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The role of anoxia in the decay and mineralization of proteinaceous macro-fossils

Actualistic experiments have quantified rate of anaerobic decay and associated mineralization around proteinaceous macro-organisms. Carcasses of the polychaete worm Nereis and the eumalacostracans Nephrops and Palaemon were buried in airtight glass jars filled with sediment and water from marine, brackish, and lacustrine environments. Over a period of 25 weeks the contents were examined to determine the state of decay and were chemically analyzed to monitor early diagenetic mineralization (two methods for such analysis are reviewed). Decay processes were active in the experimental conditions despite anoxia and had virtually destroyed the carcasses within 25 weeks. However, decay-rate in the sulfate-reducing marine system was greater than in the methanogenic freshwater environments. Petrological and geochemical analyses of the organic remains identified discrete layers of authigenic iron monosulfide (a pyrite precursor) on the surface of the decaying Nephrops cuticle within weeks of initiating the experiment. Chemical analysis of decomposing flesh showed a marked increase in pore-water calcium content with time. The results clearly show that anoxia is ineffective as a long-term conservation medium in the preservation of soft-bodied fossils. However, decay-induced mineralization can be very rapid so that even a slight reduction in decay rate can lead to improved levels of fossil preservation. Traditionally, stagnation and rapid burial are considered to be the main prerequisites for the preservation of soft-bodied fossils and the formation of Konservat-Lagerstätten. Clearly these factors are only important in that they promote early diagenetic mineralization. This is the only way to halt information loss through decay.