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Current knowledge on the origin and structure of Estonian kukersite kerogen.

If we cannot determine the truth then we should follow the most probable.

R. Descartes, 1637

More than three decades have passed since the UN Symposium on the development and utilization of oil shale research in Tallinn in 1968. During this time considerable changes in our understanding of the kerogens have taken place. They are caused by the convergence of the research, integration of research methods and extensive generalization of the experimental data. In this short report, it is not possible to discuss these developments. Therefore let as concentrate on the problems of origin and structure of Estonian kukersite kerogen, a topics open to debate during last decades, and on the ways the time as the best judge has made decisions on them.

Origin of Kerogen

Origin of Estonian Kukersite Kerogen [1]

In 1917 botanist (sic!) M. Zalessky [2], studying microscopically oil shale from Kukruse stage, introduced the term 'kukersite'. He described the oval bodies in kerogen, interpreted them as the remains of an extinct microorganism, and named this supposed organism Gloeocapsamorpha prisca due to morphological similarity with extant cyanobacterium Gloeocapsa quarter-nata Kutzing occurring on moist rocks. About forty years later it was shown that these oval bodies are in fact cavities consisting of carbon dioxide, hydrogen and methane and interpreted as gas bubbles without any biological meaning [3, 4]. However, further microscopic research confirmed Zalessky's observations [5-8], and now there exists a general agreement between kerogen researchers that a line of Ordovician kerogens is derived from G. prisca.

The corresponding oil shales are termed as kukersites [9]. Along with the microscopic investigation including electron microscopy exceeding [10.sup.4]-fold magnification, the flash-pyrolytic (characteristic hydrocarbon pattern) and spectral data contribute to this agreement. The remains of Gloeocapsamorpha prisca consist of bright yellow, spherical to ovoid globules 20-500 [micro]m in diameter [6]. These globules consist of a uniform or concentrically layered wall surrounding a number of ovoid cavities. These cavities, located in distinct order, are believed to represent the non-fossilizing cells of a colonial microorganism.

Different types of colonies give rise to several morphotypes identified in various kukersites, in our context most relevant are "closed-wall phenolrich" and "open-wall phenol-poor" morphotypes from Estonian and North American (Wisconsin shales), respectively [6]. Their thick resistant organic walls (sheaths) are presumably the main source of kerogen. It consists of phenolic compounds (absorbance at 3050, 3200-3500 [cm.sup.-1] in the IR spectra, 5-alkyl- and 5-alkenyl-benzene-1,3-diols in flash pyrolysates) [10, 11], which may have played a role analogues to that of the modern UV-blocking cyanobacterial pigment scytonemine. A recent study of microfossil-rich part of Estonian and Guttenberg kukersite kerogens using microscopic, spectral, pyrolytic and oxidative methods resulted in a reconstruction of n-alkylresorcinol-based polymer composing the G. prisca microfossils for these two kukersites (Fig. 1 [8]).

[FIGURE 1 OMITTED]

In rich fauna of kukersite beds described by several researchers ([7] and citation therein) the inorganic skeletal material predominates. Moreover, elemental mapping demonstrates the fundamental role of authigenic mineralization in the preservation and replication of faunal remains in the presence of clay minerals [12]. Thus, the contribution of faunal remains to the formation of kerogen seems to be rather low.

Mineral matter accompanying the kerogen comprises mainly the minerals of limestone and clay, e.g. calcite, dolomite marl with some minor admixtures, e.g. pyrite, gypsum [4, 13]. The source of carbonate-rich minerals is the debris of fauna and precipitation from the seawater. These both sources of calcium carbonate are augmented through intensive photosynthesis, i.e. light-enhanced calcification and the shift of the carbonate solubility equilibrium in the seawater respectively [14]. However, this relationship is not so straightforward, in fact, the continental discharge controlled the carbonate accumulation in the littoral zone [15].

Redox Conditions and Microbial Mats in Ordovician Epicontinental Sea

Early Paleozoic including Ordovician phytoplankton activity was governed by organic-walled plankton, i.e. cyanobacteria [16] and sulfur bacteria [17] inhabiting in oxic and anoxic (euxinic) photic zones, respectively.

As shown in a recent detailed study of a single continuous core from the late Middle Ordovician [18], sediments deposited in oxic period are characterized by high pristane/phytane ratio, low homohopane index and low gammacerane ratio and abundant presence of G. prisca remains. (One must keep in mind the possibility of local anoxia discussed in [1, 18].) Sediments, in which contrary values of redox indicators are observed and whose organic matter consists of arylisoprenoids derived from sulfur bacteria, have been deposited in anoxic period. During the latter siliciclastic lithology (including argillites) dominated over carbonaceous ones. Fundamental changes in the dominant phototrophic sources of organic matter are probably related to the changes in ocean circulation and temperature regime.

