Recurrent palaeo-wildfires in a Cisuralian coal seam: A palaeobotanical view on high-inertinite coals from the Lower Permian of the Paraná Basin, Brazil

Distribution and abundance of charcoal in coal seams (in form of pyrogenic macerals of the inertinites group) have been considered as a reliable tool to interpret the local and regional palaeo-wildfire regimes in peat-forming depositional environments. Although the occurrence of inertinites is globally well documented for the Late Palaeozoic, the description of palaeobotanical evidence concerning the source plants of such charcoal is so far largely missing. In the present study, we provide the first detailed analysis of macro-charcoal preserved in the Barro Branco coal seam, Rio Bonito Formation, Cisuralian of the Paraná Basin, Santa Catarina State, Brazil. Charcoal, in form of macro-charcoal and inertinites, was documented in all the six coal-bearing strata that compose the succession, confirming the occurrence of recurrent palaeo-wildfires during its deposition. Reflectance values indicated a mean charring temperature reaching ~515°C (and up to 1,045°C in excess) and the macro-charcoal exhibits anatomical features of secondary xylem of Agathoxylon. Combination of results derived from palaeobotanical and petrological data demonstrates that gymnosperm-dominated vegetation was repeatedly submitted to fire events and reinforced the hypothesis that Gondwanan mires were high-fire systems during the Cisuralian.


Introduction
The distribution and abundance of fossil charcoal in coal seams, which is well documented for the Late Palaeozoic in form of pyrogenic inertinites [1], is a reliable tool to understand palaeowildfire events and interpret certain palaeoenvironmental conditions of the original peatforming systems [2,3]. However, compared to coal petrological studies on inertinites, studies on palaeo-wildfire records based on palaeobotanical evidence (like anatomical analysis of PLOS  Overlying a medium-grained sandstone (0.40 m), the Barro Branco coal seam is exposed as a 2.85 m succession consisting of 6 carbonaceous levels, each only a few centimetres thick (named here as Carbonaceous 1-6) interbedded by 5 medium-grey siltstones ( Fig 2B).
In the field, 10 hand samples were collected from each outcropping level and taken to the Laboratório de Paleobotânica e Evolução de Biomas, Museu de Ciências, Universidade do Vale do Taquari-Univates (LPEB/MCN/Univates) for analysis under stereomicroscope (Zeiss Stemi 2000-C). Plant remains exhibiting characteristics of macro-charcoal (� 2.0 mm; black colour; silky lustre and; black streak on touch) [16,17,18], were mechanically extracted with the aid of forceps and needles. Subsequently, the plant remains were mounted on stubs with adhesive tabs, coated with gold, and investigated under Scanning Electron Microscope (SEM-Zeiss EVO LS15) at the Parque Científico e Tecnológico do Vale do Taquari (TECNOVATES-Univates).
Anatomical features were measured with the use of the software ImageJ [19] from digital images. All the macro-charcoal and inertinite bearing rock samples collected are stored in the LPEB/MCN/Univates Palaeobotanical Collection under accession numbers PbUMCN1163-1168.
Maceral and reflectance analyses were conducted on the macro-charcoal containing samples at the Laboratório de Análise de Carvão e Rochas Geradoras de Petróleo of the Universidade Federal do Rio Grande do Sul (UFRGS) by using the standard preparations for optical analysis [20]. The maceral analysis was based on 500 observation points [21] and classified according to the International Committee for Coal Petrology [22,23,24] standards. The reflectance values were determined according to [25] and charring temperature was estimated based on the inertinite reflectance average, calculated by the mathematical formula [ o C = 184 + 118 x (light reflectance %)] [26].

Macro-charcoal overview
Macro-charcoal, ranging between 3-49 mm in width and 5-111 mm in length, was recovered from each of the six carbonaceous levels exposed at the Porongos outcrop ( Fig 2B). The fragments show slightly abraded edges ( Fig 3A) and are impregnated by pyrite (Fig 3B). Compression is slight and no Bogenstrukturen could be observed. No macro-charcoal was found in the greyish siltstones.

Macro-charcoal anatomy
Under SEM, the fragments show well-preserved anatomical details ( Fig 3C) as well as homogenized cell walls (Fig 3D). It was possible to differentiate three anatomical patterns named as Porongos Charcoalified Wood Type 1, 2 and 3 (Table 1).

