Introduction

Today Sphagnum (peat mosses) is one of the most important and widespread genera of bryophytes represented by c. 300 accepted species, which are mainly distributed in temperate, predominantly cool temperate regions (Michaelis 2011, 2019). The present diversity is thought to have resulted from a Miocene radiation (Shaw et al. 2010; Michaelis 2019). However, phylogenetic analyses suggest that the origin of the genus dates back much earlier (Bechteler et al. 2023). It is therefore not surprising that putative Sphagnum ancestors showing the very characteristic leaf anatomy date back into the Paleozoic where they have been described from the Permian (e.g., Neuburg 1958; Ignatov 1990; Ignatov and Maslova 2021), the Mississippian (Hübers and Kerp 2012) and even the Ordovician (Cardona-Correia et al. 2016).

Modern Sphagnum mosses form characteristic spores, which are widely preserved and recognized in Cenozoic sediments. Similar forms potentially attributable to the Sphagnum stem group appear as early as the Late Triassic (Döring et al. 1966). They become rather widespread in the Late Cretaceous and Paleogene, especially in the early Eocene, then showing a considerable diversity (Döring et al. 1966). Most of the fossil sphagnoid spore species described thus far have originally been attributed to Stereisporites Pflug in Thomson and Pflug 1953. However, the genus is invalid since Thomson and Pflug (1953) chose S. stereoides as the type species, which was already used as type species of Sphagnumsporites Raatz 1937 (Riegel and Wilde 2016) as validated by Potonié (1956). Hence, the correct authorship for Sphagnumsporites is Raatz 1937 ex Potonié 1956.

As many as 73 different species of the genus Stereisporites have been described from the Central European Late Cretaceous and Eocene alone and attributed to 17 subgenera (Krutzsch 1963; Döring et al. 1966). Few of these subgenera have since been raised to generic rank such as Tripunctisporis (Potonié 1970; Herngreen et al. 1986; Jameossanaie 1987; Riegel and Wilde 2016), Distancoraesporis (Srivastava 1972) and Distverrusporis (Jameossanaie 1987). In view of the variability and the many transitions within Sphagnum-type spores even in a single sample, it seems rather unlikely that each of the formally described fossil spore species may correspond to a separate parent species. Nevertheless, the diversity among fossil Sphagnum-type spores suggests that the diversity of Sphagnum in the past was about as great as or even greater than at present.

For the present study of the lower seams in the Schöningen Southfield opencast mine (Helmstedt Lignite Mining District, Lower Saxony, Germany), we distinguish three genera among Sphagnum-type spores (Inglis et al. 2015): Tripunctisporis, characterized by three distinct pits in the distal polar thickening and a cingulum of variable width, Distancoraesporis with a more or less distinct triradiate distal thickening and Sphagnumsporites without any distal structure, thus, smooth on proximal and distal side (Fig. 1). Depending on the quality of preservation or lack of distinct characters, 10–30% of the Sphagnum-type spores remain indeterminate. The diagnostic but quite variable triradiate distal structure in Distancoraesporis (Krutzsch 1963) is often faint and the distinction from Sphagnumsporites, therefore, somewhat arbitrary. Triradial distal ridges similar to those observed in Distancoraesporis have also been figured for several modern species of Sphagnum, such as, e.g., S. imbricatum, S. strictum, S. tenellum, S. fallax, and S. molle (Cao and Vitt 1986), mostly with bifurcating ridges. In the Schöningen section, however, distal ridges in Distancoraesporis are not bifurcated. In contrast, Tripunctisporis represents a clearly delimited extinct type of sphagnoid spores with variation mainly concerning overall size and width of the margin.

Fig. 1
figure 1

Sphagnum-type spores from the lower Eocene Schöningen Formation; a–d Tripunctisporis (proximal view on the left side, distal view on the right side); a, c Seam 5, sample 6c; b, d Seam 5, sample 6b; e–h Distancoraesporis (proximal view on the left side, distal view on the right side); e–g Seam 1, sample 8, h Main Seam; i–l Sphagnumsporites (distal view); i Seam 1, sample 10, j Seam 1, sample 8, k, l Seam 5, sample 5; scale bar: 10 µm