Kerogen of Estonian black shale (alum shale) formed probably at such anoxic period. This euxinic more aromatic kerogen, enriched ca six-fold with nitrogen in comparison to the kukersite kerogen, is deposited in the same area and yields upon pyrolysis products which do not show the characteristic features of these of G. prisca-derived kerogens, and is believed to be formed by cyanobacteria [19]. This argillaceous metal-rich amorphous kerogen contains the remains of graptolite Dictyonema pointing to the largely dysoxic conditions.

We speculate here that the dominant role in its formation could have been played by green sulfur bacteria using in the process of photosynthesis hydrogen sulfide as electron donor. The elevated nitrogen content can be the result of intensive nitrogen fixation suppressed by the increasing atmospheric oxygen concentration [20] or better protection via adsorption on siliciclastic minerals [21] (the analysis of this topic is beyond the borders of this report).

Multilayered microbial communities (microbial mats) along with tropical rain forests and coral reefs are among the most productive ecosystems in the world [11, 22]. Very intense carbon turnover (high production/biomass ratio) in microbial communities is an important difference between these ecosystems. It is noteworthy that cyanobacteria have played the major role in bringing about the increase in free oxygen in the Earth's atmosphere from far less than 1 % to about 20 %, and massive deposits of kerogen (e.g. Green River Formation [21] and petroleum [20]) are believed to be formed by masses of decayed cyanobacteria.

Thus, kukersites (and possibly Estonian alum shale as well) are believed to be formed mainly by a massive accumulation of a single type of organisms, analogously to the formation of boghead, torbanite and tasmanites. The three latter originate from microalgae [16, 23]. The occurrence of these kerogens has strongly contributed to the formulation of selective preservation pathway of kerogen formation. However, until G. prisca is not biologically firmly defined it is not possible to link the chemistry of Estonian kukersite kerogen to some certain extant organism.

Biological Identification of G. prisca

Currently G. prisca is thought to be derived from a marine photoautotroph. The modern mat-forming cyanobacterium Entophysalis [21] (kingdom Monera) appears to be the morphologically nearest living organism to G. prisca. Its outermost thick sheaths have the highest preservation potential, especially following chelation by trace elements (the actual cell membrane as opposed to the sheats and its contents are completely degraded soon after the death). There is a complete continuum in morphology from the larger benthic mat colonies to the smaller planktonic ones [11].

Cyanobacteria, formerly and misleadingly called 'blue-green algae' are photosynthetic bacteria (in which chlorophyll a occurs along with characteristic pigments known as phycobilins), ancestors of red and green algae [20]. At present, the classification of cyanobacteria is not unequivocal, this phylum is classified according to phenotypic characteristics rather than the 16S rDNA phylogeny as such a scheme is not yet possible [24].

Some researchers suggest G. prisca to be freshwater/brackish green microalgae related to Botryococcus braunii [6, 25] (Phylum Chlorophyta, kingdom Protista) and the polymorphism of G. prisca and abundance of 5-alkyl-1,3-benzenediols could be the result of various salinity levels [6]. Noteworthy, green algae is unable to fix atmospheric nitrogen, many cyanobacteria among them Gloeocapsa have this possibility [5], corresponding ability of G. prisca is unknown.

Chemical Structure of Kerogen

Carbon Skeleton and Functional Groups

For generalization of numerous data on this matter, we used the method of modeling designing the two-dimensional chemical structure of kerogen (Fig. 2 [26]). The obtained structure was checked by simulation of its [sup.13]C CP MAS NMR spectrum. We note that quantification of the NMR spectra of low-aromaticity kerogens is reliable under certain conditions of the experiment [27].

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The comparison of carbon types in the model and intact kerogen is shown in Fig. 3. We can observe the consistency of both data demonstrating the well-known low aromaticity of Estonian kukersite kerogen.

The distribution of aromatic carbon types again shows the adequacy of the model (Fig. 4). On the contrary, the structure proposed for G. prisca microfossils in [8] (right columns) consists of ca twice quantity of oxygenated carbons compared to these of the intact kerogen and no protonated carbons in the meta position to oxygenated carbons. The question arises, whether the proposed structure does not represent the real structure or the microfossils do not represent the whole kerogen (the yield of the used dichloromethane float is not shown by the authors of this serious research article).

