Stable carbon isotope and n‑alkane distributions in sediment cores from saline and freshwater Gabu lakes, southeast Nigeria: environmental implications

Abstract

Studies on the utilization of molecular and isotopic proxies for the characterization of organic matter (OM) sources and environmental conditions in lakes have been well-documented globally. Nevertheless, inland lacustrine salt basins remain less well-studied, particularly in tropical sub-Saharan Africa. In this study, we quantified OM sources and evaluated the effects of salt deposit and barite mining on the distribution of n -alkanes in saline and freshwater Gabu lakes sediment cores, southeast Nigeria using elemental, bulk carbon isotope ratios and biomarker distributions. Our results indicate a system inundated with comparable proportions of vascular plant leaf waxes (mean = 56.6%) and submerged/floating macrophytes (mean = 41.1%) with minor contribution from algae/photosynthetic bacteria (mean = 2.3%). The scenario indicates a shallow water system that predominantly preserved long chain n-alkanes derived from vascular plant leaf waxes and macrophytes. The capacity of macrophyte to biosynthesize long chain n-alkanes most likely reflects adaptation of these organisms to partial exposure to the atmosphere. The occurrence in moderate abundance of C17 and C19 n-alkanes and the near absence of other low molecular weight (LMW) n-alkanes in the saline lake was linked to the effect of salt stress. The absence of LMW n-alkanes in freshwater lake may be associated with slow barite hydrolysis likely to cause acidification. Our results have demonstrated that long chain n -alkanes of terrestrial and submerged/floating macrophyte origins are better preserved under conditions of low salinity and acidification in inland shallow lakes than those derived from algae/bacteria