Shells of freshwater bivalves are commonly used for radiocarbon dating late Pleistocene archaeological and vertebrate fossil sites, thus providing important constraints on late Pleistocene human dispersal and megafauna extinction hypotheses. The reliability of bivalve shells for dating rests partly on the ease with which subsequent diagenetic alteration can be recognized; typically, wherein original shell aragonite is replaced by calcite in meteoric environments. Here we document late Pleistocene freshwater bivalve shells wherein meteoric diagenesis involved syntaxial overgrowth of aragonite cement on original aragonite shell biocrystals. Aragonite cement was identified in situ using Raman microspectroscopy and formed rather than calcite as a result of unusually high Mg:Ca ratios in local groundwaters. Thus, altered shells contain diagenetic 14C, rendering their dates unreliable, but they may slip past common vetting techniques because (1) epitaxial cements are not readily apparent petrographically because they are in optical continuity with adjacent biocrystals; (2) X-ray diffraction indicates that no calcite is present; (3) alteration is not apparent in cathodoluminescence studies; and (4) stable isotopes of C and O are difficult to interpret in shells that originate in terrestrial-meteoric environments. Hence, although freshwater with a high Mg:Ca ratio is not common, groundwater chemistry should be considered before accepting bivalve-based radiocarbon dates uncritically. More broadly, meteoric diagenesis in carbonate rocks is generally characterized by the dissolution of aragonite or its conversion to calcite. Our data show that such is not invariably the case, even in fully terrestrial, freshwater systems.