Neoproterozoic carbon isotope excursions are commonly attributed to changes in the global fraction of organic carbon burial associated with climate instability and/or oxygenation. Here we show that carbonate sediment deposited during the ca. 661 – <651 Ma Cryogenian non-glacial interlude between the Sturtian and Marinoan glaciations exhibit lateral offsets in carbonate-carbon isotope values from coeval units by as much as 10‰. Within the Thorndike submember of the Cryogenian succession in the Panamint Range, California, USA, carbonate-carbon isotope values can be linked to a laterally discontinuous dolomitization front: limestones exhibit δ13Ccarb values of ∼+4 to +9‰, whereas values of stratigraphically equivalent dolostones are consistently lower, between ∼-4 and +4‰. Field observations and analyses of clasts from the overlying Marinoan glacial diamictite show that the offset in δ13Ccarb values resulted from pre- to syn-Marinoan dolomitization. Further, δ44/40Ca and δ26Mg data indicate that this isotopic variability resulted from sediment-buffered diagenesis. We propose that extremely positive δ13Ccarb values record local primary productivity within restricted platform surface waters and/or oxygenated pore fluids and negative values reflect anaerobic remineralization of organic carbon within sediment pore waters. In this scenario, neither the original calcite/aragonite nor subsequent dolomite precipitates of the Thorndike submember record δ13Ccarb values that are representative of global Cryogenian seawater, and instead they archive the evolution of local dissolved inorganic carbon pools.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science
- Death Valley
- carbon isotopes