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Aside from Schöningen (Riegel and Wilde 2016), Tripunctisporis is reported from central Europe (e.g., Döring et al. 1966; Krutzsch and Mibus 1973; Herngreen et al. 1986; Riegel and Wilde 2016). There are several records from North America, e.g., from South Dakota (Stanley 1965), North Dakota (Kroeger 1995), New Mexico (Jameossanaie 1987), and Louisiana (Gregory and Hart 1995). In addition, Tripunctisporis has also been reported from the southern hemisphere, e.g., South America (Patagonia: Clyde et al. 2021), Australia (Carpenter et al. 2015; Macphail et al. 1994) and New Zealand (Raine et al. 2011). These reports indicate that Tripunctisporis reached a nearly global distribution within a relatively short stratigraphic range from the Late Cretaceous (Campanian and Maastrichtian: e.g., Stanley 1965; Krutzsch 1970; Krutzsch and Mibus 1973; Herngreen et al. 1986; Carpenter et al. 2015) to the early Eocene (Ypresian: Krutzsch 1970; Riegel and Wilde 2016; this paper). But at most sites, it does not occur as abundantly as at Schöningen. Besides, the Schöningen record of Seam 6 appears to be one of the youngest known thus far.

In a previous study (Riegel and Wilde 2016) of a small seam of limited extent, we were able to demonstrate for the first time the existence of a Sphagnum peat bog in the Schöningen section. There, Tripunctisporis was highly dominant among sphagnoid spores and associated with pollen of oligotrophic and acidophilic plants, e.g., Droseridites (Droseraceae/Nepenthaceae) and Ericipites (Ericaceae) as well as the remains of other organisms such as the rotatorian Habrotrocha and the fungal parasite Tilletia, both known from modern Sphagnum peat bogs. Striking, too, was the abundance of charcoal particles including charred fern tracheids, Sphagnum leaf remains and spores, which indicated that fires were largely indigenous to the peat bog.

In the present study, we trace the distribution of sphagnoid spores within other seams of the Schöningen section in order to detect any consistent correlation between sphagnoid spore abundance and charcoal, which may throw light on the possible control of fire regimes and known thermal events on the development of coal seams. Our study aims to present evidence regarding the type of environment that supported the growth and proliferation of the sphagnoid parent species of Tripunctisporis. Of particular interest is the relation of Tripunctisporis to a distinct mire and coal lithofacies type and its possible adaptation to a certain fire regime.

Geological setting

The Paleogene sediments of the Helmstedt Lignite Mining District occur in two rim synclines paralleling the NW–SE-trending Helmstedt–Stassfurt salt wall on both sides (Fig. 2; Look 1984; Brandes et al. 2012). As the result of an interplay between subsidence, terrestrial sediment input and sea level, fluctuations up to 400 m of alternating marginal marine, mostly estuarine, and terrestrial sediments have been accommodated including 12 continuous and several thin local lignite seams (Fig. 3; Riegel et al. 2012, 2015; Osman et al. 2013). This multiple alternation of marginal marine and terrestrial deposits including coal seams appears to be unique worldwide for the early Cenozoic.

Fig. 2
figure 2

(modified after Brandes et al. 2012; Lenz et al. 2021)

a Paleogeographic map of northwestern Europe during the early and middle Eocene (adapted from Ziegler 1990) showing the area of the Helmstedt Lignite Mining District (‘H’) at the southern coast of the Proto-North Sea; b cross-section through the study area, showing the Helmstedt–Stassfurt salt wall and related synclines

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Fig. 3
figure 3

Stratigraphy of the lower Eocene succession in the Western Synclines at Schöningen. The age model for the succession is based on K/Ar-ages (Gramann et al. 1975; Ahrendt et al. 1995), nannoplankton zones (Gramann et al. 1975), dinoflagellate zones (Köthe 2003; Lenz et al. 2022) and palynological zones Pflug (1952, 1986). Position of thermal events as well as carbon isotope data are taken from Lenz et al. (2022): PETM Paleocene Eocene Thermal Maximum, ETM1/2/3 Eocene Thermal Event 1/2/3, H2 H2-event; EECO Early Eocene Climatic Optimum

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Based on dinoflagellate cyst ranges, scattered radiometric dates, carbon isotope excursions and by matching the sequence with established eustatic sea level curves (Köthe 1990; Ahrendt et al. 1995; Riegel et al. 2012; Lenz et al. 2022), the entire coal bearing section of the Helmstedt Lignite Mining District at Schöningen is now known to begin in the latest Paleocene (late Thanetian: Lenz et al. 2022) and is terminated by a widespread marine transgression in the late middle Eocene (Bartonian: Gramann et al. 1975) (Fig. 3). A complete section ranging from the latest Paleocene into the early middle Eocene, thus comprising the entire early Eocene (Ypresian) and recording most of the early Paleogene thermal events was exposed for many years in the opencast mine Schöningen Southfield (Riegel et al. 2012; Lenz et al. 2022).

