It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum group metals, the high density and tendency of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.
42.
However I have had considerable difficulty getting citations for the minor terms, e . g . biosphere-> fossil fuels, biosphere-> crustal rocks, terrestial biosphere-> oceans, oceans-> fossil fuels, etc . ( I suppose one could technically define all fossil fuels as coming from carbon in crustal rocks, but I would prefer to seperate terrestial and oceanic carbon sources .)
43.
However I have had considerable difficulty getting citations for the minor terms, e . g . biosphere-> fossil fuels, biosphere-> crustal rocks, terrestial biosphere-> oceans, oceans-> fossil fuels, etc . ( I suppose one could technically define all fossil fuels as coming from carbon in crustal rocks, but I would prefer to seperate terrestial and oceanic carbon sources .)
