Hypersaline conditions during deposition of the calcare di base revealed from archaeal di- and tetraether inventories

The Calcare di Base is a widespread Messinian carbonate formation in the Mediterranean, especially in Sicily and Calabria, Italy. The environmental conditions under which these limestones were deposited, their formation mechanism and the timing of deposition are a matter of debate. In order to shed new light on their formation, the lipid biomarker inventories of Calcare di Base from four locations in Sicily and Calabria were studied following petrographical and mineralogical characterization of the limestone samples. The mineralogy is chiefly calcite, the microfabric is mostly peloidal and partly laminated or brecciated. Deposition took place at high salinity as confirmed by numerous pseudomorphs after halite. Long chain n-fatty acids were the most abundant lipid biomarkers in samples from all sites, but isoprenoid alcohols were also abundant. The most prominent archaeal compounds were diphytanyl glycerol diethers (DGDs) and glycerol dibiphytanyl glycerol tetraethers (GDGTs). Among these, archaeol and GDGT-0 (caldarchaeol) predominated, respectively. Other GDGTs were virtually absent. Minor amounts of C20-25DGD (extended archaeol), a biomarker for halophilic archaea, were detected in all samples. The δ13C values of caldarchaeol (avg. -20‰) and archaeol (avg. -11‰) varied widely, indicating that these compounds derived from distinct archaeal precursor lipids. Based on lipid biomarker patterns, the Calcare di Base had been previously considered to represent a normal marine deposit, in contrast to the evaporitic gypsum and halite deposits in Messinian successions. The archaeal biomarker inventories from the Calcare di Base samples reported here, in conjunction with the occurrence of pseudomorphs after halite, reveal that deposition took place in a hypersaline environment.