Salt Shaker

30 Mar 2018

At ... an excellent link sent in by Jovan - primeval salt deposit shakes up ideas on how the atmosphere got its oxygen (by Nola Taylor Redd). An ancient deposite of 'sea salt' drilled from a geological basin in Russia is providing dramatic new clues on how Earth's 'early' atmosphere became oxygen rich - allowing life to evolve. This salt deposit is a billion years older than other salt deposits we are told. Its age puts it smack in the middle of the Great Oxygenation Event - which is when oxygen began to dominate atmospheric chemistry. The study is published in the journal Science (March 23rd 2018). The salt, we are led to believe, is made up of minerals left behind after water has evaporated - sea water. These evaporative minerals are our most direct way of sampling ancient sea waters, it continues. Within a 3km long cylindrical core extracted from the Russian site (lead author comes from Princeton University in the US but a co-author is from St Andrews University in Scotland) there was a 600m thick deposit of sulphate rich minerals, included sodium chlorite (the crystalline progenitor of common table salt. Its immense size and geochemical markers, we are told, suggest it formed in ocean water. However, they then say the thickness of the deposit reveals the process occurred over and over again, gradually building up the deposit over time. There is no way you can from that much from just evaporating one batch of sea water, it continues, indicating the basin filled with sea water that evaporated multiple times - but is this really necessary. Might the origin of the salt be from some other process? In other words catastrophism might explain the salt deposit - but not a uniformitarian process.

The analysis of the salt deposit is said to show roughly 20 per cent as many sulphates as the modern oceans. Sulphate (or sulfate) concentration in sea water is said to be a key tracer of how much oxygen is in the atmosphere - and how it gets there in the first instance. The piece ends up telling us in that in usual circumstances, and especially if sea flooded the basin on multiple occasions, the sulphate would be dissolved in the next flooding event - redissolving the previous evaporites (and erasing the record by laying down a new layer of minerals) - 'for some unknown geological reason these were preserved, and they were a little bit unexpected ...'. The real interest in the deposit is that it seems to show a dramatic transition - a sudden jump in oxygen levels. 'Our climate observations favour a dramatic transition' we are told. All they have to do now is decide where all the oxygen originated - an asteroid strike perhaps, or a welling up from Earth's interior, perhaps a global volcanic event?