Lipid biomarkers archived in guano accumulations from French caves

International audienceGuano are accumulations over time of materials issued from bat excretions. Although they constitute potential archives of past environmental changes, they are yet unexplored, at least from an organic geochemistry point of view. Here we discuss biomarker imprints from 15 guano samples recovered from various French caves. Our results are discussed in terms of source of biomarkers, conditions of preservation (guanos constitute a hotspot of acidic conditions in carbonate caves) and mechanisms of transfer of molecular biomarkers from their source to the guano deposit.Samples were collected from guano layers or guano cores recovered from eight caves located in southern France. Miguro (MIG-013 and MIG-014; ~5500 yrs cal BC), Escalans (ESA-19 and ESCA-20; from 4400 to 6000 yrs cal BC), Demoiselles (Demoi-004; dating in progress) and Brouzet (BROU-3; dating in progress) are located in Hérault. Tourtouse (TOURT-1, TOURT-5 and TOURT-10; recent) and L'Herm (Lherm-4, Lherm-8 and Lherm-11; dating in progress) caves are located in Ariège. Fées (FEES-001 and FEES-003) and Roquette (ROQ-002) caves are located in Gard. The age of samples ranges from recent to > 45000 yrs BP.n-Alkanes were abundant (from 2 to 36μg/g) in all samples except ESCA-19 and ESCA-20 and MIG-013 (<1 μg/g). They range from n-C11 to n-C37 with a maximum at n-C29, an Average Chain Length comprised between 27 and 29.5, and a clear odd-over-even predominance (Carbon Preference Index from 2 to 8) except for Lherm-8, MIG-013 and MIG-014, FEES-001 and FEES-003 and ESCA-20. These criteria are typical for n-alkanes derived from vascular plants. In addition, high CPI indicate that in most cases n-alkanes did not suffer strong alteration.The contribution of vascular plant material to the guano is also attested by abundant and diverse C-3 oxygenated pentacyclic triterpenes that indicate and input from angiosperms. β- and α-amyrins and amyrenones, taraxerone were found in all Lherm samples, in MIG-013 and in TOURT-1. Friedelin was detected in BROU-3, Demoi-004, and ROQ-002. Miliacin, a pentacyclic triterpene methyl ether reputed specific of Gramineae (Jacob et al., 2005) and in certain cases of Panicum miliaceum (Bossard et al., 2013) was detected at low abundances (0.1 μg/g) in BROU-3 and ROQ-002. ESCA-19, Lherm-8 and Lherm-11, TOURT-1, TOURT-5 and TOURT-10 contained low levels (