Using δ15N and δ13C and nitrogen functionalities to support a fire origin for certain inertinite macerals in a No. 4 Seam Upper Witbank coal, South Africa
Introduction
The Witbank Coalfield, situated in the Main Karoo Basin of South Africa, contains Permian Ecca coals typically rich in inertinite (Falcon, 1986, Hagelskamp and Snyman, 1988, Cadle et al., 1993, Snyman and Botha, 1993, O'Keefe et al., 2013). Two viewpoints currently exist for the origin of the inertinite macerals. Earlier workers attributed the inertinite to the incomplete degradation of plant matter as a result of a cool, temperate climate along with aerial oxidation (Falcon, 1986, Hagelskamp and Snyman, 1988, Cadle et al., 1993, Snyman and Botha, 1993). Glasspool, 2003a, Glasspool, 2003b rejected this, instead arguing that the macerals are pyrogenic, based on the description of fossil charcoal associated with coals of the Witbank Coalfield. Macroscopic charcoal associated with clastic sedimentary rocks of the Vryheid Formation (Vereeniging Coalfield, South Africa) has also been documented by Jasper et al. (2013). Despite this, the now firmly entrenched view that the inertinite macerals present in the coals are oxidation- and cold climate-derived continues to persist and is often cited in the published literature on South African coals (e.g., Van Niekerk et al., 2008, O'Keefe et al., 2013). The controversy stems, in part, from the glacial/post-glacial climatic conditions prevailing in Gondwana in contrast to the tropical conditions of Laurasia (Falcon, 1986, Snyman and Botha, 1993). The Gondwana conditions were previously assumed to have hindered the development of extensive wild and/or peat fires. The presence of charcoal in the sediments contradicts this view.
The stable isotope composition of coal (e.g., δ15N and δ13C) reflects the composition of the source vegetation as well as changes resulting from early diagenesis and subsequent coalification processes (Spiker and Hatcher, 1987, Rimmer et al., 2006, Skrzypek et al., 2010, Suto and Kawashima, 2016, Valentim et al., 2016). For iso-rank bituminous coal samples, δ13C values rise with an increase in density, concomitant with an increase in inertinite content (Rimmer et al., 2006, Valentim et al., 2016). This reflects the aromatization of the coal macromolecule with the loss of the heavier 13C atoms, suggested to be related to the preferential loss of aliphatics (Rimmer et al., 2006).
Coal macerals formed through the degradation of plant matter by organisms may not possess anatomical structures (Guo and Bustin, 1998, ICCP, 1998, Hower et al., 2013, Richardson et al., 2012, O'Keefe et al., 2013). At the same time, bacterial degradation preferentially eliminates the isotopically lighter 14N (Spiker and Hatcher, 1987, Rimmer et al., 2006, Skrzypek et al., 2010). Based on these observations, it is implicit that macerals formed through bacterial degradation, such as collotelinite and collodetrinite, may be structureless and have higher δ15N values relative to their inertinite counterparts. With limited degradation, macerals derived through charring may possess anatomical structures (Scott, 1989, Scott, 2010, Jones and Chaloner, 1991, Guo and Bustin, 1998, Scott and Glasspool, 2007), and possibly have a higher 14N content (Turekian et al., 1998, Bird and Ascough, 2012). In the absence of post-formation heating, for example, resulting from the emplacement of igneous intrusions, the δ15N and δ13C signatures imparted on botanical precursors during early coal formation processes remain unaffected until the meta-anthracite rank (Valentim et al., 2016, and References therein). This implies that these isotopes can be used to constrain the early coal formation pathways for macerals present in iso-rank bituminous coal samples (Spiker and Hatcher, 1987, Rimmer et al., 2006, Valentim et al., 2016).
