METHODOLOGY FOR EVALUATING HEAVY MINERAL HYDRAULIC EQUIVALENCE WITHIN SOUTH AFRICA’S MOODIES GROUP AS FIRST STEP IN BIOSIGNATURE SEARCH

Oxygenic photosynthesis was likely not the first microbial metabolism. Phototrophs possibly produced other biproducts before oxygen. The geologic record contains many proxies that O₂ levels remained low until about 2.5 Ga. Unless oxygenic photosynthesis evolved very rapidly, oxygen build up must have started earlier. How long before 2.5 Ga is our question. To do this, we inquired into whether heavy minerals (e.g., zircon, rutile, chromite, pyrite, and apatite) in these sediments were in hydraulic equivalence. If so, we can calculate their expected sizes using Stoke’s Law. If oxygen levels were high enough in these waters, redox sensitive grains like pyrite could chemically erode and be smaller than expected, while chemically resistant minerals like zircon and rutile would be unaffected. We examined the Moodies Group in the Barberton Greenstone Belt (South Africa) where microbially induced sedimentary structures (MISS) have been reported from this, the oldest reported siliciclastic environment, a shallow delta from 3.2 Ga. We scanned cut rock slabs with a Bruker µXRF (10 µm pixel size) and produced datasets with point count data for individual elements (e.g., Zr, Ti, Cr, S, P) as proxies for the heavy minerals listed above. We used a Python implementation of Lindeberg's watershed-based algorithm to detect and outline the extent of local bright pixel regions (presumably grains) in the µXRF scans. We created a set of rules, including: 1) defining local maxima (relative to the background), 2) defining the prominence of these maxima (relative to their neighbors), 3) smoothing to simplify peak structure, and 4) zeroing-out pixels below a threshold. Current results suggest that the lag deposits fail to be hydraulically equivalent, but further testing and searching for independent verification will strengthen our arguments. What is evident is that biofilms appear to grow on these mineral lags, suggesting that a hiatus in deposition may control times that are more conducive to microbial growth. In Mars’ Jezero Crater, ancient biofilms could be preserved from a clastic, watery environment. Heavy mineral lags would be detectable with the Perseverance Rover’s Planetary Instrument for X-ray Lithochemistry (PIXL) and make a clear choice for further biosignatures testing.