Organic geochemistry and paleoenvironment of the Early Eocene "Pesciara di Bolca" Konservat-Lagerstätte, Italy

Exceptional preservation of fossils in so-called Konservat-Lagerstätten requires specific depositional regimesexcluding disturbance of bottom sediments by either wave actions and currents or by benthic fauna. We heredescribe a depositionalmodel for the Eocene “Pesciara di Bolca” Konservat-Lagerstätte based on sedimentological,paleoecological, and detailed organic geochemical results. Sedimentswere deposited in a lagoonal-like basinwithstagnant bottomwaters located on an extended carbonate platform thatwas sheltered from open marinewatersby a submarine threshold. Run-off from nearby land areas provided nutrients to support an algal communitydominated by diatoms.No fossil diatomshells have been identified, but evidence for their presence is given by thehigh abundance of highly branched isoprenoids in extractable bitumens. Influx of terrigenous organic matter intothe lagoon occurred in particular during deposition of the basal fish-bearing level L1. Here not only plantmacrofossils, amber, spores and pollen but also the lipid composition indicated notable input of land plants via thepresence of n-C24 to n-C32 carboxylic acids, long-chain n-alkanes (n-C27, n-C29, n-C31) and angiosperm waxtriterpenoids. The redox regime in generalwas strongly reducing as evidenced by the high concentration of sulfurvs. organic carbon, excellent kerogen preservation as shown by high hydrogen indices, and lowpristane/phytanebut high phytane/n-C18 ratios. Thewater columnwas highly stratified with anoxic saline bottomand fresh surfacewaters. Euxinic conditionswith free reduced sulfur present in the photic zone could only be detected in sedimentsfrom the L1 horizon via traces of aromatic carotenoids derived from green sulfur bacteria (chlorobiaceae), whichutilize H2S in anoxygenic photosynthesis. The depositional regime is thus comparable to the lithographiclimestones of Solnhofen but based on biomarker evidence lacks the high salinities postulated for the latter.Biomarker composition indicates that best preservation conditions prevailed in the basal part of the studiedsection (0–7 m above datum) but declined upon deposition of the upper part. We interpret the body of thePesciara as a parasequence of the 4th order (0.01–0.5 Ma), with the lower part representing a relative sea-levellowstand and the upper part a relative sea-level highstand.