Arp et al. bring a geomicrobiological dimension to bear on ancient microbially mediated calcium carbonate precipitation. The basic bone of contention is whether this precipitation was triggered by the metabolism—photosynthesis—of the cyanobacteria presumed to be involved, or took place in dead material during degradation. I have been struggling with this issue since finding calcified filaments in Holocene stromatolitic intraclasts (Pratt, 1979). The notion of postmortem calcification that I entertained for Girvanella (see Pratt, 1984, 1995, for Tubiphytes and the Epiphyton-Renalcis plexus) was based purely on circumstantial, petrographic evidence.


Arp et al. submit that the polysaccharide-dominated composition...

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