Lockwood over at Outside The Interzone is successfully ensuring that the Acccretionary Wedge is rising phoenix-like from the ashes. The mission for this month is … “Where and when would you most like to visit to witness and analyse an event in Earth’s history?”.
I suppose that like most geologists I’d like to visit the K/Pg boundary to see if it was the volcanoes or the asteroid/comet ‘wot dun it’ but leaving aside the obvious I’d like to travel back to the Lower Cambrian to answer the question – “Why is this rock red?”
These are the Caerfai Bay Shales, from their type locality in Caerfai (pronounced care-vahy) Bay, Pembrokeshire, South Wales. The Lower Cambrian was actually known as the Caerfai until International Commission on Stratigraphy got their grubby hands on it. These siltstones and mudstones are marine, they contain bioturbation and the very occasional ostracode Indiana lentiformis fossils. So why are they red? This colour is characteristic of haematite, with oxidised iron, typical of oxidising terrestrial environments. Marine silts tend to have reduced iron and are greenish coloured.
The story I was spun twenty years ago was that the haematite was detrital, from the erosion of lateritic soils. The major problem with this is that Wales in the Lower Cambrian was at a latitude about 70°S, well out of the tropical weathering zones.
More recent work would indicate that the haematite is of biogenic origin but this in turn means that something very strange is happening to the ocean water chemistry at the time. Not only must the sea water become very iron rich but we are also at the time when there is a major change from high magnesium concentrations to low magnesium. Prior to this was an ‘Aragonite Sea’ with the primary organic carbonate precipitates being aragonite and high-magnesium calcite, subsequently it is a ‘Calcite Sea’ of low-magnesium calcite.
So, we have a major change in sea water chemistry. We also have the ‘Cambrian Explosion’ occurring at precisely that same time. Up to this point in the Cambrian we have the ‘small shelly fauna’ but it is now that the triboltes and echinoderms kick in. This surely cannot be a coincidence.
So I’d like to take my time machine back to the Lower Cambrian to find out just what is causing the chemistries of our oceans to change as it may well have had a very significant influence on life on Earth.