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Let us turn to the largest group of carbons, the aliphatic ones (Fig. 5). Unfortunately, methylene and methine groups, and especially the formers in the chains and cycles, cannot be distinguished by the NMR spectra. In accordance with the designed model, up to 80 % of the methylene groups are located in chains with the average length about 7.5 methylene groups. Based on statistical calculations, strong dominance of methylene groups in cycles was suggested [28]. Our model studies taking into account especially the high straight to branched chain ratio in oxidation products of kerogen showed that a different interpretation of the elemental composition of kerogen is possible as well. This result shows the less extent of cross-linking in the kerogen structure, a fact consistent with the low maturity of this kerogen.

The determination of the methylene group distribution between chains and cycles in amorphous solids is a complicated problem and needs further research, especially the dipolar-dephasing behavior of the kerogen in the [sup.13]C NMR MAS experiment. By our experience it cannot be reduced only to the determination of the ratio of "mobile"/"rigid" carbons based on comparison of "protonated" and "non-protonated" spectra as follows from [29].

The distribution of oxygen functionalities is shown in Fig. 6. Based on the stability of model sec.-dialkylethers of 1,3-benzenediols in the conditions of alkaline KMn[O.sub.4] oxidation [30] the suggested dominating role of such structural unit in the structure of Estonian kukersite kerogen [31] is unrealistic but aliphatic ether bonds are probably involved in the cross-linking. This new knowledge is reflected in the model consisting of free phenolic hydroxyl groups and dialkyl ether bonds. The presence or absence of aryl-alkyl monoethers needs further research.

[FIGURE 6 OMITTED]

As for the chemical determinations of functional groups in the kerogen there is a growing evidence on limitations of these methods caused by the three-dimensional structure of the kerogen [30, 32]. The latter is so far badly understood.

Formation of Phenols Present in Retort Oil

We return now to Fig. 3 and turn attention to the carbon attached to oxygen shown in the lower part of the columns. Simple calculation shows that the quantity of oxygen bound to aromatic carbon (ca 4.5 % from the total carbon) is high enough to form pyrolytic oil abundant of phenols (with average molecular mass e.g. ca 200) and other oxygen compounds. Nearly the same quantity of oxygen is bound to the aliphatic carbon. From this oxygen only a minor part is located in alcoholic hydroxyl groups, and it will be consumed totally by the formation of pyrolytic water (3-5 % from the kerogen). Therefore, the transformation of this oxygen into phenolic hydroxyl groups in the process of kerogen pyrolysis as supposed in [33] is unrealistic.

The pathways of phenol formation upon pyrolysis of the kerogen and an abundance of aromatic carbons in its structure, intensively discussed in fifties and sixties [4, 34] are nowadays relatively clear, the phenols originate from the phenolic structures already present in the kerogen, they are released and modified via further transformations in the process of pyrolysis (see [35] as well). These processes and especially the glass-liquid transition of the kerogen need further research. Notably, trace amounts of 5-alkyl-1,3-benzenediols can be detected in the oxidative degradation products of the kerogen at the temperature as low as 75[degrees]C [30].

Conclusions

1. Microscopic and geochemical data evidence that Estonian kukersite kerogen is formed along with other kukersites from a single extinct photosynthetic microorganism which seems to be related morphologically to cyanobacteria but chemically to green algae.

2. This kerogen represents a highly aliphatic macromolecule. According to NMR data, the ratio of aliphatic and aromatic carbons is ca 4-5, tentative ramification of known data on the distribution of aliphatic carbon shows the predominance of secondary carbon in the alkyl chains.

3. Heteroatomic functional groups are represented mostly by oxygen-containing ones, more than two third of these groups are hydroxyl and alkoxy groups. These groups are attached to aromatic and aliphatic carbons, quantitative estimation of these structural elements is only tentative.

4. For further interpretation of available data on the structure of kerogen, the knowledge of its three-dimensional structure is necessary.

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[33.] Aarna, A.J., Urov, K. Study of the structure of oil shale kerogens using dehydrogenation by sulfur. Report 2 // Tallinna Polutehnilise Instituudi Toimetised (Transact. Tallinn Polytech. Inst.) A-228. Tallinn, 1965. P. 27-40 [in Russian].

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[35.] Lille, U. On the origin of 5-alkyl-1,3-benzenediols in the retort oil of Estonian kukersite // Oil Shale. 1999. Vol. 16, No. 3. P. 301-307.

* Presented at Symposium on Oil Shale in Tallinn, Estonia, November 17-21, 2002.

U. LILLE **

Institute of Basic and Applied Chemistry at Tallinn Technical University 15 Akadeemia St., Tallinn 12618, Estonia

** e-mail: lille@chemnet.ee
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