Carbonate-Associated Sulfate: Assessment of and Use as an Isotopic Proxy for Global Sulfur Cycling During End-Permian Mass Extinction
Open Access
- Author:
- Riccardi, Anthony
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 14, 2007
- Committee Members:
- Lee Kump, Committee Chair/Co-Chair
Michael Allan Arthur, Committee Chair/Co-Chair
Katherine Haines Freeman, Committee Member
Carlo G Pantano, Committee Member - Keywords:
- Sulfur Isotopes
Mass Extinction
Geochemistry - Abstract:
- Carbonate associated sulfate (CAS) has recently become an important proxy for determining change to the global sulfur cycle through time. Variations in the isotopic composition of CAS during the late Permian have been utilized to suggest the presence of a euxinic ocean which may have helped trigger a mass extinction event. Samples from three marine sections spanning the Permian-Triassic boundary were analyzed: Meishan and Shangsi (Southern China) and Demirtas (Turkey) for CAS isotopic composition. Rapid variations in the ƒÔ34SCAS with extremely 34S depleted values in the Permian were found at all three sections. A decrease in ƒ´13C (ƒÔ13Ccarb-ƒÔ13Corg) was also measured at Meishan and Shangsi, but not Demirtas coincident with the largest pulse of extinction. The patterns of isotopic shift indicate a shallow unstable chemocline overlying euxinic deep water which periodically upwelled into the photic zone. This release of sulfide may have stimulated a bloom of phototrophic sulfur oxidizing bacteria. These bacteria would have produced elemental sulfur globules depleted in 34S, which were deposited in the sediment and later oxidized and incorporated as CAS. The variations in ƒÔ34SCAS between the three marine sections suggest intermittent euxinic conditions in the surface waters which varied by location helping to trigger the largest mass extinction of the Phanerozoic. These rapid variations in the isotopic composition of CAS during the latest Permian associated with the Permian-Triassic boundary mass extinction event have raised concern over the susceptibility of the CAS isotopic proxy to diagenetic alteration. Chemical and spectroscopic analyses were performed on both modern and ancient samples to determine the resistance of CAS to alteration. Fourier transform infrared spectroscopy of powdered carbonate suggests CAS is present within the calcite mineral lattice. Examinations of CAS isotopic values in a modern euxinic environment were performed at meromictic Fayetteville Green Lake (near Syracuse, NY). Lake water and sediment samples were analyzed for sulfate and sulfide concentration and isotopic composition. The isotopic values for CAS remained the same as those found in the overlying lake water for both the oxic and euxinic sediments suggesting CAS is a reliable proxy for overlying water sulfate isotopic composition under varying burial conditions.