The origin of n-alkanes in Santa Monica Basin surface sediment: a model based on compound-specific Δ14C and δ13C data

Abstract

Δ14C and δ13C values were measured for individual long-chain n-alkanes (C24–33) from Santa Monica Basin sediments. The data were then simulated using a three-component mixing model designed to represent the contributions of different sources. The three selected end members were petroleum, modern plant wax, and shale-derived alkanes. The model was optimized to fit the data and to determine the fractional contribution of each component. The results indicated that petroleum accounted for 12% of the alkanes in 0–2.5 cm sediment and 5% in 2.5–7.5 cm sediment. Modern plant waxes contributed 80% (0–2.5 cm) and 87% (2.5–7.5 cm), and the remaining 8% of each sample was attributed to the shale source. The 14C concentration of the modern terrestrial end member was also determined from the model. Δ14C values of ∼+235‰ for the 0–2.5 cm (post-bomb) horizon and ∼ 0‰ for the 2.5–7.5 cm (pre-bomb) horizon indicate that plant leaf waxes have a continental residence time of decades in the southern California region.