WERE CRYOGENIAN LANDSCAPES GREEN? -- AN EMERGENCE-SYNCHRONIZED MARINE CARBONATE 12C ANOMALY IN NAMIBIA FROM SEA TO SHORE

A corollary of the ‘early greening’ hypothesis is that negative C-isotope excursions (CIEs) in Neoproterozoic carbonates might result from terrestrial runoff and/or meteoric diagenesis, previously discounted as terrestrial biomass was assumed to be negligible at that time. We tested the corollary in the 0.1-km-thick Gruis Formation, which records marine regression during the last phase of rifting on the Otavi platform in northern Namibia. The Gruis Fm is riddled with exposure surfaces and marine carbonate interfingers laterally with alluvial clastics shed off dip-slopes on the uplifted footwalls of spaced crustal-scale normal faults. We obtained 13 high-resolution sections spanning 80 km across depositional strike from the Makalani dip-slope into the marine basin to the north. The formation is bounded by disconformities, yet consistent d¹³C values (±0.5) of 4.5‰ at the base and 7.5‰ top (PDB) suggest that the carbonate-rich sections are isochronous. These sections exhibit a saddle-shaped CIE, falling first by 3.5‰ and then rising by 6.5‰. If the greening corollary is correct, landward amplification of the CIE would be expected. Instead, d¹⁸O is 5-15‰ lighter in the most landward sections, but all d¹³C data fall on the same curve as the more distal sections. We conclude that the Gruis Formation provides no support for a terrestrial source of light carbon in Cryogenian time.