Physico-chemical and microbial processes governing the decay of organic substance in sediment under anoxic conditions

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To fossilize soft tissue of marine taxa it is essential that a carcass be transferred rapidly to anoxic conditions and isolated from potential scavengers. A practical way to achieve this is by burying a decaying carcass or a living sessile species in sediment. The oxidation of organic carbon to HCO3- and CO32- within the sediment will reduce the major electron acceptors in sediment and seawater, i.e. ferric iron and sulfate. The decay reactions will create local Eh and pH haloes around a carcass within which FeS sulfide, siderite, and other carbonate phases may be stabilized. These phases may then replace or template organic tissue, or encase the organic material in concretions. Within the research unit FOR 2685 the applicant will develop experimental routines that allow to investigate in-situ the anoxic decay of fish and echinoderms within a marine sediment. The major aim is to quantify the rates by which replacement can take place. While an experiment is progressing, organic material will be sampled from time to time. Inorganic precipitates will be identified by Raman spectroscopy. Bacterial communities emerging will be characterised in close collaboration with members of the research unit. It is expected that the templation by inorganic material - FeS phases, siderite, calcite - may be a matter of weeks to months. The characterisation of bacterial communities, an important aspect in the present proposal, will elucidate if pseudomorphic replacement reaction are inorganic by nature and driven only by pH and Eh gradients, or if they must be mediated by bacteria in order to be efficient.