Experimental taphonomy of fish - role of elevated pressure, salinity and pH

Abstract

Experiments are reported to reconstruct the taphonomic pathways of fish toward fossilisation. Acrylic glass autoclaves were designed that allow experiments to be carried out at elevated pressure up to 11 bar, corresponding to water depths of 110 m. Parameters controlled or monitored during decay reactions are pressure, salinity, proton activities (pH), electrochemical potentials (Eh), and bacterial populations. The most effective environmental parameters to delay or prevent putrefaction before a fish carcass is embedded in sediment are (1) a hydrostatic pressure in the water column high enough that a fish carcass may sink to the bottom sediment, (2) hypersaline conditions well above seawater salinity, and (3) a high pH to suppress the reproduction rate of bacteria. Anoxia, commonly assumed to be the key parameter for excellent preservation, is important in keeping the bottom sediment clear of scavengers but it does not seem to slow down or prevent putrefaction. We apply our results to the world-famous Konservat-Lagerstätten Eichstätt-Solnhofen, Green River, and Messel where fish are prominent fossils, and reconstruct from the sedimentary records the environmental conditions that may have promoted preservation. For Eichstätt-Solnhofen an essential factor may have been hypersaline conditions. Waters of the Green River lakes were at times highly alkaline and hypersaline because the lake stratigraphy includes horizons rich in sodium carbonate and halite. In the Messel lake sediments some fossiliferous horizons are rich in FeCO 3 siderite, a mineral indicating highly reduced conditions and a high pH. Since the advent of experimental methods in paeontological research, our understanding of taphonomic and fossilisation reactions has much improved. Today we realise how easily and rapidly organic tissue may be transformed into inorganic materials 1-6. Consensus is emerging that fossilisation reactions can take place within time frames accessible with laboratory experiments 7. The near-perfect articulation of the Pycnodontid in Fig. 1 suggests that the decision for or against preservation must have been made early, shortly after the fish died. Under ambient marine conditions-oxygenated water, normal marine salinity, and near-neutral pH-a fish so delicate would have been disarticulated or consumed by scavengers within hours to days. But what are the environmental factors most effective in retarding or preventing organic decay? If we identify those variables experimentally, we may hold the key to understanding the genesis of Konservat-Lagerstätten within which fish are prominent fossils. We report novel decay experiments with fish to understand early taphonomic pathways toward fossilisation. Parameters investigated experimentally are elevated hydrostatic pressure, elevated salinity, the role of proton (pH) and electron activity (Eh), bacterial activity, and time. We apply our results to three prominent Konservat-Lagerstätten where fossil fish are prominent species-Eichstätt-Solnhofen, Green River, and Messel. Previous taphon...

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