mechanisms of sulfur isotope fractionation during thermochemical sulfate reduction by amino acids: implications for the Mif-s record in archean rocks

Open Access
Author:
Chorney, Andrew Philip
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 30, 2014
Committee Members:
  • Hiroshi Ohmoto, Thesis Advisor
  • Matthew Scott Fantle, Thesis Advisor
  • Christopher Howard House, Thesis Advisor
Keywords:
  • sulfur
  • isotopes
  • TSR
  • Archean
  • MIF-S
  • mass independent
  • atmosphere evolution
  • oxygenation
Abstract:
The understanding of the evolution of the earth’s early atmosphere has largely been based on the behavior of the multiple isotopes of sulfur. However experiments designed to mimic fractionation in both an oxygen poor atmosphere through SO2 photolysis and a presumably oxygen rich atmosphere though thermochemical sulfate reduction (TSR) have thus far been unable to confirm a mechanism capable of creating the mass independent fractionation of sulfur (MIF-S) signatures observed in Archean age rocks. This has created much controversy in how the MIF-S record is used to determine Archean environments. However large amounts of geochemical evidence suggest at least appreciable amounts of oxygen in the atmosphere and ocean at different stages through the Archean. This has motivated us to further explore TSR as a source of MIF-S through a rigorous investigation of how chemical speciation, experimental conditions, time, and rate effect sulfur isotope fractionation. Our results show the importance of initial sulfur redox state and chemistry, temperature, and organic reductant type in creating characteristic mass dependent and independent fractionations. Including previous TSR results (Watanabe et. al., 2009 and Oduro et. al., 2011) we report increased δ34S and Δ36S fractionation in TSR products to 15.50‰ and 1.5‰ respectively and increased Δ33S range from -.22 to 2.30‰ in H2S largely through the use of mixtures of glycine and alanine. In sulfate we observe δ34S fractionation between 0.00 and 7.85‰ and Δ33S fractionation between -.15‰ and 0.0‰. By quantifying fractionation amongst different sulfur bearing products produced during experiments and collected at experiment conclusions in addition to characterizing the effects due to various experimental conditions we are also able describe a schematic framework for the isotope effects during various steps of TSR. Fractionation from the magnetic isotope effect (MIE), the vibrational energy discontinuity effect (VEDE), first described for heterogeneous reactions by Lasaga et. al, 2008, and normal stable isotope mass dependently driven kinetic isotope effects (KIE) are utilized in this schematic to describe the experimental results. The MIE and VEDE are responsible for the MIF-S signatures observed in organic polysulfides and H2S at high temperatures. Additionally the effects of these mechanisms are greatly influenced by amino acid type, specifically in experiments that use a mixture of glycine and alanine. The VEDE and KIE are responsible for δ34S fractionation with the overall effect of producing sulfide depleted in 34S at lower temperatures. The interplay of these mechanisms is responsible for Δ33S and δ34S trends observed in sulfides and sulfate samples. Based on this proposed schematic for sulfur isotope fractionation during TSR major trends and features in the Archean Δ33S vs. δ34S and Δ36S vs. Δ33S record can be interpreted. Our results confirm that MIF-S signatures observed in natural samples are a product of depositional environment and possibly reflect characteristics of reactions specific to sedimentary basin conditions and early earth’s tectonic regime.