Polyphasic carbonate precipitation in the shallow subsurface: Insights from microbially-formed authigenic carbonate beds in upper Miocene sediments of the Tertiary Piedmont Basin (NW Italy)
article i nfo 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 car- bon 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 geom- etry and are characterized by dolomitic intergranular cement, which is typified by positive δ 13 C 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 (δ 13 C: �40‰ VPDB) and various bacterial dialkyl glycerol diethers (DAGEs; δ 13 C: �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 δ 13 C 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 pro- pose a scenario for the formation of the diagenetic beds, suggesting that carbonate precipitation was the re- sult 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 geo- chemical conditions and fluid circulation patterns that prevailed in the ancient subseafloor during early dia- genesis 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.