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.