A large (11--15?) negative shift in δ13C is observed in shallow water carbonates directly beneath Neoproterozoic glacial deposits (or correlative disconformity) in northwest Namibia ascribed to a snowball Earth. Reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin, and field relations show it predates the fall in sea level associated with the Ghaub glaciation. We crudely estimate the duration of the isotopic shift as ~0.6 ¿ 106 years from a simple thermal subsidence model. Similar or larger δ13C anomalies are found directly beneath Neoproterozoic glacial units in Australia, Canada, China, Scotland, and Svalbard. After considering conventional interpretations for negative δ13C anomalies, we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale, magnitude, and geological context of the anomaly in Namibia. Counterintuitively, an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation.