Polycyclic aromatic hydrocarbons and their relationship to maturity and paleoenvironmental settings in lacustrine sediments of the Neogene Toplica Basin, Serbia

The study investigated the influence of maturity, biomass type, and depositional settings on the distribution and relative abundance of polycyclic aromatic hydrocarbons (PAHs) for lacustrine sediments collected from depths up to 1000 m of Prebreza and Čučale stratigraphic units (the northwest part of the Toplica Basin). A recently proposed benzo[ghi]perylene/(perylene + benzo[ghi]perylene) parameter, along with commonly used Phenanthrene Alkylation Index and benzo[e]pyrene/(perylene + benzo[e]pyrene) indices, pointed out differences in maturity levels between stratigraphic units by displaying a positive linear relationship with vitrinite reflectance. However, in several immature Prebreza sediments, a substantial presence of algae and/or anoxic, mesosaline/hypersaline conditions were suitable for forming β-substituted methylphenanthrenes and 6-ring benzo[ghi]perylene. Generally, high molecular weight unsubstituted PAHs (HMWPAHs), particularly perylene, predominated Prebreza sediments. Anoxic conditions appeared to be decisive for accumulating and preserving the perylene carbon skeleton in studied stratigraphic units. Besides, more intense volcanism in the Čučale unit favored combustion processes, which prompted the accumulation of low molecular weight unsubstituted PAHs (LMWPAHs), especially phenanthrene. A general prevalence of retene over cadalene in Prebreza sediments, in which alginite and liptodetrinite predominated, implied algae as retene precursor. Selective degradation of retene or hindered demethylation of 9-methylphenanthrene under anoxic and more saline environmental settings had occurred notably in the Prebreza unit, which led to the formation of 1-methylphenanthrene and/or pimanthrene (1,7-dimethylphenanthrene). Čučale sediments with substantial amounts of vitrinite macerals or saturated diterpenoids had a predominant simonellite derived from conifers. Non-degraded and well-preserved Pinaceae conifers predominated in Prebreza sediments deposited under semi-arid climatic conditions, whereas mixed degraded/non-degraded conifers characterized Čučale sediments deposited in a wide range of climatic conditions, from semi-arid to semi-humid.

hypersaline conditions were suitable for forming bsubstituted methylphenanthrenes and 6-ring benzo [ghi]perylene. Generally, high molecular weight unsubstituted PAHs (HMW PAHs ), particularly perylene, predominated Prebreza sediments. Anoxic conditions appeared to be decisive for accumulating and preserving the perylene carbon skeleton in studied stratigraphic units. Besides, more intense volcanism in the Č učale unit favored combustion processes, which prompted the accumulation of low molecular weight unsubstituted PAHs (LMW PAHs ), especially phenanthrene. A general prevalence of retene over cadalene in Prebreza sediments, in which alginite and liptodetrinite predominated, implied algae as retene precursor. Selective degradation of retene or hindered demethylation of 9-methylphenanthrene under anoxic and more saline environmental settings had occurred notably in the Prebreza unit, which led to the formation of 1-methylphenanthrene and/or pimanthrene (1,7-dimethylphenanthrene). Č učale sediments with substantial amounts of vitrinite macerals or saturated diterpenoids had a predominant simonellite derived from conifers. Non-degraded and well-preserved Pinaceae conifers predominated in Prebreza sediments deposited under semi-arid climatic conditions, whereas mixed degraded/non-degraded conifers characterized Č učale sediments deposited in a wide range of climatic conditions, from semi-arid to semihumid.

Introduction
Polycyclic aromatic hydrocarbons preserved in sediments may provide evidence about maturity and origin of the organic matter (OM) and/or can serve as tracers of paleoenvironmental changes during the sedimentary deposition (Radke et al. 1982;Jiang et al. 1998;Wen et al. 2000;Bechtel et al. 2007;Stojanović et al. 2007; Grice et al. 2009;Marynowski et al. 2014;Xu et al. 2019). Investigations of (Tan and Heit 1981;Jiang et al. 1998;Luo et al. 2006;Yunker et al. 2011;Xu et al. 2019) have suggested that substituted and unsubstituted PAHs have diagenetic/catagenetic (higher plant terpenoid or kerogen transformations) and combustion (pyrolytic) origin.