Materials and methods

We described and sampled the section at Schöningen Southfield in a great number of partial sections at various mining stages (Riegel et al. 2012). As Sphagnum-type spores are essentially restricted to lignite seams, we selected representative sections of all seams from Main Seam to Seam 6 to trace the abundance of Sphagnum-type spores with special regard to the proportion of Tripunctisporis in relation to lignite lithotypes and charcoal abundance. Several seams have been sampled at fairly high resolution using about 10 cm increments (Main Seam) or less (Seam 2 and Seam 6). Generally, one sample was taken per sediment bed (bed number). For thicker beds, however, several samples were taken, which are labeled with letters in addition to the bed number. The thin Sphagnum Seam has even been split along bedding planes covered by charcoal into 12 splits of about 1 cm thickness. Distinction of lithotypes has been carried out in the field mainly on the basis of the matrix to tissue ratio and color using a classification modified from that developed for the Lower Rhine lignites by Vogt (1981).

Lignite samples have been macerated by briefly heating with 10% hydrogen peroxide (H2O2) and subsequent dissolution of humic substances in cold potassium hydroxide (KOH, about 5%), washing and screening through a 10 µm mesh screen. Residues are stored in glycerine.

180 samples from the lower three seams and Sphagnum Seam as well as Seam 3 to Seam 6 have been studied. Results are summarized in Online Resource 1 and graphically represented separately for each seam (Figs. 4, 5, 6, 7, 8, 9, 10).

Fig. 4
figure 4

Abundance of Sphagnum spores vs. charcoal particles in the Main Seam. Note different scales: Scale for Sphagnum-type spores in total is the percentage of total sporomorph assemblage. The proportion of Tripunctisporis, Distancoraesporis and Sphagnumsporites as well as indeterminate sphagnoid spores was obtained by separately counting 100 specimens of sphagnoid spores. The abundance of charcoal particles is presented as additional percentage (in % of the total sum of pollen and spores)

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Fig. 5
figure 5

Abundance of Sphagnum spores vs. charcoal particles in Seam 1. For description of the different scales, see caption of Fig. 4

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Fig. 6
figure 6

Abundance of Sphagnum spores vs. charcoal particles in Seam 2. For description of the different scales, see caption of Fig. 4

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Fig. 7
figure 7

Abundance of Sphagnum spores vs. charcoal particles in Seam 3. In the upper part of section, B a thin rider seam (RS) is included. For description of the different scales, see caption of Fig. 4. Black bars represent samples of section A, grey bars of section B. (S) Seed layer

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Fig. 8
figure 8

Abundance of Sphagnum spores vs. charcoal particles in Sphagnum Seam. For description of the different scales, see caption of Fig. 4

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Fig. 9
figure 9

Abundance of Sphagnum spores vs. charcoal particles in Seam 5. In bed 5 a tree stump was included, which projected slightly into the clay below (layer 4). Sample 5A was taken from clay and sample 5B from lignite immediately above the tree stump, whereas sample 5C was composed of clastic/lignitic detritus taken from a adjacent small channel. For description of the different scales, see caption of Fig. 4

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Fig. 10
figure 10

Abundance of Sphagnum spores vs. charcoal particles in Seam 6. For description of the different scales, see caption of Fig. 4

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About 300 specimens of pollen and spores have been counted from at least 2 slides to establish the abundance of total Sphagnum-type spores within the assemblages. As an additional percentage, the abundance of fungal remains and charcoal particles has been determined in a semiquantitative way. The abundance of fungal remains, mostly spores, has been calculated as additional percentage to the total of pollen and spores. Similarly, the amount of charcoal particles has been determined by counting opaque to dark brown angular particles of an estimated 10 Mikron or larger size. The proportions of the three genera distinguished among Sphagnum-type spores, i.e., Tripunctisporis, Distancoraesporis and Sphagnumsporites (Riegel and Wilde 2016) as well as indeterminate sphagnoid spores were obtained by counting 100 specimens. The number has been kept limited to 100 in order to secure statistical equity among samples of greatly differing abundance and to save time. Along with this, several elements, which may be significant in environmental interpretations, such as Droseridites, Sphagnum leaf fragments, certain spore genera (e.g., Goczanisporis, Foveosporites), main type of tracheids (scalariform or pitted) and algal cysts, have been recorded.

Seam 4 has been excluded from consideration here since it lacks any Sphagnum-type spores and includes only very little charcoal (Robson et al. 2015). Therefore, it does not seem to follow long-term changes, however, it includes a number of thermophilic elements, such as, e.g., various palm pollen in frequencies not known from the other seams in the section. In addition, Seam 4 is made up of up to four benches differing in lithology and laterally split by interbeds. Thus, it appears to have been formed largely depending on local factors, edaphic or climatic, outside of the general trend.

