SECT. 3] 



BASIN SEDIMENTATION AND DIAGENESIS 



607 



Table I 



Isotope Fractionation in Estuarine Mud and Water of Milford Sound, 



New Zealand 



G 



R240/3 Surface mud 0.280 2L86 0.080 22.92 



2S = 0.218% (drywt) 

 En=- 100 mV, pH =7.20, 



3=S/"S = 22.47 



K240/5 Surface mud 0.262 21.99 0.061 22.83 



2 8 = 0.219% (drywt) 

 Eh= -l-15mV,pH = 7.10, 



"S/"S = 22.39 



-46.2 0.041 22.78 -40.4 



•36.8 0.069 22.86 -38.0 



annual rainfall of 270 in. and a very high run-off rate into the fjord. The influ- 

 ence of fresh water may also be reflected in the somewhat lower sulfate content 

 for this sample. 



The sulfate in the sediments shows a consistently higher ^^g content than that 

 in the sea-water. This cannot be explained on the basis of sulfate reduction, 

 since in that case there would be an enrichment of "^^S. Here again, as in other 

 natural environments described by Kaplan, Rafter and Hulston (1960), it 

 would appear that a reoxidation of sulfide is occurring with the formation of a 

 sulfate enriched in 32g which dilutes the ^'^S-rich sulfate. This process could 

 only occur at the surface, where a sufficiently high oxygen tension is present. 

 On this assumption, an increased depth down a sediment profile should produce 

 sulfate correspondingly increasing in 34g content. This suggestion of sulfate 

 formation by hydrogen sulfide oxidation is also reflected in the higher oxidation- 

 reduction potential as well as in the higher isotojDe ratio in sample R240/5. The 

 presence of ThiobaciUus detected in surface muds supports these conclusions. 



The isotope ratios for both the acid volatile and combustible sulfides (prob- 

 ably of the pyrite tjrpe as well as perhaps some other base-metal sulfides) show 

 an expected enrichment in ^'^S. The highest enrichment is 46.2 %o for the acid 

 volatile sulfide of sample R240/3 with respect to the sulfate in this sample. 

 This enrichment falls close to the value of 46%^ found by Feely and Kulp (1957) 

 for samples of sulfides and sulfates from the salt domes of Louisiana and Texas. 

 Since bacterial sulfate reduction in this fjord is still very active, and no other 

 reasonable explanation can be offered for the formation of such high concen- 

 trations of sulfides, we must conclude that fractionations of 46%o (or somewhat 



