The Neoproterozoic glacial countdown to the biological 'big bang'

The hospitable environment of the Earth that we know today was born between approximately 780 and 540 million years ago. During this time, the supercontinent of Rodinia had already formed and was beginning to break apart along the equator. During this break up, massive amounts of volcanism were documented, which changed the chemical composition of our atmosphere and oceans. These changes led to a series of glacial periods, of which at least two were global. These global glaciations, however, were critical for the rise of life as we know it today. After the glaciations ended, the oceans were rich in nutrients that allowed life to flourish. As life flourished, the oxygen concentration in the atmosphere increased dramatically, which allowed more and more complex life to develop – a positive feedback. With the increasing oxygen concentrations and the emergence of multi-cellular life, a critical point was reached 541 million years ago: the biological ‘big bang’. This big bang marks when the biological diversity of life increased dramatically. dr Ashley Gumsley, photo Łukasz Bera

This global consensus, however, is not without controversy. Many of the mechanisms which allowed for the build-up of atmospheric oxygen and the development of multi-cellular life, remain poorly understood. One of the keys necessary to understand this controversy better is context through accurate and precise geochronology (i.e., age dating), on key intervals across this time period. This will assist when used in conjunction with other studies on geochemical and geobiological cycles and indicators.

Several areas exist where rocks from this time period are preserved, and one of the best place for rock remnants is in Namibia and South Africa. These remnants include a variety of sedimentary and volcanic rocks. However, many contradictory correlations have been made on the glacial rocks (i.e., diamictites), preserved there. This includes their timing, and whether they are from four, three or two glaciations. Resolving these timings and correlations was the goal of this study, using a combination of mostly geochronological techniques. This is needed, as the four glaciations documented may be the product of structural complexity. This is important, as some of the diamictites are controversially interpreted to have formed before and after the known global glaciations: the Sturtian and Marinoan. This complicates any modelling of the geochemical and geobiological cycles and indicators of the Earth at this time. As diamictites are very diagnostic units, they can be used in the rock record as tracers.

dr Ashley Gumsley, photo Łukasz Bera In this project, it was shown that the existence of this pre-Sturtian glaciation is incorrect. This is due to the remnants of one of these massive magmatic provinces being shown to be quite complex, being composed of multiple events following the same pathways over a protracted time period. It was found to be composed of magmatic conduits varying in age from 1508 million years ago, to 717 million years ago, and other ones emplaced 508 million years ago, highlighting the complexities of using cross-cutting relative age relationships to determine the absolute age of the sedimentary (glacial) units.

Additionally, new, massive magmatic events were also identified within this study, which adds key data on the placement of southern Africa within the supercontinent Gondwana (the successor of Rodinia) during this whole time period, and which will help us resolve the paleogeography from this tumultuous period in time, as well as identify further glacial units elsewhere.