The SPSS statistics package as well as PAST 4.12 (Hammer et al. 2001) were used for graphical representation and numerical analysis. Due to non-normality of data for correlation analysis, Spearman’s rank-order correlation coefficients have been calculated as a non-parametric alternative to Pearson’s product moment correlation. Data for and results of correlation analyses are presented in Online Resources 2 and 3.

All data, on which graphs and interpretations are based, are summarized in Online Resource 1. Here, the charcoal particles larger than about 10 µm are listed as additional percent of total palynomorphs next to the data concerning Sphagnum-type spores and the proportion of different Sphagnum-types distinguished in this study (i.e., Tripunctisporis, Distancoraesporis, and Sphagnumsporites). In addition, certain palynofacies elements, which were considered significant in coal facies interpretation such as Sphagnum leaf fragments, Droseridites, charred scalariform and pitted tracheids, are shown as they were encountered during counts of Sphagnum spore types.

Results

Tripunctisporis in Main Seam

In the 11 m thick Main Seam, Sphagnum-type spores begin to appear in accountable numbers rather late, in the top 3 m quickly rising to 37% (bed 85), from which values they recede gradually toward the top (Fig. 4). However, Tripunctisporis is nearly lacking and represented by only rare singular occurrences, e.g., in samples 25a and 27b, in which Sphagnum-type spores in general are very rare. A number of samples did not even include sufficient Sphagnum-type specimens to allow a statistically significant differentiation between sphagnoid taxa. Hence, a continuous representation through a seam section was not possible for the Main Seam.

Tripunctisporis in Seam 1

In Seam 1, the distribution of Sphagnum-type spores is highly variable. They are rare in the lower part, but suddenly rise to a maximum of 37% of total assemblage at about midseam to continue upward but with considerable setbacks (samples 7a and 8; Fig. 5). The proportion of Tripunctisporis appears to be greater in samples in which Sphagnum-type spores are less abundant. However, an isolated peak of 30% is reached in bed 8 in the upper part of the seam (Fig. 5).

Tripunctisporis in Seam 2

In Seam 2, Sphagnum-type spores begin to occur continuously in significant numbers at about 80 cm above the base (sample 10b; Fig. 6). In the first rise of Sphagnum-type spores, to about 30% Tripunctisporis is still missing and Distancoraesporis highly dominant (sample 12 b).

An intermittent decline of Distancoraesporis is compensated by Sphagnumsporites in bed 13. At this point, Tripunctisporis begins to appear in recordable numbers and reaches a first peak at the top of bed 16. In the succeeding pale lithotypes of beds 17–19, Tripunctisporis suddenly disappears again, but returns equally suddenly with high values (22%) immediately thereafter.

In the upper more homogeneous woody part of the seam, Tripunctisporis remains at a low level fluctuating around 3% of all Sphagnum-type spores and even disappears at some levels (bed 23b and 23c). Only when charcoal horizons become more common toward the top of the seam Tripunctisporis reaches dominance among Sphagnum-type spores with values up to 50% at certain levels. However, Distancoraesporis maintains a dominant role throughout Seam 2 except for this terminal phase.

Tripunctisporis in Seam 3

Seam 3 is divided into a lower relatively light-colored (60 cm) and an upper dark part (55 cm) with a marked seed layer (S) separating the two (Fig. 7). A thin rider seam (about 30 cm) is locally developed. Sphagnum-type spores are very rare to absent in the lower part of Seam 3 and in the rider seam. In the dark upper part of Seam 3 they are abundant only at one level (13.7%). There, Distancoraesporis is by far the dominant Sphagnum-type spore (up to 62%). Sphagnumsporites is second averaging about 17%. The record of Sphagnum-type spores marks a striking change in facies within Seam 3 including a change from Tricolpopollenites liblarensis- to Tricolporopollenites cingulum-dominated assemblages (discussed below).

Tripunctisporis in Sphagnum Seam

We have previously considered the thin Sphagnum Seam as a typical example of an Eocene Sphagnum peat bog (Riegel and Wilde 2016) and referred to Southern Hemisphere restionad bogs as a potential modern equivalent since restionad pollen (Milfordia spp.) is common, albeit not abundant. The occurrence of Milfordia is combined with high values of charcoal and Sphagnum-type spores. For the first time up-section Tripunctisporis greatly dominates among Sphagnum-type spores, more than in any of the 8 seams studied, averaging 77% throughout the seam with a peak at 87% (Fig. 8).

Tripunctisporis in Seam 5

Seam 5 is only about 1 m thick and divided by a clay parting into a lower part of light-colored matrix lithotype and an upper part of dark mixed lithotype (Fig. 9). Sphagnum-type spores are almost totally absent from the light-colored lower part of the seam, but abundant in the darker upper part. Moreover, Tripunctisporis is the clearly dominant sphagnoid spore type in the dark upper part of the seam, while Tripunctisporis is largely replaced by Distancoraesporis and Sphagnumsporites in the overlying clays.

