Revised estimate of δ34S for marine sulfates from the Upper Ordovician: data from the Williston Basin, North Dakota, U.S.A.

Analyses for δ34S of 13 bedded, marine anhydrite samples from the “C” anhydrite member of the Red River Formation (Upper Ordovician) in the North Dakota portion of the Williston basin represent an addition of δ34S data to a portion of the S isotope age curve with few data. Previously published estimates of δ34S for Upper Ordovician marine sulfates apparently are limited to 4 samples from the Saskatchewan portion of the same basin. An adjusted mean value of +25.5‰ was calculated for all known Upper Ordovician δ34S determinations. This value is approximately 2 to 3‰ lighter than the previous estimate, which suggests that δ34S of the world ocean during the Upper Ordovician may have been lighter than previously thought. However, because all δ34S data are from one sedimentary basin, additional S isotopic data from several globally-distributed evaporite basins are needed to evaluate this hypothesis and further constrain δ34S for the Upper Ordovician. Similar re-examination of other portions of the S isotope age curve with limited amounts of data may increase our understanding of the secular variation in δ34S.