Overview of the different mechanisms of OM preservation in marine environments
The process of degradation-recondensation of OM (Figure 1. 1) is considered to be quantitatively the principal mechanism of OM preservation (Tissot and Welte, 1978; Larter et al., 1979). It includes the degradation of biological polymers (biopolymers) such as proteins and polysaccharides (polymeric carbohydrates), leading to the production of a variety of labile biological monomers, for example amino acids, simple sugars, or fatty acids (Burdige and Gardner, 1998). These labile organic components are then recombined into more refractory compounds by complex condensation reactions. The common condensation reaction is named ‘geopolymerisation’ or ‘humification’, a general term for the process by which humic substances form. These humic substances are considered to be amorphous and hydrophilic, and refractory with respect to both chemical and biological degradation (Burdige, 2007). One well-studied example of geopolymerisation is the Maillard reaction. This reaction allows the formation of refractory compounds, known as melanoidins, by a sugar-amino acid condensation reaction (Rashid, 1985), and it has been observed that synthetic melanoidins produced in the laboratory show some similarity to marine humic substances (Frimmel et al., 1988).
This term describes the selective preservation (Figure 1. 1) of part of vascular plants and algae through their production of lignins and highly aliphatic macromolecular material which is insoluble, non-hydrolyzable, and resistant to biological degradation (Leeuw and Largeau, 1993; Gelin et al., 1999). These refractory molecules include (Burdige, 2007)
❖ Algaenans: algal cell wall components consisting of long-chain aliphatic compounds with hydroxyl or ester functional groups.
❖ Cutans: nonhydrolyzable components of the cuticles of higher plants.
❖ Lignins: class of phenolic compounds found exclusively in vascular plants.
They likely represent a very small fraction of the initial biomass produced by marine and terrestrial organisms. However, as they are selectively preserved, they are concentrated in the sedimentary OM pool during diagenesis (Tegelaar et al., 1989).
Physical protection of OM
OM can be physically protected by both inorganic and organic matrices. It has been observed in many studies that OM is preferentially associated with fine-grained sediment particles (clay minerals) (Premuzic et al., 1982; Keil et al., 1994; Mayer, 1994). However, the role of these fine-grained sediment particles in OM preservation is not fully understood. Based on observations from different studies (Hedges and Keil, 1995; Ransom et al., 1997; Mayer, 1999; Bock and Mayer, 2000; Arnarson and Keil, 2001; Kennedy and Wagner, 2011; Kennedy et al., 2014), OM-mineral interactions appear to involve physical protection of the OM in small mesopores, either on mineral surfaces or in between mineral grains. For instance, a covariation of mineral surface area (MSA) and TOC across multiple scales of variability was observed by Kennedy and Wagner (2011) and Kennedy et al. (2014) suggesting an influence on organic carbon burial by detrital clay minerals controlled MSA. Concerning OM physical protection by organic matrix, it would occur through encapsulation of reactive OM within insoluble, hydrolysis-resistant organic matrices such as algaenans (Knicker, 2004).
In addition to that, Lalonde et al. (2012) determined, using an iron reduction method previously applied to soils, the amount of organic carbon associated with reactive iron phases in sediments of various mineralogies collected from a wide range of depositional environments. Their results suggest that associations between organic carbon and iron can promote the preservation of organic carbon in sediments.
Natural sulfurization (Figure 1. 1), by creating sulphur-bounds between the organic molecules, leads to the formation of a sulphur rich organic residue that has a lower susceptibility to be degraded (Sinninghe Damste and Leeuw, 1990). It is the chemical process by which organo-sulphur compounds are formed through the incorporation of inorganic sulphur into functionalized lipids and carbohydrates (Aycard et al., 2003). This mechanism is enhanced in marine anoxic-euxinic environments, where the lack of oxygen leads bacteria to use dissolved sulphate (SO42-) as electron acceptor to oxidize low molecular weight organic compounds (Canfield, 1994). This process, called bacterial sulphate reduction (BSR), permits the formation of hydrogen sulphide, as illustrated in the following equation involving formaldehyde (CH2O):
2CH2O + SO42- ➔ H2S + 2HCO3- (Westrich, 1983)
Hydrogen sulphide (H2S) is the principal sulphide product of BSR, but it is possible to form other sulphides and polysulphides via the partial oxidation of this hydrogen sulphide (Vairavamurthy et al., 1995). In the presence of reactive iron minerals, including oxyhydr(oxides) iron, free sulphide reacts first with iron to form iron sulphides, such as pyrite. The remaining sulphide is then incorporated into OM to form organo-sulphur compounds. In this classical view, significant sulfurization of OM can only occur if iron minerals have not reacted with all sulphides present (Tribovillard et al., 2015). A direct way of incorporation of dissolved sulphates into OM (assimilatory sulphate reduction) has also been identified. But this process is considered secondary compared to the incorporation of sulphides and polysulphides (Vairavamurthy et al., 1995).