Several studies (Soma et al. 1996;Silliman et al. 1998;Elias et al. 1997;Qin et al. 2008;Marynowski et al. 2013;Szczerba and Rospondek 2010) have substantiated that anoxic conditions and saline paleoenvironments can be highly suitable for the accumulation of 1-MP and perylene. Apparently, these conditions may influence demethylation-isomerization reactions of MPs and/or are responsible for preserving the perylene carbon skeleton. Besides, the abundance of retene, simonellite, or cadalene may be climatically governed since their presence relates to the occurrence of specific coniferous gymnosperm taxa, which may preferentially settle in humid or arid regions (Hautevelle et al. 2006).
Considering all previous facts regarding PAHs, the main goal of this study is to find reliable maturity indicators, predict their source and origin, and inspect their distribution dependence on distinct paleoenvironmental settings for selected Prebreza and Č učale lacustrine sediments.

Study area
The location of the Toplica Basin is in southern Serbia (Fig. 1a). A hilly to mountainous relief with a moderately continental climate characterizes the study area (Perišić et al. 2004). The Toplica Basin represents a northeast-southwest trending intramontane tectonic depression filled with Miocene freshwater and coalbearing sediments, as well as Pliocene and Quaternary freshwater sediments, which are covering an area of * 20 km 2 (Malešević et al. 1974;Burazer et al. 2020). The northeast and north Precambrian metamorphic rocks, along with the Jurassic diabase on the east, surround the basin (Fig. 1b). Freshwater units, namely Č učale (thickness 600 m), Prebreza (thickness 570 m), Clastic (thickness 100 m), and Quaternary (thickness 10 m), are a part of Neogene of the Toplica Basin (Malešević et al. 1974;Burazer et al. 2020).
During and after Neogene, the formation of longitudinal and diagonal gravitational faults on the western and northern edges, as well as in the middle parts, characterize this tectonic depression (Malešević et al. 1974). These faults are of greater importance for mutual relations of sedimentary formations. The sedimentation process in the Č učale unit started during Lower Miocene with older basal series in upper parts and continuing with younger coal-tuffaceous and fine-grained bituminous series in bottom parts of this unit along with tuffaceous and bituminous fine-grained series in the upper part (Burazer et al. 2020). On the other hand, the depositional process in the Prebreza unit occurred during Middle Miocene. The Prebreza unit transgressively overlies the Č učale Fig. 1 Burazer et al. 2020) unit and consists of three clastic series: fine-grained (older), medium-grained (younger), and coarsegrained (the youngest) (Burazer et al. 2020). Gravel, sand, sandy clay with very thin marly layers are characterizing the Clastic Upper Miocene-Pliocene series, which transgressively overlies the Prebreza unit (Burazer et al. 2020).
Moreover, the presence of analcime in Prebreza and Č učale stratigraphic units relates to the volcanic activity. However, a more pronounced presence of analcimized tuffs, tuffites, and tuffaceous components characterize Č učale sediments, particularly at the depth interval from 747 to 754 m (Burazer et al. 2020). Alluvial flows are mainly responsible for the transportation of a significant portion of fine-grained andesitic volcanic and volcanoclastic rocks into basin from south marginal parts (Burazer et al. 2020). Volcanoclastic material originates mostly from the Lece Magmatic Complex (Malešević et al. 1974;Dragić et al. 2014;Burazer et al. 2020).

