Biomarker and carbon isotope data from the 1st Mid-Polish and 2nd Lusatian lignite seams of the Konin Basin (Poland) re-evaluated to reconstruct vegetation dynamics during the Miocene Climatic Optimum

Bulk geochemical data

4.1.

The total organic carbon (TOC) contents of fossil wood remains (xylites) from MPLS-1 vary from 32.5 to 53.6 wt% (Table 1). Higher TOC values were obtained from the xylites of the LLS-2 (Lubstów, 53.6–60.2 wt%), despite similar degrees of organic matter decomposition (e.g., gelification, cellulose decay) and thermal maturity (Bechtel et al., 2007, 2020). Average TOC contents are lowest in the xylites from the Adamów deposit (Table 1). Low TOC contents (< 40 wt%) of the samples TX-01, JX-02, and of xylites of the Adamów mine most probably reflect impurities from inherent detritic lignite (Bechtel et al., 2020). Variable TOC abundances of detritic lignite samples (16.7–58.8 wt%; Table 2) from MPLS-1 are caused by dilution of organic carbon by mineral matter (Bechtel et al., 2019), reflecting periods of increased fluvial runoff (e.g., crevasse splay; Widera, 2016).

The yields of extracted cellulose from most xylites of the Lubstów deposit (LLS-2) were reported as low as 4.5–7.1 wt%, except for two samples with cellulose yields exceeding 30% (Table 1). These data indicated low to advanced degrees of cellulose decomposition by fungi and bacterial activities (Schleser et al., 1999; Bechtel et al., 2007). Cellulose yields of fossil wood remains from MPLS-1 ranged between 1.6 and 24.2% (dry weight; Table 1). The highest cellulose yields were recorded from xylites of the Tomisławice mine. Considering the original cellulose contents of wood (40–50%; Pettersen, 1984), the data indicate an advanced degree of cellulose decomposition. However, impurities due to inherent detritic lignite must be taken into account as an additional factor responsible for the low cellulose yields.

Molecular composition of lipids

4.2.

The n-alkanes contents of the xylite samples from the Adamów, Jóźwin IIB, and Tomisławice deposits (MPLS-1) vary from 75 to 440 μg/g TOC (Table 1). Lower n-alkane concentrations of xylites were obtained from the Lubstów deposit (LLS-2), varying from 31 to 122 μg/g TOC (average content: 61 μg/g TOC). Detritic samples of MPLS-1 are characterised by comparable n-alkane concentrations as the corresponding xylites (71–353 μg/g TOC; Table 2). However, relative abundances of n-alkanes are higher in the hydrocarbon fractions of detritic lignite samples due to reduced abundances of terpenoid hydrocarbons (Fig. 3C). The n-alkane patterns of all samples are dominated by long-chain (> n-C₂₇) homologues with a marked predominance of odd carbon-numbered n-alkanes (Fig. 3), indicating their origin from plant-waxes and the immature character of organic matter (Eglinton & Hamilton, 1967). The carbon preference index (CPI, according to Bray & Evans, 1961) values of xylite samples vary between 3.0 and 8.9; whereas slightly higher CPI (3.6–10.0) were obtained from detritic lignite samples of MPLS-1.

Fig. 3.

Hopanoids are present in elevated concentrations in the extracts of the xylites collected from the investigated lignite deposits (Table 1; Fig. 3). Average concentrations of hopanoids vary between 65 μg/g TOC (Tomisławice mine) and 109 μg/g TOC (Jóźwin IIB mine). The concentrations of hopanoids of detritic samples from MPLS-1 fall within a comparable range (42–111 μg/g TOC). The 17α,21β(H)- and 17β,21β(H)-type hopanes from C₂₇ to C₃₁ were identified. The C₂₈ hopanes are missing, and the 17α,21β(H)-C₃₁ hopane (22R) predominates, especially in detritic lignite samples (Fig. 3). In addition to the hopanes, the C₂₇ and C₃₀ hop-17(21)-enes are present (Philp, 1985). Likewise, relative abundances of hopanes and hop-17(21)-ene are higher in the detritic lignite samples (AD-02; Fig. 3C).

The aromatic sesquiterpenoids curcumene, cuparene, and cadalene (Grantham & Douglas, 1980; Simoneit & Mazurek, 1982) are present in the extracts of fossil wood remains (xylite) from MPLS-1 in low to moderate concentrations (3.4–91.5 μg/g TOC; Table 1). In the Lubstów deposit very low contents of sesquiterpenoids (cadalene and 5,6,7,8-tetrahydrocadalene) were found in the range of 0.5–3.8 μg/g TOC. Cadalene predominates in all samples (Figs. 3B, C; 4A).

