Isotopic and trace metal geochemistry of calcite, gypsum, and pyrite as proxies for ancient life and environments

Open Access
Author:
Mansor, Muammar
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 24, 2017
Committee Members:
  • Matthew Fantle, Dissertation Advisor
  • Jennifer Macalady, Committee Chair
  • Christopher House, Committee Member
  • Li Li, Outside Member
Keywords:
  • Achromatium
  • pyrite
  • Fe isotope
  • biosignature
  • sulfur isotope
  • molybdenum
  • biofilms
  • reactive transport modeling
  • gypsum
  • calcite
Abstract:
Geoscientists reconstruct the past in order to understand the future. Minerals and associated geochemical (isotopic and trace metal) signatures act as tools for geoscientists to decipher past processes that led to Earth’s biogeochemical evolution. In this thesis, the geochemical signatures contained within calcite, gypsum and pyrite were characterized through a multidisciplinary approach involving genomics, field sampling, laboratory experiment and diagenetic modeling to understand the primary processes (abiotic and biotic) that led to specific geochemical signatures. These signatures are then interpreted as either biosignatures or as environmental indicators for the ancient Earth. Chapter 2 focuses on the unusual calcite (CaCO3) inclusions precipitated intracellularly within giant bacteria called Achromatium, which have the potential to be used as biosignatures. Single-cell genomics were utilized to shed light on the bacteria’s metabolisms and genetic machinery in forming intracellular calcite. Two potential functions were proposed for this unusual metabolism. First, calcite inclusions may have been deposited as ballasts to anchor the cells from being swept away by water flow. Second, intracellular calcite precipitation may be a novel way to generate free protons and ATP to sustain life. Limitations in current techniques hamper geochemical characterizations of the calcite inclusions; nonetheless, our progress in understanding Achromatium have paved the way for culture-based, omic or in silico modeling techniques that will be useful for interpreting specific signatures of microbial calcite. In Chapter 3, we will travel deep into the subsurface to look for gypsum (CaSO4.2H2O) minerals coated by slimes of sulfur-oxidizing microbes. The microbes consumed hydrogen sulfide to sulfate, imparting unique sulfur isotopic (δ34S) signatures in gypsum that are spatially recognizable at the cave room scale. Similar spatial isotopic patterns in subsurface systems may be utilized as biosignatures, subject to tighter constraints of physico-chemical diagenetic effects to gypsum deposits over million-year timescales. From Chapter 4 to 7, we will shift our attentions towards pyrite (FeS2) minerals. In Chapter 4, we will encounter submerged microbial “ropes” containing abundant pyrite (and its mineral polymorph, marcasite) that are potentially analogous to microbial biofilms in the anoxic early Earth. Combined with morphological, trace metals and Fe isotopic (δ56Fe) analysis, we suggest that the FeS2 minerals were formed through both Fe-oxides in the bedrock as well as dissolved Fe from the water, with significant catalysis induced by microbial sulfur cycling. Biofilm-associated FeS2 minerals are clearly different from sedimentary FeS2 in terms of morphology, mineralogy, trace metal and Fe isotopic compositions, rendering them potentially useful as a biosignature. Additionally, their differences suggest that care must be taken to interpreting geochemical signatures from Precambrian pyrite, which has traditionally been based solely on sedimentary pyrite. In Chapter 5 and 6, we will travel back to the laboratory and use defined experimental studies to constrain factors affecting the chemical signatures of pyrite. Chapter 5 is focused on molybdenum (Mo) content in pyrite as potential proxies for Mo content in ancient waters, with direct relevance towards testing the Mo scarcity hypothesis that impeded eukaryotic evolution during the Proterozoic. Experimental data show that the initial dissolved Mo concentrations are linearly correlated with Mo contents in pyrite. This relationship is validated for use in diagenetic pyrite under oxic to sub-oxic water column, but overestimates the Mo concentrations calculated from pyrite deposited under euxinic water columns. Correcting for these biases as much as possible, we determined that Proterozoic seawater had Mo concentrations between 1-50 nM and are typically higher than the 5 nM Mo limit for nitrogen fixation. Consequently, the Mo scarcity hypothesis cannot readily explain the lack of eukaryotic evolution in the middle Proterozoic. In Chapter 6, Fe isotopic analysis of laboratory-formed pyrite showed that the apparent fractionation associated with pyrite precipitation are affected by the relative rates of pyrite precipitation to isotopic exchange rate between FeS mineral and aqueous FeS at low temperatures (< 40⁰C) and between aqueous FeS and pyrite at high temperatures (> 80⁰C). In euxinic sediments where diagenetic pyrite formed, pyrite δ56Fe may be used to constrain pyrite precipitation and burial rates, with implications to oxygen sinks particularly in the Precambrian. In Chapter 7, a diagenetic model is developed in order to characterize the effects of multiple pyrite precipitation pathways to pyrite δ56Fe in marine sediments. Model outputs suggest that pyrite likely formed from at least two pathways: one mediated by aqueous Fe(II) and another mediated by surface-bound Fe(II). The model’s framework can easily explain the typical ~7‰ variation of individual pyrite grains within the same sediment. Additionally, bulk pyrite δ56Fe that leans toward more positive values are characteristic of Fe(III)-dominated system while bulk pyrite δ56Fe that leans toward more negative values are characteristic of Fe(II)-dominated system.