Palaeoenvironment and taphonomy
High inertinite contents in coals have frequently been reported from several Cisuralian coal deposits all over Gondwana ( Fig 5) and in the Paraná Basin coal seams contents are variable, reaching >70% in some cases [1,27,28]. For the Barro Branco coal seam, a medium inertinite value of 14.6% was reported [15].
Macro-charcoal occurrences are also increasingly being reported for Gondwana (Fig 5), confirming that fire was a significant component of many terrestrial ecosystems on this palaeo-continent during the Cisuralian [4,5,6,29]. In general, the Late Palaeozoic is considered a high-fire period of Earth's history, as, due to considerably elevated atmospheric oxygen concentrations, the ignition and spread of wildfires were primarily controlled by atmospheric composition and not as during other periods by climate [2,6,30,31,32,33,34]. Under such conditions, fire events would have been frequent events even in ever-wet biomes such as mires [2,3,6,33].
The co-occurrence between high-inertinite contents and macro-charcoal has been demonstrated in Gondwanan coal-bearing strata [4,5], but analysis of both types of data for the same samples are still rare. Examples of such analysis come from the Cisuralian deposits of the Bonito mine I (Bonito coal seam, Rio Bonito Formation, Santa Catarina State, Brazil) [36] and Dhanpuri Coal Mine (Barakar Formation, Sohagpur Coalfield, Madhya Pradesh, India) [6]. Although a pyrogenic origin of fossil charcoal, or fusain, in clastic sediments as well as in coals and lignites has been widely accepted for the northern continents [37,38,39], the origin of the high-inertinite contents in the Permian Gondwanan coal-bearing strata remained an unresolved matter of debate amongst many coal-petrologists and palaeobotanists. Using integrated methods, it was demonstrated that the medium content of 42.2% inertinites in coals from the Dhanpuri Coal Mine in India was coincident with the occurrence of macro-charcoal, indicating that palaeowildfires reached the mire during the deposition of this peat [6]. The occurrence of macro-charcoal in the six carbonaceous levels of the Porongos outcrop indicates that fire was a recurrent element in the depositional system studied here. However, inertinite contents vary from 1.0% in Carbonaceous Level 4 up to 18.2% in Carbonaceous Level 3 (= 7.4% medium).
Modern peat-forming environments are susceptible to surface or smouldering ground wildfires, especially during seasons of severe drought or during longer periods of reduced water tables [40,41,42,43,44]. Under these conditions, surficial burning of previously deposited peat layers can be represented by a continuous layer of charred material [45]. Such surface fires might ignite smouldering ground fires [46] and produce large quantities of charred remains [17,18]. Such continuous bands, rich in charred material, including abundant macrocharcoal and inertinites have not been found in any of the six carbonaceous levels investigated here. Therefore, in the studied area, there is no clear evidence to support an autochthonous surface or a smouldering ground burning of peat-forming material inside the mire.  Table and S3 Table). Map adapted from [35]. The slightly abrasions observed on the edges of most macro-charcoal recovered from the six carbonaceous levels might indicate that such charred material was transported into the mire. This suggests that the fire events occurred some distance away from the place of final deposition. As the size of the macro-charcoal remains investigated here ranges from fragments of 3 mm x 5 mm to relatively larger fragments of 49 mm x 111 mm, they were transported inside the depositional environment via hydraulic flow and not by wind [18,47]. That water transport may have resulted in a selective bias in favour of macroscopic charred wood remains, as no other charred plant organs were recorded in the six carbonaceous levels studied here [17,48]. This taphonomical interpretation is congruent with the high mineral content present in all six carbonaceous levels, which suggest that the water influxes frequently transported sediments from an external source into the mire. Although macro-charcoal was transported by water and wind might also have acted and transported the minor charcoal particles into or within the mire. Such particles are petrographically represented by inertodetrinites, which are present in all the six carbonaceous levels. These fine charcoal particles can be lifted into the air and transported over the long distances, and might be formed by crown fires [17,49,50].
A reliable determination of the transport distance of the charred remains studied here is difficult. In modern environments, the presence of macro-charcoal in forest soils as well as lake and peat-forming deposits is usually considered as an indicator of a local wildfire event [51,52,53,54,55,56,57]. As the fossil assemblages studied here are composed predominantly of macro-charcoal remains (some of them of relatively large size), a hypautochthonous origin for such charred material might be suggested. This interpretation may be supported by the presence of only slight abrasions on the edges of the macro-charcoal from all six carbonaceous levels, which suggest that these fossils were transported only over the short distances [58]. Therefore, it seems that these recurrent palaeo-wildfires events occurred in the vicinity of the Barro Branco coal seam peat-forming environment. The lack of charred remains in the massive shale levels might be the result of an absence of palaeo-wildfires during the deposition of these sedimentary horizons or it might be a result of a taphonomical bias, since no other plant fossil remains where documented in these sedimentary layers. However, it is well-know that charcoal can be preserved in non-carbonaceous silty-grained sediments [18,58].

