Comings and Goings of the End-Cryogenian Ice Sheet: A Stratigraphic Study of the Pre-, Syn-, and Post-Glacial Deposits, South Australia.

Abstract

[1] Between 1000 and 542 million years ago, the geologic record shows evidence for the progressive rise in atmospheric oxygen, large changes in the carbon cycle, and extreme climatic states. This era was punctuated by the `Sturtian' (710 million years ago) and `Marinoan' (635 million years ago) glaciations, where ice may have extended to low-latitudes near the equator. These glaciations are of particular importance because their severity may have set the stage for the radiation of complex animal life. In the pages that follow, I use a combination of field observations and chemostratigraphic data to study sedimentary rocks in South Australia that document the comings and goings of the younger Marinoan ice-sheet near the paleo-equator. This research yields new insights into the origin of pre-glacial perturbations to the carbon cycle (Chapter 2), the nature of syn-glacial incision and deposition (Chapter 3), and the style of subsequent deglaciation (Chapter 4).

[2] Carbonates preceding the Marinoan glacial succession record an 18 permil negative shift in the d13C of carbonate around the world. This `Trezona' isotopic anomaly is the largest d13C shift in Earth history and its origin and timing with respect to the glaciation remain controversial. The d13C anomaly could record a dramatic reorganization of Earth's carbon cycle and be linked causally to the initiation of Marinoan ice-house conditions. Alternatively, the d13C anomaly might record secondary fluid alteration following carbonate deposition. I present detailed sedimentological observations paired with chemostratigraphic data to show that the Trezona anomaly was recorded prior to the local glacier advance and late-stage burial diagenesis, and that d13C recovery toward 0 permil was synchronous with the appearance of icebergs in the tropics. This work provides the first systematic assessment of all the competing hypotheses offered to explain the Trezona d13C anomaly.

[3] Glacial deposits generated during the Marinoan Glaciation have received little attention in recent years because research has emphasized the geochemistry of the pre- and post-glacial carbonate successions. However, these enigmatic low-latitude glacial sediments hold important information regarding the style of regional glaciation, as well as sedimentological details, that provide clues about global nature of the glaciation. The syn-glacial Elatina Fm records an impressive array of facies at different water depths across the ARC and provides a unique opportunity to study the comings and goings of a low-latitude ice sheet within South Australia. I present detailed sedimentological observations coupled with high-resolution geochemical data throughout the pre- and syn-glacial sediments of the Elatina glaciation to document the onset of the glaciation across the basin, quantify the degree of erosion, identify the provenance of sediment and clasts, and determine temporal variability in chemical weathering to test predictions of the snowball Earth model. This work provides the first geochemical analysis of the Elatina diamictites, reinterprets the sedimentary fold test for the low-latitude paleolatitude interpretation, and presents detrital zircon U-Pb ages data that may have implications for correlation of the Elatina glaciation to other low-latitude glacial deposits.

[4] Sedimentologically and geochemically distinctive carbonate sequences consistently drape the glacial deposits associated with the Marinoan ice age record the deglaciation. An isochronous model proposes that cap dolostones were deposited synchronously as a blanket around the world regardless of water depth, whilst a diachronous model proposes that deposition tracked glacioeustatic flooding during deglaciation. I present a comprehensive carbon isotope dataset that supports neither a uniquely isochronous or diachronous model, and given the high frequency spatial variability of d13C values, temperature cannot be the dominant control on the isotopic variability of the cap dolostone. This work provides the first test of the cap dolostone models using sedimentological and d13C data taken from across a basin in different water depths.