Local Environmental Controls on Sulfur Isotope Ratios in Marine Sedimentary Iron Sulfide Minerals
Date of Award
Doctor of Philosophy (PhD)
The controls on the bulk sulfur isotopic composition of marine sedimentary iron sulfides (often referred to simply as pyrite; δ34Spyr) are poorly understood. Nevertheless, many have employed δ34Spyr in efforts to reconstruct the past operation of the biogeochemical sulfur cycle, from the planetary scale down to individual microbial metabolisms. This practice has been thrown into doubt by a growing body of evidence that suggests δ34Spyr is strongly controlled by local environmental conditions. This dissertation describes efforts to determine the mechanisms responsible for the link between local environmental conditions and δ34Spyr. In order to do this, we developed novel laboratory procedures and analytical techniques to facilitate the measurement of grain-specific δ34S values.
Firstly, by determining the major controls on the Raman spectrum for pyrite, we demonstrate that laser Raman microprobe analysis is an effective tool for distinguishing between pyrite and other minerals (e.g., marcasite) on a grain-specific basis. This tool is used extensively throughout this dissertation, prior to any grain-specific geochemical analyses.
Secondly, we present procedures for physical isolation, mounting and grain-specific sulfur isotopic analysis of marine sedimentary iron sulfides. The sulfur isotope analysis procedure utilizes secondary ion mass spectrometry (SIMS) to generate images of sulfur ion (32S- and 34S-) abundance in mineral grains. These procedures are tested on isotopically homogeneous hydrothermal iron sulfides and some examples of modern and Cretaceous-aged sedimentary iron sulfides. We find that the overall procedure produces distributions of accurate and precise grain-specific δ34S values that record the range of sulfur isotopic compositions of the fluid(s) from which the iron sulfide grains precipitated. This information can be used to infer local environmental conditions associated with iron sulfide mineral formation.
Thirdly, we apply the SIMS procedure to Pleistocene-aged pyrite grains sourced from the Gulf of Lion, in order to understand the driver(s) of large magnitude (75‰) stratigraphic oscillations in δ34Spyr over 100-kyr glacial-interglacial cycles. We find that the δ34Spyr changes at this site were the result of increased sedimentation rate during glacial intervals, which restricted diffusive communication between water column and sediment pore waters and led to higher δ34Spyr values. In contrast, the activity of sulfate reducing microbes showed no discernible response to the changing environmental conditions.
Finally, we apply the SIMS procedure to Cretaceous-aged iron sulfides from Demerara Rise, in order to understand the driver(s) of a secular decrease in δ34Spyr values toward the onset of the Cenomanian-Turonian Ocean Anoxic Event (OAE-2). We find that the relative abundances of different iron sulfide minerals and textures change over this interval, but that the isotopic compositions of these different minerals/textures are nearly invariant. More 34S-depleted sulfide was incorporated into iron sulfide minerals during OAE-2, likely because of an increase in the rate of iron sulfide formation relative to the rate of sulfate reduction.
Overall, these findings suggest that depositional conditions can control δ34Spyr by modulating the flux of sulfate into sediment pore waters, although the relative kinetics of iron sulfide formation and sulfate reduction can also have a major influence.
Chair and Committee
David A. Fike
Alexander S. Bradley, Jeffrey G. Catalano, Itay Halevy, Jill D. Pasteris,
Bryant, Roger Nicholas, "Local Environmental Controls on Sulfur Isotope Ratios in Marine Sedimentary Iron Sulfide Minerals" (2019). Arts & Sciences Electronic Theses and Dissertations. 1817.
Permanent URL: https://doi.org/10.7936/d5xb-dr05