It is interesting to note that, in certain depositional environments, sulfurization can outcompete pyrite formation. Indeed, in the uppermost surface sediments of organic-rich, sulphide-dominated marine sedimentary systems, sulfurization of labile organic compounds, such as humic acids (Francois, 1987; Ferdelman, 1988; Ferdelman et al., 1991), can precede the formation of pyrite (Mossmann et al., 1991; Vairavamurthy et al., 1992; Vairavamurthy et al., 1995; Filley et al., 2002; Werne et al., 2008; Riedinger et al., 2017).
Formation of sulphur species in marine sediments
Marine sediments commonly contain solid-phase sulphur species. This is due to the fact that oceans represent one of the largest reserves of sulphur in the form of dissolved sulphate (SO42-) and the principal output of this sulphur, following a range of biogeochemical reactions or evaporation, is sedimentation (Vairavamurthy et al., 1995). Excluding sulphate minerals like gypsum that directly precipitate from evaporating seawater, two principal types of sulphur species are dominant in marine sediments, pyrite and organo-sulphur compounds, both containing sulphur in its reduced form. A diagrammatic representation of the principal processes allowing their formation is presented in Figure 1. 2.
Sedimentary pyrite (FeS2) forms by reaction of reduced sulphur species (HS-, H2S) with reactive iron (Berner, 1970; Vairavamurthy et al., 1995; Baudin et al., 2007). The reduced sulphur species are produced by bacterial sulphate reduction (BSR), a process which can only occur under anoxic conditions in marine waters or sediments. Due to the lack of oxygen in the environments, sulphate-reducing bacteria use dissolved sulphate as electron acceptor to oxidize low molecular weight organic compounds, releasing reduced sulphur species (HS-, H2S) (Canfield, 1994). Reactive iron or reduced iron is formed via the reduction of iron oxide principally provided from terrigenous supply to the marine environment (Berner, 1984). Two principal types of pyrite particles can be found in marine sediments, framboids and euheudral crystals (Love and Amstutz, 1966). Framboid pyrite formation is generally thought to occur through the formation of metastable iron sulphides, such as greigite (Fe3S4) and mackinawite (FeS) (Sweeney and Kaplan, 1973; Morse and Cornwell, 1987; Baudin et al., 2007; Hunger and Benning, 2007), whereas euhedral pyrite would be formed via direct precipitation (Giblin and Howarth, 1984). Pyrite formation, both framboids and euheudral crystals, occurs generally within the sediment during early diagenesis (diagenetic pyrite). Moreover, in euxinic marine basins, pyrite can also form in the water column (syngenetic pyrite) as well as below the sediment-water interface (Raiswell and Berner, 1985; Wilkin and Barnes, 1997). Syngenetic pyrite particles are characteristically smaller and less variable in size relative to diagenetic pyrite particles, reflecting shorter average growth periods due to hydrodynamic instability in the water column (Wilkin and Barnes, 1997).
Other trace metals, such as copper (Cu), lead (Pb), zinc (Zn), can co-precipitate together with iron sulphide minerals or form their own stable metal sulphides during early diagenesis (Jacobs et al., 1985; Huerta-Diaz and Morse, 1990). It is therefore possible to find in marine sediments some traces of other sulphide minerals such as chalco-pyrite (CuFeS2), galena (PbS), or sphalerite ((Zn,Fe)S).
The formation of organo-sulphur compounds is the second output of sulphur from the marine column water (Berner and Raiswell, 1983). This process, called natural sulfurization, occurs via two basic pathways, assimilatory and dissimilatory (Vairavamurthy et al., 1995; Werne et al., 2008).
• Assimilatory pathway:
This pathway describes the direct incorporation of dissolved sulphates into the organic matter followed by its reduction to sulphides. This assimilatory sulphate reduction is done by autotrophic organisms, for example, algae in the photic zone of the water column and chemosynthetic bacteria in hydrothermal systems. It is believed that organo-sulphur compounds formed in this way are quantitatively negligible, due principally to the fact that most of the assimilatory sulphur is contained in proteinaceaous substances, which hardly survive degradation to the later stages of diagenesis.
