“…These comparatively lower δ 13 C values in the cortices are clearly related to microbial activity; the preferential uptake of 12 C-enriched compounds during microbial mineralization leads to the precipitated carbonate being more depleted in 13 C than the source material (e.g., Lee et al, 1987). The cortex δ 13 C values suggest incorporation of organically derived carbonate ions (Luzon et al, 2009), but they are too high to be related to sulfate-reduction (commonly δ 13 C < -12‰) and too low to be related to fermentation or methanogenesis (commonly δ 13 C > +5‰; Irwin et al, 1977;Pueyo et al, 2011). Similar δ 13 C values (-7‰ to -10‰) have been obtained from a calcifi ed freshwater stromatolite (Souza-Egipsy et al, 2006) and a laminated biofi lm (Shiraishi et al, 2008), where photosynthesis driven by cyanobacteria has been identifi ed as the main process inducing calcite precipitation.…”