Hood, Ashleigh1, Penman, Donald2, Lechte, Maxwell3, Wallace, Malcolm1, Giddings, Jonathan1 and Planavsky, Noah2
1School of Earth Sciences, University of Melbourne, Parkville, Australia; 2Department of Geology and Geophysics, Yale University, New Haven, USA; 3Department of Earth and Planetary Sciences, McGill University, Montreal, Canada
Perhaps the most extensive ice age in Earth’s history, the Neoproterozoic was host to two globally-extensive glaciations from ~717-635 Ma. There has been ongoing debate as to the extent of ice coverage over the oceans during this time. The ‘snowball Earth’ theory suggests that during this time the Earth’s surface was completely frozen, with marine ice sheets that sealed off the Earth’s oceans from its atmosphere (Hoffman et al., 1998, Science 281, 1342-1346). A central tenet of this hypothesis is that carbonate could not precipitate from syn-glacial seawater due to a lack of alkalinity influxes into ice-covered, isolated oceans. These oceans would instead be dominated by chemical exchange with mid-ocean ridge volcanic systems, developing low pH (resulting in carbonate dissolution) and low Mg/Ca ratios (Hoffman et al., 1998).
However, carbonate sediments are present in both of the Neoproterozoic (Sturtian and Marinoan) ice ages in global glacial successions. Further, Sturtian sediments of the Adelaide Fold Belt, South Australia have sedimentological evidence for dolomite precipitation from syn-glacial seawater. Discrete beds of dolomite and dolomitic silt (with a carbonate content of up to 72 %) are present throughout the Sturtian Yudnamutana Subgroup, including in horizons over one kilometre stratigraphically below post-glacial strata. These carbonates may be deformed by sedimentary processes (e.g. dropstone emplacement) demonstrating their syn-sedimentary precipitation. Euhedral dolomite crystals appear to replace detrital (silicate) minerals and show no evidence of a detrital core, indicating an authigenic origin for the dolomite. The mid-Sturtian Warcowie Dolomite Mb. shows increasing carbonate content towards the upper bed surface, suggesting a seawater source for the dolomitising fluid.
The dolomite mineralogy of these syn-glacial sediments, as well as the post-glacial ‘cap carbonates’, is inconsistent with ‘snowball Earth’ Mg-poor glacial ocean conditions and implies that magnesium cycling (i.e. continental weathering) must have been active during the ~57 million year Sturtian glaciation. Based on these observations, a recent Precambrian geochemical model (PreCOSCIOUS) was modified to test the links between Sturtian carbonate mineralogy and environmental conditions. We found that around 9% of present continental chemical weathering during glaciation (i.e. not fully isolated oceans) is consistent with sedimentological observations and further supports the long timescale of glaciation.
Ashleigh Hood is a Lecturer and DECRA Fellow at the University of Melbourne. Her research focuses on the evolution of life and environmental conditions on the early Earth. This work is very much a joint effort from all authors, Don, Max, Malcolm, Jon and Noah.