Effects of Paleolatitude on Coal Quality--Model for Organic Sulfur Distribution in United States Coal

Ronald H. Affolter, Gary D. Stricker

In the conterminous United States, most Carboniferous peats accumulated at latitudes of 0°-15°S, Cretaceous Rocky Mountain province peats at 30°-45°N, Tertiary northern Great Plains peats at 40°-55°N, and Tertiary Gulf Coast peats at 30°-40°N. Alaskan Cretaceous and Tertiary peats accumulated at latitudes above 70°N. A comparison of paleolatitudes calculated from paleomagnetic poles and organic sulfur contents for more than 7,000 coal samples indicates that the higher the latitude in which a peat swamp developed, the lower the mean organic sulfur content of the subsequent coal (correlation coefficient -0.4; significant at the 99% confidence level). Mean organic sulfur contents range from 0.90% (range = 0.01-5.08%, standard devi tion = 0.56) in low-latitude Carboniferous coal to 0.25% (range = 0.01-1.41%, standard deviation = 0.23) in high-latitude Alaskan Cretaceous and Tertiary coal.

Although many factors may affect the overall content of organic sulfur, the data suggest the latitude at which the original swamp developed plays an important role. The amount of incident solar radiation that a peat swamp receives during its development varies with latitude and would affect the average temperatures in the swamp. Much of the sulfur that entered the coal-forming environment as sulfate was reduced by the action of sulfate-reducing bacteria. Temperature is one of several controls that can affect the rate of microbiological activity, and sulfate reduction is generally inhibited by lowering the temperature about 5-10C°. The inhibition of sulfate-reducing bacterial activity could suppress the amount of reduced sulfur available for incorporation in the peat as organic su fur.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.