Fig. 4.

The non-aromatic diterpenoids found in xylites of the Lubstów mine (Fig. 3A) consist of compounds of the pimarane-type (pimaradiene, isopimarane, pimarane), the abietane-type (norabietane and abietane), as well as compounds of the tetracyclic series (ent-beyerane, 16α(H)-phyllocladane, 16β(H)-phyllocladane; Hagemann & Hollerbach, 1979; Noble et al., 1985; Philp, 1985). In the xylites from MPLS-1, additionally isonorpimarane and norpimarane have been identified, but pimaradiene and ent-beyerane are missing (Fig. 3B). Pimarane-type diterpenoids predominate in most of the xylite samples. In contrast, 16α(H)-phyllocladane predominates in most of the detritic lignite samples taken from the Adamów, Jóźwin IIB and Tomisławice deposits (Fig. 3C). Exclusively abietane-type diterpenoids were identified in the aromatic hydrocarbon fractions of xylite samples from the Lubstów deposit (Fig. 4A). Simonellite was present in the highest abundances in most samples. In the samples (xylites and detritic lignite) from MPLS-1, aromatic diterpenoids (e.g., dehydroabietane, 18-norabietatriene, tetrahydroretene, simonellite, and retene; Philp, 1985) were found in the hydrocarbons (Figs. 3B, C) and partly in the low-polar (ketone) fractions (Figs. 4B, C). Ferruginol and dehydroferruginol (Otto et al., 1997) occurred in the low-polar fractions of all samples from the Adamów, Jóźwin IIB, and Tomisławice deposits (Figs. 4B, C). These compounds are not identified in fossil wood samples from the Lubstów mine, as only the hydrocarbon fractions were analysed (Bechtel et al., 2007).

The average concentrations of all diterpenoid compounds quantified in the extracts from xylites (Table 1) are in the range of 359 μg/g TOC (Lubstów) and 692 μg/g TOC (Adamów). Much lower average concentrations have been found in detritic lignite samples from the investigated deposits within MPLS-1 (62–95 μg/g TOC; Table 2). As terpenoid compositions provided valuable information about palaeovegetation (Otto et al., 1997; Fabiańska & Kurkiewicz, 2013), abundances of pimarane, abietane, and phyllocladane type diterpenoids, as well as of ent-beyerane and ferruginol plus dehydroferruginol, relative to the total diterpenoid concentrations, were calculated (Tables 3, 4). Average relative abundances of diterpenoid biomarkers in the lipids from xylites of the investigated deposits (Table 3), and from detritic lignite samples of the Adamów, Jóźwin IIB, and Tomisławice mines (Table 4) are illustrated in Figure 5. Pimarane, abietane, and phyllocladane type diterpenoids predominate. Ferruginol and dehydroferruginol, not found in the Lubstów xylites, occur in variable abundances in the xylites from MPLS-1 (Fig. 5). The diterpenoid ent-beyerane has been identified only within the Lubstów samples. The Lubstów samples show, on average, higher relative abundances of abietane-type diterpenoids and lower relative abundances of phyllocladane-type diterpenoids than the xylites from MPLS-1 (Fig. 5). The detritic lignite samples from the Adamów, Jóźwin IIB, and Tomisławice deposits are characterised by low average abundances of abietane- and high relative contents of phyllocladane-type diterpenoids.

Fig. 5.