Samples
In accordance with changes in macroscopic lithology and sediment type determined by petrographic microscopy and X-ray diffraction (Burazer et al. 2020), the study investigated 38 selected Prebreza and Č učale sediments from the borehole BL7, located in the northwest part of the Toplica Basin (Fig. 1). Sediments were collected from depths up to 1000 m. A detailed lithological characterization of investigated sediments is given in the study of Burazer et al. (2020). This study applied Soxhlet extraction and gas chromatography-mass spectrometry (GC-MS) to extract and analyze substituted and unsubstituted PAHs in studied sediments. Before the Soxhlet extraction, sediments were crushed using a pestle and mortar and later sieved through a 63-lm sieve.

Organic geochemical analysis
To obtain the soluble OM (bitumen), the Soxhlet extraction method was performed on 38 selected Prebreza and Č učale sediments with an azeotrope mixture of methanol and dichloromethane (1:7.6, v:v) for 36 h. The elemental sulfur was removed during the extraction process by adding some copper to the mixture. The saturated fraction was primarily isolated using n-hexane via column chromatography (adsorbents: SiO 2 and Al 2 O 3 ). Afterward, the elution of the aromatic fraction from the bitumen was done with benzene.
Subsequently, the aromatic fraction was analyzed by the gas chromatography-mass spectrometry (GC-MS) technique. A gas chromatograph Agilent 7890A GC (HP5-MS capillary column, 30 m 9 0.25 mm, 0.25 lm film thickness, Helium carrier gas 1.5 cm 3 min -1 ) coupled to Agilent 5975C mass selective detector (70 eV) was used. The column was heated from 80 to 300°C, at a rate of 2°C min -1 , and the temperature of 300°C was maintained for an additional 20 min. Afterward, the temperature of 300°C was rapidly increased to 310°C, at a rate of 10°C min -1 , and the final temperature of 310°C was maintained for 1 min. The determination of individual peaks was achieved by comparisons with literature data and mass spectra (library: NIST5a). The relative abundance of polycyclic aromatic hydrocarbons was determined by integrating peak areas using GCMS Data Analysis software in the appropriate mass chromatograms. Based on their relative abundance, aromatic PAH parameters were calculated. The mass fragmentograms of the aromatic fraction used in this study were m/z 178 for phenanthrene, m/z 183 for cadalene, m/z 192 for methylphenanthrenes, m/z 202 for fluoranthene and pyrene, m/z 206 for dimethylphenanthrenes, m/z 219 for retene, m/z 228 for benz [a]anthracene, m/z 237 simonellite, m/z 252 for benzo[e]pyrene, benzo[a]pyrene, and perylene, m/ z 255 for dehydroabietane, as well as m/z 276 for benzo[ghi]perylene. Table 1 demonstrate the distribution and relative abundance of identified substituted and unsubstituted PAHs in representative samples of Prebreza and Č učale units. Additionally, structures of substituted and unsubstituted PAHs, namely Cad, P, 1-, 2-, 3-, 9-MPs, Fla, Pyr, 1,7-, 1,3-, 3,9-, 2,10-, 3,10-DMPs, Re, BaAn, Sim, BePy, BaPy, Pery, and BghiP are presented in ESM1.
Besides, Prebreza sediments have shown an overall predominance of retene over simonellite and cadalene, which is supported by relatively high Re/Cad and low Sim/Cad ratios (Re/Cad up to 5.70 and avg. 2.33, Sim/ Cad under 1, Table 2). However, higher portions of cadalene and simonellite characterize various Č učale sediments, most prominently samples BL7/31 and BL7/49, in which Sim/Cad or Cad/(Re ? Sim) ratios have reached a maximum (Sim/Cad up to 9.19 and Cad/(Re ? Sim) up to 3.52 Table 2, Fig. 3b). Additionally, the conifer wood degradation index (CWDI) is averaging at 0.65 and 0.19 for Prebreza and Č učale units, respectively (Table 2).  (Table 3).
