REDOX CONDITIONS IN ENVIRONMENTS WITH EARLY EUKARYOTES FROM THE 1.4 GA BELT BASIN, USA DETERMINED FROM IRON MINERALOGY

Iron chemistry and mineralogy in sedimentary rocks provide a valuable record of ancient redox processes, but questions still remain as to the O₂ content of Proterozoic basins after the rise of oxygen during the evolution and early diversification of eukaryotes. To better ascertain the paleoenvironmental behavior of iron in Precambrian samples that typically bear post-depositional overprints, we developed an approach that pairs the microscale textural techniques of light and electron microscopy, magnetic microscopy, and (synchrotron-based) microprobe X-ray spectroscopy with sensitive bulk rock magnetic experiments. Samples were collected from excellently preserved shales and siltstones—focused on formations containing stem group eukaryotic fossils—in the eastern stratigraphic sections of the ~1.4 Ga lower Belt Group, Belt Supergroup, Montana, USA. Primary iron phases were typically affected by post-depositional processes in open fluid-rich conditions including recrystallization of iron sulfide phases and formation of base metal sulfides and nano-phase pyrrhotite. The presence of nano-phase pyrrhotite highlights the low-temperatures at which the conversion from pyrite to pyrrhotite can occur. Primary records of redox chemistry were preserved in shallow hematite-rich red beds, (sub)micron-sized detrital magnetite grains, Fe-rich carbonate cements, and recrystallized early diagenetic pyrite framboids. Magnetite was quantified using magnetic techniques at 1 to 8 ppm in sharp contrast with previous bulk extraction analyses that suggested far higher contents of 0.06 to 0.51 wt% magnetite. Detrital magnetite and hematite represented an important flux of highly reactive iron to the Belt Basin and their preservation highlights the oxic nature of shallow waters as well as fairly oxic deep waters and pore-fluids. Iron carbonate cements and pyrite framboids formed in anoxic and sulfidic pore waters, potentially extending into deep portions of the water column episodically or in sub-basins. Although distinct from the prior interpretations of ferruginous waters based on iron speciation results, this more oxygenated redox reconstruction is consistent with paleontological identifications of diverse groups of (aerobic) eukaryotes and previous interpretations of sulfur isotope data.