Neoproterozoic Snowball Earth and the 'Great Unconformity,

Date: 
Friday 25 May 2018 - 14:00 to 15:00
Location: 
NOC Southampton - Seminar Room (104/13) (Waterfront Campus).
Speaker: 
Tom Gernon (OES)

The Neoproterozoic “Snowball Earth” hypothesis remains a subject of lively debate and scrutiny among geoscientists. Two important yet unresolved issues are the origin of thick cap carbonate sequences (>1 km), and evidence (or lack thereof) for a global erosive event of unprecedented scale.

During glaciations, the oceans gained massive amounts of alkalinity, culminating in massive cap carbonate deposition on deglaciation. Changes in terrestrial runoff associated with both breakup of the Rodinia supercontinent and deglaciation can explain some, but not all of the requisite changes in ocean chemistry. Submarine volcanism along shallow ridges formed during supercontinent breakup potentially made an important—but overlooked—contribution to changes in ocean chemistry during Snowball Earth glaciations. Monte Carlo simulations show that widespread basalt alteration under near-global sea-ice cover could lead to Ca2+ and Mg2+ supersaturation over the course of the glaciation that is sufficient to explain the volume of cap carbonates deposited. Furthermore, conservative estimates of phosphorus release are sufficient to explain the observed P:Fe ratios in banded iron formations from this time. This large phosphorus release may have fuelled primary productivity, which in turn would have contributed to atmospheric oxygen rises that followed Snowball Earth episodes. 

One test of the Snowball hypothesis that has not been explored is whether evidence exists for deep glacial erosion. I will present new evidence from a range of sources (including the Moon) for Neoproterozoic glacial erosion totaling a global average of several vertical kilometers. I propose that this rapid erosion produced the ‘Great Unconformity’, a very prominent surface observed globally in the rock record. Together, these results provide new constraints on the geochemical and sedimentary environment in which the first multicellular animals evolved and diversified in the "Cambrian explosion".

 

 

Seminar category: 
Friday seminars