Special attention was paid to the effect of the clay parting on the distribution of sphagnoid spores. The section at the studied site included a tree stump in the lower part, which projected slightly into the clay parting due to differential compaction. Three successive samples (samples 5A, 5B, 5C) were taken from immediately above the tree stump, which closely resemble the sphagnoid spectrum of the upper part of the seam.

Tripunctisporis in Seam 6

As in all studied seams of the section Sphagnum-type spores are rare to absent in the basal part of Seam 6 (about 60 cm; Fig. 10). As far as counts of sphagnoid spores in this part were possible Distancoraesporis and Sphagnumsporites are the dominant types. As sphagnoid spores rise to high levels Tripunctisporis becomes dominant, however, with a slight delay. From there upward, the dominance of Tripunctisporis persists to the top of Seam 6 except for an interval of very light-colored to pale lithotypes at about midseam (samples 24–28), in which Sphagnum-type spores are almost absent. The upper part of Seam 6 is particularly thin-bedded to laminated with fine detritus of charcoal on bedding planes. In this interval, Tripunctisporis averages nearly 78% of total Sphagnum-type spores (averaging in this interval about 15% of total assemblage).

General considerations

Charcoal abundance varies greatly within each seam and generally with coal lithotype color. Light colored to pale lithotypes are poor or even devoid of charcoal, dark lithotypes variably rich in charcoal. Very thin charcoal horizons or drapes of charcoal on bedding plains often cannot be separated from the adjacent dark lithotypes, such as in the upper part of Seam 6, in which charcoal values are persistently high. Nevertheless, Tripunctisporis correlates well with charcoal in the upper seams, in sharp contrast to the lower three seams (Fig. 11, see below).

Fig. 11
figure 11

Scatterplot matrix between Sphagnum-type spores and charcoal in the swamp facies (Main Seam to Seam 2) and the bog facies (Seam 3, Sphagnum Seam, Seam 5, Seam 6). The scatterplot was provided using the SPSS statistical package and combines the bivariate plots of all possible pairs of comparison. Spearman’s (non-parametric) rank-order correlation coefficients are calculated with PAST 4.12 (Hammer et al. 2001). Red marked comparisons indicate significant negative correlations with p values < 0.05 and green marked comparisons significant positive correlations. In contrast, correlations that are not highlighted in color are not statistically significant

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From the data presented above, a number of trends in the distribution of Sphagnum-type spores can be recognized within the nearly 100 m thick section from Main Seam to Seam 6 representing about 3.2 Myr of the latest Paleocene to early Eocene, thus covering 6 hyperthermal events and including the Early Eocene Climatic Optimum (EECO, Lenz et al. 2022).

Within the section, the abundance of Sphagnum-type spores generally increases from Main Seam to Seam 6 with the exception of Seams 3 and 4. Besides, within the individual seams Sphagnum-type spores tend to be more abundant in the upper part, e.g., during late stages of peat mire formation.

The proportion of Tripunctisporis generally rises parallel to that of sphagnoid spore abundance: Tripunctisporis is virtually missing in the Main Seam, common in the upper part of Seam 1, dominant to the top in Seam 2, rare to absent in Seams 3 and 4, abundant in the upper part of Seam 5 and dominant in the Sphagnum Seam and through most of Seam 6. The distribution of Tripunctisporis is highly dependent on coal lithofacies. It is generally rare to absent in light-colored lithotypes and most abundant in dark lithotypes rich in charcoal.

Due to the rare occurrence of Tripunctisporis in the lower seams of the Schöningen Formation (Main Seam to Seam 2) and its dominance in most of the upper seams (Sphagnum Seam to Seam 6), separate correlation analyses were made for both seam groups (Fig. 11). It can be seen that in the lower seams, all three recorded Sphagnum-type spores do not correlate with the abundance of charcoal particles. Only when looking at the total number of Sphagnum-type spores, there is a significant but weak positive correlation with charcoal (Spearman’s rs 0.40, p < 0.001, Online Resource 2). In contrast, however, the upper seams show a significant positive correlation of Tripunctisporis with the number of charcoal particles (rs 0.65, p < 0.001, Online Resource 3). Distancoraesporis (rs − 0.72, p < 0.001) and Sphagnumsporites (rs − 0.58, p < 0.001), on the other hand, have a significant negative correlation with charcoal. Therefore, the association of the total number of Sphagnum-type spores in total with charcoal is significant but only weak in the upper seams (rs 0.36, p < 0.003).