• Dissimilatory pathway:
This more prominent process for organo-sulphur compound formation occurs, like pyrite formation, during early diagenesis and under anoxic-euxinic conditions. This mechanism involves the reduction of dissolved sulphate via BSR. It is commonly assumed that the reduced sulphur species produced (HS-, H2S) are firstly used for pyrite formation in the presence of reactive iron minerals, and the remaining reduced sulphur species are incorporated between the organic molecules (intermolecular sulphur incorporation) and into the organic molecules (intramolecular sulphur incorporation) to form organo-sulphur compounds. However, recent works (Mossmann et al., 1991; Vairavamurthy et al., 1992; Vairavamurthy et al., 1995; Filley et al., 2002; Werne et al., 2008; Riedinger et al., 2017) have shown that the generally accepted paradigm of reactive iron minerals outcompeting organic molecules in the presence of dissolved sulphide does not necessarily hold true. As shown by Poulton et al. (2004), iron minerals have different reaction half-lives with dissolved sulphide even under ideal, experimental conditions, while some organic molecules are more susceptible to fast sulfurization than previously thought (Raven et al., 2016a).
Different molecular structures of organo-sulphur compounds are found in marine sediment kerogens (Sinninghe Damsté et al., 1988; Uteyev, 2011):
❖ Sulphides and poly-sulphides: Their formation is related to intermolecular sulfurization which corresponds to the formation of (poly-) sulphide between organic molecules.
❖ Thiols: Thiols are characterized by the presence of an R-SH functional group in their molecular structure which occurs via intramolecular sulphur incorporation.
❖ Thiophenes: The formation of the thiophene molecular group is due to intramolecular incorporation of sulphur into a cyclic or aromatic organic molecular structure.
Other sulphur species
Sulphate minerals only precipitate in closed basins with a negative water balance, where water input is reduced and the evaporation rate is significant (Vairavamurthy et al., 1995). The common sulphate minerals in marine shale are gypsum (CaSO4, 2H2O) and anhydrite (CaSO4) (Jensen et al., 1998), but also barite (BaSO4). Barite is an ubiquitous sulphate mineral in marine sediments which is formed throughout the oceanic water column in association with decaying OM and biogenic debris (Dehairs et al., 1980; Bishop, 1988; Dymond et al., 1992; Arndt et al., 2009). It can also undergo diagenetic redistribution in reducing sediment columns (Von-Breymann et al., 1992; Stamatakis and Hein, 1993; Griffith and Paytan, 2012; Henkel et al., 2012). Syngenetic or diagenetic barium sulphate can be relatively well-preserved and abundant in marine shales, and can co-occur with pyrite and organic S compounds (Baudin et al., 2007), but it is often concentrated in diagenetic fronts that can be easily recognised (Torres et al., 1996; Riedinger et al., 2006).
• Elemental sulphur:
Elemental sulphur deposits are commonly, but not invariably associated with sulphate minerals. Microbial sulphate reduction is, indeed, considered to be the key process for the precipitation of elemantal sulphur (Davis and Kirkland, 1970; Anadón et al., 1992; Peckmann et al., 1999). Elemental sulphur is not stable in most marine sediments given that it is highly transformed into sulphate or sulphide by organisms such as bacteria (Canfield and Raiswell, 1999; Philippot et al., 2007).
Geochemical properties of carbon and sulphur as descriptors of OM preservation in marine environments
Carbon and sulphur occurrences in marine sedimentary depositional environments can be used as indicators of paleoenvironmental conditions responsible of OM preservation in sediments. Indeed, OM preservation in marine sediments is controlled by numerous environmental factors, such as bottom-water oxygen content, primary production, rain rate of organic carbon and overall sedimentation. Carbon and sulphur relationships have proved useful in the characterisation of marine sedimentary depositional environments. For instance, a relationship between total or pyritic sulphur (S) and organic carbon (C) called “normal marine” sediments (those accumulating under oxygenated water masses) was established in numerous studies (Berner, 1970; Sweeney, 1972; Goldhaber and Kaplan, 1974; Berner, 1982). Their results show that sediments from normal oxygenated waters are characterized by a C/S ratio of 2.8. Samples presenting lower ratios (C/S< 2.8) reflect euxinic environments (Leventhal, 1983; Raiswell and Berner, 1985; Leventhal, 1987). Samples presenting much higher ratios (C/S>5) were found in non-marine freshwater sediments (Berner and Raiswell, 1984).
