Onset and Deglaciation of Cryogenian Snowball Earth

Cryogenian Snowball Earth glaciations are some of the most extreme changes in Earth’s climate through its history. Decades of work have established the global ubiquity of Snowball Earth sedimentary deposits and their synchroneity in time, yet key questions remain concerning the potential causes for global glaciations and their drastically different durations and expressions in the rock record. This dissertation addresses these questions and proposes explanations for the onset of global glaciation and how the abrupt termination of a Snowball event could explain the differences in chemical sediments seen between the two Cryogenian Snowball events. In Chapter 1, I evaluate the emplacement of a large igneous province as a potential trigger for the Sturtian Snowball Earth (717-661 Ma). This work was done in collaboration with Francis A. Macdonald, Mark D. Schmitz, Robert H. Rainbird, Wouter Bleeker, Barra A. Peak, Rebecca M. Flowers, Paul F. Hoffman, Matthew Rioux, and Michael A. Hamilton. Previous geochronology has suggested a rough coincidence of glacial onset with one of the largest magmatic episodes in the geological record, the Franklin large igneous province. I show that chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology on zircon and baddeleyite from sills associated with the paleo-equatorial Franklin large igneous province in Arctic Canada record rapid emplacement between 719.86 ± 0.21 and 718.61 ± 0.30 Ma ago, 0.9 to 1.6 Ma before the onset of widespread glaciation. Geologic observations and (U-Th)/He dates on Franklin sills are compatible with major post–Franklin exhumation, possibly due to development of mafic volcanic highlands on windward equatorial Laurentia and increased global weatherability. After a transient magmatic CO2 flux, long-term carbon sequestration associated with increased weatherability could have nudged Earth over the threshold for runaway ice-albedo feedback.In Chapter 2, I address whether there is evidence for glaciations in the 50 Myr prior to the Sturtian glaciation by examining the proposed ca. 750 Ma Kaigas glaciation. I evaluate this hypothesis at the eponymous location with detailed stratigraphy and geochronology through the Kaigas, Rosh Pinah, and Numees formations on the western margin of the Kalahari craton in southern Namibia. This work was done in collaboration with Francis A. Macdonald, Emily F. Smith, Jahandar Ramezani, and Nicholas Swanson-Hysell. We find that glacial deposits previously assigned to the Kaigas Formation are instead ca. 717-661 Ma diamictites of the Sturtian Numees Formation. Pre-Numees strata, including the Kaigas Formation, host facies associations diagnostic of fan delta deposition along an active normal fault. Interbedded volcanic rocks in the Rosh Pinah Formation overlying the Kaigas Formation were dated with U-Pb CA-ID-TIMS on zircon at ca. 752 Ma. These Tonian deposits are interpreted as being deposited in an active rift basin without evidence for glaciation. Rosh Pinah magmatism could be correlative with the Mount Rogers Complex in Virginia, USA, consistent with a scenario of the Kalahari craton actively rifting from Laurentia and associated terranes within 20º of the equator at the time. We conclude that, at least in marine settings, evidence for low-latitude glaciation is limited to the 717-635 Ma Cryogenian Period. In Chapter 3, I propose an explanation for why the Sturtian and Marinoan glaciations differ. This work was done in collaboration with Mark D. Schmitz and Francis A. Macdonald. I present new geological mapping, measured stratigraphic sections, and U-Pb zircon geochronology and geochemistry on Cryogenian successions exposed in the Naukluft Nappes of Namibia. Stratigraphic sections measured in the context of geological mapping document glacial deposition on a slope setting below the ice grounding line. Deglaciation is marked by an abrupt coarsening, boulder-sized dropstones, and the appearance of volcaniclastic deposits. Large lithic fragments within the volcaniclastics and detrital zircon provenance using laser ablation split stream mass spectrometry suggest a local source. CA-ID-TIMS on these units yielded a weighted mean 206Pb/238U date of 635.84 ± 0.22/0.29/0.71 Ma, which is interpreted as a depositional age. We suggest that glacio-isostatic unloading reactivated the formerly rifted passive margin. This age overlaps with dates near the top of Marinoan glacial deposits on the Swakop terrane, Australia, and South China. We suggest that all of these dates record deglaciation and that deglacial volcanism associated with isostatic unloading provided a positive feedback for both albedo and CO2 that shortened the Marinoan glaciation.

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