Preservation and taphonomy

The Soom fossils show a remarkable level of preservation, sometimes with three-dimensional features of the soft tissues intact.  As the majority of the fauna was nektonic or nektobenthic, some carcasses must have sunk rapidly to the sea floor after death;  there is no evidence to suggest any significant transport took place before the carcasses were deposited.  Scavengers would have been excluded from the sediment surface during those periods when the bottom water conditions were dominantly anoxic or euxinic.  However, decomposition of the soft tissues by anaerobic bacteria would have destroyed the carcasses completely unless rapid mineralisation occurred.  The soft tissues and some of the hard tissues are now preserved in clay minerals, and the high degree of fidelity of replication, sometimes even conserving sub-cellular features, strongly suggests that clay minerals were directly involved in the replacement of animal tissues.  There is no evidence for the occurrence of any preliminary phase of mineralization involving phosphate, pyrite or calcite.  The highly acidic geochemical environment at the time of deposition would have aided clay mineral/organic interactions whilst militating against phosphate, pyrite or carbonate interactions with the organic material (Gabbott 1998).

Colloidal clay particles have an affinity for organic substrates in the presence of cations.  This affinity, coupled with the small (1 µm - 1 nm) particle size of colloidal clays, offers the potential for high fidelity soft tissue replication.  In the Soom Shale basin, detrital illite and kaolinite and possibly authigenic kaolinite probably existed as colloidal suspensions in the pore and bottom waters.  These particles would probably have had net negative charges under the low pH conditions present, as would any organic substrate (i.e. the soft tissues of animals).  Therefore, before any interaction between the clay minerals and the organic substrate could have occurred, their mutual electrostatic repulsion had to be overcome.  Attractive forces, either chemical or physical, could then have operated. For example, in the presence of  electrolytes such as Na+ and Ca2+ clay minerals would have been able to approach the organic substrate closely enough to bond by van der Waals and/or hydrogen bonding.  In this way, colloidal clay minerals could have nucleated by flocculation and subsequent adsorption onto specific organic substrate templates so that the soft tissues were replicated before they were destroyed.
The labile, decay-susceptible tissues such as muscle  show the most detailed preservation, whereas more recalcitrant organic tissues such as the chitin of eurypterid carapaces are also replaced by clays but not to the same level of detail.  In addition to the clay replication, there is some organic preservation in the form of carbon films, which  can be seen, for example, in the conodont eyes, the body of the naked agnathan and the Siphonacis spines;  this form of preservation is rarer than the clay replication.  In some cases, most notably the conodont eye capsules, both organic and clay mineral preservation co-occur where the clays have coated the surface of the organic substrate.  It is interesting that it is the most recalcitrant organic biomolecules that show organic preservation, so perhaps the reactivity of the organic substrate controlled the process of clay mineralization with the labile tissues preferentially acting as templates for the clay particles that eventually replaced them.  During diagenesis, the clays have been modified from their original compositions and electron microprobe analyses show them now to be in the illite and mixed layer clay solid solution series.
While the acidic bottom waters appear to have promoted the deposition of the clay paricles, they served to dissolve completely the skeletal biominerals of the animals.  The aragonite of the orthocone cephalopods, the calcite of the trilobites and articulate brachiopods, and even the apatite of conodont elements and lingulate brachiopod shells has all disappeared. The demineralisation of the apatite, in particular, is indicative of very low pH waters.

The unusual early diagenetic history of the hard and soft parts of the organisms was largely controlled by the composition of the organic and sediment matter supplied to the sea floor, which in turn controlled the Eh-pH conditions of the ambient waters.  The presence of large quantities of organic matter is evident from the masses of algal material on many bedding planes.  Decomposition of this organic matter by sulphate reducing bacteria in an environment with limited reactive iron oxides would have produced pore and bottom waters rich in H2S.  This acidity may have been exacerbated by oxidation of the H2S to form sulphuric acid.  The sparsity of carbonate-bearing minerals meant there was no available pH buffer, allowing large scale dissolution of the biominerals, inhibiting phosphatization and pyritization of the soft tissues but encouraging clay minerals to deposit on these organic templates and replace them

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