Neoproterozoic Glaciation

The Snowball Earth hypothesispredicts synchronous glaciation at low-latitudes, which end as a result of the syn-glacial build-up of pCO2, leading to a super-greenhouse aftermath. The prediction of synchroneity is testable with U/Pb geochronology on zircons from volcanic rocks inter-bedded with Neoproterozoic glacial deposits. Although “Marinoan” glacial deposits from multiple continents have been dated at 635 Ma, older, “Sturtian” glacial deposits have yielded an array of ages between 750 and 660 Ma. Furthermore, previous studies found evidence for low-latitude deposition only in Marinoan glacial deposits. This led some authors to suggest that the Neoproterozoic glaciations were not Snowball events, but that the Neoproterozoic merely represented a glacial period similar to the Pleistocene.  

We began our attempt to calibrate the Neoproterozoic Period on the Kalahari craton, along the Orange River, which marks the border between South Africa and Namibia.  There, glacial deposits had been reported to underlie a 741±6 Ma rhyolite flow, representing the oldest age constraint on Neoproterozoic glaciation. We remapped the region and discovered that the conglomerate of the Kaigas Formation (Fm) that is below the dated rhyolite is a debrite that was miscorrelated with glacial deposits of Numees Fm. The Numees diamictite is stratigraphically above the 741 Ma rhyolite, and can be correlated with carbon and strontium isotope (d13C and 87Sr/86Sr) chemostratigraphy to Sturtian glacial deposits around the globe. This age model is important not only because it narrows the age constraints on the Sturtian glaciation, but also because banded iron formation (BIF) is present in the Numees Fm. BIF’s return to the rock record in the Neoproterozoic after a billion year absence is an iconic feature of the Snowball Earth hypothesis, thought to represent the buildup of ferrous iron in anoxic, ice-covered oceans.  Work by myself and colleagues in the Kalahari, Death Valley, and NW Canada demonstrates that the reappearance of BIF was not widespread, but focused both in space and time, occurring exclusively in Sturtian-age glacial deposits that formed in grabens with active volcanism [e.g. Fig. S3 of 16] .  These geological associations suggest that the combination of lowered sea-level during glaciation, restriction in narrow, actively extending basins, favorable Fe/S ratios in the ocean, and enhanced subaqueous volcanism, conspired to produce this enigmatic facies [15, 33].

After refining the maximum age constraints on putative Sturtian glacial deposits in the Kalahari, we focused  our efforts in NW Canada where we identified evidence for grounded marine ice-sheets with interbedded volcanic rocks.  With colleagues at Boise State University, we then dated a rhyolite dome that underlies the glacigenic strata at 717.4 ± 0.1 Ma, and dated a tuff within these glacial deposits at 716.5 ± 0.2 Ma, providing the first age constraints on the onset of the Sturtian glaciation [16].  Finally, we dated sills of the Franklin large igneous province (LIP), which host a robust low-latitude paleomagnetic pole, to 716.3 ± 0.5 Ma, indistinguishable to our syn-glacial age, demonstrating that Sturtian glacial deposits extended to the equator [16], as had been shown previously for the Marinoan glaciation.

The synchroneity of the Franklin LIP and the onset of the Sturtian glaciation suggested a possible link between the two, consistent with the Fire and Ice hypothesis, which proposes that the low latitude breakup of the supercontinent Rodinia, followed by the implacement of LIPS at low-latitude, increased CO2 consumption via enhanced weatherability and plunged the Earth into a global glaciation [e.g. 16].  To test the Fire and Ice hypothesis and to better constrain the duration of the Sturtian glaciation in strata that lack interbedded volcanic rocks, we began a coupled Re-Os geochronology and Osi and Sr isotope chemostratigraphy campaign in NW Canada. With colleagues at MIT and Durham University, we found that the oceanic Osi and Sr isotopes trend toward unradiogenic, mantle-like values going into the Sturtian glaciation, and spike to extremely radiogenic values in the cap carbonate sequence [32].  These data are further consistent with initiation of the Sturtian glaciation via the emplacement and subsequent weathering of flood basalts at low-latitude, followed by glacial scouring and extreme weathering of the continents during deglaciation in a super-greenhouse. Moreover, we dated the Sturtian cap carbonate to 662.4 ± 3.9 Ma (Re-Os isochron).  Coupled with the 717.4 ± 0.1 Ma and 716.5 ± 0.2 Ma ages bracketing the onset [16], these dates represent the first set of age constraints on both the onset and demise of a Neoproterozoic glaciation from a single margin. Together with the recalculation of Re-Os ages from Australia and existing U-Pb zircon ages, these data suggest a minimum duration of ~50 Myr for the Sturtian glaciation, and globally synchronous deglaciation at ca. 662 Ma [32]. We are now building off of these results from NW Canada, and utilizing the unique preservation of weakly deformed organic-rich limestone in Mongolia to construct high-resolution Neoproterozoic-Cambrian 87Sr/86Sr and Osi curves tied to U-Pb and Re-Os geochronology. Our initial results from Mongolia are in line with the data from NW Canada [32], and suggest major mantle input to the ocean proceeds glaciation, and that the Neoproterozoic increase in 87Sr/86Sr is not a linear trend as previously proposed, but is instead stepwise, with rapid rises after glacial events, consistent with extreme weathering in the super-greenhouse aftermath of global glaciation.