A geochemical study of Cryogenian (late Neoproterozoic) limestones from western Mongolia is presented, including δ 1 3 C (carbonate and kerogen), δ 1 8 O, 8 7 Sr/ 8 6 Sr and rare earth element data. Carbon isotope compositions confirm the existence of anomalously high δ 1 3 C in Cryogenian seawater with δ 1 3 C c a r b >+10%%. Positive δ 1 3 C excursions are explained conventionally by elevated rates of organic carbon burial. However, four additional factors appear likely to have contributed to δ 1 3 C excursions during the Cryogenian: (1) increased δ 1 3 C of carbon input (δ i ) due to the exposure and weathering of carbonate platforms during sea-level falls, (2) increased carbon isotopic discrimination between carbonates and organic carbon (Δδ 1 3 C) due to microbial reworking of primary organic matter, (3) locally elevated δ 1 3 C in restricted basins and (4) enhanced vertical gradients in seawater δ 1 3 C due to redox stratification. All four factors may have contributed to similar positive δ 1 3 C excursions at other times, and may help to explain the almost ubiquitous association of eustatic regression, and cooling, with high δ 1 3 C during the late Neoproterozoic and early Palaeozoic.