Natural and Human-Induced Impacts on the Origins and Distributions of Fatty Alcohols in Sediment Cores from Cross River System, Nigeria

Abstract

Tropical rivers are important pathways for transporting organic carbon to the ocean, yet the use of molecular markers to distinguish natural inputs from human-induced effects in these dynamic systems remains limited. This study evaluated the distribution and possible sources of fatty alcohols in two 50 cm sediment cores collected from the upper and lower sections of the Cross River system, south-eastern Nigeria. The sediment cores were sectioned at 5 cm intervals and analysed for fatty alcohol biomarkers using gas chromatography-mass spectrometry. Seventeen fatty alcohol compounds, ranging from n-C12ol to n-C28ol, together with phytol, were identified. The upper system was characterised by comparatively higher average abundances of middle-chain fatty alcohols, particularly n-C18ol (1283 +- 2.1 ng/g TOC), n-C20ol (683 +- 2.2 ng/g TOC), n-C22ol (3486 +- 3.1 ng/g TOC) and n-C24ol (2923 +- 3.0 ng/g TOC), suggesting a substantial contribution from submerged macrophytes under relatively favourable light conditions. Long-chain fatty alcohols, represented mainly by n-C26ol (2961 +- 2.6 ng/g TOC) and n-C28ol (2897 +- 2.2 ng/g TOC), were also more abundant in the upper system, indicating stronger terrestrial organic matter input. In contrast, the lower system showed relatively greater influence of short-chain fatty alcohols in selected core intervals, which may reflect enhanced aquatic biological inputs. Down-core variations in fatty alcohol ratios indicate that sedimentary fatty alcohol origins and distributions were controlled by aquatic productivity, terrestrial organic matter supply and post-depositional alteration. The findings suggest that fatty alcohols can provide useful information on organic matter sources in tropical riverine and estuarine sediments when site-specific environmental controls and anthropogenic disturbances are carefully considered.