Polyphasic carbonate precipitation in the shallow subsurface: insights from microbially-formed carbonate beds in Upper Miocene sediments of the Tertiary Piedmont Basin (NW Italy).

Authigenic methane-derived carbonates hosted in upper Miocene slope sediments of the Tertiary Piedmont Basin (NW Italy) are studied by a multidisciplinary approach including petrography, stable oxygen and carbon isotopes of carbonates, as well as lipid biomarkers in order to explore the relationship between microbial activity and carbonate precipitation in the shallow subsurface. The studied rocks show a bed parallel geometry and are characterized by dolomitic intergranular cement, which is typified by positive δ13C values as high as+6.2‰VPDB. A striking feature of some dolomite beds is an intricate network of septarian-like cracks filled with both injected sediments and polyphasic carbonate cements. Prokaryotic molecular fossils in the dolomite beds comprise archaeol (δ13C: −40‰ VPDB) and various bacterial dialkyl glycerol diethers (DAGEs; δ13C: −30‰ VPDB), strongly suggesting that dolomite precipitation took place at the interface of the zones of archaeal methanogenesis and bacterial sulphate reduction. In contrast, extremely negative δ13C values of carbonate cements (as low as −56.3‰ VPDB) and various archaeal and bacterial molecular fossils (e.g. pentamethylicosane (PMI): −106‰ VPDB) are recorded in the crack-filling carbonate cements. These cements precipitated due to anaerobic oxidation of methane coupled to sulphate reduction. We propose a scenario for the formation of the diagenetic beds, suggesting that carbonate precipitation was the result of three microbially-driven processes (sulphate reduction, methanogenesis, and, finally, anaerobic oxidation of methane). This unusual sequence was a consequence of a dynamic change of environmental geochemical conditions and fluid circulation patterns that prevailed in the ancient subseafloor during early diagenesis of the unconsolidated sediments. Anaerobic oxidation of methane, which usually predates methanogenesis during increasing burial, postdates methanogenesis in case of the septarian-like beds after the beds were affected by crack formation induced by overcritical pore pressure, allowing the ingress of sulphate-rich water from above and methane-rich water from below.