The chemical and isotopic compositions of carbonates are commonly used as proxies for ancient seawater or paleoenvironments. Iron formation (IF) carbonates have been used as evidence for an anoxic, Fe(II)-rich Archean and Paleoproterozoic ocean and high atmospheric CO2 contents. It has been proposed, however, that microbial Fe cycling dominates the chemical and isotopic compositions of IF carbonates, suggesting less direct applicability as an oceanic proxy. Here were use an isotope tracer that is not affected by biological processes or isotopic fractionation, the radiogenic 87Rb-87Sr system, to test the applicability of IF carbonates as a paleoenvironmental proxy. We focus on the 2.5 Ga Campbellrand platform, Transvaal Basin, South Africa, that records a shift from Ca-Mg carbonates to IF carbonates during a marine transgression. When coupled with previously determined Fe, C, and O isotope compositions, it becomes clear that the IF carbonates studied here do not reflect seawater compositions, but instead record extensive microbial Fe cycling in the soft sediment prior to lithification. These results question the use of IF carbonates to infer seawater compositions and paleoenvironmental conditions, including estimates for atmospheric CO2 contents.