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.