Discussion

Following previous authors (e.g., Döring et al. 1966; Herngreen et al. 1986; Riegel and Wilde 2016) and based on its unique morphology and limited occurrence, we consider Tripunctisporis as a distinct genus representing the spores of extinct species of Sphagnum or, at least, of a closely related genus. This is supported by relatively well-preserved remains of Sphagnum leaflets in samples, in which Tripunctisporis predominates almost exclusively (Riegel and Wilde 2016). In contrast, Distancoraesporis seems to be at least related to some modern species of Sphagnum showing triradiate ridges on the distal surface (Cao and Vitt 1986). Furthermore, Tripunctisporis may represent a species of Sphagnum adapted to certain environments, since Tripunctisporis is restricted to certain coal lithofacies in the Schöningen section characterized by thin bedding and high charcoal content.

Floristic differentiation: from swamp to bog facies

It becomes clear from the data presented that the three lower seams, i.e., Main Seam, Seam 1 and Seam 2 differ significantly from the seams higher up in the section, i. e. Sphagnum Seam, Seam 5 and Seam 6 with regard to the quantitative composition of palynological assemblages in general and the frequency and distribution of Tripunctisporis and charcoal in particular. This confirms our earlier statement largely based on field observation regarding the difference in the type of vegetation and fire regime (Riegel et al. 2012). The change in vegetation and environment is best illustrated by distinguishing two contrasting coal lithotypes, respectively, peat forming facies, a swamp facies represented in the lower three seams, Main Seam, Seam 1 and Seam 2, and a bog facies represented by Seam 3, the Sphagnum Seam, the upper parts of Seam 5 and Seam 6.

The swamp facies is largely characterized by a dominance of inaperturate pollen of Cupressaceae s.l. (incl. former Taxodiaceae) with Nyssapollenites as a consistent companion (Fig. 12, Tab. 1). This association is generally regarded as representing a Nyssa/Taxodium resp. Nyssa/”Taxodiaceae” swamp forest commonly initiating seam formation in Neogene and Paleogene lignites (e.g., Thomson 1956; Teichmüller 1989; Wilde and Riegel 2022). Other than in modern swamps, in which Nyssa-Taxodium swamp forests grow somewhat remote from the coastline (Riegel 1965; Spackman et al. 1969; Ewel 1992), it closely accompanies the estuarine shore in Seam 1 at Schöningen (Lenz et al. 2021). Rather pronounced is the occurrence of Labrapollis sp., a typical Eocene pollen (Krutzsch and Vanhoorne 1977) of unknown botanical affinity in the lower part of the Main Seam (Table 1). Frequent associates are monolete spores (Laevigatosporites spp., Polypodiaceae) and the small tricolpate/tricolporate pollen of the Tricolpopollenites liblarensis group resp. Tricolporopollenites cingulum group, the latter representing a closed-canopy angiosperm mire forest (Thomson 1956; Pflug 1952; O’Keefe et al. 2005; Lenz et al. 2021). Isolated peaks of Leiotriletes-type spores (Schizaeaceae), Alnipollenites (Alnus) and Plicapollis pseudoexcelsus (Juglandaceae?) suggest a certain patchiness accompanying early peat forming stages. In this type of swamp facies, Sphagnum-type spores are mostly rare and Tripunctisporis is nearly absent.

Fig. 12
figure 12

Pollen diagram of characteristic taxa of the Main Seam that represents the typical pollen assemblages of the swamp facies. For legend of lithology, see Fig. 4

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Table 1 Average percentage values of important palynomorphs and palynofacies elements in Seam 6, Sphagnum Seam and Main Seam
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As peat growth continues, the Nyssa/Taxodium type of swamp facies is replaced, sometimes very suddenly, by an angiosperm vegetation represented by myricaceous pollen of the Triporopollenites robustus/rhenanus group as the dominating element (Table 1). This in turn may be locally replaced by Pompeckjoidaepollenites subhercynicus. These changes occur in all three seams of the swamp facies group. Thus, they are likely to represent a natural succession taking place as a consequence of peat aggradation independent of climate change, but rather controlled by water and nutrient availability (Lenz et al. 2021). This change is particularly pronounced in the Main Seam, where it coincides with a strong negative δ13CTOC excursion interpreted as the PETM onset (Lenz et al. 2022), which amplified the change.

In general, Sphagnum-type spores are common only in the upper part of each of the swamp facies seams. The proportion of Tripunctisporis still remains at a low level in the Main Seam. Moderate, but fluctuating frequencies are reached in Seam 1 and Seam 2. Tripunctisporis becomes abundant only in terminal stages of Seam 2.

In the late stages of peat formation in the three seams of the swamp facies, the size and frequency of charcoal increases with sizeable chunks of charcoal concentrated in horizons up to a few centimeters in thickness. They begin to appear in the Main Seam, but similar to the distribution of Sphagnum-type spores they become more pronounced in Seam 1 and near the top of Seam 2.

