Date of Award

Spring 5-15-2019

Author's School

Graduate School of Arts and Sciences

Author's Department

Earth & Planetary Sciences

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

The long-term redox state of Earth’s surface environments is governed by the relative burial proportions of redox-sensitive carbon and sulfur species throughout geologic time. The sulfur isotopic signature of seawater sulfate (δ34SSO4) traces the relative proportions of sulfur buried as sulfide (commonly pyrite) and sulfate (as evaporites, barite or carbonate-associated sulfate) and weathering source to the ocean. Thus, δ34SSO4 is a powerful tool to understand the evolution of redox conditions and the marine biogeochemical sulfur cycle. Due to the spatial and temporal extent of carbonates in the rock record, carbonate-associated sulfate (δ34SCAS) is a common proxy for the isotopic composition of ancient seawater sulfate. However, deep-time δ34SCAS datasets are inherently variable and many paired δ34SCAS-δ34Spyr records display discordant signals that cannot be reconciled by established interpretations of the sulfur cycle. Here we investigate the effects of depositional and diagenetic controls on CAS by combining sedimentologic observations, bulk isotopic records and micron-scale investigations using µ-X-ray Fluorescence imaging (µ-XRF) and X-ray Absorption Near Edge Structure (XANES) spectroscopy into the abundance and speciation of sulfur in ancient carbonates.

We demonstrate the effect of shallow- vs. deep-water depositional processes that generate heavier and more variable vs. lighter and more stable bulk δ34SCAS-δ34Spyr records over timescales considered too short to reflect perturbations to the marine sulfate reservoir from early Silurian carbonates. Shallow-water environments have higher (more variable) sedimentation rates that limit the exchange of pore fluid sulfate with seawater sulfate. Due to microbial sulfate reduction in the sediment, and lack of connectivity to seawater, pore fluid sulfate evolves under closed-system conditions. However, site-specific depositional environment information is important to constrain, as these effects differ across a platform, depending on the energy of the environment, where high energy (paleoshoreline) and low sedimentation (deep water) environments record the most seawater-like values. Combined with δ18OCAS, ancient δ34SSO4 can be determined based on the least evolved δ34SCAS-δ18OCAS and most stable δ34SCAS-δ34Spyr values.

Micron-scale investigations of carbonate petrographic thin sections using µ-XRF and XANES spectroscopy demonstrate the variable abundance and speciation of sulfur between carbonate components (fossils, grains, cement and micrite). Sulfate is present in all carbonate components but, based on the timing and location of precipitation, is variable within a sample. Micron-sized sulfide minerals are present in micrite and are often partially oxidized. The relative proportions of fossils, grains, cement and micrite, and thus the abundance and source of sulfate, therefore, varies between different depositional settings. The sulfate in extant and well-preserved fossil brachiopods is inorganic sulfate (as CAS) with little contribution from organic sulfate esters suggesting epifaunal brachiopods are a relatively robust archive of local seawater δ34SSO4. Depositional and early marine diagenetic conditions inherently impact δ34SCAS-δ34Spyr values and must be assessed for all carbonate successions prior to interpreting sulfur isotopes, in order to disentangle local signal from global trends.

Language

English (en)

Chair and Committee

David A. Fike

Committee Members

Alexander S. Bradley, Jeffrey G. Catalano, Michael J. Krawczynski, Samuel M. Webb,

Comments

Permanent URL: https://doi.org/10.7936/h0z2-v510

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