Doctor of Philosophy (PhD)
First Committee Member
Second Committee Member
Stephen M. Rowland
Third Committee Member
Elisabeth M. Hausrath
Fourth Committee Member
Fifth Committee Member
Number of Pages
The Early Mississippian K–O (Kinderhookian-Osagean) δ13C excursion or TICE (mid-Tournaisian carbon isotope excursion) is characterized by an anomalous carbonate carbon isotope value of ≥5‰ that has been documented from numerous stratigraphic sections across the globe. This δ13C excursion coincides with global cooling and sea-level fall and predicts a significant change in seawater chemistry. However, sulfate sulfur isotope data across the K-O δ13C excursion reported in previous studies seem not to be responsive to the carbon cycle. Likewise, a recent study has documented a unidirectional increase in nitrogen isotopes across this excursion, which is not anticipated considering the amount of organic carbon burial required to form the prominent positive δ13C excursion and its resultant oxygen increase and global cooling. This study aims to understand the coupling between carbon, nitrogen, sulfur and oxygen cycles at this critical transition.
This research uses Early Mississippian carbonate successions from Star Range (SR), Mountain Home Range (MH), and Pahranagat Range (PR) that indicate peritidal, shallow subidal to deep subtidal depositional environment in the Great Basin of western USA. Samples from these sections are analyzed for carbon isotopes (δ13C), sulfur isotopes (δ34S), nitrogen isotopes (δ15N) and oxygen isotopes (δ18O). Additionally, SR samples are analyzed for fluid inclusions, REEs, major and trace element concentrations. Stratigraphic records from PR and MH sections indicate that the outer shelf section (PR) may be more representative of seawater isotope record. The δ34SCAS records a ≥7‰ positive anomaly near the peak of the K-O δ13C excursion. Numerical modeling suggests that pyrite burial rates 5–10 times higher than that of the modern ocean are required to produce the observed δ34SCAS anomaly in a sulfate-rich Early Mississippian ocean. The aerial and volumetric expansion of sulfate reduction and pyrite burial was likely fused by abundantly available organic matter at the peak of the K-O δ13C excursion when oxygen minimum zone (OMZ) in the ocean has substantially expanded. At the falling limb of the K-O δ13C excursion, coupled negative shifts in δ34SCAS and δ18OCAS imply increase of sulfide reoxidation in the ocean and pyrite-derived riverine sulfate input, in response to global cooling, sea-level fall, and oxygenation resulted from enhanced organic carbon and pyrite burial, resulting in gradually decreasing the volume of OMZ. Such a change in ocean redox is reflected in the carbon and nitrogen cycles. Stratigraphic records indicate that the deeper-water outer shelf section (PR) show double spikes in both δ13Ccarb and δ13Corg with a magnitude up to 7‰ and a negative shift down to 4‰ between the peaks which is more representative of the seawater isotope record. However, considering the sensitivity of δ15N to redox conditions of depositional environments, the coupled δ13Ccarb – δ15N pattern from the MH section may better record the isotope signature of the oceanic nitrate (NO3–) reservoir, while the decoupled δ13Ccarb – δ15N in the PR section reflects involvement of local N-biochemical cycling in periodically developed suboxic-euxinic environments. The increase of δ15N toward the peak of the K-O δ13C excursion may record the expansion of oxygen minimum zone (OMZ) in the ocean that promotes water-column denitrification and 15N enrichment in the marine nitrate (NO3–) reservoir. The decrease of δ15N at the falling limb of the K-O δ13C excursion reflects the shrink of the OMZ and reduces the water-column denitrification in response to more oxygenated and cooler oceans resulting from enhanced organic carbon burial.
The significant amount of organic carbon burial during Early Mississippian results global cooling and possibly glaciation which is partly supported by the presence of highly depleted δ18O values, down to –34‰, in SR samples. Block samples from SR and PR are studied for carbon and oxygen isotopes, fluid inclusion, REEs, major and trace elements to investigate the origin and processes involved during carbonate diagenesis. These geochemical data suggest that the carbonate diagenesis took place from a diagenetic fluid originated form freshwater. Importantly, SR samples are from close to or below the unconformity surfaces which may have been altered by glacial melt water with δ18O values of ≤ –15‰. Then, it could have been further modified by a burial diagenesis with geothermal temperature < 200°C to produce the highly depleted δ18O values down to –34‰. Collectively, carbon, nitrogen, sulfur and oxygen isotope records from Early Mississippian carbonates show a coupled variation in response to the environmental change during this time.
Carbonates; Carbon isotope; Early Mississippian; Nitrogen isotope; Oxygen isotope; Sulfur isotope
Climate | Geology | Sedimentology
Maharjan, Dev Krishna, "Environmental Changes across the Early Mississippian Carbon Isotope Excursion" (2017). UNLV Theses, Dissertations, Professional Papers, and Capstones. 3005.
Available for download on Friday, May 15, 2020