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

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 (