Origin of cap carbonates: An experimental approach

• Analogical experiments related to Neoproterozoic cap carbonate precipitation
• New synthetic model of the cap carbonate formation based on an inorganic phenomenon
• Aragonite is the precursor of dolomite forming the basal unit of the cap carbonates.
• Seawater equilibrated with atmosphere and sediments is the main source of carbonates.

An experimental study was carried out to assess the abiotic natural reactions associated with carbonate precipitation leading to the formation of Neoproterozoic cap carbonate sequences. For this purpose, synthetic seawater solutions were prepared reflecting the presumed composition of Neoproterozoic seawater. These solutions were heated to 40 °C, undergoing degassing and/or evaporation, while monitoring their chemical composition (sulphate concentration, Mg/Ca molar ratio) and identifying the precipitated minerals. The experimental results are in good agreement with previous studies, allowing us to constrain the type of carbonate precipitates formed in the ocean during the Snowball Earth aftermath. Among the phases precipitated during heating of the solutions, Mg-bearing carbonate minerals (i.e. Mg-calcite, magnesite and dolomite) remain of minor importance compared to Ca-carbonates (especially aragonite). Consequently, the basal dolostone units of the cap carbonates (i.e. cap carbonates s.s.) were originally in the form of CaCO3, and evolved spontaneously toward dolomite.Cap carbonates s.l. (i.e. cap limestones) containing aragonitic cement could result from the degassing of the whole ocean and the concomitant precipitation of aragonite. The calculated thicknesses of carbonates formed in this way are of the order of several metres, in line with the field observations. Although this process of abiotic carbonate precipitation does not rule out biologically-mediated carbonate formation, it may represent an alternate route to produce carbonate, particularly in environments which are not yet colonized by bacteria after the global ice melting.