NEOPROTEROZOIC GLACIATIONS AND EVOLUTION: NEW DATA FROM SOUTH CHINA

Geochronometric data suggest that at least three glacial intervals – Sturtian, Marinoan, and Gaskiers in geochronological order – occurred in the Neoproterozoic [1]. At least two intervals of glacial deposits occur in Neoproterozoic successions in South China. The youngest and most widespread Nantuo glacial deposit overlies Mn-rich dolomite of the Datangpo Formation and underlies the fossil-rich Doushantuo Formation. The Nantuo diamictite has been variously correlated to Sturtian or Marinoan diamictites elsewhere. High-resolution d¹³C chemostratigraphic data of the Nantuo cap carbonate, along with ⁸⁷Sr/⁸⁶Sr ratios and sedimentary features, suggest that the Nantuo is Marinoan in age. Radiometric constraints indicate that the Nantuo (and Marinoan) glaciation must have started after 663 +/- 4 Ma (new U-Pb zircon age from a tuffaceous bed in the Datangpo Formation) and ended before 599 +/- 4 Ma (Pb-Pb age from the overlying Doushantuo Formation; [2]). Sedimentary record of the Gaskiers glaciation is absent in South China. However, a sequence boundary and a negative d¹³C excursion in the uppermost Doushantuo Formation may record the physical and geochemical disturbance associated with the Gaskiers glaciation. Limited geochronological data also support that much of the Doushantuo Formation predates the short-lived Gaskiers glaciation. If so, the Doushantuo Lagerstätte, where both micrometazoans and multicellular algae are preserved, represents a window between two Neoproterozoic glaciations. This implies that the origin of animals may have occurred between or before the last two Neoproterozoic glaciations and that the disappearance of Doushantuo-type acritarchs may have been related to the Gaskiers glaciation.

[1] Bowring, S., Myrow, P., Landing, E., Ramezani, J., and Grotzinger, J., 2003, Geochronological constraints on terminal Neoproterozoic events and the rise of metazoans: Geophysical Research Abstracts, v. 5, p. 13219. (http://www.cosis.net/abstracts/EAE03/13219/EAE03-J-13219.pdf)

[2] Barfod, G.H., Albaréde, F., Knoll, A.H., Xiao, S., Télouk, P., Frei, R., and Baker, J., 2002, New Lu-Hf and Pb-Pb age constraints on the earliest animal fossils: Earth and Planetary Science Letters, v. 201, p. 203-212.