Systematic variations in C, O, and Sr isotopes and elemental concentrations in Neoproterozoic carbonates in Namibia: implications for a glacial to interglacial transition

Abstract

We analyzed C, O, and Sr isotopic compositions of Neoproterozoic cap carbonates overlying a glacial deposit in Namibia to trace environmental changes following the glaciation. The three isotopic records showed coherent and phased changes that corresponded to the stratigraphic variation. A lower rhythmite unit had three intervals, which were distinguished by isotopic variations and mineral type. In the basal interval, δ13C increased exponentially from a negative value (−4.3‰, Pee Dee Belemnite [PDB]). δ13C values were relatively constant in the middle interval, and increased again to positive values in the top interval, indicating a phased removal of 13C-depleted carbon from the surface ocean. The gradual increase to larger values (>6‰) in the upper stromatolite unit was interpreted to reflect a change in seawater composition resulting from biological activity after environmental recovery from the glacial episode. δ18O values and Sr isotopic ratios decreased in the basal interval and approached stable values in the middle interval. They increased in the upper interval and fluctuated in the stromatolite unit. On the basis of geochemical screening criteria for detecting diagenetic alteration of Sr isotopes, the middle interval of the rhythmite unit, with a minimum 87Sr/86Sr ratio of 0.70685, was interpreted to have retained the primary value, or to have the least-altered Sr isotopic composition. The basal interval, which had ratios (∼0.718) at its base as high as those of old, sialic rocks, was interpreted as having altered Sr isotopic ratios. The high Sr isotopic ratios at the base of this interval gradually decreased to near the minimum value at the top of the interval, and other isotopic compositions and trace element concentrations showed systematic variations. Such systematic variations, except for the variation in Na concentration, can be explained by a progressive diagenetic fluid–rock interaction. Alternatively, they might reflect a compositional transition from a local, transport-limited water following the glaciation to global, well-mixed seawater.