Changes toward the top of the swamp facies seams include algal cysts with an affinity to the Zygnemataceae (e.g., Tetraporina, cf. Mougeotia). Filaments of Zygnemataceae are known to form extensive mats in peat forming shallow freshwater sites (periphyton), which have a relatively sparse vegetation cover and may seasonally fall dry such as in the marshes of the Florida Everglades (Riegel 1965; Spackman et al. 1969; Kushlan 1992). This seemingly contradictory coincidence of open water and flooding versus increasing dryness and wild fire takes place as the water table in the swamp rises in response to the rising sea level in the adjacent estuary (Spackman et al. 1969), followed by intermittent dry spells, during which flammable fuel load may accumulate (Myers 1992). Along with this development, climate-controlled changes of vegetation take place, however, without changing the mode of seam formation in general.

The bog facies of seams higher up in the section is in sharp contrast to the swamp facies in botanical as well as lithologic respect. Here, inaperturate pollen and Nyssapollenites together with their common associate Labrapollis are virtually absent (Fig. 13, Table 1), while Sphagnum-type spores are dominant and now mainly represented by Tripunctisporis. The associated fern flora is completely different and represented by other spore taxa such as Goczanisporis baculatus, a small trilete spore of unknown botanical affinity with a characteristic distal baculate sculpture (see Riegel and Wilde 2016). The species is infrequent even in the bog facies, but highly restricted to it. More frequent is a group of spores with a foveolate distal exine, e.g., Foveotriletes crassifovearis. Similarly, Leiotriletes microadriennis is a common trilete spore in the bog facies, but rare in the swamp facies. Other common Paleogene fern spores such as Cicatricosisporites (Schizaeaceae) are notably rare at Schöningen except for a very short interval near the base of Seam 6. This brief appearance is associated with a first rise of charcoal in the seam including a number of scalariform tracheids. At Cobham, Collinson et al. (2009) reported extremely high percentages of Cicatricosisporites together with a very high abundance of fern charcoal which have, therefore, been linked to a low diversity fire prone plant community of ferns and some woody angiosperms. At Schöningen, this is rapidly substituted by dark lithotypes with other ferns, mainly Polypodiaceae, and an increasing number of Tripunctisporis.

Fig. 13
figure 13

Pollen diagram of characteristic taxa of Seam 6 that represents the typical pollen assemblages of the bog facies. For legend of lithology, see Figs. 9 and 10

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Bisaccate pollen is strikingly rare at Schöningen (Hammer-Schiemann 1998). But even at this low level, there is a significant difference in abundance between lignites and interbeds, reflecting the absence or presence of a closed-canopy acting as a filter for bisaccate pollen (Jardine and Harrington 2010). A tenfold increase of bisaccates in Seam 6 over the Main Seam, for instance, is convincing evidence that all bog seams were lacking a canopy.

Among the angiosperm pollen of the bog facies are all those including the closest living relatives which characterize modern ombrogenous peat bogs, e.g., Droseridites, Milfordia and Ericipites, previously highlighted in the Sphagnum Seam (Riegel and Wilde 2016; Table 1).

Ericipites represents the Ericaceae, a large family mainly of herbs and shrubs of worldwide distribution generally growing in nutrient-poor soils. Many of the modern representatives are known to be highly dependent on mycorrhiza to compensate for nutrient deficiency (e.g., Overbeck 1975; Richards 1987; Küster 1990). Although low in actual numbers Ericipites in Seam 6 greatly exceeds those found in forest mire seams and may, therefore, be considered a true member of the bog facies. Therefore, much of the numerous fungal spore material associated with the bog facies in our section may be even attributable to mycorrhizal fungi. Furthermore, fungi are active decomposers in Sphagnum peat (e.g., Overbeck 1975).

Milfordia is the pollen of Restionaceae, a small family of rush-like monocotyledons, which was widely spread in the northern hemisphere during the Paleogene, but is today essentially restricted to the southern hemisphere. Modern Restionaceae have a broad tolerance of very wet to dry conditions in nutrient-poor environments (e.g., Campbell 1975; Clarkson et al 2004) often associated with fires (Heywood 1993). This agrees well with the numerous charcoal drapes on thin-bedded lignite in our section. Thus, our previous comparison of the Sphagnum Seam with the southern hemisphere restiad bogs (Riegel and Wilde 2016) can now be extended to each of the bog facies seams.

