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

Abstract

Exceptional preservation of fossils in so-called Konservat-Lagerstätten requires specific depositional regimes excluding disturbance of bottom sediments by either wave actions and currents or by benthic fauna. We here describe a depositional model for the Eocene “Pesciara di Bolca” Konservat-Lagerstätte based on sedimentological, paleoecological, and detailed organic geochemical results. Sediments were deposited in a lagoonal-like basin with stagnant bottom waters located on an extended carbonate platform that was sheltered from open marine waters by a submarine threshold. Run-off from nearby land areas provided nutrients to support an algal community dominated by diatoms. No fossil diatom shells have been identified, but evidence for their presence is given by the high abundance of highly branched isoprenoids in extractable bitumens. Influx of terrigenous organic matter into the lagoon occurred in particular during deposition of the basal fish-bearing level L1. Here not only plant macrofossils, amber, spores and pollen but also the lipid composition indicated notable input of land plants via the presence of n-C 24 to n-C 32 carboxylic acids, long-chain n-alkanes ( n-C 27, n-C 29, n-C 31) and angiosperm wax triterpenoids. The redox regime in general was strongly reducing as evidenced by the high concentration of sulfur vs. organic carbon, excellent kerogen preservation as shown by high hydrogen indices, and low pristane/phytane but high phytane/ n-C 18 ratios. The water column was highly stratified with anoxic saline bottom and fresh surface waters. Euxinic conditions with free reduced sulfur present in the photic zone could only be detected in sediments from the L1 horizon via traces of aromatic carotenoids derived from green sulfur bacteria ( chlorobiaceae), which utilize H 2S in anoxygenic photosynthesis. The depositional regime is thus comparable to the lithographic limestones 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 studied section (0–7 m above datum) but declined upon deposition of the upper part. We interpret the body of the Pesciara as a parasequence of the 4th order (0.01–0.5 Ma), with the lower part representing a relative sea-level lowstand and the upper part a relative sea-level highstand.