Generally, a higher proportion of perylene relative to all unsubstituted PAHs characterizes Prebreza and the upper section of the Č učale unit, above 821 m (Pe (%) up to 80.70 and 69.95, Table 3). On the other hand,   Fig. 3d). The Fla/ (Fla ? Pyr) parameter averaged 0.49 and 0.50 for Prebreza and Č učale units, whereas the Fla/Pyr ratio indicated a small predominance of fluoranthene over pyrene in both studied stratigraphic units (avg. 1.00 and 1.01, respectively, Table 3).

Thermal maturity
The previous discussion about distinct maturity levels of the OM deposited in Prebreza and Č učale units included the application of Rock-Eval, vitrinite reflectance, and various molecular parameters. In this case, the selectivity of defined maturity indices that employ phenanthrene derivatives such as MPI-1, MPI-2, MPI-3, MPR, and PAI, as well as BeP/(BeP ? Pe) and B[ghi]Pe/(B[ghi]Pe ? Pe) parameters, has been considered (Tables 2, 3; Radke et al. 1982;Stojanović et al. 2007;Marynowski et al. 2014;Zakrzewski et al. 2020). Figure 4 shows the alteration of several selected parameters alongside vitrinite reflectance. MPI-1, MPI-2, PAI, BeP/(BeP ? Pe), and B[ghi]Pe/ (B[ghi]Pe ? Pe) parameters demonstrate a substantial positive linear relationship with vitrinite reflectance (Fig. 4), whereas MPI-3 and MPR parameters correlate poorly (r = 0.02 and r = 0.04). Even though maturity levels of Prebreza sediments range between immature and early mature (Burazer et al. 2020), trends of MPI-1, MPI-2, and PAI (Table 2, Fig. 4a, b, c) are indicating that samples BL7/12, BL7/20, BL7/ 25, and BL7/27 have displayed unexpectedly high values of almost all phenanthrene parameters, suggesting the predominance of thermally more stable bsubstituted MPs. At the early stages of maturation, MPs presence may reflect the alkylation pattern inherited from precursor biomass or salinity influence on their rearrangements (Otto et al. 2003;Qin et al. 2008;Szczerba and Rospondek 2010;Xu et al. 2019). A significant presence of algal precursor biomass deposited under mesosaline/hypersaline conditions characterizes Prebreza sediments (Burazer et al. 2020), meaning that the alkylation pattern originates from algae or that higher salinity of the paleoenvironment may have prompted rearrangements of MPs, therefore preferentially yielding b-substituted MPs (Qin et al. 2008;Szczerba and Rospondek 2010).  Although the maturity of Č učale sediments gradually transitioned from early-mature to mature, a rather abrupt decline in values of phenanthrene parameters occurred between 712 and 764 m (Fig. 4a, b, c, Table 2). As mentioned earlier, the presence of an entirely analcimized tuff series at the depth interval from 747 to 754 m in the Č učale unit may suggest intense volcanic processes. Intense volcanism is responsible for abnormal heating rates that may prompt combustion processes, thus leading to a higher production of phenanthrene instead of MPs, which probably is the case for this particular depth zone (P (%) up to 52.37,  Fig. 3d; Xu et al. 2019;Zakrzewski et al. 2020). Interestingly, the sample BL7/19 characterizes easily soluble salt deposits, a relatively abundant phytane (Pr/Ph under 1), as well as the highest content of b-carotane (BC (%) 23.37), implying strongly anoxic and saline paleoenvironmental conditions (Didyk et al. 1978;Sinninghe Damasté et al. 1995;Burazer et al. 2020 Marynowski et al. (2014) have also documented that the substantial amount of perylene, even in higher thermal transformation zones, reflects the presence of terrestrial and/or fungal precursors or anoxic depositional settings. Judging on the maceral composition presented in Burazer et al. (2020), a higher contribution of terrigenous and fungal precursor biomass characterize samples BL7/49 and BL7/64, respectively, thus proposing a possible source of perylene. It seems that the accumulation rate of perylene from these biological precursors is higher than its degradation rate to benzo[e]pyrene and benzo[ghi]perylene. Besides, less suitable suboxic conditions, which have characterized the depositional period of these samples (Burazer et al. 2020), have lowered actual concentrations of perylene, preventing a potential conversion of some of its portions into either benzo[e]pyrene or benzo[ghi]perylene (Soma et al. 1996;Silliman et al. 1998 Table 3 The results of specific organic geochemical parameters calculated from the distribution and relative abundance of phenanthrene, fluoranthene, pyrene, benz [a] Table 3, Fig. 4e, d).
