Abstract
Four recent sediment cores (0-30 cm long) from Afam (AF), Mangrove (MG), Estuary (ES) and illegal Petroleum refinery (PT) sites of the Imo River, Southeastern Nigeria were analyzed to characterize the sources and distribution of organic matter (OM), as well as examine their historical trends of deposition and assess human-induced changes in the last ca. 5 decades using biomarker approach. Radionuclides 210Pb and 137Cs were used to assign approximate dates to each section of the cores. Evaluation of proxy parameters such as carbon preference index (CPI, 2.01 - 2.19), carbon number maximum (Cmax, 29, 31) and atomic C/N (16.51-31.32) for the most recent top layers (0-5 cm) revealed greater wash-in of land-derived organic matter (OM), attributable to the recent rise in water height. The bottom layer (PT1, 25-30 cm,) of the PT core deposited ca. 1964-1972 exhibited a CPI of 0.97 and pristane/phytane (Pr/Ph, 3.75), suggesting that oil bunkering/illegal refinery activity had begun in the region ca. 8 years after the first commercial discovery of oil in Nigeria in 1956. The occurrence in high abundance of heptadecane in the middle layer (ES4, 10-15 cm) of the ES almost corresponded with the period of eutrophication that blocked the waterway in the late 1980s. Measurement of a marked unresolved complex mixture at the near-top layer (AF5, 5-10 cm) of the AF indicated that the heaviest contamination by petroleum hydrocarbons occurred at ca. 1997-2005. This time frame coincided with the period of intensive bunkering and oil pipeline vandalism by Niger Delta militant groups.
Highlights
Allochthonous and autochthonous organic matter (OM) are deposited in the ocean margins and shelves rivers at a rapid rate (Saliot et al 2001)
The three coring sites still provided evidence of sedimentary and historical events. These events were characterized by a change in grain size/bulk density at the top layers (SI-1) and inundation of the lower layers by petroleum hydrocarbons/algal OM in the last few decades, respectively
Source and historical variations of aliphatic hydrocarbons in subsurface sediment cores collected from four locations (Mangrove, Afam, illegal petroleum refinery and Estuary) of Imo River, SE Nigeria in the last ca. 5 decades were determined
Summary
Allochthonous and autochthonous organic matter (OM) are deposited in the ocean margins and shelves rivers at a rapid rate (Saliot et al 2001). Several studies showed that OM transported by tropical rivers/estuaries are mainly degraded terrestrial materials (Ittekkot 1988). The understanding of OM sources, distribution and fate in aquatic environment have received significance scientific attentions (Jaffé et al 2005). Aliphatic hydrocarbons are biomarkers substantially used to classified OM origins from terrestrial to marine environment (Camacho-Ibar et al 2003; Oyo-Ita et al 2010, 2016). As a major contributor to OM flux, tracing the sources of aliphatic hydrocarbons in the study area will help in the understanding of global dynamics of OM delivery. The present study is part of the regional project aimed at understanding the sources, transport and fate of lipid biomarkers in the Niger Delta drainage basin
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