University of Bristol EARTH SCIENCES

Rate of decay is a critical factor in the preservation of fossils, as it generates mineralisation. Experiments on decay are important in determining which factors promote preservation. This of course is not straightforward, but there are established parameters. Invertebrates tend to decay through the same stages regardless of the circumstances (i.e. variations of salinity, oxygen, and temperature etc) (Briggs 1995).

Too much decay leaves nothing to be preserved. Based on the evidence of the fossils alone, we can speculate on how long the mineralisation process is likely to take. Experiments, however, can provide data on the time and conditions required ( Briggs 1995).

What does it involve?

Carcasses are dried with blotting paper, weighed, and each specimen is then transferred to a screw cap experimental vessel with 50-150mls of artificial seawater, usually inoculated with natural water and or sediment from a known source e.g. the Tay Estuary, Scotland, as a source of bacteria. The next stage is to setup the vessels with varying degrees of oxic to completely anoxic conditions. This is done by means of e.g. for oxic conditions covering the vessel with sterile cotton and tissue and securing, through to sealing the vessel with a screw cap and degassing the artificial seawater overnight before sealing the vessel inside an aluminium bag. All experiments are then incubated at 20 C ± 0.5 C for periods ranging from 2 to 30 days before sampling.

Experiments on the polycheate worm Nereisshowed that decay was significantly slower where oxygen was completely absent from the outset than where oxygen was initially available. This appeared to be the result of differences in the rate of decay of the more volatile constituents of the carcass. Similar experiments on shrimps, however, showed no significant differences whether oxygen was present or absent at the outset. This contrasting result is likely to reflect the differences in the composition and structure of the carcass of worms and shrimps.

These experiments can be used as one measure of amount of decay. Five gross stages of decay were identified using polychaete worms, but the degree of variability in an individual organism means that this approach is necessarily crude. It does, however, have the advantage that it can be related directly to fossils.

Gel stabilisation experiments

New experimental methods have been developed at the University of Bristol to determine rates of decay.These experiments use an experimental vessel that contains a basal gel on which the carcass is placed. Water is added to sufficiently cover the carcass and a futher layer of gel is added to cover the carcass and the water. The top of the experimental vessel is then sealed with a teflon cover. Fine micro electrodes have been developed and are placed within the carcass. These allow researchers to profile the carcass and determine levels of pH, sulphide and oxygen through depth, space and time. This allows closer monitoring of the dynamics of decay. The use of a gel coring device allows samples to be taken at various stages throughout the experiment. It is then possible to determine the number of bacteria and also the distribution of cations within the carcass. The results of these experiments are extremely promising and have taken our understanding of the processes of decay and mineralisation further. The results of this experiment are awaiting publication.