosphorus cycle.

[1] Kato et al. (2011) Nat. Geosci. 4, 535-539. [2] Yasukawa et al. (2016) Sci. Rep. 6, 29603. [3] Matsunami et al. (2024) Ore. Geol. Rev. 175, 106338. [4] Ohta et al. (2020) Sci. Rep. 10, 9896.