ORCID
https://orcid.org/0000-0001-7363-3990
Date of Award
9-1-2023
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
The environmental and biological factors that influence the precipitation and composition of geologic proxies are often assumed to be minimally variable relative to the spatial and temporal domain of individual samples (ca. millimeters to centimeters). While this presumption may apply to many pelagic environments, it may not be appropriate for methane cold seep sites. Methane cold seeps are localized environments that can transport large quantities of dissolved methane - a potent greenhouse gas - toward the seafloor and, in turn, sustain high rates of anaerobic oxidation of methane by sulfate reduction (AOM-SR). The concurrent transport of anoxic seep fluids, diffused seawater sulfate, and AOM-SR activity can produce steep gradients in the concentration and isotopic (δ³⁴S) composition of dissolved sulfate near the seafloor. The precipitation of methane-derived authigenic carbonate (MDAC) polymorphs (e.g. aragonite and calcite; CaCO3) and δ³⁴S values of reduced and oxidized sulfur in authigenic pyrite (FeS2) and barite (BaSO4), respectively, have been used gauge the magnitude of past AOM-SR activity and infer paleo methane seep conditions. However, there is growing evidence these geologic proxies can possess considerable mineralogic, textural, elemental, and isotopic variability over small spatial scales (micrometers to millimeters). These microscale heterogeneities can potentially result from either steady-state or non-steady-state conditions depending on the precipitation interval of a given sample. The overarching aim of this dissertation is to empirically evaluate the potential for increased environmental understanding derived from microscopic (< 1 mm) δ³⁴S heterogeneities in authigenic seep barite (δ³⁴SBaSO4) and pyrite (δ³⁴SPyr) obtained by in-situ secondary ion mass spectrometry (SIMS) measurements paired with petrographic analyses. By coupling petrographic and δ³⁴S assessments, the relative timing of microscale heterogeneities can often be inferred and used to further constrain the geochemical evolution of the precipitation environment. SIMS δ³⁴S analyses also reveal unprecedented degrees of microscale δ³⁴SBaSO4 variability and 34SBaSO4 enrichment compared to previous bulk δ³⁴SBaSO4 measurements. Additionally, a novel 1D biogeochemical model was constructed to describe the reactive transport of advected methane and barium and diffused sweater sulfate and to investigate potential causes of measured microscale δ³⁴SBaSO4 heterogeneities. The model explicitly tracks the distillation of porewater sulfate due to AOM-SR activity and both the precipitation depth and δ³⁴SBaSO4 value of seep barite. Modeling results indicate that the stratigraphic interval of barite precipitation is often narrow (millimeters to centimeters) and that large δ³⁴SBaSO4 variability (> 90‰) can arise from steady-state precipitation conditions. Moreover, both barite precipitation rates and ³⁴S enrichment are sensitive to minor – and potentially foreseeable – changes in precipitation conditions. Model results indicate that bulk sampling methods may potentially obscure microscale δ³⁴SBaSO4 variability produced under steady-state and non-steady-state precipitation conditions. Similar to seep barite, SIMS analyses reveal that δ³⁴SPyr values can vary significantly within an individual seep sample (< 1” diameter). We find that SIMS δ³⁴SPyr values can differ significantly between pyrite hosted in calcitic- and aragonitic- MDAC. By assessing authigenic seep pyrite with respect to textural layering in MDAC phases, the relative timing of δ³⁴SPy variability can often be inferred. We present a qualitative model for how δ³⁴SPyr values, petrographic features, and carbonate-associated sulfate (CAS) concentrations are potentially interrelated by temporal changes in AOM-SR activity and seawater diffusivity. Successive changes in CAS intensity were measured in aragonitic-MDAC using mono-energy micro-X-Ray fluorescence (µ-XRF) mapping. We ultimately suggest that successive changes in SIMS δ³⁴SPy values and CAS intensity can be used to distinguish the effects of porosity and AOM-SR activity on porewater ³⁴S enrichment. At first glance, the measured and modeled degrees of microscale δ³⁴S variability in seep pyrite and barite could call into question the utility of these proxies for inferring past seep conditions. However, under steady-state precipitation conditions, modeling results indicate that even large aliquots (e.g. 1 cm3) that incorporate a wide range of δ³⁴SBaSO4 values (e.g., > 60‰) can still return precipitation-weighted δ³⁴SBaSO4 values that are similar to the modal ³⁴SBaSO4 value of precipitates (± 5‰). Additionally, SIMS δ³⁴SPy analyses seem to support established interpretations of MDAC mineral phases. There also appears to be a relationship between microscale textural variability in aragonitic-MDAC and seep barite that correlates with proportionately microscale changes in CAS concentrations and ³⁴SBaSO4 values, respectively. This apparent relationship may indicate that petrographic analyses can be used to rapidly screen authigenic seep minerals prior to chemical and isotopic analyses. Collectively, we suggest that these microscale textural and compositional heterogeneities can provide new and valuable insights into the geochemical evolution and dynamic nature of active methane seep environments.
Language
English (en)
Chair and Committee
David Fike
Recommended Citation
Wood, Robert Seth, "A multiproxy approach to deciphering the geochemical evolution of cold seep environments" (2023). Arts & Sciences Electronic Theses and Dissertations. 3145.
https://openscholarship.wustl.edu/art_sci_etds/3145