On Nov 21, 11:32 pm, Weatherlawyer wrote:
On Nov 21, 8:24 pm, Alastair wrote:

The carbon that gets subducted is the Great Barrier Reef and the White
Cliffs of Dover. The latter actually stretches from Beer, on the
Dorset/Devon border to Malta and up into the Dolomites. There is
plenty of limestone that has not been buried yet. Not all volcanoes
pump out CO2, but since all the limestone gets formed on the sea bed,
it is in the right place to get subducted.

Yes Calcium carbonate contained in shellfish exoskeletons will go into
an abyss quite easily but then what provided all the sulphur? Besides,
the narrator didn't make it plain that he was talking about
lime"stone" subduction.

It plays a part. Limestone locks up a heck of a lot of the Earths CO2.
Without it we would have a considerably higher atmospheric pressure
and a composition more like that of Venus.

There is also expected to have been a fair amount of carbon in the raw
material that the Earth was formed from based on analysis of meteorite
composition.

I suppose that that is because he didn't want to get dragged into a
chicken and egg argument about where the free calcium came from to
supply the carbonate in the first place.

Calcium is an abundant terrestrial element. See for example:
http://www.daviddarling.info/encyclopedia/E/elterr.html

And calcium bicarbonate is relatively water soluble on a geological
timescale. Liquid water makes a big difference.

Regards,
Martin Brown