Characterization of authigenic carbonates from Huoshaogang landfill, Guangzhou, China: implication for microbial metabolism

Authigenic carbonates from a landfill leachate collection system (LCS) resulting from early diagenesis mediated by microbes, are unique archives of the biogeochemical processes and environmental conditions inducing the carbonate formation. This paper reports the results of mineralogical, petrological, and geochemical studies of the carbonates collected from the LCS of Huoshaogang landfill in Guangzhou, China. The carbonates from the LCS pipes and the equalization basins occurred as laminated crusts and porous precipitates, respectively. The authigenic minerals consist mainly of low- and high-Mg calcites. Sedimentary fabrics likely linked with microbial metabolism are developed, including stromatolites, ooids, clots, peloids, botryoidal cements, and framboidal pyrites. In addition, putative microbial textures, including spheres, filaments, and rods, are preserved in the carbonates. The δ¹³C and δ¹⁸O values of the carbonates range from +7.34 to +18.21 ‰ and from −8.64 to −4.76 ‰ (V-PDB), respectively, indicating that the carbon is derived mainly from the residual CO₂ after methanogenesis, and the oxygen is predominantly from meteoric waters. The Ce/Ce* values of carbonate phases revised by eliminating La effects demonstrate no or positive Ce anomalies, suggesting that the carbonates were precipitated in a reducing environment. The bacterial degradation of waste organic matter to CH₄ and CO₂ would significantly increase the alkalinity and pH of leachate and decrease the content of SO₄ ²⁻, and thus promote calcite crystal nucleation on the surface of microbial cells and subsequent precipitation under reducing conditions. The occurrence of high-Mg calcite precipitating later with incremental δ¹³C values implies the increasing trend of Mg/Ca ratios and δ¹³C values of dissolved inorganic carbon in the leachate during stabilization of the closed landfill.