Calcification and Silicification: Fossilization Potential of Cyanobacteria from Stromatolites of Niuafo‘ou's Caldera Lakes (Tonga) and Implications for the Early Fossil Record

Abstract

Calcification and silicification processes of cyanobacterial mats that form stromatolites in two caldera lakes of Niuafo'ou Island (Vai Lahi and Vai Si'i) were evaluated, and their importance as analogues for interpreting the early fossil record are discussed. It has been shown that the potential for morphological preservation of Niuafo'ou cyanobacteria is highly dependent on the timing and type of mineral phase involved in the fossilization process. Four main modes of mineralization of cyanobacteria organic parts have been recognized: (i) primary early postmortem calcification by aragonite nanograins that transform quickly into larger needle-like crystals and almost totally destroy the cellular structures, (ii) primary early postmortem silicification of almost intact cyanobacterial cells that leave a record of spectacularly well-preserved cellular structures, (iii) replacement by silica of primary aragonite that has already recrystallized and obliterated the cellular structures, (iv) occasional replacement of primary aragonite precipitated in the mucopolysaccharide sheaths and extracellular polymeric substances by Al-Mg-Fe silicates. These observations suggest that the extremely scarce earliest fossil record may, in part, be the result of (a) secondary replacement by silica of primary carbonate minerals (aragonite, calcite, siderite), which, due to recrystallization, had already annihilated the cellular morphology of the mineralized microbiota or (b) relatively late primary silicification of already highly degraded and no longer morphologically identifiable microbial remains.

What do we really know about early diagenesis of non-marine carbonates?

“…Further, poorly crystallized Mg-silicates are now also often found and recognized in association with EOM and microbial cell walls (Arp et al, 2003;Bontognali et al, 2010Bontognali et al, , 2014Kremer et al, 2012;Burne et al, 2014;Zeyen et al, 2015). In these cases, and tightly associated to the processes outlined in Section 2.2.1, they may play a role in the formation, transformation and replacement of metastable carbonate phases.…”

“…Episodic surface inflow and groundwater discharge influence pH, pCO 2 , Mg/Si and the salinity of the fluids, resulting in the precipitation of kerolite, stevensite, sepiolite, palygorskite, hectorite or saponite (Warren, 2016). Furthermore, authigenic clay minerals enriched in Mg form in recent, hypersaline lacustrine and palustrine settings with high pH, as early evaporative precipitates (Eugster and Jones, 1979;Fisher, 1988;Calvo et al, 1999), where they occur in close association with carbonates (Calvo et al, 1995;Freytet and Verrecchia, 2002;Arp et al, 2003;Kremer et al, 2012;Burne et al, 2014;Tosca and Masterson, 2014;Pace et al, 2016).…”

“…1, 2). Non-marine carbonates are primarily located in continental settings, with some on islands (Benzerara et al, 2010;Kremer et al, 2012;Ionescu et al, 2015). The following deposits are included in this review under this heading (Fig.…”

Calcification and Diagenesis of Bacterial Colonies

“…Biomineralized micro-organisms appear more resistant to diagenesis than non-biomineralized ones [26][27][28][29]. For instance, biologically mediated calcification of bacteria, which is involved in the formation of stromatolites or microbialites [30][31][32], has been described as a key process contributing to their fossilization [27,29,30,33].…”

“…For instance, biologically mediated calcification of bacteria, which is involved in the formation of stromatolites or microbialites [30][31][32], has been described as a key process contributing to their fossilization [27,29,30,33]. The propensity of biogenic organo-mineral assemblages to overcome degradation during diagenesis might be dependent upon a number of parameters, including the paleoenvironmental and diagenetic conditions.…”