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 shale s of the 2.7 Ga Jeerinah Formation, Hamersley Group, Australia. The analyse s were performed using the Nd-YAG laser microprobe method. The delta(13)C((PDB)) values of organic matter range from -38.3 parts per t housand to -35.1 parts per thousand with an average of -37.0 parts per thou sand (44 analyses). These carbon isotope compositions support the previousl y proposed hypothesis that methanogens and methanotrophs were involved in t he 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 v ariable delta(34)S values within a micro-scale area; approximately 6 to 7 p arts 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 th e Jeerinah shales were formed as a result of the sulfate reduction. This su ggests that the 2.7 Ga Hamersley ocean contained appreciable amount of diss olved sulfate, opposing to the previously popular H2S-rich ocean model. Bec ause of no evidence for the hydrothermal alterration on the examined sample s, pyrites in the 2.7 Ga Jeerinah shales were most likely formed by the bio logical sulfate reduction in sediments.