Factors contributing to high δ13C values in Cryogenian limestones of western Mongolia

A geochemical study of Cryogenian (late Neoproterozoic) limestones from western Mongolia is presented, including delta(13)C (carbonate and kerogen), delta(18)O, Sr-87/Sr-86 and rare earth element data. Carbon isotope compositions confirm the existence of anomalously high delta(13)C in Cryogenian seawater with delta(13)C(carb)>+10 parts per thousand. Positive delta(13)C excursions are explained conventionally by elevated rates of organic carbon burial. However, four additional factors appear likely to have contributed to delta(13)C excursions during the Cryogenian: (1) increased delta(13)C of carbon input (delta(i)) due to the exposure and weathering of carbonate platforms during sea-level falls, (2) increased carbon isotopic discrimination between carbonates and organic carbon (Deltadelta(13)C) due to microbial reworking of primary organic matter, (3) locally elevated delta(13)C in restricted basins and (4) enhanced vertical gradients in seawater delta(13)C due to redox stratification. All four factors may have contributed to similar positive delta(13)C excursions at other times, and may help to explain the almost ubiquitous association of custatic regression, and cooling, with high delta(13)C during the late Neoproterozoic and early Palaeozoic. (C) 2002 Elsevier Science B.V. All rights reserved.