Fossil shark teeth record a microscale pathway for ferromanganese oxide nucleation and Li–Ni enrichment in deep-sea biogenic substrates

Marine ferromanganese crusts and nodules are major reservoirs of critical metals considered essential for a low-carbon economy, yet the mechanisms governing their initial formation remain poorly documented. Here, we integrate Micro-CT, SEM-EDS, EPMA, LA-ICP-MS, TEM, Raman spectroscopy, and XANES to reconstruct the initial stages of Fe–Mn oxide (FMO) formation in fossil shark teeth from the Rio Grande Rise (southwestern Atlantic Ocean). We document a four-stage FMO mineralization sequence preserved within dentin tubules: (1) bioapatite alteration and organic matter degradation, potentially associated with endolithic bioerosion; (2) precipitation of nanoscale goethite onto dentin tubule walls, associated with phosphate release and pH buffering; (3) Mn oxidation and precipitation of lithiophorite, a layered Mn(III–IV) oxide enriched in Li (~ 650 µg g –1 ) and Ni (~ 37,000 µg g –1 ) overgrowing goethite, with lithiophorite-bearing domains characterized by a high average Mn oxidation state of ~ 3.6; and (4) selective redistribution of REY into altered bioapatite, while Ce remains preferentially associated with FMO, followed by late clay-rich infilling. These results highlight the importance of biogenic substrates and confined dentin microenvironments in early FMO nucleation and phase-specific metal redistribution and retention, providing a microscale model for metal partitioning during the initial stages of deep-sea FMO mineralization.