Even-n-alkane/alkene predominances in surface sediments from the Calabar River, SE Niger Delta, Nigeria

We report the geochemical characterisation of hydrocarbons extracted from surface sediments of the Calabar River and coastal soils, SE Niger Delta, Nigeria using gas chromatography–mass spectrometry (GC–MS). As a result, a special group of organic compounds prevalent in the entire study area was identified. It consists of aliphatic hydrocarbons (7.3–22.2% of the total lipids) with several distinctive chemical features. These include a high abundance of even numbered n-alkanes (n-C₁₂–n-C₂₆, maximising at n-C₁₈, n-C₂₀ and n-C₂₂), n-alk-1-enes (n-C_14:1–n-C_26:1, maximising at n-C_18:1 and n-C_20:1), giving rise to Carbon Preference Indices (CPIs) between 0.15 and 0.82. An unresolved complex mixture (UCM) occurring in the range n-C₁₈–n-C₃₅, and the presence of hopanes indicate petroleum contamination. The predominance of even numbered n-alkanes in the Calabar River sediments are thought to be derived from inputs of different microorganisms inhabiting an oil-polluted environment and contributing to the organic matter (OM). This paper, for the first time, gives an account of the unusual predominance of even numbered n-alkanes/alkenes in surface sediments from the Niger Delta of Nigeria and thus contributes to the information on the rare occurrence of such distributions in the geosphere.