Chemostratigraphy of the Lower Arroyo del Soldado Group (Vendian, Uruguay) and Palaeoclimatic Implications

New C- and O-isotopic determinations from the Vendian lower Arroyo del Soldado Group are reported and combined with sedimentologic and biostratigraphic data. On the basis of different geochemical and petrographic criteria, the primary nature of the C- and Sr-isotopic signature is shown. Positive d13C-values characterize the mainly siliciclastic upper Yerbal Formation, which contains oxide-facies BIF and a diverse assemblage of skeletal fossils, including Cloudina riemkeae. A series of positive and negative d13C-excursions occur up-section in the overlying Polanco Formation, mainly composed of limestones and limestone-dolostone rhythmites. The transition to the overlying Barriga Negra and Cerro Espuelitas Formation, which consists of conglomerates and shales/cherts/BIF respectively, is marked by a further negative excursion. On the basis of sedimentary structures, a correlation of d13C with palaeobathymetry is established. Positive d13C -peaks are associated with high sea-level stand, high organic-carbon burial and relatively higher microfossil diversity, while negative d13C -excursions occur in carbonates with less organic matter, less microfossil diversity and are always associated to regressions. These observations can be readily explained by palaeoclimatic models which postulate that enhanced bioproductivity due to higher availability of nutrients (P, N, Fe) was the key factor controlling Neoproterozoic glaciations. The mentioned models are discussed in view of the new data. The occurrence of at least four cold periods in the upper Vendian is envisaged. These cold periods led to sea-level fall and possibly glaciation at higher latitudes. The absence of glacigenic rocks associated to negative d13C-excursions in the Arroyo del Soldado Group is probably due to the tropical setting of the basin. Finally, the upper Vendian age of the lower Arroyo del Soldado Group is confirmed, on the basis of C- and Sr-isotopes. The onset of carbonate deposition at the base of the Polanco Formation is estimated at 580 Ma by comparison with existing global isotopic curves.