20 - 



E 



0) 



i_ 

 o 



a 

 a 



40 - 



60 



0.10 



0.20 



% Carbon by Weight >38/j 



Figure 2. The depth profile of elemental carbon (charcoal) in a Lake Michigan sedi- 

 ment. The ages were determined by Pb-210 geochronology. 



On the other hand, one component of the sediments, quartz, the source of which is 

 dominantly natural, decreased markedly in concentration in the recently deposited 

 strata, as compared with pre-industrial revolution values. The fall-off in concentra- 

 tion amounted to about 20 percent and resulted from the increased fluxes of fly ash 

 and associated debris to the deposit site from industrial activities. 



There are a continually increasing number of studies illustrating similar environ- 

 mental histories of pollutants based upon records in sediments. Of importance is the 

 knowledge that the historical records are available, especially for those substances 

 that are recognized as toxic well after the time of their initial introduction to the envi- 

 ronment. In addition, knowledge about the persistence of organic materials in the 

 marine environment can be drawn from their persistence in the sediments. 



THE STEPPING STONES 



In the past three decades, during which the problems of marine pollution have 

 been identified, knowledge about the oceans has grown dramatically. A part of this 



