T80 Advances in Non-Traditional Stable Isotope Measurements and Utility as Proxies in Modern and Paleo-Settings

Monday, 10 October 2022
8:00 AM - 12:00 PM
506

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

Paper number: T80-4
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

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 δ13Ccarb from the primary dissolved inorganic carbon (DIC) of the global ocean (or decouple δ13Ccarb from platform DIC, if different from the global ocean). This study pairs δ44/40Ca data with Sr/Ca and δ13C to examine the potential effects of fluid or sediment buffered diagenesis on δ13C in bulk carbonate. The δ44/40Ca 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 δ44/40Ca 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 δ44/40Ca value from the excursion peak (δ13C of +5‰ to +7‰) is -1.16‰ (relative to seawater; n=6, 2SD=0.06‰). The average δ44/40Ca value for baseline δ13C (0‰ to +1‰) is –1.14‰ (n=2, 2SD=0.04‰). Thus, preliminary data show no covariation between δ44/40Ca and δ13C or δ44/40Ca and Sr/Ca. Using the Ahm et al. (2018) diagenetic model, our δ44/40Ca and Sr/Ca data indicate predominantly sediment buffered diagenesis if aragonite was the primary mineral and seawater δ44/40Ca=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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-380109

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