In-situ U-Pb dating of early marine carbonate cements constrains the age of the late Ediacaran lower Nama Group, Namibia

The middle-late Ediacaran (∼580 to >533 Ma) saw the emergence and early diversification of animals (metazoans), but the tempo of this event is obscured by a paucity of datable ash beds and uncertainties in global stratigraphic correlation through specific intervals. The Nama Group, Namibia, is of fundamental importance as this succession preserves a diverse terminal Ediacaran fossil assemblage that includes the first appearance of metazoan biomineralization, a key evolutionary innovation. The precise age of the lowermost Nama Group is unknown, however, with best estimates based on global chemostratigraphic correlation and inferred depositional rates suggesting that the onset of deposition was between ca. 555 and 551 Ma. Here we use uranium-lead (U-Pb) radioisotopic dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), to date early marine pseudomorphed calcite cements from the lower Nama Group. The analyses yield an absolute age of 549.3 ± 9.8 Ma and constitute the first independent time constraint for the lowermost Nama Group. The result is consistent with current chemostratigraphic age models and validates the use of in-situ U-Pb dating of early marine carbonate cements to constrain depositional ages.