Organic geochemistry and paleoenvironment of the Early Eocene “Pesciara di Bolca” Konservat-Lagerstätte Reconstruction of past environments depends on the interpretation of fossil remains, most of which are shelly and thus induce a preservation bias in that non-shelly faunas are rarely preserved and available for investigation. Under exceptional conditions even soft- bodied organism may be preserved in exceptional detail in so-called Fossil-Lagerstätten. These are divided into concentration versus conservation deposits, the latter synonymous to Konservat-Lagerstätten, are related to two major processes: obrution (rapid burial) or stagnation. While the former leads to an entombment of preferentially benthic organisms by rapid or catastrophic sedimentation, the latter also includes water- or even airborne fauna embedded in sediments accumulating in a low hydrodynamic energy regime with reduced water mixing, characteristic of stagnant basins .
Transcript
Organic geochemistry and paleoenvironment of the Early Eocene “Pesciara di Bolca” Konservat-Lagerstätte
Reconstruction of past environments depends on the interpretationof fossil remains, most of which are shelly and thus induce apreservation bias in that non-shelly faunas are rarely preserved and available for investigation. Under exceptional conditions even soft-bodied organism may be preserved in exceptional detail in so-called Fossil-Lagerstätten. These are divided into concentration versus conservation deposits, the latter synonymous to Konservat-Lagerstätten, are related to two major processes: obrution (rapid burial) or stagnation.While the former leads to an entombment of preferentially benthicorganisms by rapid or catastrophic sedimentation, the latter alsoincludes water- or even airborne fauna embedded in sedimentsaccumulating in a low hydrodynamic energy regime with reducedwater mixing, characteristic of stagnant basins .
Konservat-Lagerstätten while depending on exceptional circumstancesof accumulation and preservation are not evenly distributed in timeand space. Peaks in outcrop-normalized occurrence of Konservat-Lagerstätten are coupled to the prevalence of particular environments or evolutionary events. In the Cambrian the lack of invertebrate grazers or deep bioturbators, in the Carboniferous the widespread high productivity regimes in coastal delta plains may have favoured preservation. The Jurassic yields high numbers of Konservat-Lagerstätten due to the extended shelf areas on the NW-margin of the Tethyan Ocean. Here either extensive carbonate platforms with intra-platform depressions surrounded by microbial or sponge reefs restricted circulation and thus favoured deposition of fine-grained carbonates in Plattenkalk-facies of the Solnhofen type, or transgressive black shales of the Posidonia Shale type led to exceptional fossil preservation. It is thus of importance to understand the depositional environment and its influence on the modes of fossil preservation when a genetic model of a particular Konservat-Lagerstätte is developed.
The Pesciara Fossil-Lagerstätte, together with the neighbouringMonte Postale, is the most outstanding fossiliferous outcrop in thesurrounding of the Bolca village, about 25 km north-east of Verona.In a few square kilometres several other well-known localities are present, such as Monte Purga, Vegroni, and Spilecco, but only for the last one a true exposure is still maintained.The Pesciara beds are world-wide known since the 16th centuryfor their exceptionally preserved fossil fish, but the extraordinary fossil content includes also snakes, birds, insects, land plants, amber, algae, rare jellyfish, molluscs, corals, and foraminifera.The magnificent quantity and quality of fossil fishes focused theattention of the scientists. Amazingly enough, not so much interestwas addressed to establish the geological features (stratigraphy, tectonics, sedimentology, geochemistry, etc.) of the area. The age of the Pesciara di Bolca limestone, previously supposed to be just across the boundary between Early and Middle Eocene, has been only very recently re-assessed as late Early Eocene.
The Pesciara Fossil-Lagerstätte, quarry and mine.
The Pesciara Fossil-Lagerstätte, paleogeographic reconstruction
Schematic geological map of M. Postale : 1 Scaglia Rossa Formation, 2 SpileccoLimestone, 3 volcanics, 4 massive limestone (lithofacies 1), 5 bedded/laminated limestone (lithofacies 2 and 3) (a outcrops, b interpretation)
The Pesciara Fossil-Lagerstätte, quarry and mine.
The Pesciara Fossil-Lagerstätte, museum, quarry and mine.
Microfacies of the M. Postale 1 succession:a Typical microfacies of Spilecciano, consisting of macro-foraminifera and planktic foraminifera-bearing packstone with crinoid fragments. b Nummulitid and ortofragminid-bearing packstone with coralline algae, bryozoans, echinoderms, miliolids and peloids.