The influence of paleoenvironmental settings on the distribution and relative abundance of PAHs and their source.
Volcanic activity, biogenic or combustion origin of PAHs Jiang et al. (1998), Scott (2000, and Xu et al. (2019) have suggested a close association between volcanic processes and large-scale paleo-wildfire events, which are responsible for the formation of combustionderived unsubstituted PAHs. The volcanism products, such as analcimized tuffs, tuffites, and tuffaceous components, are present along the lithostratigraphic column of the borehole BL7. However, a more frequent occurrence of analcimized tuffaceous components, as well as the presence of an entirely analcimized tuff series at the depth interval from 747 to 754 m, characterize the Č učale unit. Therefore, the difference in volcanism intensity between studied stratigraphic units may influence the distribution and relative abundance of individual unsubstituted PAHs. A general predominance of high molecular weight unsubstituted PAHs (HMW PAHs ), particularly perylene, has characterized a large group of sediments from the Prebreza unit (Pe (%) avg. 50.07, LMW PAHs / HMW PAHs under 1, avg. 0.85, Table 3, Fig. 3c). Nevertheless, samples BL7/19, BL7/20, BL7/22, BL7/ 23, BL7/25, and BL7/26, in which low molecular weight unsubstituted PAHs (LMW PAHs ) prevail, phenanthrene becomes more dominant (P (%) up to 38.21, avg. 21.99, LMW PAHs /HMW PAHs over 1, Table 3, Fig. 3c). The study of Suzuki et al. (2010) has suggested that if perylene contents exceed 10% of the total contents of unsubstituted PAHs, as is the case for Prebreza sediments, combustion processes are unlikely to be the cause of perylene formation but rather diagenetic transformations of precursor biomass. Besides, Venkatesan (1988) and Wang et al. (2014) have proposed a perylene index as a parameter for distinguishing biogenic from combustion perylene by investigating its prevalence over other pentacyclic PAH isomers, and this parameter has further substantiated its biogenic origin in the Prebreza unit (Perylene index (%) avg. 88.13, Table 3, Fig. 3d). Moreover, Wakeham et al. (1980), Tan et al. (1996), Jiang et al. (1998), Nizzetto et al. (2008, and Parinos et al. (2013) have proposed that phenanthrene may arise through both diagenetic transformations and combustion processes of precursor biomass. Possible mechanisms for forming biogenic phenanthrene are diagenetic transformations of steroids or dealkylation of pimanthrene and retene under anoxic conditions. Intriguingly, a predominance of steroids, pimanthrene, or retene characterize most of Prebreza sediments (S/H over 1, 1,7-/(1,3-? 3,9-? 2,10-? 3,10-DMPs) avg. 1.68, and Re/Cad avg. 2.33, Table 2; Burazer et al. 2020), meaning that suggested pathways for the production of biogenic phenanthrene are highly probable. Nevertheless, judging on PAI, Fla/(Fla ? Pyr), and Fla/Pyr diagnostic ratios collectively, a large number of Prebreza sediments falls in the combustion-derived zone (PAI under 1, Fla/(Fla ? Pyr) over 0.50, and Fla/ Pyr over 1, Tables 2 and 3, Fig. 5a; Yunker et al. 2002;Liu et al. 2005;Xu et al. 2019). Regardless, it should be cautious when assessing the origin of PAHs with the PAI diagnostic ratio, since several factors, such as the origin of the OM, thermal maturity, or depositional settings, may influence the relative abundance of MPs (Qin et al. 2008;Szczerba and Rospondek 2010;Xu et al. 2019). On the contrary, Fig. 5b (Xu et al. 2019). Nevertheless, some portions of fusain macerals (fusinite and semi-fusinite (vol.%) 4.80 and 1.00, respectively; Burazer et al. 2020) have characterized this sample, implying that volcanism may have triggered small-scale paleofire events, which are responsible to a certain extent for the formation of LMW PAHs , specifically phenanthrene, via combustion processes (Robinson 1991;Longyi et al. 2012;Pickel et al. 2017).

