Distribution of Biogenic- Derived Polycyclic Aromatic Hydrocarbons in Sediment Cores from Gabu Lake, South East Nigeria: Diagenetic and Environmental Implications

Abstract

Polycyclic aromatic hydrocarbons (PAHs) in lake sediments can originate from both natural and human-related sources. In tropical environments, biogenic PAHs often predominate and provide valuable information on organic matter transformation, environmental conditions, and ecosystem history. However, data on the distribution and diagenesis of biogenic PAHs in tropical African lake sediments remain limited. A comparative study of 80‑cm sediment cores from Gabu Lake—GS near the shoreline and GC from open water—was conducted using gas chromatography–mass spectrometry to trace the diagenesis of natural precursors forming biogenic polycyclic aromatic hydrocarbons (PAHs). Prominent compounds identified included two isomers each of octahydrochrysene and tetrahydrochrysene, along with mono‑ and di‑aromatic lupeol derivatives. GS core contained higher levels of natural and diagenetically produced PAHs, likely due to proximity to diverse vascular plant stands along the catchment. Variable concentrations of lupeol and α‑/β‑amyrin derivatives across intervals suggest aromatization occurred within micro‑layers of the cores or in watershed under anoxic conditions and later transported by runoff. A greater conversion of triene (mono‑aromatic) to pentene (di‑aromatic) lupeol‑derived PAHs in GS indicates stronger microbial oxidation compared to GC. Initial selective aromatization of lupeol preceded transformations in α‑ or β‑amyrin. The dominance of biogenic PAHs over anthropogenic ones highlights the sediments’ good quality and the lake’s healthy ecosystem.