In order to address the question of whether charring contributed to the formation of certain inertinite macerals present in these Witbank coals, vitrinite-rich and inertinite-rich samples were prepared through density-fractionation using a medium rank C bituminous coal (termed the “parent coal”) from the No. 4 Seam Upper, Witbank Coalfield. The δ15N and δ13C values as well as the concentrations of nitrogen functionalities for the three samples were determined, and subsequently used to constrain the early origin pathways for the dominant macerals in the density fractionated samples. A necessary assumption is that charring of Permian coal-forming vegetation fractionated the stable isotopes in a similar manner to what is observed in laboratory experiments using modern plants. In addition to the measured isotopic values for the Witbank coals, the discussion also considers the relative shifts in δ15N and δ13C values previously observed by others during laboratory experiments on various modern woods. The nitrogen functionalities present in the samples: namely N-pyridinic, N-pyrrolic, N-quaternary, and N-oxide complexes, were also analysed, and used in conjunction with the stable isotope results.
Carbon-13 cross-polarization magic-angle-spinning solid-state nuclear magnetic resonance (13C CP-MAS SS NMR) and electron spin resonance (ESR) results for the coal samples used in the present study were reported in previous publications (Moroeng et al., 2018a, Moroeng et al., 2018b). These results (see Section 4.3) were interpreted to be consistent with a fire-origin for specific inertinite macerals. Thus, this study attempts to provide additional evidence for a fire-origin for certain inertinite macerals occurring in the No. 4 Seam Upper, based on a single coal sample and density fractionated products thereof. To the authors’ knowledge, the present paper is the first to report on both the δ15N and δ13C values for South African coal. Although δ13C values and nitrogen functionalities have been reported (e.g., Gröcke et al., 2009, Suto and Kawashima, 2016, Phiri et al., 2017, Phiri et al., 2018), the previous studies do not focus on the origin of macerals in the coals.
Section snippets
Sample characterization
The run-of-mine medium rank C bituminous Permian Ecca coal sample (No. 4 Seam Upper) used in this study was obtained from a mine operating in the Witbank Coalfield, South Africa. Following crushing, density fractionation was undertaken to create a vitrinite-rich (RD = 1.3 g/cm3) and an inertinite-rich (RD = 1.8 g/cm3) sample following South African National Standard (SANS) 7936 (2010), and a portion of the original coal was set aside to represent the “parent sample”. Witbank coals generally
General characterization
The maceral nomenclature adopted herein follows that of the International Committee for Coal and Organic Petrology for bituminous coals (ICCP, 1998, ICCP, 2001, Pickel et al., 2017). The parent sample comprises of 41.6 vol% total vitrinite and 48.5 vol% total inertinite (Table 1). The density fractionated samples comprise of 81.3 vol% vitrinite and 62.6 vol% inertinite, respectively. Collotelinite and collodetrinite – with minor proportions of corpogelinite, telinite, and pseudovitrinite – are
δ13C signatures of Witbank coals and C3 plants
The δ13C signatures of organic matter derived from C3 plants is controlled by local conditions such as plant species, topographical position, latitude/altitude, soil moisture content, irradiance, and the degree of reuse of respired CO2 (Bird and Ascough, 2012, and References therein). Glossopterid gymnosperms (among others such as heterosporous lycopods) were the most important Gondwana coal-forming plants (Falcon, 1986, Glasspool, 2003a, Glasspool, 2003b, Jasper et al., 2013, Ruckwied et al.,
Conclusions
Certain inertinite macerals present in the coal assessed in this study are likely to have a fire origin, contrary to the aerial oxidation pathway proposed by earlier workers. The coal sample was density fractionated to create a vitrinite-rich and an inertinite-rich sub-sample. Collodetrinite and collotelinite were abundant in the vitrinite-rich sample (81 vol% total vitrinite), whereas the inertinite-rich sample was dominated by fusinite, semifusinite, and inertodetrinite (63 vol% total
Acknowledgements
Dr. H. Dorland is acknowledged for assistance with obtaining the sample used in the study. Mr. T.V. Makhubela is thanked for assistance with the de-carbonation procedure. Dr. C. Reinke and Mr. A. Mangs are thanked for assistance with and the XRF analysis. Dr. B.J. Wood (Centre for Microscopy and Microanalysis, The University of Queensland, Australia) is gratefully acknowledged for assistance with the XPS analysis. The first author acknowledges the Research, Education and Investment (REI) Fund
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