2.1. Sediments used in taphonomic experiments with Artemia salina.
We buried A. salina in five different fine-grained sediments (four clays and artificial silica) in order to assess the relative importance of different components and properties of the sediment for the preservation of SBO. Sediments of similar mineralogy are common in Lagerstätten (Anderson et al., 2018). However, we made no attempt to model real sediment types involved in Lagerstätten formation; rather, we sought to better understand the general principles of SBO fossilization. The main criterion for choosing sediments was the purity of the available samples which was crucial for the subsequent analysis and interpretation.
The initial chemical composition of the sediments, measured by X-ray fluorescent analysis, is shown in Table 2. The kaolin came from the Polog deposit (Ukraine) and contained 97% of kaolinite in the purified sample (see Naimark et al., 2016a for details). The clinochlore originated from the Akhmetievskoe deposit (Ural mountains) and was given to us from the collection of Fersman Mineralogical Museum (Moscow). The chamosite originated from the Kursk Magnetic Anomaly, the collection stored in the museum of the Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (Moscow). The montmorillonite was from Tikhmenievskoe deposit (Sakhalin), from the collection of one of the authors (N.B.). Artificial silica was precipitated from perlite by thermal decomposition (150o C) with NaOH and consequent neutralization by sulfuric acid yielding porous, amorphous siliceous particles. We did not use glass beads or sand as a model for siliceous sediment (like in Newman et al., 2019) because glass beads and sand are chemically highly inert.
Non-sterile artificial sea water (ASW; brand “Tetra”) was prepared according to the instructions for this brand. We checked its pH (7.4-7.8) and salinity (24‰); the chemical composition was assessed by X-ray fluorescent analysis (Table 2). For the freshwater experiment with montmorillonite we took tap water and kept it open under non-sterile conditions in a glass jar for a week to obtain a stable microbial consortium.
To prepare the sediments, all samples of minerals were ground finely in an agate mortar. Next, they were mixed in settled tap water and equilibrated for half a minute, allowing large and/or heavy particles to gravitate to the bottom. Then the upper portion (approximately 2/3 of the suspension) was decanted. This upper portion was dried, ground again and mixed in ASW (or tap water in the case of montmorillonite) in proportion of 3g/100 ml. The resulting suspensions were actively mixed with bubbling from an air pump for 48 hours to remove clay pellets.