Sulfur and carbon isotope analyses of the 2.7 Ga Jeerinah Formation, Fortescue Group, Australia

Sulfur and carbon isotope ratios have been determined on carbonaceous shales of the 2.7 Ga Jeerinah Formation, Hamersley Group, Australia. The analyses were performed using the Nd-YAG laser microprobe method. The delta(13)C((PDB)) values of organic matter range from -38.3 parts per thousand to -35.1 parts per thousand with an average of -37.0 parts per thousand (44 analyses). These carbon isotope compositions support the previously proposed hypothesis that methanogens and methanotrophs were involved in the carbon cycles in the 2.7 Ga Hamersley ocean. Sulfur isotope compositions are ranging from +0.4 parts per thousand to +10.2 parts per thousand with an average of +4.6 parts per thousand (90 analyses). A notable feature is variable delta(34)S values within a micro-scale area; approximately 6 to 7 parts per thousand variations of delta(34)S values were detected within 5 x 5 min areas. Such delta(34)S variations clearly indicate that pyrites in the Jeerinah shales were formed as a result of the sulfate reduction. This suggests that the 2.7 Ga Hamersley ocean contained appreciable amount of dissolved sulfate, opposing to the previously popular H2S-rich ocean model. Because of no evidence for the hydrothermal alterration on the examined samples, pyrites in the 2.7 Ga Jeerinah shales were most likely formed by the biological sulfate reduction in sediments.