The origin of n - alkanes in Santa Monica Basin surface sediment : a model based on compound - speci ® c 14 C and d 13 C data

C and dC values were measured for individual long-chain n-alkanes (C24ÿ33) from Santa Monica Basin sediments. The data were then simulated using a three-component mixing model designed to represent the contributions of di€erent sources. The three selected end members were petroleum, modern plant wax, and shale-derived alkanes. The model was optimized to ®t 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 C concentration of the modern terrestrial end member was also determined from the model. C 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. # 2000 Elsevier Science Ltd. All rights reserved.