SULFUR, NITROGEN, AND CARBON IN THE BIOSPHERE 



189 



S, 10 14 tons 



11.35 



T 

 K 



P 



IP 



M 



D 

 S 

 



6 34 S . 32 g 0/oo 



Fig. 1 Sulfur-isotope age curve, from Holser and Kaplan. 2 5 Computed total 

 quantities of oceanic sulfur, based on an assumed constant oceanic volume, 

 have been added by Hitchcock and Wechsler. 1 (After Hitchcock and 

 Wechsler. 1 ) 



temperatures in the Permian and Cretaceous periods, which I find counter- 

 intuitive. 



Holser and Kaplan 5 suggested that there is a shifting balance between the 

 weathering of sulfur from sediments, which they assume to be chiefly shales, and 

 the deposition of sulfide in the sediments: at times when more light sulfur leaves 

 the ocean than enters it, the concentration of sulfur in the ocean falls, and the 3 

 S of the remaining sulfate rises. (The scale showing the quantity of sulfur has 

 been added to the diagram, Fig. 1, by Hitchcock and Wechsler.) This, too, is an 

 oversimplified model because it neglects evaporite sulfur, assumes an ocean of 

 constant volume, and makes predictions about the oxygen content of the 

 ocean— atmosphere system which are hard to verify. If we notice that, during 

 high rates of deposition of 32 S and sulfide, the mean redox state of the ocean is 

 lower so that black shales can be both formed and preserved in the open ocean, 

 we are suggesting oscillation in the relative proportions of anaerobic environ- 

 ments. If that suggests intensified recycling of biological elements, there are 

 interesting implications for the terrestrial ecology of the Devonian, the 