Aromatic angiosperm-derived triterpenoids are present in some samples of xylites from the Lubstów mine in very low abundances insufficient for peak integration (Fig. 4A; Table 1). In most xylites from MPLS-1, the following oleanane, ursane, and lupane type triterpenoids were identified in very low abundances in the hydrocarbon fractions: des-A-oleanenes, des-A-lupane, olean-12-ene, olean-13(18)-ene, and urs-12-ene (Philp, 1985; ten Haven et al., 1992; Logan & Eglinton, 1994; Rullkötter et al., 1994). The aromatic triterpenoid hydrocarbons included 24,25-dinoroleana-1,3,5(10),12-tetraene (Fig. 3B), 24,25-dinoroleana-1,3,5(10)-triene, 24,25-dinorlupa-1,3,5(10)-triene (Spyckerelle et al., 1977; Wakeham et al., 1980; Wolff et al., 1989), as well as tetramethyl-octahydro-picenes (Fig. 4B) and trimethyl-tetrahydro-picenes. Total concentrations of non-hopanoid triterpenoids in the fossil wood remains (xylite) are low compared to the contents of diterpenoids (di-/(di- + tri-)terpenoid ratios > 0.84 in most samples; Table 1) and were suggested to represent impurities from inherent detritic lignite (Bechtel et al, 2020). In contrast to the xylites from the Adamów, Jóźwin IIB, and Tomisławice deposits, angiosperm-derived triterpenoids were present in high abundances in the hydrocarbon (Fig. 3C) and low-polar fractions (Fig. 4C) of detritic lignite. Their concentrations (32–159 μg/g TOC) vary over a comparable range as the diterpenoid contents of the samples (10–293 μg/g TOC; di-/(di- + tri-)terpenoid ratios: 0.18–0.70; Table 2).

Carbon isotopic composition of xylites, cellulose and detritic lignite

4.3.

The δ¹³C values of the fossil wood (FW; xylites) from the Lubstów deposit vary between −22.8 and −25.7‰ (Table 1; Fig. 6A). The average carbon isotopic composition (δ¹³C = −24.5‰) is nearly identical to that reported for xylites from the Adamów (δ¹³C = −25.3‰), Jóźwin IIB (δ¹³C = −25.3‰), and Tomisławice (δ¹³C = −24.5‰) mines (Table 1; Fig. 6B). Carbon isotope ratios of extracted cellulose (Cell) from xylites of the Lubstów mine are in the range from −19.5 to −22.6‰ (Table 1; Fig. 6A), whereas in the samples from MPLS-1 are in the range from −19.9 to −22.7‰ (Table 1; Fig. 6B). An overall trend towards lower δ¹³C values of cellulose with decreasing cellulose content was observed in the Lubstów xylites (Fig. 6A; Bechtel et al., 2007). On average, δ¹³C of cellulose is enriched by approximately 3.6‰ relative to those of the xylites. The δ¹³C values of the xylites fall within the same range as those of the TOC in detritic lignite samples collected from the corresponding sections of MPLS-1 in the Konin Basin (−24.7 to −26.2‰; Table 2).

Fig. 6.

Insights into palaeovegetation and mire types

5.1.

The occurrence of xylitic lignite has been interpreted to reflect peat formation within wet- to dry forest swamps, whereas detritic lignite is suggested to indicate swamp bush to wetland environments with shrubs, grasses, and herbs as peat-forming vegetation (Teichmüller, 1989). Based on geochemical data obtained within Polish lignite deposits, Fabiańska & Kurkiewicz (2013) found indications for grasses and herbs as the peat-forming vegetation of detritic lignites, whereas xylitic lignites showed geochemical characteristics typical of conifers. These findings are supported by higher relative abundances of n-alkanes from leaf waxes and lower amounts of terpenoid biomarkers in the detritic samples, compared to the lipid composition of fossil wood fragments (xylite). Furthermore, terpenoid composition of detritic lignite samples reflects significant contributions from angiosperms (variable di-/(di- + tri-)terpenoid ratios; Table 2) in contrast to xylites, originating from conifers (high di-/(di- + tri-)terpenoid ratios; Table 1).

The phenolic abietane ferruginol was identified in most conifer families, especially in species of Cupressaceae and Podocarpaceae, but seems to be absent in Pinaceae (Otto & Wilde, 2001). Abietane type diterpenoids are considered as diagenetic products of abietic acid, preferably found in Pinaceae, or have been derived from phenolic derivatives (i.e., ferruginol; Otto et al., 1997). Recently, ferruginol was identified as one of the major characteristic markers for the Taxodioideae conifer subfamily of Cupressaceae (Simoneit et al., 2019). However, abietic acid has also been found in extracts from MPLS-1 samples (Bechtel et al., 2020). Pinaceae have been characterised by abundant pimarane and abietane type diterpenoids (Otto & Simoneit, 2001; Otto & Wilde, 2001). Tetracyclic diterpanes (kaurane and phyllocladane type) were identified in Cupressaceae, Araucariaceae, and Podocarpaceae families (Noble et al., 1985; Otto & Wilde, 2001; Otto et al., 2002).

