Fingerprinting local controls on the Neoproterozoic carbon cycle with the isotopic record of Cryogenian carbonates in the Panamint Range, California

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 δ¹³C_carb 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 δ¹³C_carb values resulted from pre- to syn-Marinoan dolomitization. Further, δ^44/40Ca and δ²⁶Mg data indicate that this isotopic variability resulted from sediment-buffered diagenesis. We propose that extremely positive δ¹³C_carb 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 δ¹³C_carb values that are representative of global Cryogenian seawater, and instead they archive the evolution of local dissolved inorganic carbon pools.