Petrological and geochemical characteristics of xylites and associated lipids from the First Lusatian lignite seam (Konin Basin, Poland): Implications for floral sources, decomposition and environmental conditions

Highlights

• Xylites are affected by weak gelification and advanced cellulose decomposition.

• δ¹³C of xylites reflect the extent of cellulose decomposition.

• Xylites represent wood remains of the same species of conifers in the lignite.

• Fluctuating δ¹³C and δ²H of lipids within the seam are caused by water availability.

Abstract

Single pieces of fossil wood fragments (xylites) were collected from the middle Miocene First Lusatian lignite seam at the Adamów, Jóźwin IIB and Tomisławice opencast mines and are characterized by maceral variety, cellulose contents and their molecular and isotopic composition. Biomarker composition of xylites and δ¹³C of their total organic matter, lipids and cellulose are used to provide insights into woody plant community and the effects of wood decomposition.

The investigated xylites represent fragments of fossil wood from conifers, most likely species of Cupressaceae, indicated by terpenoid biomarkers characteristic for conifers and by the δ¹³C values of the extracted cellulose. This conclusion is confirmed by paleobotanical data highlighting Taxodium and Nyssa as the main elements of the wet forest swamps. Due to the wet swamp habitat and the higher-decay resistance exclusively wood fragments of conifers are found in the lignite seam. Minor abundances of angiosperm-derived triterpenoids in the xylites are explained by impurities from inherent detritic lignite.

The xylites are characterized by minor to moderate extents of gelification, but elevated to high cellulose decomposition. The relationship between δ¹³C values of xylites and their cellulose contents reflects wood decomposition removing preferentially the ¹³C-enriched compounds, but decomposition did not affect the δ¹³C of cellulose. Despite of similar δ¹³C of xylites and detritic lignite, differences in isotopic composition of hopanoids argue for slightly different microbial communities involved in the decomposition of the respective OM. Thus, we conclude that wood decomposition proceeded in a freshwater environment under acidic conditions by fungi and bacteria.

Variations in water availability during growth periods of the conifers are suggested as the most probable cause for the observed minor variations in isotopic composition of plant lipids. The positive relationship found between δ²H and δ¹³C of plant biomarkers, and cellulose of xylites can be explained by the ability of vascular plants to minimize evapotranspiration during dryer phases resulting in plant OM enriched in ¹³C and ²H. The significant differences in δ²H between diterpenoids of different structural types and n-alkanes are most likely caused by differences in isotopic fractionation during lipid biosynthesis.

Introduction

Information about environmetal changes over Earth’s history (e.g., air temperature, humidity, pCO₂, δ¹³C of atmospheric CO₂) have been obtained from the stable isotope composition of cellulose extracted from single tree rings and wood fragments (Feng and Epstein, 1995, Schleser, 1995, Arens et al., 2000). However, their interpretation with respect to climate change is complicated by interfering processes like inter-species and intra-species variations in δ¹³C and postdepositional changes due to different decay resistance of macromolecules (Benner et al., 1987, Lücke et al., 1999, van Bergen and Poole, 2002, Marynowski et al., 2007). The combination of biomarker and stable isotope analyses has been successfully applied to reconstruct changes in vegetation and the environment of lignite deposition (van Bergen and Poole, 2002, Poole et al., 2006, Bechtel et al., 2008), as well as in carbon cycling and isotope fractionation processes (Bechtel et al., 2008, Jahren and Sternberg, 2008).

The δ¹³C values of individual lipids are frequently used to identify their biological sources and biosynthetic pathways during synthesis (Pancost et al., 2007, Đoković et al., 2018). In a recently published database from peats, biomarkers were related to climate and changes in vegetation (Naafs et al., 2019). Vascular plants adjust stomatal aperture to minimize water loss during warm and dry periods (van Bergen and Poole, 2002, Jahren and Sternberg, 2008, Bechtel et al., 2008) which also affects the isotopic composition of newly formed biomolecules. Therefore, ¹³C-enriched OM (i.e., cellulose, n-alkanes, and diterpenoids) of xylites may indicate dryer conditions during wood formation (and vice versa). The additional influences of environmental and vegetational changes on the molecular and isotopic systematics of modern plants and plant-derived lipids were addressed by Diefendorf and Freimuth, 2017, Diefendorf et al., 2019. Further postdepositonal effects like the effect of increasing maturation on the isotopic composition of lignin (Lee et al., 2019) have also to be considered.

Hydrogen isotope analyses on lipids have been shown to shade light on influences of climatic factors on δ¹³C of plant biomarkers. The hydrogen isotopic composition of water in the environment mainly depends on temperature, evaporation and precipitation. The δ²H values of individual lipids have been shown to reflect the isotopic composition of the source water modified by isotopic fractionation during the biogeochemical pathway (Sessions et al., 1999). Previous studies by Yang and Huang (2003) revealed a similar difference between δ²H values of alkanes from angiosperm leaves from the Miocene Clarkia deposit (USA) and sediment water (apparent fractionation) as observed in modern species (123‰; Sachse et al., 2004, Sachse et al., 2006). Therefore, hydrogen isotope ratios of leaf wax-derived n-alkanes are frequently used in paleohydrology and climate research (Sachse et al., 2012).

Our study is focused on the fossil wood remains (xylites) collected from the First Lusatian lignite seam of the Konin Basin in Poland, deposied during the mid-Miocene Climatic Optimum. Based on the results of previous investigations, grasses and herbs were suggested as the peat-forming vegetation of detritic lignites, whereas xylitic lignites showed geochemical characteristics typical of woody conifers (Fabiańska and Kurkiewicz, 2013). This interpretation was supported by biomarker and δ¹³C data from xylites of the Lubstów mine within the Konin Basin (Bechtel et al., 2007). Changes in biomarker composition and δ¹³C of xylite from the Lubstów mine, related to gelification and cellulose decomposition of wood remains, have also been found (Bechtel et al., 2007).

Palynomorphs of taxa representing a swamp forest environment (Taxodium, Nyssa, Alnus and Liquidambar) as well as of shrubs from peat bogs have been found within the lignite seams of the Konin Basin. Pollen elements of the coniferous (e.g., Sequoia, Pinus) and deciduous forest (e.g., Betula, Fagus, Quercus and Ulmus) indicated the presence of dryer habitats within the mire (Sadowska and Giża, 1991, Piwocki and Ziembińska-Tworzydło, 1997, Kasiński and Słodkowska, 2016, Worobiec et al., 2020). In a previous study, detritic lignite samples from identical sampling locations as the xylites, studied here, (i.e., three opencast deposits within the Konin Basin) have been investigated by Bechtel et al. (2019). The discussion of the lipid composition and carbon isotope data of detritic lignite in combination with the results of paleobotanical studies highlighted the varying contribution of gymnosperms vs. angiosperms to the peat-forming vegetation. Furthermore, the possible contribution of biomarker data to gain information on climate and implications about the effects of changes in vegetation and carbon cycling on δ¹³C of lignite samples within the peat have been addressed.

In this study, single pieces from larger xylites were selected from the middle Miocene First Lusatian lignite seam at approximately equal distances along profiles from the same opencast mines within the Konin Basin as investigated for detritic lignite samples in our previous study (Bechtel et al., 2019). The aim of the study is to reveal information about the effects of wood decomposition and environmental conditions on stable isotope composition of woody plant OM. Carbon and hydrogen isotope ratios of cellulose, plant wax and resinous lipids are reported, and are discussed for their suitability as environmental proxies. Finally, we aim for an improved understanding of palaeoenvironmental changes during peat accumulation from our combined application of molecular and isotopic proxies.