Charring temperature and palaeo-wildfire classification
Experimental studies demonstrated that the reflectance values of charred plant tissues increase with increasing of charring temperatures [18,59,60,61,62]. Therefore, reflectance values of charcoal and inertinites have been used to estimate palaeo-wildfire temperatures [3,6,60,63,64,65]. The average of the inertinite reflectance values from the six carbonaceous levels studied ranged from 1.88 (%) to 2.79 (%), and this indicates an estimated medium charring temperature that range from 405 to 513˚C for these recurrent palaeo-wildfires ( Table 2). As the average of the vitrinite reflectance values from the six carbonaceous levels are relatively low, ranging from 0,692 (%) to 0,791 (%) ( Table 2), coalification did not affect inertinite reflectance [63]. However, the maximum reflectance value of 7.3%, indicates a charring temperature of 1,045˚C.
Traditionally, wildfires have been classified into surface, ground and crown fires [17,46,66,67]. Surface fire burns dead plant material derived from litter as well as living shrubby and herbaceous plants, and have a comparably low temperature [17,46,66,68]. This type of fire produce most of the macro-charcoal [18], and might ignite smouldering ground fires, which burn organic-rich soil layers beneath the surface litter at low temperatures [17,46,69] and can last from days to years [70]. In contrast, crown fires have high temperatures, burn living vegetation from canopy as well as understorey trees [17,46,66,71], and produce less macro-charcoal [18].
As the charred remains from the six carbonaceous levels studied here were transported, an insitu observation of the depositional characteristics of these repeated wildfire events is not possible. Therefore, it is difficult to classify the fire type that produced the charred materials as well as indicate the fire intensity. However, maximum burning temperatures reached up to 1,045˚C and suggest that at least in some cases intensive heat acted on the wood. Based on the predominance and relatively large size of the macro-charcoal remains in the six carbonaceous levels studied here, we speculate that they are result of repetitive surface palaeo-wildfires, as such a fire type produce most of the macro-charcoal [18,72]. However, high-temperature crown fires might have also played a (subordinate) role in forming the macro-charcoal within the six carbonaceous levels.
Considering the taphonomical interpretation presented above, these charred remains produced by such palaeo-wildfires were transported and hydrodynamically sorted only over a short distance into the mire. This might explain the low abundance of macro-charcoal and inertinites in the six carbonaceous levels studied here, as not all charred remains produced by these fires may have reached the mire. It has been suggested that much of the inertinites present in Permian Gondwana coals resulted from low temperature surface palaeo-wildfires [17]. Some of these palaeo-wildfires occurred outside the mire and may have been followed by increased soil erosion, which could explain the high mineral content present not only in the coal seams analysed here, but also in many other Permian Gondwana coal deposits [17].

