Unconventional rare earth element archives in the energy transition era: The Cenozoic organic sediments from west coast of southern India

Organic-rich sediments represent vital components of Earth's geochemical cycles, acting both as potential hydrocarbon and coal reservoirs and as unconventional archives for critical metals such as rare earth elements (REEs). With the growing emphasis on clean energy technologies, the Cenozoic organic deposits of India have gained renewed significance; however, those from the southern state of Kerala remain understudied. This study investigates lignite and associated carbonaceous sediments from the Cheruvathur and Warkalli Formations using a multi-proxy approach integrating organic petrography, infrared spectroscopy, stable carbon isotopes, and REE geochemistry. The lignite exhibits huminite dominance with Type III kerogen, deposited in a wet, mesotropic bog forest swamp under anoxic conditions. The mineral assemblage, dominated by kaolinite, quartz, illite, montmorillonite, feldspar, and pyrite/marcasite, reflects strong chemical alteration in a reducing environment. The δ 13 C values (−25.1‰ to −27.3‰) indicate a C 3 angiosperm source and deposition in tropical to subtropical swamp settings. REE patterns reveal LREE enrichment in carbonaceous shales and HREE enrichment in lignite, with distinct Ce, Eu, and Gd anomalies associated with provenance and redox conditions. The findings provide new insights into the paleoenvironmental evolution of Kerala's Cenozoic basins and highlight their potential as unconventional REE-bearing resources in the context of the global energy transition. • Multi-proxy investigations of lignite and associated carbonaceous sediments are carried out. • Carbonaceous sediments are LREE-enriched, and the lignite exhibits relative HREE enrichment. • Implications on paleo-ecology and environment from mineralogy, geochemistry and isotopic study. • Evaluation of potential for REE exploration pathways.