STRATIGRAPHY, PETROGRAPHY, AND CARBON ISOTOPE VARIABILITY AMONG STROMATOLITES DURING THE 2.2 GA LOMAGUNDI-JATULI EVENT

Around 2.2 Ga, marine carbonates record a pronounced positive carbon isotope excursion (δ13Ccarb) known as the Lomagundi-Jatuli Event (LJE). In most marine carbonates, δ13Ccarb varies between -5 and +5 ‰. In contrast, LJE carbonates reach values of +10 to +15 ‰. The causes for the LJE are debated, but are typically connected with the preceding Great Oxidation Event ~2.4 Ga. Some schools of thought argue for δ13Ccarb enrichment in localized evaporative basins, while others support δ13Ccarb changes across the global ocean.

To test the LJE hypotheses, this study provides new stratigraphic, petrographic, and δ13Ccarb data from multiple stromatolites across Michigan's Upper Peninsula. The Kona Dolomite represents an evaporitic environment, while the Randville was deposited in an open shoreline where water flow is unrestricted. Previous bulk isotope studies show that Kona deposits are δ13C-enriched (+5 to +10 ‰), while Randville deposits are δ13C-depleted (+0 to +3 ‰). The Kona and the Randville Dolomite are relatively closer geographically, and are typically interpreted as correlating deposits.

One understudied aspect of the LJE is short term changes in δ13C. Global carbonate records show a ~200 million year period of heavy marine δ13C, but how did isotopic values shift over thousands of years? To address this question, this study investigates finely-layered stromatolites recording short-term shifts in marine chemistry. Part of our study examines the Kona and Randville Dolomite on a finer scale, observing how δ13C changed over smaller time periods. If the LJE is tied to global ocean chemistry, δ13C in different stromatolite layers should be relatively homogenous. If LJE excursion in the stromatolites represents a local evaporative basin, local environmental shifts would potentially create more pronounced δ13C shifts between layers. In either case, our data could help future studies resolve the LJE regional vs. global debate.