370 REINERS 



are first buffered by the atmosphere and secondarily by land plant biomass 

 together with marine-solution reactions. On the basis of Broecker's report in this 

 volume, marine solution and sedimentation is probably the master control of the 

 cycle in the long term (circa 10 years). 



Where more than one estimate is given for a pool size or flow rate, the 

 estimate supplied by the Brookhaven group is preferred. Discrepancies in these 

 estimates are testimony to our lack of definitive data on the carbon cycle. It 

 should be of particular interest that these discrepancies seem to be greatest for 

 organic components of the system. 



The quality of estimates on the flux rates in Fig. 1 varies widely. In general, 

 rates for physically controlled fluxes and especially atmospheric exchanges are 

 probably the most accurate, and biological rates are the least accurate. Better 

 accuracy is often related to higher levels of homogeneity of process over the 

 earth's surface. There is no analog to mixing to ease the sampling problems of 

 terrestrial processes, such as primary production, forest cutting, or frequency of 

 fire. 



It is conceivable that the present carbon cycle may have existed as an overall 

 steady state for periods of time, but known episodes of vulcanism and mountain 

 building plus changes in area of land and sea and changes in climate suggest that 

 the cycle may have shifted repeatedly throughout geological history. The extent 

 of such shifts might be calculated from the geological evidence (see Broecker, 

 this volume). There is little doubt that the steady state is being disturbed in our 

 era, if only due to fossil-fuel combustion. Increases in atmospheric C0 2 are 

 documented in this volume by Ekdahl and Keeling. Accelerated forest cutting, 

 burning, decomposition of soil organic matter, and erosion are likely to be 

 reducing pools of carbon on land. At the same time, eutrophication may be 

 increasing organic pools in fresh and coastal marine waters. Man's activities are 

 undoubtedly increasing rates of rock weathering, 4 ' 5 but we do not know the 

 significance of these perturbations compared with variation in weathering rates 

 throughout the Pleistocene and over the longer term of the Cenozoic with its 

 relatively high orogenic activity. 



CRITICAL RESEARCH TOPICS 



The net effects of all the influences described in the preceding section are 

 difficult to appraise because inventories and flux rates are poorly known. In 

 many instances processes are insufficiently understood to provide models for 

 predictions. Clearly, estimates of all pools and pathways need refinement, but 

 the Brookhaven group believed that the critical research recommended in this 

 section would yield the most understanding per unit effort. 



The Atmosphere 

 Increases in C0 2 



We are fortunate that the best data on the carbon cycle are also the most 

 critical — measurements of the increase in C0 2 content of the atmosphere. 





