MODERN PROCESS STUDIES WITH HALF AN EYE ON SNOWBALL EARTH: INSIGHTS AND LIMITATIONS

My present preoccupation with contemporary processes arose from a desire to seek modern analogues for the carbonate-tillite association which is one of the central pieces of geological data in the Snowball Earth debate.

What happens when glaciers crush and transport carbonate rocks in contemporary environments? In terrestrial settings, this makes an already reactive material equilibrate with glacial meltwater extremely quickly: so much so that its rate of dissolution can be limited by the degree of turbulence of the water. It is not possible to find a calcite-free dolomitic substrate today and the fast dissolution kinetics of calcite give it an importance way beyond its abundance. In arid terrestrial settings, photosynthesis and evaporation play a key role in carbonate precipitation. In the Arctic ocean detrital (Palaeozoic) carbonates (mostly dolomite) dominate, but despite this, secondary carbonate is limited to local intermediate-Mg calcite in "Heinrich-like" ice-rafted deposits.

The lack of young metastable marine carbonates undergoing glacial transport today is a crucial difference from Neoproterozoic times and highlights the special nature of the latter. Terrestrial, glaciolacustrine carbonates, enriched in ¹⁸O, from Neoproterozoic Svalbard appear consistent with the aridity of the Snowball, but also have to be reconciled with the thick glacial deposits around them which on the face of it require warm-based glaciers. An issue concerning the ending of Snowball Earth is how to effect a rapid and permanent lowering of high atmospheric CO₂ levels since carbonate weathering would still exceed that of silicates by orders of magnitude, but this process does not result in permanent removal of CO₂.

Apart from the special features of the dolomite rock flour mentioned above, the surprising thing is how readily the physical and chemical attributes of the Neoproterozoic glacial and carbonate sediments can be matched today (albeit by introducing analogues from diverse parts of the globe). To match the strangeness of the climatological shifts, why are the rocks themselves not more strange (more consistently arid for the Snowball hypothesis, or more strongly seasonal for the high obliquity hypothesis)?