Many researchers have used C vs. S plots to help characterize ancient depositional environments (Anderson et al., 1987; Leventhal, 1987; Suits et al., 1993; Riboulleau et al., 2003; Rimmer et al., 2004). However, some problems have been noted in using this relationship in iron-poor rocks, such as carbonates and sandstones, where the complete sulfidization of iron may limit the total solid-phase mineral sulphide content of the rock (Berner, 1984; Pratt, 1984). Also, in rapidly accumulating organic-rich sediments, where the amount of dissolved sulphate can become exhausted and limit the formation of pyrite, a linear C vs. S relationship is not always observed (Murray et al., 1978; Devol and Ahmed, 1981). Other problems in the interpretation of C vs. S can arise if the amount of metabolizable organic carbon is limited (Raiswell and Berner, 1985), OM has been lost by heating (Raiswell and Berner, 1986), or unusual water chemistry is present (Tuttle and Goldhaber, 1993).
Organic sulphur occurrences in sediments is indicator of OM sulfurization, an important OM preservation mechanism. Indeed, in marine anoxic-euxinic environments, natural sulfurization can play a significant role in organic carbon preservation (Sinninghe Damsté et al., 1988; Sinninghe Damste and Leeuw, 1990). For instance, Boussafir et al. (1995) found, using transmission electron microscopy, that sulfurization of OM does play an important role in the preservation of OM in Kimmeridge Clay Formation sediments (Dorset, UK). The impact of natural sulfurization on OM preservation has been observed by Lückge et al. (2002) who show that sulfurization of OM prevents its further decomposition in sediments from the continental margin of Pakistan. The role of natural sulfurization on OM preservation was also observed by Tribovillard et al. (2015) in their comparative study of two Jurassic sediment successions (Boulonnais, France). In this study, the enhanced OM preservation of one formation compared to a second has been attributed to differences in natural sulfurization controlled by differences in reactive iron availability. However, other studies, conducted on anoxic sediments from the Cariaco Basin (Venezuela) (Aycard et al., 2003) and on the anoxic formation from Kössen (Hungary) (Vetö et al., 2000), where natural sulfurization is enhanced, show no positive correlation between organo-sulphur compound concentrations and the preservation of OM. In these studies, the OM preservation was not essentially attributed to natural sulfurization but was attributed to the mechanism of degradation-recondensation (Aycard et al., 2003) and to high primary productivity (Vetö et al., 2000).
Carbon and sulphur occurrences in sediments can be indicators of bottom water oxygenation, an important environmental controlling factor of OM preservation, and OM sulfurization, an important OM preservation mechanism. This requires relevant carbon and sulphur parameters, such as: Total Organic Carbon (TOC); Hydrogen Index (HI); Oxygen Index; OM maturity (Tmax); total sulphur (STotal); pyritic sulphur (SPyrite); and organic sulphur (SOrganic). These different carbon and sulphur parameters will be determined using Rock-Eval 7S, a new model of Rock-Eval technologies. Indeed, this new facility permits the analysis of sulphur species in addition to organic and inorganic carbon determination. With Rock-Eval 7S, the combined variations of carbon and sulphur parameters in rocks can be quantified rapidly (on average 2 hours) and simultaneously in a large number of samples. The Rock-Eval 7S principles are described in the following section.
Rock-Eval 7S principle
Being developed by IFP Energies nouvelles and Vinci-Technologies, the Rock-Eval 7S is the latest model of Rock-Eval technology. It has the same functionalities as the current version (Rock-Eval 6), complemented by a new system adapted for sulphur detection (Figure 1. 3). The Rock-Eval 7S method can be used on a wide range of rock and oil types, such as source rocks, reservoir rocks, recent sediments, crude oils and petroleum distillates. This analytical method can be summarized in two main stages, a pyrolysis followed by an oxidation (Figure 1. 3).
During the pyrolysis stage, the sample is heated under a flux of nitrogen, according to a pre-determined temperature program. The temperature can be raised from 100°C to 800°C at different heating rates, from 1°C/min to 50°C/min. The pyrolysis effluents are separated in three parts. One part is carried to a Flame Ionization Detector (FID) for the quantification of hydrocarbons. Another part is carried to an Infrared detector (IR) for the quantification of CO2 and CO. The last part passes through a combustion chamber where the reduced sulphur effluents are oxidized into SO2. This SO2 effluent is then measured by an Ultra-Violet detector (UV).
