Constraints on Neoproterozoic ocean chemistry from δ^(13)C and δ^(11)B analyses of carbonates from the Witvlei and Nama Groups, Namibia

Abstract

High-resolution δ^(13)C data have been collected from shallow marine carbonates of the Witvlei and Nama Groups of south-central Namibia. The highest amplitude carbon isotopic excursion of the >600m composite section spans a range in δ^(13)C of over 12‰, with the lowest value of -9.4‰ at -550-560 Ma. The δ^(13)C values preceding this most negative spike range from -2‰ to -6‰. Positive values of o13C (0-4‰) are indicated only for the uppermost -250m of section. A duration of up to 10 Myr for the negative excursion is suggested based on estimates of sediment accumulation rates. The temporally equivalent Shuram Fm. of Oman provides a global correlation to the negative δ^(13)C record in Namibia by showing a light carbon anomaly over 100' s of meters of section and minimum δ^(13)C values of-10‰(Burns and Matter, 1993). Unlike the short-lived (<l Myr) negative excursion in δ^(13)C associated with the PC/C boundary (Grotzinger et al., 1995), the shallow-water, slowly accumulating, ordinary platform facies in Namibia and Oman show exceptionally long-lived, high amplitude δ^(13)C anomalies. This observation excludes the possibility that this negative δ^(13)C excursion represents a pulse from a light carbon reservoir, predicted by models invoking upwelling or release of methane hydrates. Additionally, the mechanism for generation of isotopically-depleted carbonates must differ from that of the "snowball earth" (Hoffman et al., 1998), due both to their significantly greater isotopic depletion and slow rather than uncoventionally rapid sediment accumulation rates. Preliminary analyses of δ^(11)B compositions in the Namibian carbonates show a positive correlation with δ^(13)C values. Because δ^(11)B in carbonate is a proxy for seawater acidity (lower δ^(11)B corresponds to lower pH), we infer that the -550-560 Ma negative spike in δ^(13)C reflects a decrease in seawater pH.