Aragonite derived from marine molluscs is evaluated as the source for microcrystalline carbonate cements of limestone–marl alternations (LMA). Calculations demonstrate that extremely low levels of mollusc-derived aragonite, well below the production rates of molluscs in modern marine settings, could have provided sufficient carbonate to cement examples of Ordovician, Silurian and Jurassic LMA in non-tropical or tropical settings. It is likely that even in the Palaeozoic molluscs provided sufficient carbonate entirely to source microcrystalline cements of LMA. Autochthonous molluscan aragonite is the only viable aragonite precursor for LMA microcrystalline cements of cool-water settings where temperatures precluded calcified algae and abiotic carbonate precipitation. In ‘calcite seas’ where Mg:Ca ratios inhibited both abiogenic aragonite precipitation and aragonite generation by calcified algae, molluscan aragonite was again the most likely main contributor. In some epeiric seas where brackish wedges switched off the shallow-water carbonate mud factories molluscan aragonite is the parsimonious source of carbonate for LMA microcrystalline cements.