Taxonomical affinities and palaeoecological considerations
Although the charcoalified remains exhibit exceptional well-preserved internal anatomical details, due to their fragmentation and possible alterations at the time of charring, it is possible to establish a general taxonomic affinity for them. In charcoalified wood types 1 and 2 only the secondary xylem is preserved. In contrast, charcoalified wood type 3 exhibits part of the primary xylem, which is rarely preserved in Late Palaeozoic Gondwana woods [73]. This preserved part is small, and therefore, it seems more appropriate to establish the taxonomic affinity of charcoalified wood type 3 on the basis of secondary xylem, as more anatomical features such as tracheid pitting, rays and cross-field pitting are preserved in this tissue.
The three wood types identified here are rather similar in their secondary xylem characteristics and exhibit a typical gymnospermous anatomical pattern. The major anatomical differences between the secondary xylem of charcoalified wood types identified here, is that wood type 1 has tracheids with 1-2 seriate pitting and rays which are 2-7 cells high, while wood type 2 has tracheids with 1-4 seriate pitting and rays which are 3-12 cells high and wood type 3 has rays with 2-4 cells high (Table 1). These charcoalified wood types may not represent natural taxa, and it is quite possible that they are different morphological stages of distinct parts or ontogenetic stages of the same taxon [74] The presence of tracheids bearing contiguous uniseriate and alternate multiseriate bordered pitting and the absence of axial parenchyma, allows for a generic classification of the secondary xylem of three wood types as Agathoxylon Hartig [75,76,77]. This taxonomical definition is more reliable for wood type 2 and 3 as both have araucarioid cross-field pitting, which is a typical anatomical pattern of Agathoxylon fossil wood secondary xylem [75,76,77]. This wood anatomical pattern is widespread in Late Palaeozoic and Mesozoic deposits of different geographical regions of the both hemispheres [5,73,77,78,79,80]. It is related to several gymnosperm groups such as Cycadales, Caytoniales, Glossopteridales, Cordaitales, Voltziales, Ullmanniales, Cheirolepidiaceae and Araucariaceae [81,82].
Macro-charcoal remains presenting secondary xylem with an Agathoxylon anatomical pattern have already been reported from several Lower Permian coal-bearing deposits of the Rio Bonito Formation [4,7,8,83,84]. In the Faxinal coalfield (a locality of the Rio Bonito Formation), macro-charcoal remains with such an anatomy occur in association with abundant Glossopteris leaves, suggesting a biological connection between both [8,84]. However, until now there is no unequivocal evidence, which could definitively indicate that to which group or groups of gymnosperms these Agathoxylon-like charcoals from the Rio Bonito Formation might really belong.
As the charcoalified assemblages of the six carbonaceous levels studied here are composed exclusively of the Agathoxylon type of wood, it is possible to infer that plants possessing such an anatomical pattern were the most important components of the biomass responsible for the maintenance of these repetitive palaeo-wildfires. The uniformity of the vegetation in the charcoalified assemblages of the six carbonaceous levels might suggest that these plants were well adapted to grow in an environmental, experiencing the repeated and recurrent palaeo-wildfires events.
Fire-adaptation is an ecological-evolutionary trend, observed in modern environment vegetation that is submitted to regular wildfires [85,86,87,88], and has been considered as a key-factor in the evolution of Palaeozoic early conifers [89]. In the Faxinal coalfield, which can be considered as a palaeoenvironment disturbed by palaeo-wildfires and volcanic activity [8,90], the presence of an abaxial trichome complex on Glossopteris pubescens leaves, a general xeromorphic feature of leaves in a wide variety of taxa, has been mentioned as a possible mechanism providing insulation against the heat produced by fire [91]. However, until now there is no clear evidence of any fireadapted morphological structure in wood remains from Rio Bonito Formation, and further discoveries of fossil wood might shed some light about this complex ecological-evolutionary relation.

Conclusions
Based on the data and interpretations presented here, the following conclusions regarding the charcoalified assemblages from the six Barro Branco coal seams can be drawn: 1. The presence of macro-charcoal as well as inertinites in all six carbonaceous levels, provide the first evidence of the occurrence of repeated palaeo-wildfires during the deposition of the Barro Branco coal seam.
2. These recurrent palaeo-wildfires are related to the high palaeo-atmospheric O 2 concentrations proposed for the Lower Permian.
3. The macro-charcoal remains were transported inside the mire depositional system via hydraulic flow, and such water transport may have resulted in a selective bias in favour of charred wood remains. However, wind blow could also have acted and transported minor charcoal particles (inertodetrinites) inside the mire. 4. A hypoautochtonous origin for the macro-charcoal of all six levels might be suggested, and therefore, these repeated palaeo-wildfires events occurred in the vicinity of the Barro Branco coal seam peat-forming. 5. The reflectance values from the inertinites of all six levels indicate an estimated charring temperature reaching up to 1,045˚C. This high charring temperature represents intense fire acting in the depositional system. 6. The study of the macro-charcoal from the six carbonaceous levels indicates the presence of secondary xylem with an Agathoxylon-like anatomical pattern.
7. The non-significant variation of the wood anatomy in the charcoalified assemblages throughout the six levels suggest that gymnospermous plants bearing Agathoxylon-like secondary xylem were one of the most important components of the biomass responsible for the maintenance of these recurrent palaeo-wildfires.  Table. Published records of inertinites in Lower Permian coals. Data based on [1,2,3,4,5], and additional sources not mentioned in these previous compilations. (DOCX)