RADIOGENIC AND STABLE SR ISOTOPE RECORDS REVEAL CHANGES IN WEATHERING AND CARBONATE BURIAL RATES FOLLOWING MARINOAN GLACIATION (Invited Presentation)

The long-term evolution of seawater radiogenic Sr isotope ratios (⁸⁷Sr/⁸⁶Sr) recorded in marine carbonate rocks has been used to reconstruct changes in the balance between hydrothermal and weathering input fluxes to the ocean. However, ⁸⁷Sr/⁸⁶Sr ratios are less valuable for constraining Sr output fluxes, which occur primarily via marine carbonate burial. Recent work has shown that modern marine carbonates (both biotic and abiotic forms) preferentially incorporate lighter Sr isotopes relative to seawater. Therefore, unlike ⁸⁷Sr/⁸⁶Sr ratios, the stable strontium isotope composition (δ^88/86Sr) of seawater is sensitive to global carbonate burial rates. Here, we present the radiogenic and stable Sr isotope composition of two sections of the Maieberg Formation, the cap carbonate sequence to the Marinoan glaciation in northern Namibia, in order to test for evidence of changes in the balance between continental weathering and carbonate burial rates and their relationship to carbon cycle variation during this time.

The Maieberg Formation is a ~250-400 m thick transgressive to highstand carbonate sequence that is accompanied by a negative δ¹³C excursion. δ¹³C values reach a low of -5.5‰ at approximately the level of the maximum flooding surface, above which they gradually rise, approaching 0‰ near the top of the formation. δ^88/86Sr values, which shift sympathetically with δ¹³C, gradually decrease from ~0.35‰, reaching a low of 0.18‰ near the level of the maximum flooding surface, before gradually rising to 0.51‰. However, ⁸⁷Sr/⁸⁶Sr ratios for the same samples show a strong negative correlation with δ^88/86Sr and δ¹³C values. Because the lowest δ^88/86Sr values correspond to the highest ⁸⁷Sr/⁸⁶Sr ratios (up to 0.7119), increased continental weathering provides the most likely explanation. Additionally, δ^88/86Sr values of the Maieberg carbonates are significantly higher than the Phanerozoic average. These data suggest that the negative δ^88/86Sr shift resulted from a greater increase in continental weathering compared to carbonate burial; however, the Neoproterozoic ocean was characterized by higher carbonate burial rates than the Phanerozoic.