Redox conditions and salinity
Numerous studies have provided substantial evidence about the relationship between paleoenvironmental conditions and the presence of both substituted and unsubstituted PAHs (Budzinski et al. 1995;Soma et al. 1996;Elias et al. 1997;Silliman et al. 1998;Qin et al. 2008;Szczerba and Rospondek 2010;Marynowski et al. 2013). Anoxic and mesosaline to hypersaline conditions have characterized the deposition of Prebreza sediments, while anoxic and mesosaline to freshwater conditions followed the depositional process of Č učale sediments. Presumably, these paleoenvironmental differences may control the distribution and relative abundance of some PAHs, particularly those that relate closely to a specific precursor material, such as MPs, DMPs, retene, cadalene, and perylene.
Furthermore, Soma et al. (1996) and Silliman et al. (1998) have indicated that the formation of perylene does not strictly relate to specific precursor biomass but instead to the anoxic environment, which is highly suitable for the preservation of its carbon skeleton. The concentrations of perylene have exceeded the total contents of unsubstituted PAHs in a large number of sediments from both Prebreza and Č učale units (Pe (%) up to 80.70 and 69.65, Table 3), suggesting that anoxic conditions have indubitably contributed to its accumulation and preservation. As mentioned earlier, even the slightest oxygen availability (suboxic conditions, BL7/ 49 and BL7/64) or higher maturity levels in the Č učale unit can disrupt the accumulation of perylene (Pe (%) under 10,  (Jiang et al. 1998;van Aarssen et al. 2000;Haberer et al. 2006;Hautevelle et al. 2006). Besides, their presence may suggest specific  (Simoneit et al. 1993;Jiang et al. 1998;Otto and Wilde, 2001;Haberer et al. 2006;Hautevelle et al. 2006;Xu et al. 2019). However, studies of Elias et al. (1997), Wen et al. (2000), Hautevelle et al. (2006), and Romero-Sarmiento et al. (2010) have proposed other sources of retene and cadalene, such as aquatic, bacterial, and fungal. Moreover, a possible loss of retene, through its partial degradation under anoxic conditions, may preferably lead to the production of either pimanthrene, 1-MP, or phenanthrene (Simoneit et al. 1986;Wakeham et al. 1980;Tan et al. 1996;Szczerba and Rospondek 2010). As explained previously, Prebreza sediments consist mainly of algal precursor biomass, whereas microbiologically reworked mixed with some portions of terrigenous OM characterize Č učale sediments. Therefore, all of these factors appear to have mutually governed the presence of retene, cadalene, and simonellite, and in this context, the study has applied Re/Cad, Sim/Cad, and Cad/(Re ? Sim) ratios to investigate their potential source and paleoclimate inferences (Table 2, Fig. 3b).