The observed terpenoid composition indicates gymnosperms (i.e., conifers) as sources of wood fragments from LLS-2 and MPLS-1, most probably for predominant contributions from Cupressaceae (Bechtel et al., 2020). High abundances of abietane- and pimarane-type (tricyclic) diterpenoids may reflect enhanced contributions of wood fragments from Pinaceae in the Lubstów deposit (Fig. 5). The results are in agreement with palaeobotanical data, as elements of swamp forest (e.g., Taxodium, Glyptostrobus) and dry coniferous forest (e.g., Sequoia, Pinus) have been found within LLS-2 (Ciuk & Grabowska, 1991). Recently, lower tetracyclic/tricyclic diterpanes ratios have been interpreted to indicate decreasing contribution of Cupressaceae and increased input of Pinaceae in Miocene lignites from the coal mining district of Lusatia in eastern Germany (Kojić et al., 2021). Therefore, our data provide evidence for more frequent development of drier habitats during LLS-2 accumulation in comparison to MPLS-1 formation, as reflected by higher relative abundances of abietane-type diterpenoids and lower tetracyclic compounds in the Lubstów deposit (Tables 3, 4; Fig. 5). Higher average abundance of phyllocladanes relative to the sum of concentrations of diterpenoids was found in the xylites and detritic lignite samples from the MPLS-1 deposits (especially from the Adamów deposit; Fig. 5), indicating the predominant contribution of Cupressaceae and a raised (ground)water table during peat formation.

While diterpenoids have been considered as markers for gymnosperms (i.e., conifers; Otto & Wilde, 2001), oleanane, ursane, and lupane type triterpenoids have been reported to occur in angiosperms (Karrer et al., 1977; Sukh Dev, 1989). The variable di-/(di- + tri-)terpenoid ratios (Table 2) in the detritic lignite of MPLS-1 indicate mixed vegetation. In the case of MPLS-1, the occurrence of lupeol in detritic samples (Bechtel et al., 2019) suggests the contribution of Betulaceae to peat formation (Hayek et al., 1989). The results are in agreement with previous palaeobotanical data (Sadowska & Giża, 1991; Piwocki & Ziembińska-Tworzydło, 1997; Kasiński & Słodkowska, 2016). However, degradation of terpenoids during diagenesis must be taken into account because diagenetic alteration may result in erroneous estimates of relative contributions of plant families (Diefendorf et al., 2015).

Environment and mechanisms of wood decomposition

5.2.

Information on wood decomposition can be inferred from the degree of gelification, cellulose yields, and the hopanoid composition of lipids extracted from woody macrofossils. All xylite samples from LLS-2 and MPLS-1 were characterised by low to moderate gelification (Bechtel et al., 2007, 2020). In general, the Jóźwin IIB xylites show a higher degree of gelification than those from the other deposits. Cellulose yields of the xylites between 1.6 and 34.4 wt% indicate low (sample Lub 1) to high (sample JX-02) degrees of degradation. Consistent with their higher extent of gelification, several fossil wood fragments from the Jóźwin IIB mine are characterised by low cellulose yields (6.1 wt% on average; Table 1; Fig. 6B). Cellulose decomposition has been attributed to the activities of fungi (Benner et al., 1987), as well as microbial activities under aerobic and anaerobic conditions (Benner et al., 1984; Bechtel et al., 2007). Elevated hopanoid concentrations were observed in xylites with low cellulose yields (< 10 wt%) within the Jóźwin IIB mine, indicating enhanced bacterial activity (Bechtel et al., 2020), likely promoted by a raised (ground)water table.

Increased relative abundances of hopanoids in the hydrocarbon fractions of the detritic lignite samples (Fig. 3C) reflect enhanced bacterial activity in water-logged depositional environments. The detritic lignite samples from the Tomisławice mine yielded slightly higher hopanoid concentrations (49–111 μg/g TOC) than those from the other deposits (Table 2). The moderate hopanoid concentrations and low sulphur contents of all samples investigated (Bechtel et al., 2007, 2019, 2020), as well as the predominance of the C₃₁ aβ-hopane (22R) indicate freshwater, acidic mire conditions (Casagrande, 1987; Inglis et al., 2018).

Insights from carbon isotopic composition of lignite and fossil wood

5.3.

