USING MINERALS TO MAP THE MARINE CHEMOCLINE: A PROTEROZOIC BASIN CASE STUDY

The marine iron chemocline records the trajectory of Earth’s surface oxygenation and the availability and cycling of essential elements like Fe, Mn, Cr, Ni, Co, and V throughout the Precambrian. However, pinpointing the depth, movement, and temporal evolution of marine chemoclines remains difficult in Paleoproterozoic terranes, though important for evaluating potential critical mineral and natural hydrogen gas resource development. Bulk geochemical and isotopic datasets paint a broad picture of chemocline evolution across millions of years and multiple basins, but often lack the ability to track nuanced shorter-scale temporal records expressed at the local and regional scale. Our recent mineral specific work in the Animikie Basin, Minnesota hints that texturally early authigenic minerals record a prominent iron chemocline, which in turn pinpoints basin redox stratification. We leverage mineralogical and texture-specific geochemical datasets to document systematic diagenetic reactions from which we decipher early mineralogy and depositional conditions that link to the geologic framework. We further examine mineral chemistry records of texturally early Fe³⁺ and Fe²⁺ containing assemblages to map where drill core datasets directly intersect the iron chemocline throughout the stratigraphic section. Combined mineral specific data and sedimentary characteristics of Animikie basin chemical sedimentary units record transitions from oxidized conditions at the the base of the section, to reducing conditions mid-section, with a return to oxidized conditions toward the top of the section. Stratigraphic variability in mineralogy and geochemical signals may reflect diachronous lithofacies stacking during sea level change. Additionally, texture-specific geochemical data separated from co-occurring banded and granular couplets reveal disparate depositional processes. Banded couplets record seawater geochemical signals while granular couplets record a snapshot in time that may or may not be in equilibrium with the overlying water column due to seafloor silicification and granule formation processes. Overall, mineral records directly identify the marine chemocline in this Proterozoic case study and may offer a cost-effective way to assess redox-sensitive critical mineral resource formation and natural hydrogen gas producing reactions.