4.1. Decay-induced chemical processes in the sediment.
Decay in the sediment produces a range of chemical reactions that can
enhance the preservation potential of SBO. A necessary prerequisite for
many of them to start is the fine-grained texture of the sediment that
slows the diffusion of oxygen and dissolved ions (Allison, 1988). Clay
particles are usually of fine and ultrafine size, and therefore clays
are expected to enhance the preservation potential of a buried carcass.
Other sediments such as fine-grain silica in our experiments can also
trigger reactions necessary for preservation.
The arrested diffusion results in increasing concentrations of different
ions around the decaying body. Chemical gradients thus established can
form a spot-like or layer-like morphology in the sediment (Figure 1).
Although this chemical heterogeneity has been often overlooked in the
discussion of SBO preservation, it invokes at least two conditions that
can favor preservation. The first one is the fast accumulation of
mineralizing agents (e.g., ions of Ca, Mg, Al, Si, Fe, P) released via
acidic or alkaline hydrolysis of the sediment. The second one is the
peak of the pH at the water/sediment interface (Table 4). Similar peaks
can be found in the topmost layers of some modern organic-rich marine
sediments (Zhu et al., 2006). If there is no bioturbation, this peak of
the pH can sometimes induce the formation of calcium-rich sealing cement
at the water/sediment interface (Naimark et al., 2016a). Surface
cementation of this kind is a widely recognized phenomenon in SBO fossil
localities, although its origin is poorly understood (Gaines et al.,
2012). Our results suggest that it may result from the increased pH in
the top layer of organic-rich sediment.
The repertoire of mineralizing ions depends on the type of sediment. Its
chemical and physical properties determine how the pH will change in
response to organic decay, and which elements will leach. We found a
good concordance between the transformation of the sediments and the
composition of cations permeating the carcasses. Therefore, the
diversity of primary host sediments is expected to produce a diversity
of the resulting elemental content of SBO fossils.
Five experimental sediments led to quite different degrees of
preservation of the model soft-bodied organism (Table 3). It does not
necessarily mean that the same sediments would provide the same degrees
of preservation in natural environments. Their potential to preserve
soft carcasses depends on other factors as well. The montmorillonite
experiment is revealing in this sense. It is known that soft bodied
fossils are rare in rocks containing montmorillonite or its hydrated
product illite (Anderson et al., 2018), apparently because
montmorillonite is comparatively stable in the common burial conditions.
In our experimental conditions, however, the pH in the montmorillonite
sediment became alkaline (Table 4), thus triggering the dissolution of
the montmorillonite and mineralization and preservation of the buried
soft-bodied organisms.
More research is needed to elucidate the particular factors mediating
the decay and preservation of SBO in different sediments. However, our
results imply that different fine-grained sediments (such as the clays
and the artificial silica we used) can, under some conditions,
facilitate the formation of specific microenvironments and chemical
gradients in the vicinity of the buried carcassess that can affect their
preservation potential in a variety of ways.