THE APPLICATION OF CALCIUM ISOTOPES TO UNDERSTAND THE EFFECTS OF DIAGENESIS ON CARBON ISOTOPE TRENDS IN ANCIENT CARBONATES: AN EXAMPLE FROM THE EARLY MISSISSIPPIAN

A +5‰ to +7‰ δ¹³C excursion is preserved globally in lower Mississippian strata (ca. 353 Ma). This is linked to a cooling interval as part of a long-term shift from greenhouse to icehouse conditions at the onset of the Late Paleozoic Ice Age. It is thought to represent increased primary productivity and organic carbon burial, possibly linked to the rise of land plants. However, investigating the burial of organic matter using the recorded δ¹³C in carbonates remains unclear because of potential diagenetic alteration of marine bulk carbonate δ¹³C. Diagenesis can decouple shallow platform δ¹³C_carb from the primary dissolved inorganic carbon (DIC) of the global ocean (or decouple δ¹³C_carb from platform DIC, if different from the global ocean). This study pairs δ^44/40Ca data with Sr/Ca and δ¹³C to examine the potential effects of fluid or sediment buffered diagenesis on δ¹³C in bulk carbonate. The δ^44/40Ca and Sr/Ca measured in carbonates are also sensitive to the primary mineralogy (calcite or aragonite), thus influencing how we interpret these proxies.

We measured δ^44/40Ca and Sr/Ca of bulk carbonate across this stratigraphic interval in samples from the Confusion Range, Utah, Pahranagat Range, Nevada, and Dinant Basin, Belgium. Clumped isotopes were also measured for the Confusion Range section (conodont alteration index, CAI = ~1.5-2.0) but measured temperatures in the bulk carbonates were too high to indicate fluid or sediment buffered diagenesis, and instead indicate solid state reordering has occurred. Initial results show that the average δ^44/40Ca value from the excursion peak (δ¹³C of +5‰ to +7‰) is -1.16‰ (relative to seawater; n=6, 2SD=0.06‰). The average δ^44/40Ca value for baseline δ¹³C (0‰ to +1‰) is –1.14‰ (n=2, 2SD=0.04‰). Thus, preliminary data show no covariation between δ^44/40Ca and δ¹³C or δ^44/40Ca and Sr/Ca. Using the Ahm et al. (2018) diagenetic model, our δ^44/40Ca and Sr/Ca data indicate predominantly sediment buffered diagenesis if aragonite was the primary mineral and seawater δ^44/40Ca=0‰. These results suggest the δ¹³C excursion of up to +7‰ is not an artifact of diagenesis. However, it is unclear if the δ¹³C excursion represents global seawater DIC, changes in shallow shelf DIC, or a combination.