Stable carbon isotope and n-alkane distributions in sediment cores from saline and freshwater Gabu lakes, Southeast Nigeria: Environmental implications

Abstract

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 indicate a shallow water system that predominantly preserved long chain n -alkanes derived from vascular plant leaf waxes and macrophytes. The capacity of macrophyte to biosythesize long chain n-alkanes most likely reflects adaptation of these organisms to partial exposure to the atmosphere. The occurrence in moderate abundance of C 17 and C 19 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 was associated with barite hydrolysis and acidification accompanying mining activity. Our results have demonstrated that long chain n -alkanes of terrestrial and submerged/floating macrophyte origins are better preserved under conditions of salinity and acidification in inland shallow lakes than those derived from algae/bacteria.