THE BIOSPHERE 3 



Data for the number of meteorites striking the earth each year are readily 

 available, but data for their total mass are not. If we assume an average mass of a 

 hundred kilograms for 500 meteorites per year, we shall almost certainly be 

 greatly overestimating the total. About 1% of the meteorites are carbonaceous 

 chondrites with up to 5% carbon. The amount of carbon in all other classes of 

 meteorites, including class 2 and class 3 chondrites, is negligibly small in 

 comparison. This leads to the miniscule figure of 25 kg gained each year by this 

 route. 



Most of the cosmic material reaching the earth, however, does not fall in the 

 form of meteorites big enough to be collected and counted. It comes as fine dust 

 and has been estimated at from 500,000 to 5,000.000 tons each year 

 (Patterson ). Accepting the higher figure and assuming that the proportion of 

 carbon in the dust is similar to that in the meteorites suggest that 2500 tons of 

 carbon might reach the earth each year. Actually, of course, the recognizable 

 dust is only the metallic part. It would not be unreasonable to double the 

 estimate on this account to 5000 tons/year. Even if there are undiscovered errors 

 of as much as two or three orders of magnitude in this computation, the gain of 

 carbon from outer space is too small to be geochemically important. The 

 qualitative carbon chemistry of chondrites is fascinating for the insight it gives us 

 into the chemical history of some astronomical body other than our own planet, 

 but, as a quantitative feature of the carbon cycle on earth, apparently meteorites 

 can be ignored. 



During the past few years, some of the most interesting developments in 

 carbon geochemistry have involved the analysis of radiocarbon data in 

 accordance with simple dynamic models of the ocean— atmosphere system. 

 Despite the success of this simple approach, I will enter a plea for attention to 

 some of the more difficult parts of the carbon cycle. In the chain of events from 

 the death of a terrestrial plant through its incorporation in the litter and soil 

 humus, transformation of some of it into the carbonic acid of soil water that is 

 largely responsible for the chemical weathering of rocks, and transfer of the 

 resulting solutions to the sea, there are few simple equilibria and few pools or 

 fluxes that can be measured easily, but these processes are as important as they 

 are difficult to measure. 



Measurements of diffusive flux across the sea surface have shown it to be 

 very large, and this largeness occasionally leads to the neglect of other important 

 fluxes in the system. For example, Takahashi, 4 in an otherwise interesting and 

 informative article on carbon dioxide in the sea and atmosphere, says that the 

 carbon dioxide lost from surface ocean water by photosynthesis and biological 

 carbonate precipitation is replaced by diffusive gain from the atmosphere, which 

 is of approximately the same size. He maintains that the contribution from river 

 water is probably on the order of V 500 of photosynthetic consumption by 

 marine plants and may be neglected in discussing the source of the carbon 

 removed from the sea biologically. 



