The role of anaerobic microbes in carbonate sediment diagenesis

In carbonate sediments the early diagenetic dissolution of aragonite and the reprecipitation of the dissolved CaCO₃ as calcite cement potentially alter sedimentological, geochemical, palaeoecological and palaeoclimatic information. The differential diagenesis of both carbonate minerals results from different thermodynamic stabilities in the redox zones within the sediment created by microbial decay of organic matter. This process can form CaCO₃-rich (limestone) and CaCO₃-depleted (marl) beds (diagenetic beds or diabeds), severely impacting access to original environmental information and the temporal resolution of the geologic record. To date, this diagenetic process has not been documented in situ due to drilling and sampling method limitations. Therefore, it is yet unknown in which zone the main dissolution and cementation take place. In this laboratory experiment, sulphate reducing bacteria (Desulfosarcina variabilis) and methanogenic archaea (Methanococcus maripaludis) were separately cultivated in aragonite, calcite, and an aragonite-calcite mixed sediment in marine medium for (a) 4 or (b) 10 weeks to evaluate which microorganisms are responsible CaCO₃ dissolution. The 36 samples, including reference samples without microbes and the pure original carbonate material, were analysed with light microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX) on sediment and on the precipitates of the pore water. While the visual examination of preservation was not conclusive, EDX analysis of the pore water precipitates yielded higher Ca contents in the 10-week samples with Desulfosarcina variabilis in aragonite and aragonite-calcite mixed sediments. This indicates that aragonite likely  primarily dissolves in the sulphate reduction zone. Combined with petrographic evidence from the geologic record we can establish a chronological order of dissolution and precipitation processes.