3.6. Accumulation of iron.
The accumulation of iron in the buried carcasses needs additional
clarification. SBO fossils are often pyritized to a varying extent, and
the deposition of iron has been considered among the main mechanisms for
SBO preservation (e.g., Schiffbauer et al., 2014). Two of our
experimental sediments, chamosite and montmorillonite, showed
interesting signatures of iron accumulation associated with the
carcasses. Some exhumed carcasses from these sediments were scattered
with small spherules either attached to the organic surface or located
inside the thin integuments (Figure 8). These spherules varied widely in
size, which is common for mineral particles but unusual for bacterial
pseudomorphs (Schopf, 1976). According to the SEM/EDX point analyses
(Figure 8 C, F) the spherules contained Fe and S in an atomic ratio of
approximately 1:3, which is consistent with ongoing pyritization
(Rickard, Luther, 2007).
Mineralogical STA analysis confirmed that, in the presence of the buried
nauplii, the montmorillonite disintegrated almost completely within 5
years (Figure 7 A). The iron ions diffused from the mineral sediment
upward along the pH and oxygen gradients and accumulated in the
interfacial layer between the oxygen-depleted and oxygenated phases.
When concentrated in this layer, Fe2+ transformed to
Fe3+ in a form of ferrihydrite, turning the coloration
of this layer to brightly red (Figure 1 M) and inducing a decrease of pH
from 10.9-11.0 to 9.63-9.70 (Table 4). Ferrihydrite became stabilized by
silicate ions that were released into the media due to the alkaline
dissolution of montmorillonite (Rozalén et al., 2008).
Importantly, organic tissues served as a template for the nucleation of
the released iron compounds. We failed to find such iron-rich spheres in
the loose sediment around the carcasses.
In the chamosite, where the initial iron content was high (Table 2) and
iron subsequently leached from the mineral in the presence of the buried
nauplii (Figure 7 B), all iron-rich spherules were also adhered to
organic tissues (Figure 8). The results imply that such pyritization,
even if it occurs rapidly, does not necessarily provide good
preservation, because in our chamosite-based system the preservation ofA. salina was generally poor.