Quantity and isotopic composition of reduced sulfur in Cretaceous organic-rich shales

• Main Author: Bonnell, Linda Marie
• Other Authors: Anderson, Thomas F.
• Format: Text
• Language: English
• Published: 1990
• Subjects: Geochemistry; Pacific; North Atlantic
• Online Access: http://hdl.handle.net/2142/21367

Over 200 samples of mid-Cretaceous organic-rich sediment were examined to determine abundance and isotopic composition of reduced sulfur. Mid-Cretaceous pelagic sediments, particularly those of the North and South Atlantic, are characterized by high organic carbon contents relative to the modern average and should contain significant amounts of reduced sulfur, primarily pyrite. However, existing models of sulfur-carbon-iron relationships cannot account for the distribution of pyrite in mid-Cretaceous shales. Pyrite distribution in these sediments can better be described in terms of: (1) the quality of organic matter available and its effect on reduction rate, (2) physical sorting during sediment transport, and (3) the absence of reactive iron coincident with hydrogen sulfide. Sulfur isotopic evidence shows sulfate reduction rates were slow compared to those reported in studies of modern sediments. Slow reduction rates can be attributed to the refractory organic matter available to sulfate reducing bacteria. The limited amount of metabolizable organic matter along with slow sedimentation rates apparently led to slow sulfate reduction and to isotopic fractions between sulfide and sulfate ranging from $-$36% to $-$66%. Three localities from the western North Atlantic display decreasing sulfur/carbon (S/C) ratios and sulfur-isotopic compositions with increasing distance from the continent. Sediments at these are turbidite deposits. During transport pyrite settled in proximal parts of the turbidite deposit whereas fine-grained organic matter travelled to distal localities. S/C ratios should decrease with transport distance, as was observed. Turbidite deposits are widespread throughout the mid-Cretaceous Atlantic and on basement highs in the Pacific and could have played a significant role in the distribution of pyrite in these sediments. A third factor which should have limited pyrite formation in mid-Cretaceous sediments is the lack of reactive iron coincident with hydrogen sulfide. Low sulfide concentration in the pore-waters may have been insufficient to titrate available iron as iron monosulfide and other reduced iron minerals may have formed. Thus iron was no longer available for reaction with sulfide and evolving H$\sb2$S could have reacted with organic constituents. This process could explain the predominance of organic sulfur compounds over pyritic sulfur in many mid-Cretaceous sediments.