Shallow water redox conditions from Permian-triassic boundary microbialites: the rare earth element and iodine geochemistry of carbonates from Turkey and South China

Open Access
Author:
Loope, Garrison Richard
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 12, 2012
Committee Members:
  • Lee Kump, Thesis Advisor
Keywords:
  • Mass extinction
  • Permian
  • Triassic
  • REE
  • rare earth elements
  • anoxia
  • iodine
Abstract:
Rare earths and yittrium (REY), iodine, and major elements are determined in two carbonate sections spanning the Permian-Triassic transition in Demirtas, Turkey and Cili, South China. We use major oxides to identify non-seawater REY sources such as siliciclastics, Fe-oxides, phosphates and diagenetic fluids. Additionally, we employ Y/Ho ratio, La anomaly, and light rare earth element depletion to identify which samples preserve a seawater-like REY distribution. Despite siliciclastic input, we find the Permian-Triassic boundary microbialites in both sections contain a negative Ce anomaly (average Pr/Pr*=1.07) indicative of deposition in an oxic environment. These boundary microbialites have their base at the extinction horizon and are widespread within the Tethyan region. In the Cili section, the underlying Permian limestone also preserves a seawater-like REY signature with a negative Ce anomaly. This indicates that the water column was oxygenated both before and after the extinction event. The Permian limestone in the Demirtas section does not preserve a seawater-like REY distribution, so the absence of a Ce anomaly cannot be used to distinguish prevailing redox conditions during deposition. However, in these samples, we find the presence of a diverse Permian benthic community sufficient to identify deposition in an oxic environment. The geochemical evidence for a continuously oxic environment during the deposition of the boundary microbialite presented in this study strongly supports work done using ostracods as redox indicators within the boundary microbialite in South China. These microbialites have been proposed as a disaster facies caused by the exclusion of grazers. Although anoxia is one of the suggested mechanisms for the exclusion of grazers, we find that it is not supported by geochemical evidence.