Mineralogical and isotopic signatures of kaolin regolith: Paleoclimate insights from the Kuba area, Ropp Complex (central Nigeria)

The Kuba kaolin regolith provides insights into weathering processes during soil formation on a biotite-granite protolith and their relationships to paleoclimate conditions. Mineralogical and geochemical properties of the regolith were analyzed using X-ray diffraction, inductively coupled plasma mass spectrometry, X-ray fluorescence, differential scanning calorimetry, and stable isotopes (ẟ¹⁸O, ẟ²H). The regolith consists mainly of kaolinite and illite, representing a tropical weathering regime of sub-recent age. Kaolinite persists throughout, with a slight increase in moisture availability near the surface. Isotope values (ẟ¹⁸O: +9.2 to +16.2 ‰; ẟ²H: −83 to −40 ‰) suggest an interplay between meteoric and hydrothermal fluids, with formation temperatures ranging from 47 °C to 65 °C. These values exceed typical surface weathering temperatures, indicating low-temperature hydrothermal alteration or deep circulation of heated groundwater, rather than purely supergene processes. Evidence for climate transitions includes elemental leaching trends and isotopic shifts, indicating alternating wet-dry cycles. A paleotemperature decrease from the top to the base of the section also indicates a shift from a warmer, humid climate to cooler conditions supporting a broader climatic evolution. Elemental losses (Na, Ca, K, Mn, Mg) and relatively stable TiO₂ and Al₂O₃ suggest selective leaching. Chondrite-normalized rare earth elements (REE) patterns display strong light rare earth elements (LREE) enrichment, a negative Eu anomaly, and a positive Ce anomaly in some sections, signaling oxidative weathering and feldspar alteration. High weathering indices at depth CIA (chemical index of alteration) >90, MIA (mineralogical index of alteration) >90) suggest intense alteration driven by prolonged fluid interaction rather than temperature alone. Thermal analysis supports kaolinite's stability under the prevailing conditions, making it a key indicator of the weathering environment. The profile's signatures reflect a dynamic climate history marked by Late Cenozoic climatic transitions and significant influences from the African Humid Period on weathering processes.