Trace and rare earth element signatures in microcrystalline aragonite as indicators of oil vs. methane seepage

Oil seeps are important hydrocarbon sources to the ocean and sustain diverse chemosynthesis-based ecosystems. Distinguishing oil- from methane-dominated seeps in the geological record is crucial for reconstructing the evolution of chemosynthetic communities, yet remains challenging. Authigenic carbonates precipitating at seeps offer a valuable archive of the composition of seep fluid. While previous work has focused on fibrous aragonite cement, the geochemical potential of volumetrically dominant microcrystalline analog remains underexplored. Here, we present a comprehensive study of the petrography and stable isotope as well as element composition of oil-derived seep carbonates from the northern Gulf of Mexico (sites GC232, GC185), compared to methane-derived seep carbonates from the northern Gulf of Mexico (site AT340) and the South China Sea (site GMGS2-08). All studied samples are dominated by microcrystalline aragonite. Carbon isotope data (average δ¹³C = −20.9 ± 4.5 ‰; VPDB) and the presence of oil residues agree with crude oil degradation as the main carbon source of the oil-seep carbonate. Compared to methane-seep carbonates, oil-seep carbonates exhibit higher rare earth elements (REE) contents, slight light-REE enrichment, and elevated contents of uranium (U), molybdenum (Mo), zinc (Zn), and cobalt (Co), signatures linked to microbial oil oxidation. These element patterns preserved in microcrystalline aragonite offer a novel proxy for identifying oil seepage in the rock record, particularly in cases where fibrous cement is absent.