Fig. 1.

Fe speciation and S-isotopic data from the Sheepbed Formation and C-isotope data from the Gametrail Formation in the Shale Lake section, Northwest Territories, Canada. Correlative sections at Stelfox Mountain (32) and Sekwi Brook (26) are shown for reference. Note the shift in FeHR/FeT ratio between 138 and 160 m at Shale Lake. The dashed line separates anoxic deep water at >0.38 from oxic deep water at <0.38 (47). Fe speciation data of filled circles were measured by using the method in ref. 47, and the data of open circles were measured by using the method in ref. 52.