A Volcanic End to Terminal Neoproterozoic Glaciation?

Field observations and newly acquired carbon and sulfur isotope evidence from carbonates atop diamictites of the Fauquier Formation, and equivalents, in northern Virginia yield valuable insights into global environments during the evolution of Ediacaran-grade animals, and the termination of the youngest Neoproterozoic ice age. The transgressive succession of diamictite, sandstone, siltstone, and organic-rich carbonate in the Fauquier Formation was deposited in a rift environment between 702-705 Ma, the age of underlying Robertson River rhyolites, and 564 +/- 9 Ma, the age of overlying Catoctin Formation flood basalts. A strong negative-to-positive δ13C trend (from -5\permil at the base to +3\permil at the top of the composite section) is recognized in the carbonates, which is typical of post-glacial Neoproterozoic cap carbonates worldwide, supporting earlier claims that the diamictite at the base of the unit is glacial in origin. A significant enrichment of 13C in co-existing organic matter suggests carbon limitation associated with photosynthetic blooms in the aftermath of the Fauquier ice age. This cap carbonate is also unique insofar as its uppermost beds are interlayered with lava flows of the Catoctin Formation. The Catoctin is a regionally extensive unit associated with large scale rifting from Southern Virginia to Newfoundland. Carbonate sedimentation and igneous activity were clearly contemporaneous, as the soft Fauquier sediments were deformed and altered by the lavas. This intimate sedimentary and igneous association suggests that the end of the ice age may have been forced by a sudden increase in greenhouse capacity caused by volcanic degassing of CO₂. In addition, the radiometric determination is the best-known constraint for the age of the post-glacial carbonate and the conformable diamictite beneath, thereby providing an upper bound for Neoproterozoic glaciation. Notably, an ash layer in Newfoundland interbedded with a diverse assemblage of complex Ediacaran fossils is also around 565 Ma. If related to the same rift related igneous province, the observations made in northern Virginia suggest that environmental changes in the immediate aftermath of terminal Neoproterozoic glaciation, and the radiation of macroscopic animals, are causally linked.