Role of Sulfur in the Preservation of Organic Molecules in Hydrocarbon Source Rocks of Contrasting Compositions

  • Yusuf Abubakar

Student thesis: Phd

Abstract

Abundance of carbon dioxide (CO2) in the atmosphere is known to escalate greenhouse conditions/climate warming. The only way to naturally mitigate this devastating phenomenon is by massive sequestration of CO2 in sediments as either carbonate rocks or organic matter (OM). Preservation through sulfurisation is considered one of the important pathways through which OM can be stored in the geosphere for millions of years. However, the formation of iron sulfides, primarily pyrite (FeS2), is suggested to be a competitor to this process. Therefore, it remains unclear what factors are critical in the preservation of OM through sulfurisation and the stability of the sulfurised OM in the sedimentary record. There is a dearth of, particularly, spatial information on sulfur (S) and iron (Fe) inventories (oxidation states) at high resolution that could improve our understanding of sulfurisation and FeS2 formation processes as well as the environmental conditions during time of deposition. In this project samples from model organic and S-rich basins, e.g. the Kimmeridge Clay (KCF), Monterey (MF) and Whitby Mudstones Formations (WMF), as well as laboratory synthesised materials, products of sulfurised glucose reacted with Fe (II) chloride, were robustly analysed using a wide range of organic/inorganic geochemical, petrographic and probe analyses as well as state-of-the-art synchrotron-based techniques (X-ray fluorescence imaging and X-ray absorption near edge structure). For the first time, S and Fe oxidations states were spatially-resolved both at low and high resolutions. Results indicate widespread presence of sulfurised OM in the KCF, in line with relatively low influx of reactive Fe and the presence of stable euxinic conditions in the water column over a relatively long period of time, promoting the preservation of substantial amounts of OM through sulfurisation. In contrast, the presence of sulfurised OM was transitional in the MF, in line with fluctuating redox conditions in the water column over time likely due to profuse biological production, and less significant in the WMF likely due to the abundance of reactive Fe outcompeting the formation of sulfurised OM. The reaction of sulfurised glucose with Fe indicate, for the first time, that some S in the sulfurised glucose were utilised by Fe to form metastable FeS materials which were subsequently converted to minerals with co-ordination of S and Fe that is similar to the co-ordination environment and oxidation state of S and Fe in S-2 Fe sulfide minerals (likely pyrrhotite or troilite). Some of these S-2 minerals further converted to form traces of a more stable S-1 iron sulfides likely FeS2. However, there was still sulfurised glucose present at the end of the experiment, suggesting that potentially specific parts of the S originally bound to the glucose were lost during the experiments. The metastable FeS materials were either formed from breaking of weak S-S bonds while strong C-S bonds were stable and could be preserved for millions of years. It can currently not be excluded that if the experiments were run for a longer period of time more/all S in the sulfurised glucose would be stripped off to form iron sulfides. Therefore, reaction between OM and S could be said to be partially reversible in the presence of reactive Fe species but further experiments are required to arrive at full bound conclusions. This project for the first time introduced the use of novel synchrotron technologies that could be potentially used to study the oxidation states of S, Fe, and other elements from other depositional settings such as ancient and recent oxic and dysoxic milieus and other laboratory synthesised material such as sulfurised humic and fulvic type OMs.
Date of Award1 Aug 2021
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorRoy Wogelius (Supervisor), Kevin Taylor (Supervisor), Victoria Coker (Supervisor) & Bart Van Dongen (Supervisor)

Keywords

  • Whitby Mudstone Formation
  • Organic Matter Sulfurisation
  • Pyrite Precipitation
  • X-ray Imaging
  • X-ray Absorption Near Edge Structure
  • Kimmeridge Clay Formation
  • Monterey Formation

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