EXPERIMENTAL TAPHONOMY: THE EFFECTS OF SEDIMENT GRAIN SIZE ON MICROBIAL AND MINERAL FILM GROWTH ON TISSUES

Biofilms are linked to high fidelity preservation by rapidly colonizing organisms and precipitating mineral films. To better understand the role of biofilm activity, attention must also be given to the entombing sediments that affect diffusion rates limiting nutrient availability to microbial communities.

Here, we analyzed how varying sediment grain sizes affect the growth and development of microbial and mineral films on hard and soft tissues of buried organisms. Marine snails (Nassarius vibex) were euthanized, buried in different grain sizes (silts and clays, very fine sands, and fine sands) of granitic sediment and a mixture of artificial seawater. The experiment ran for 56 days with samples harvested at 9 distinct time intervals. Samples were analyzed using light microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy to identify timing of biofilm and mineral film development in each grain size.

Darkening of the sediments was observed in all grain sizes, and began within 24 hours at the sediment-water interface. These zones grew the fastest in the coarser-grained fine sands, where increased dissolution of the CaCO₃ snail shell was also observed; unlike finer-grained samples where dark zones grew slower and most of the CaCO₃ shell was left intact. The finest-grained samples had a higher degree of soft tissues remaining, and CaCO₃ shell preservation at each time interval of the study. This is most likely the result of a decreased amount of microbial activity due to the low porosity and compaction of the finer-grained sediments. Porosity differences also explain the faster rates in which the black zones, believed to be anoxic zones created by microbial respiration, grew at increased rates in higher-porosity, coarser sediments. This research provides better understanding of the role of sediment grain size on the preservation of soft tissues, and helps constrain the taphonomic processes responsible for fossilization.