CARBON ISOTOPE VARIABILITY IN 2.2 GA KONA DOLOMITE STROMATOLITES DURING THE LOMAGUNDI-JATULI EVENT

Between 2.2 and 2.0 Ga, global marine carbonates record one of the most pronounced positive carbon isotope excursions (δ13Ccarb) in the planet’s history, known as the Lomagundi-Jatuli Event (LJE). In most ancient and modern marine carbonates, δ13Ccarb varies between -5 and +5 ‰. In contrast, LJE carbonates reach peak δ13Ccarb values of +10 to +15 ‰. The causes for the LJE are still debated, but are typically connected with the preceding Great Oxidation Event ~2.4 Ga. One school of thought argues for δ13Ccarb enrichment in local evaporative basins on multiple continents, while others support δ13Ccarb changes across the global ocean.

One relatively understudied aspect of the LJE is δ13Ccarb variability over shorter timeframes. In other words: global carbonate records show a ~200 million year period of heavy marine δ13Ccarb, but how did isotopic values shift over hundreds or thousands of years? To address this question, this study investigates stromatolite deposits from the ~2.2 Ga Kona Dolomite in Michigan’s Upper Peninsula. The Kona Dolomite records the early LJE, with heavy δ13Ccarb values between +5 and +10 ‰ (Bekker et al., 2010). Kona deposits contain well-preserved, finely-layered stromatolite fossils. Stromatolite layers represent “snapshots” of carbonate precipitation on yearly to millennial scales, recording short-term shifts in marine chemistry.

This study provides new petrographic and δ13Ccarb data from multiple Kona stromatolites to test the global vs regional LJE hypotheses. If the LJE is tied to global ocean chemistry, then δ13Ccarb in Kona stromatolites should be relatively homogenous. If LJE excursion in the Kona Dolomite represent a local evaporative basin, then local environmental shifts would potentially create more pronounced δ13Ccarb shifts over shorter timeframes.