Phosphorus cycling and REE enrichment in pelagic clay: Insights from a coupled Nd–P mass-balance approach

“REE-rich mud” has attracted attention as an unconventional resource for critical rare-earth elements (REE) used in green and high-tech industries [1]. It is a type of pelagic clay characterized by high REE (especially heavy REE) concentrations. In REE-rich mud, biogenic calcium phosphate (BCP; fish-bone apatite) plays a key role as a major host phase of REE, linking sedimentary REE enrichment to marine phosphorus (P) cycling and biological productivity [2]. Preliminary Nd–P one-box mass-balance analyses [3] suggested that fish-derived P burial may constitute an important component of total P burial in pelagic realms and that variability in P cycling may therefore be a major control on the conditions favorable for REE-rich mud formation. This motivates a reassessment that explicitly accounts for oceanographic processes which regulates nutrient supply and redistribution.

In this study, we develop a Nd–P mass-balance model that represents the ocean in a subdivided, coupled-reservoir framework to account for internal transport and redistribution. The framework tracks major P cycling and burial pathways, including burial associated with organic matter, authigenic phases (Ca-phosphate and Fe-bound P), and fish debris (BCP), together with neodymium (Nd) as a representative REE.

Using this framework, we aim to examine how redistribution of nutrients and Nd influences inferred BCP burial contributions and, by extension, the conditions favorable for REE-rich mud formation. We will conduct sensitivity and scenario experiments on internal transport and biological productivity within the ocean, and discuss implications for linking Earth-system processes to REE-rich mud genesis.

[1] Kato et al. (2011) Nat. Geosci. 4, 535–539. [2] Ohta et al. (2020) Sci. Rep. 10, 9896. [3] Matsunami et al. (2025) AGU Annual Meeting 2025, PP24B-08.

1: School of Engineering, Univ. of Tokyo, 2: ORCeNG, Chiba Institute of Technology