The carbon isotopic composition of lignite has been shown to vary in response to changes in the peat-forming vegetation (Bechtel et al., 2008), in δ¹³C of atmospheric CO₂ and climatic changes (Ahrens et al., 2000; Jahren & Sternberg, 2008). In previous studies, a major influence of the contribution of gymnosperms (i.e., conifers) versus angiosperms has been shown (Lücke et al., 1999; Bechtel et al., 2008, 2019), due to differences in isotopic fractionation between leaves and atmospheric CO₂ and lipid biosynthesis (Diefendorf et al., 2011). This relationship is reflected in the positive correlation between the di-/(di- + tri-)terpenoid ratios and δ¹³C of detritic lignite in samples from MPLS-1 (Table 2; Fig. 7). The δ¹³C values of fossil wood remains (xylites) vary over comparable ranges as the lignite samples. Because all xylites represent pieces of conifer wood, variations in δ¹³C towards lower values are caused by cellulose decomposition and impurities from inherent detritic lignite, as indicated by decreased di-/(di- + tri-)terpenoid ratios (< 1.0; Fig. 7).

Fig. 7.

The positive relationship between cellulose yield and δ¹³C values of total organic matter from xylites (δ¹³C_FW; Table 1; Fig. 6) indicates the progressive depletion of wood in ¹³C during cellulose decomposition (Benner et al., 1987; Lücke et al., 1999). The obtained δ¹³C data of wood cellulose support the origin of xylites from woody conifers (Lücke et al., 1999; Stock et al., 2016), as already indicated by their terpenoid biomarker composition. Varying differences in δ¹³C of xylites and extracted cellulose (2.3–4.6‰; Fig. 6) are in general agreement with those obtained for Miocene lignites and xylites from the Lower Rhine Embayment in Germany (3.8‰; Lücke et al., 1999; Stock et al., 2016).

Because the xylites were derived from similar conifer sources (e.g., most likely from Cupressaceae and minor Pinaceae) inter-species variations are unlikely to be responsible for the observed variations in δ¹³C (between −19.5 and −22.7‰) of cellulose. Small fluctuations in δ¹³C within the lignite seam (< 1‰) of each mine may be caused by variations in δ¹³C of CO₂ available to the plants, indicated by minor variations of δ¹³C of atmospheric CO₂ during the Miocene Climatic Optimum (Zachos et al., 2001; Tipple et al., 2010). The observed differences in δ¹³C of cellulose were most probably caused by water availability to plants due to the adjustment of the stomatal aperture (Jahren & Sternberg, 2008; Diefendorf & Freimuth, 2017). This interpretation is supported by the data presented by Bechtel et al. (2020).

A slightly higher average isotopic composition of wood cellulose from the Lubstów deposit was obtained (δ¹³C_Cell = −20.8 ± 1.0‰), in comparison to the xylites from MPLS-1 (δ¹³C_Cell = −21.4 ± 0.8‰). Only the Tomisławice xylites yielded a nearly identical average δ¹³C value of cellulose, as obtained from the Lubstów deposit. Several xylites from the Tomisławice and Lubstów mines are characterised by high cellulose yields. Therefore, an influence of cellulose yields on their δ¹³C values cannot be excluded. This factor, and the fact that only 7 samples have been investigated from LLS-2 in contrast to 27 xylites from MPLS-1, should be considered for the discussion of the outlined minor difference in average δ¹³C values of cellulose from xylites of LLS-2 and MPLS-1, respectively. However, comparable tendencies in average δ¹³C of wood cellulose over the Miocene Climatic Optimum (17–15 Ma) were obtained from fossil wood of lignite deposits within the Lower Rhine Basin (Jordan, 1995; Fig. 8). The influence of decreasing air temperatures during uppermost Miocene and Pliocene on δ¹³C of cellulose from tree stumps of the Lower Rhine Embayement (Fig. 8) is obvious (Jordan, 1995). However, only minor temperature fluctuations during the Miocene Climatic Optimum (17–15 Ma) are unlikely to result in the observed differences in δ¹³C of cellulose from LLS-2 and MPLS-1. GDGT-based palaeotemperature proxies obtained from Miocene lignites (18–14 Ma before present) revealed a warm climate during deposition of the peats, shifting towards lower temperatures (2–3 °C) during deposition of the uppermost seam Garzweiler, i.e., at about 13–11 Ma before present (Stock et al., 2016).

Fig. 8.

The data may imply an increased (ground)water table in the mires during MPLS-1 peat accumulation induced by periods of increased precipitation. This interpretation is consistent with the higher average abundance of abietane-type relative to phyllocladane-type diterpenoids in the LLS-2 xylites compared with the MPLS-1 samples, suggesting a greater contribution of Pinaceae from relatively drier habitats.