Long term evolution of petroleum biomarkers in mangrove soil (Guadeloupe)

In March 1986, plots of a peaty mangrove were delimited and polluted by Arabian Light crude oil. Superficial sediments were sampled (0–5; 5–10 and 10–20 cm core section) in 1987, 1990, 1994 and petroleum hydrocarbons were analysed by FID-GC, high resolution GC-MS and MRM GC-MS. One year later, no significant evolution of the petroleum chemical composition was noted. Four years later, n-alkanes had strongly decreased and isoprenoids were predominant. Eight years later, isoprenoids were severely degraded and biomarkers were more or less altered as follows in the declining order of importance: steranes > hopanes > bicyclic terpanes > triand tetracyclic terpanes > diasteranes > norhopanes and triaromatic steroids > C-ring monoaromatic steroids. For steranes, the following order of degradation can be proposed C26 > C27 > C28 > C29 > C30 with the stereochemical degradation sequence aaa 20R > αββ 20(R + S) > ααα 20S. Regarding pentacyclic terpanes, a decreasing removal is observed in the order C35 > C34 > C33 > C32 > C31 with a preferential degradation of the 22R epimers compared to the 22S epimers. The C30-17α(H), 21β(H)-hopane appears more sensitive to weathering than its higher homologues. We also observed that there was selective degradation with drimanes/homodrimanes, diasteranes/steranes and norhopanes/hopanes. A light depletion of the C20–C21 triaromatic steroids can be noted whereas no degradation of C-ring monoaromatic steroids is observed. Biomarkers, widely used in geochemistry, can be also used to follow the weathering of petroleum in natural conditions and can be considered as environmental markers.