After the pyrolysis stage, the residue of the sample is transferred into an oxidation oven, where it is heated under a flux of air, following a predefined temperature program. The temperature can be raised from 100°C to 1200°C with different heating rates, from 1°C/min to 50°C/min. It is important to note that above 750°C the flux of air is replaced by a flux of nitrogen to initiate sulphate thermal degradation. The released CO and CO2 are continuously measured using the IR detector. The released sulphur effluents, being already in oxidized form (SO2), are directly recorded by the UV detector.
Table of contents :
Chapter 1: INTRODUCTION
1.1- POTENTIAL APPLICATIONS OF A NEW METHOD FOR PYRITIC SULPHUR AND ORGANIC SULPHUR QUANTIFICATION USING ROCK-EVAL 7S
1.1.1- Petroleum exploration and production
1.1.2- Paleoenvironmental/Paleoclimate reconstruction
1.1.3- Coal exploration and production
1.1.4- Soil pollution
1.2- OM PRESERVATION IN MARINE SEDIMENTS: THE LINK TO GEOCHEMICAL PROPERTIES OF CARBON AND SULPHUR
1.2.1- Environmental factors controlling OM preservation in marine sediments
1.2.2- Overview of the different mechanisms of OM preservation in marine environments
1.2.3- Formation of sulphur species in marine sediments
1.2.4- Geochemical properties of carbon and sulphur as descriptors of OM preservation in marine environments
1.3- ROCK-EVAL 7S
1.3.1- Rock-Eval 7S principle
1.3.2- Rock-Eval 7S sulphur speciation: state of the art
1.4- PHD OBJECTIVES
1.4.1- First objective: Developing and validating new analytical methodology on Rock-Eval 7S for sulphur quantification and speciation
1.4.2- Second objective: Using Rock-Eval 7S carbon and sulphur parameters as descriptors of OM preservation
Chapter 2: ROCK-EVAL 7S CALIBRATION STUDIES
2.2- ROCK-EVAL 7S STotal CALIBRATION
2.2.3- STotal calibration results
2.3- ROCK-EVAL 7S CHARACTERISATION OF THE MAIN TYPES OF SULPHUR SPECIES FOUND IN MARINE SEDIMENTS
2.3.1- Samples and methodology
2.3.2- Rock-Eval 7S characterisation of the main sulphur species found in marine sediments
Chapter 3: PYRITIC AND ORGANIC SULPHUR QUANTIFICATION IN ORGANIC RICH MARINE SEDIMENTS USING ROCK-EVAL 7S
3.1- CHAPTER INTRODUCTION
3.2- DEVELOPMENT OF A NEW METHOD FOR PYRITIC SULPHUR AND ORGANIC SULPHUR QUANTIFICATION USING ROCK-EVAL 7S
3.2.2- Samples and methods
3.3- VALIDATION OF THE NEW ROCK-EVAL 7S METHODOLOGY FOR PYRITIC SULPHUR AND ORGANIC SULPHUR QUANTIFICATION ON ORGANIC RICH SEDIMENTS
3.3.2- Samples and methods
3.3.3- Results and discussions
3.4- CHAPTER CONCLUSIONS
Chapter 4: APPLICATION OF ROCK-EVAL 7S TO THE DESCRIPTION OF OM PRESERVATION OF JURASSIC BLACK SHALES FROM NORTH YORKSHIRE, DORSET AND SOMERSET
4.2.1- Dorset section: Kimmeridge Clay Formation
ABOUSSOU Anabel – Doctoral thesis 2018
4.2.2- North Yorkshire section: Grey Shale Member
4.2.3- Somerset section: Blue Lias
4.3- ROCK EVAL 7S RESULTS
4.3.1- Dorset section: Kimmeridge Clay Formation (Appendix J and N)
4.3.2- North Yorkshire section: Grey Shale Member (Appendix J and N)
4.3.3- Somerset section: Blue Lias (Appendix J and N)
4.4- CONTRIBUTION OF NEW ROCK-EVAL 7S DATA IN THE DESCRIPTION OF OM PRESERVATION
4.4.1- Dorset section: Kimmeridge Clay Formation
4.2.2- North Yorkshire section: Grey Shale Member
4.2.3- Somerset section: Blue Lias
Chapter 5: SUMMARY AND OUTLOOK
5.2.1- Development of Rock-Eval 7S for SPyrite and SOrganic quantification
5.2.2- Using the Rock-Eval 7S carbon and sulphur properties as descriptors of OM preservation in DIONISOS sedimentary basin model