title: Geochemical Analysis of Carboniferous Carbonates and Implications for Ocean Oxygenation
Creators: name: Aragon, Abel Joseph
Publisher: name: George Mason University
Year: 2024
Subjects: subject: Geochemistry
subject: FOS: Earth and related environmental sciences
subject: NATURAL SCIENCES::Earth sciences::Exogenous earth sciences::Sedimentology
subject: Lower Missippian
subject: Stable Isotopes
subject: Isotopes Excursions
subject: Carbon Isotopes

description: The Carboniferous Period (358－298 Ma) was an important time in Earth history, where events such as the expansion of large terrestrial forests, the recovery of marine and terrestrial life after the Late Devonian extinction events, and significantly low carbon dioxide levels changed the trajectory of life and climate. This change is partly reflected in the carbon isotope record, where there is a major positive perturbation in the Tournaisian δ13C record, known as the TICE or K-O excursion. This excursion is well-recorded globally, with prominent sections in North America, China, Belgium, and Russia. In North America, the most prominent records of this excursion are on the western open ocean margin, but little is known about TICE expression in the interior epeiric seas of North America and how epeiric sea processes can modify global δ13C signals. In this study, we generate a new δ13C curve for the Tournaisian of the Williston Basin from three cores in North Dakota that cover the majority of the Lodgepole Limestone and the Mission Canyon Limestone. Samples from these cores (the Sjol, TI-W, and Irgens cores) were cut and polished into thick sections. Specific fabrics, such as gray mudstone, gray wackestone, skeletal fragments, and black mudstone and marlstone were targeted when drilling for carbonate powders. Diagenesis was evaluated by visual inspection of the degree of primary textural preservation and through a regression analysis between δ13C and δ18O values, where little correlation was found, thus signaling a lack of isotopic resetting in these samples. Importantly, in the three cores, a sharp rise in δ13C from ~0‰ to ~+5‰ is recorded, followed by a prolonged peak where δ13C values reach nearly +9‰. This is followed by a progressive decline back to values of ~+1‰, which are maintained at the top of the composite section. These data affirm that the TICE excursion was found in the Williston Basin, with δ13C values reaching a remarkable and prolonged peak of nearly +9‰. Compared to pre-existing records from around the globe, the peak of δ13C values during the TICE excursion in the Williston Basin is substantially higher, and the Williston Basin δ13C record does not capture the two-pronged nature of the excursion that is seen in the open ocean sections of Nevada. These differences are ultimately due to: (1) local epeiric sea organic carbon burial in restricted, low-oxygen environments that modified the global δ13C signal, and (2) a nearly 270 ft. stratigraphic gap in our sampling of the TI-W core, which we plan to fill in with future sampling. Observed differences in δ18O values between the Williston Basin and global records are also due to enhanced evaporation and high salinity waters prevalent in the Williston Basin. This new expanded TICE record from the Williston Basin emphasizes the important influences of epeiric sea processes on the δ13C record and lays the groundwork for future research into Carboniferous Earth history.