Furthermore, a general prevalence of retene over cadalene in Prebreza sediments, as well as in the upper section of the Č učale unit, in which alginite and liptodetrinite are dominant macerals, implies either algae or bacteria as precursors of retene (Re/Cad over 1, Table 2; Fig. 6a; Wen et al. 2000;Pickel et al. 2017;Burazer et al. 2020). Besides, it may also suggest the origin of 1-MP and pimanthrene in these sediments since some portions are directly resulting from the decomposition of retene (Table 2; Wakeham et al. 1980;Tan et al. 1996;Szczerba and Rospondek 2010). However, the vascular source of retene should not be discarded, especially for samples enriched in vitrinite and sporinite macerals or above-mentioned saturated diterpenoids, and those samples are BL7/19, BL7/20, BL7/39, BL7/42, and BL7/47 (Table 2, Fig. 6a, total vitrinite (vol.%) up to 53.10 and sporinite (vol%) up to 33.30; Burazer et al. 2020). The accumulation of retene in sedimentary environments can also be a consequence of seasonal climate variations (Jiang et al. 1998;van Aarssen et al. 2000;Haberer et al. 2006;Hautevelle et al. 2006;Cesar and Grice 2019). It is questionable to what extent the presence of retene truly reflects paleoclimate variations in this study due to its partial decomposition under anoxic conditions or the lack of vascular plants as the main source of its production. Nevertheless, for samples in which retene has a vascular origin, a potential influence of climate variations has been inspected (Table 2, Fig. 6a). Intriguingly, instead of cadalene, the predominance of retene in samples BL7/19, BL7/20, and BL7/47 with non-degraded and well-preserved woody material, deposited under semi-arid climate conditions, is observed (CWDI under 0.50, Table 2, Fig. 6a, C-value under 0.40; Cao et al. 2012;Burazer et al. 2020;Marynowski et al. 2013;Zakrzewski et al. 2020). Its prevalence also characterizes samples BL7/39 and BL7/42 with highly degraded woody material, which deposition has occurred under more humid climate circumstances (CWDI over 0.50, Table 2, Fig. 6a, C-value up to 0.63, Cao et al. 2012;Burazer et al. 2020;Marynowski et al. 2013;Zakrzewski et al. 2020). The presence of retene in a wide range of climate conditions may relate to a specific coniferous representative, which is responsible for its production. The Pinaceae coniferous family has acquired xeromorphic adaptations that allow them to adapt in arid environments, while Araucariaceae, Cupressaceae, or Podocarpaceae conifer species preferentially settle under humid climate conditions, in which the Pinaceae family is absent or poorly represented (Hautevelle et al. 2006).

Conclusion
The distribution dependence of PAHs on maturity, paleoenvironmental settings, and biomass type for 38 selected Prebreza and Č učale lacustrine sediments was the subject of this investigation. Since volcanic processes seem to have intensified during the depositional period of Č učale sediments, the origin of phenanthrene and perylene in the Prebreza unit is mostly biogenic. Undoubtedly, combustion processes are responsible for forming phenanthrene in the Č učale unit, while perylene has yet again substantiated its biogenic origin.
Anoxic conditions may prompt the demethylation process of retene towards either 1-MP or pimanthrene, whereas mesosaline/hypersaline conditions may delay the demethylation process of 9-MP, providing the possibility for its isomerization into 1-MP. Therefore, it may explain why anoxic and mesosaline/hypersaline paleoenvironmental conditions preferentially accumulated 1-MP and/or pimanthrene in Prebreza and the upper section of the Č učale unit. Besides, anoxic conditions appear to be responsible for accumulating and preserving the perylene carbon skeleton in both Prebreza and Č učale sediments.
A general prevalence of retene over cadalene in Prebreza sediments, in which alginite and liptodetrinite predominated, indicated algae as retene precursors. On the other hand, Č učale sediments with substantial amounts of vitrinite macerals or saturated Fig. 6 The change of a Re/Cad and b Sim/Cad with depth for Prebreza and Č učale sediments diterpenoids have a predominant simonellite derived from conifers. Moreover, the predominance of the specific coniferous representative may explain retene accumulation under both arid and humid periods. In arid environments, the Pinaceae family is the primary source of retene, while under more humid climate circumstances, a highly abundant retene relates mostly to Araucariaceae, Cupressaceae, or Podocarpaceae conifers.