Figure 3: Geochemical model showing the carbon isotopic difference between MT microspar and host rock formed at different water depths.
From: Molar tooth carbonates and benthic methane fluxes in Proterozoic oceans
First, the amount of CH4 required to produce a unit volume of cracks at ambient pressure and temperature was estimated. The co-production of 
related to CH4 generation was then estimated and assumed to have been used fully for MT microspar precipitation. The methanogenic 
was inadequate to precipitate enough MT microspar to fill a unit volume and the shortage was made up by (1) pore water 
(black solid line); (2) 
from MSR (
) and methanogenesis (
) with a molar ratio of 1:1, and the remaining shortage fulfilled by pore water 
(red dotted line); or (3) 
from MSR and methanogenesis with a molar ratio of 2:1, and the remaining shortage fulfilled by pore water 
(blue dashed line). Porewater 
was assumed to have a δ13C value similar to that of host rock (that is, 1‰). The δ13C value of methanogenic 
was estimated at +150‰, given a δ13C value of methyl sulphides at –30‰, a fractionation between CH4 and methyl sulphides at –60‰ and the production of 3/4 mole of CH4 and 1/4 mole of 
from each mole of methyl sulphides (equations (2) and (3)). The δ13C value of MSR 
was assumed to be –30‰. The x axis represents water depths and the y axis indicates the isotopic difference between MT microspar and host rock (δ13CMT-δ13CHR).