Microfacies of the M. Postale 1 succession:
c Previous figure in detail, with Nummulites partschi (SBZ 10).d Massive limestone (SBZ 10/11) consisting of packstone with coralline algae, bryozoans, encrusting foraminifera, discocylinids, nummulitids, miliolids and peloids, with some fibrous siliceous cement
Excellent preservation of Ceratoichthys pinnatiformis
Excellent preservation of Mene Rhomba
Excellent preservation of Eocene amber
Excellent preservation of Xanthoxylum
The Pesciara Fossil-Lagerstätte, soft tissue preservation in fish fossils.
Location map (A) and stratigraphic column (B) of the Pesciara Section.
Rock-Eval data for source and maturity identification. A) A pseudo van Krevelen diagram depicts samples as type II kerogen of low maturity, with some degradative alteration and admixture of terrigenous material.
Rock-Eval data for source and maturity identification. B) The HI versus Tmax diagram confirms a type II kerogen of low thermal maturity that has been affected by secondary alteration.
C) An improved kerogen classification diagram minimized the mineralmatrix effects yielding an averaged HI value of 495 (mg HC/g TOC) calculated from the slope of the regression line.
GC/MS mass fragmentograms (m/z 85) of four characteristic samples from the Pesciara Section, revealing aliphatic hydrocarbon distributions.
GC/MS mass fragmentograms (m/z 117) of four characteristic samples from the Pesciara Section, revealing the distribution of carboxylic acids.
Trends in biomarker distribution along the Pesciara profile: A) pristane/phytane ratio, B) phytane/n-C18 ratio, C) hydrogen index HI (mg HC/g TOC), D) δ13Corg. Open symbols represent samples 0203 and 0204, both affected by impregnation.
faintly ->laminated
Trends in biomarker distribution along the Pesciara profile: E) short/long-chain n-alkanes, F) n-C16/n-C17 ratio, G) HBI20/n-C17 ratio, H) HBI25/HBI29 ratio. Open symbols represent samples 0203 and 0204, both affected by impregnation.
Depositional setting during lowstand times, with effective threshold and permanent water stratification; these conditions are recorded in the lower part of the Pesciara Section.
Depositional setting during highstand times, with less effective threshold and temporary water stratification; these conditions are recorded in the upper part of the Pesciara Section.
Schematic diagram of the Pesciara-palaeoenvironment from carbonate microfacies.a Ideal cross section of Pesciara type-successions, with prevalent laminated mudstone, well-preserved fossils and rare bioclastic intervals. b Ideal cross section of M. Postaletype-succession, with common bioclastic levels, thinner, laminated, and discontinuous mudstone intervals, with fossils commonly poorly preserved. C Ideal cross section of the buildup type-succession, with poorly stratified bioclastic pack-grain-rudstone, rich in macro-foraminifera and miliolids.
Alternation between laminated an non-laminated wackestone controlled by restriction of circulation due to sea-level stand in response to compensation by reef growth. TST allows for open circulation before ascending reef growth starts to reduce or interrupt communication.
Schematic reef type and carbonate microfaciesdistribution.
Organic geochemistry and paleoenvironment of the Early Eocene “Pesciara di Bolca” Konservat-Lagerstätte
Exceptional preservation of fossils in so-called Konservat-Lagerstättenrequires specific depositional regimes excluding disturbance of bottomsediments by either wave actions and currents or by benthic fauna. Adepositional model for the Eocene “Pesciara di Bolca” Konservat-Lagerstätte is based on sedimentological, paleoecological, and detailedorganic geochemical results. Sediments were deposited in a lagoonalbasin with stagnant bottom waters located on an extended carbonateplatform that was sheltered from open marine waters by a submarinesill. Run-off from nearby land areas provided nutrients to support analgal community dominated by diatoms. No fossil diatom shells havebeen identified, but evidence for their presence is given by the highabundance of highly branched isoprenoids in extractable bitumen.Influx of terrigenous organic matter into the lagoon occurred inparticular during deposition of the basal fish-bearing level L1.
Organic geochemistry and paleoenvironment of the Early Eocene “Pesciara di Bolca” Konservat-Lagerstätte
Here not only plant macrofossils, amber, spores and pollen but alsothe 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 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 pr/phy-but high phy/n-C18 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 H2S in anoxygenic photosynthesis
Organic geochemistry and paleoenvironment of the Early Eocene “Pesciara di Bolca” Konservat-Lagerstätte
The depositional regime is thus comparable to the lithographiclimestones of Solnhofen but based on biomarker evidence lacks the higher salinities postulated for the latter. Lower salinities result from episodic freshening by surface run-off from adjacent land areas, due to a position of the lagoon closer to the shore-line. 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 sediment sequence. The body of the Pesciara is interpreted 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.
