Hydrochemical Key to the Genesis of Calcareous Nonlaminated and Laminated Cyanobacterial Microbialites

Theoretical geochemical considerations (Kempe and Degens, 1985) and field work at sites of active growth of in situ calcified cyanobacterial mats and biofilms (e.g., Kempe et al., 1991; Kempe and Kazmierczak, 1993; Kazmierczak and Kempe, 2006) convinced us that in the past the ocean must have been more alkaline than at present and that it was of higher CaCO₃ supersaturation (SI_Calcite > 0.8) (Kempe and Kazmierczak, 1990a, 1994; Kazmierczak et al., 2004). Two processes contributed to the higher alkalinity: (1) the slowly declining high primary alkalinity established in the Hadean ocean by binding large amounts of degassed and cometary CO₂ through silicate weathering as CO^2- ₃ and HCO^- ₃ in ocean waters (“Soda Ocean”) and (2) the effect of the export of excess alkalinity from sulfate reduction processes in stagnant marine basins (“Alkalinity Pump”) (Kempe, 1990; Kempe and Kazmierczak, 1994). The latter alkalinity source started to be effective only after enough sulfate became available in the ocean (probably during the last 1.0–0.8 Ga – for evaluation of sulfate level in the Precambrian ocean see e.g., Buick, 1992; Grotzinger and Kasting, 1993; Eriksson et al., 2005). It could be the single most important factor for short-term modifications of Phanerozoic ocean chemistry. Sudden, in geological terms, export of alkalinity from overturning anaerobic basins could cause high pH and Ca²⁺ stress upon the marine biota (Kempe and Kazmierczak, 1994; Brennan et al. 2004; Kazmierczak and Kempe, 2004b), and is associated with negative δ¹³C excursions in carbonate sequences at the Precambrian/Cambrian transition (e.g., Knoll et al., 1986; Magaritz, 1989; Magaritz et al., 1991), where biocalcification started in several phyla almost simultaneously (e.g., Lowenstam and Margulis, 1980; Lowenstam and Weiner, 1989).