Mineral composition of host sediments influences the fossilization of soft tissues

Abstract

Cambrian Lagerstätten host rocks are frequently composed of kaolinite and chlorite in varying amounts; accordingly, our goal was to study the preservation potential of crustaceans in these two clays. We conducted long-term experiments (12–18 months, the longest duration of actualistic taphonomy experiments from published literature) on the decay of Artemia salina in these clay sediments. The degree of preservation, transformed mineralogical composition of the sediments, and the elemental composition of the nauplial remains were examined. We demonstrate that the kaolinite and chlorite sediment enhanced the preservation (in the kaolinite the effect was considerably higher than in the chlorite) compared with the sediment-free control. pH inside the sediments dropped to 6.5–7.1 and was even lower (<4) around the buried carcasses, facilitating the dissolution of clays. This phenomenon was confirmed by mineralogical analyses of the experimental sediments, which showed mineralogical signatures of such dissolution and new mineral phases. According to the variations in the dissolved minerals in the sediments, different cations entered the buried remains as was shown by the multiple energy dispersive X-ray analyses. An increased level of Mg was detected in the carcasses buried in chlorite, whereas Al and Si concentrations were higher in the kaolinite; in both cases, Ca rapidly entered the decaying tissues from marine water. Bacteria underwent similar mineralization as the macroremains and apparently had no direct effect on the mineralization. The results confirmed an important role of dissolved Al ions in preservation of soft-bodied organisms in clay-dominated sediments and explained wide variation in chemical composition of their fossils.

Taphonomic experiments imply a possible link between the evolution of multicellularity and the fossilization potential of soft‐bodied organisms

“…This is in concordance with mineralogical transformations detected in the experimental sediments. The results of mineralogical analysis (STA, see Section 2) of the kaolinite and clinochlore were reported previously (Naimark, Boeva, et al, 2018; Naimark, Kalinina, Shokurov, et al, 2018). The results for the chamosite and montmorillonite are shown in Figure 7.…”

“…In order to identify core chemical pathways in SBO fossilization, we conducted a line of long-term taphonomic experiments (1.5-5 years: the longest of this type) with the model multicellular organism Artemia salina buried in different sediments. Some results were published previously Naimark, Kalinina, Shokurov, et al, 2018). They are combined with the new ones presented here ( Table 1).…”

“…Our taphonomic experiments (Table 1) imitate rapid burial of SBO under a sediment layer, which is believed to be a common condition for Lagerstätten formation. The experimental design was described in our earlier works (Naimark, Boeva, et al, 2018; Naimark, Kalinina, Shokurov, Boeva, et al, 2016; Naimark, Kalinina, Shokurov, Markov, et al, 2016; Naimark, Kalinina, Shokurov, et al, 2018). The homogeneously mixed clay suspensions were poured into high glass tubes (50–70 cm in height, 1.0–1.5 cm in diameter; as we observed in multiple experiments, this variation did not affect preservation).…”

“…Group 2 was comprised of well-shaped rigid external integument with finely preserved limbs but completely decayed internal anatomy ( Figure 2d). This group is specific to decay in sediment as it was abundant in all sediments but almost absent in the sediment-free control ( Table 3; Butler et al, 2015;Naimark, Kalinina, Shokurov, et al, 2018).…”

Taphonomic experiments imply a possible link between the evolution of multicellularity and the fossilization potential of soft-bodied organisms

“…Group 2 was comprised of well-shaped rigid external integument with finely preserved limbs but completely decayed internal anatomy (Figure 2 D). This group is specific to decay in sediment as it was abundant in all sediments but almost absent in the sediment-free control (Table 3; Butler et al, 2015;Naimark et al, 2018b).…”