Methanogenic carbonates

Methane (CH₄) is largely built from the light carbon isotope (¹²C). When emitted from the seabed, methane is oxidized by sediment-dwelling microorganisms, with one of the byproducts being methanogenic carbonate cements, themselves with large amounts of the ¹²C. A team of scientists from Poland, USA, Norway, and Sweden studied data from around 200 ancient methanogenic carbonates and concluded that their composition is shaped not only by the composition of the parent methane, but also by seawater chemistry at the time of their formation and local geological history.

Figure: A schematic section through shallow seabed showing the effects of methane-derived carbonate cementation. Cements form at or close to the seabed where worm tubes (A) and bivalve shells (B) occur, or in deeper parts of the subsurface where concrections (C) form.