IN-SITU IRON ISOTOPE ANALYSIS OF PYRITE AND ORGANIC CARBON/NITROGEN ISOTOPE RATIOS FROM THE MIDDLE PROTEROZOIC SEDIMENTS, MCARTHUR BASIN, NORTHERN AUSTRALIA

Oxygenation of Earth's surface is deeply linked to evolution of life. Many of independent evidences suggested that the Earth’s atmospheric oxidation state increased in two-stepwise: (1) from 2,400 to 2,300 million years ago, and (2) around 600 million years ago (Holland, 2002; Holland, 2006). On the other hand, ocean was mostly reducing condition during the Archean eon, whereas the Phanerozoic oceans were oxygenated as modern. Compared with research for Archean and Phanerozoic ocean, the redox status of ocean during middle Proterozoic (1.8–1 billion years ago) remains little known. However, Canfield considered that the middle Proterozoic deep ocean was globally sulphidic condition (Canfield, 1998). On the other hand, Planavsky and others considered that deep ocean was globally iron-rich anoxic condition, and sulphidic conditions are restricted to biologically productive ocean margin and restricted marginal basin (Planavsky et al., 2011).

Here we show iron isotope analysis of individual pyrite grains of middle Proterozoic sediments, mainly mudstones and black shales, from four drillcore samples (Mount Young 2, McArthur River 2, Urapunga 4 and 5) in McArthur Basin, Northern Australia. We obtained a large variation of iron isotope data from -2.45 to +2.51 ‰ in δ⁵⁶Fe values, from 178 points of pyrite grains in 15 rock specimens. The highly positive δ⁵⁶Fe values of pyrites up to +2.51 ‰, indicating partial oxidation occurred in the ocean. It suggests that the pyrite precursor minerals were formed through interaction of ferruginous seawater under anoxic conditions. The results support that the middle Proterozoic deep ocean was dominantly ferruginous conditions.

In addition, we also analyzed carbon and nitrogen isotope ratio using whole rock powder from these rock samples. From these results, we discuss about the redox condition of middle Proterozoic ocean.