Burial and reactivity of sedimentary microalgal lipids in bioturbated Mediterranean coastal sediments

Vincent Grossi (1) , Sarah Caradec (1) , Franck Gilbert (1)

1 LOB - Laboratoire d'océanographie et de biogéochimie

The fate of microalgal lipid biomarkers in marine coastal sediments when acted on by natural bioturbation processes (Carteau Bay, Gulf of Fos, Mediterranean Sea) was studied under laboratory conditions. Both dead phytoplanktonic cells (Nannochloropsis salina) and luminophores (inert fluorescent particulate tracers) were deposited at the surface of intact sediment cores which were then incubated for 22, 44 and 63 days. Sediment reworking and concentration profiles of specific lipid components of N. salina (n-alkenes, alkyl diols, sterols and fatty acids) were determined as a function of time and depth. The results show that, in the sediment investigated, bioturbation occurs essentially as a biodiffusive process and that it has a rapid and significant impact on the qualitative and quantitative record of sedimentary lipids. Whereas most of the biomarkers were detected in the entire reworked layer (0–6 cm) after 22 days, n-alkenes were never detected below 3 cm due to their low concentration and their high reactivity. For each individual lipid, the comparison of the amounts obtained from the inventories of biomarkers in the reworked zone, with the amount deposited initially at the sediment surface, allowed the determination of its extent and rate of degradation. These ranged from 72% to 99% and from 0.010 to 0.047 day-1, respectively, depending on the biomarker considered, with polyunsaturated fatty acids (PUFAs) and alkenes being degraded faster than the other components. Comparison with previous work suggests that, in biologically reworked sediments, the apparent reactivity of lipids is: (i) positively correlated with the biological mixing coefficient (Db) and, (ii) generally much higher than in non-bioturbated (anoxic) sediments. Our results also support the idea that degradation of lipids in reworked sediments involves the combined effects of aerobic and anaerobic degradation processes, but that biological mixing results in diagenetic properties more characteristic of completely oxidized conditions.

Mots clés: Lipid biomarkers; Bioturbation; Coastal sediments; Kinetics of degradation; Redox conditions; Mediterranean Sea; Gulf of Fos

Domaines: Océanographie; Ecologie, Environnement

Vincent Grossi, Sarah Caradec, Franck Gilbert. Burial and reactivity of sedimentary microalgal lipids in bioturbated Mediterranean coastal sediments. Marine Chemistry, 2003, vol. 81, pp. 57-69. ⟨10.1016/S0304-4203(02)00139-1⟩. ⟨hal-00761744⟩