The main element of the woody angiosperms in the bog facies is the myricaceous/betulaceous pollen of the Triporopollenites robustus/rhenanus group, which is also prominent in the terminal phases of the forest mire seams of the swamp facies (Table 1). In either case, they represent more acidic environments with somewhat restricted nutrient supply and may have formed small stands within or fringes around the bog. The ubiquitous small tricolpate and tricolporate pollen of the Tricolpopollenites liblarensis and the Tricolporopollenites cingulum group (Fagaceae) which dominate early and intermediate stages of the swamp facies seams, are secondary in abundance in the bog facies and likely to represent the background vegetation.

Surface fires in bogs are maintained at relatively low temperatures within the range of fire temperatures of 300–600 °C allowing plant parts to be preserved in a partially charred state (Scott et al. 2000). It is notable, for instance, that charred scalariform tracheids derived from herbaceous vascular bundles are far more common in the bog facies seams than in the swamp facies seams, which include almost exclusively fragments of tracheids with bordered pits typical in coniferous wood. This agrees very well with our field observations noting the abundance of coniferous wood in the lower seams and its total lack in the upper seams (Riegel et al. 2012).

Furthermore, slightly charred fragments of Sphagnum leaflets with their unique anatomy, which have previously been reported from the Sphagnum Seam (Riegel and Wilde 2016), have now been commonly found in the other seams of the bog facies, but never outside.

Rather striking among non-pollen palynomorphs is the abundance of fungal remains, mainly fungal spores, which sharply contrasts with the low frequency in the forest mire seams (Table 1). Up to forty times more fungal remains occur in the upper seams than in the lower seams. It is known from modern bogs that many of the bog plants secure their nutrient demand via mycorrhizal fungi (e.g., Overbeck 1975; Richards 1987; Küster 1990). Modern Sphagnum species are known to host a variety of fungi (Küster 1990; Kostka et al. 2016), often showing highly specified interactions with certain species. In addition, by far most fungi live and proliferate under aerobic conditions. Thus, the frequent drying of the peat bed in the bog seams is more prone to sustain aerobic conditions favoring fungal growth than the waterlogged conditions, which have prevailed in the swamp facies. Other palynofacies elements previously designated as fungal tubes in the Sphagnum Seam (Riegel and Wilde 2016) have also been observed in seams of the upper group along with Botryococcus.

Interestingly, Yusup et al. (2022) found in an experimental study that moderate heat and smoke can stimulate Sphagnum spore germination and the most suitable conditions are commonly reached a few centimeters below the burning surface. It is, therefore, conceivable that Tripunctisporis was similarly stimulated and viable under a high frequency fire marsh environment. The differences between the lower three seams and the seams in the upper part of the section, in which Tripunctisporis largely predominates, are highlighted in Fig. 14.

Fig. 14
figure 14

Ternary diagrams including a color map of point density showing the distribution of important spores, pollen and palynofacies elements of the bog facies of the upper seams (Sphagnum Seam, Seam 6) compared to their distribution in the Main Seam (swamp facies). a Nyssa-“Taxodiaceae” facies of the Main Seam; b Myricaceae mire of the Main Seam (see Lenz et al. 2022). The proportions are calculated using the data from Table 1

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The distribution and abundance of Tripunctisporis are paralleled by a difference in lithotype constitution: the lower three seams (Main Seam, Seam 1 and 2) consist of a highly xylitic lignite of alternating medium to dark brown lithotypes including tree stumps and sizeable charcoal layers. There, Sphagnum-type spores are largely restricted to the upper part of the seams, i.e., late stages of peat formation, which include only very little (Main Seam) to moderate amounts of Tripunctisporis (Seam 1 and 2). In contrast, the upper seams (Sphagnum Seam, Seams 5 and 6) are thin-bedded with thin but numerous charcoal drapes on bedding planes and high amounts of Sphagnum-type spores through much of the seams and a domination of Tripunctisporis (Riegel et al. 2012).

The difference between the seams of the forest mire type and the bog type is best illustrated by comparison with modern counterparts. We previously compared the terminal stages of Seam 1 with the present day Okefenokee Swamp of Georgia, USA (Lenz et al. 2021), where patches of myricaceous shrub and ponds of open water coexist side by side (Cohen 1974) and hydrology is mainly controlled by groundwater. This can be expanded now to most of the Main Seam, Seam 1, and Seam 2. The bog seams on the other hand may be compared with the rain-fed southern hemisphere restiad peat bogs, in which Restionaceae are associated with various fern taxa, Ericaceae, grasses, other herbs and Sphagnum (Whinam et al. 2012). Thus, our previous comparison of the thin Sphagnum Seam (Riegel and Wilde 2016) can be applied to Seam 5 and Seam 6 as well. It should be noted that the latter is 4 m thick and laterally quite persistent, thus demonstrating that this type of environment was not a local phenomenon but conditions leading to it existed over a long period (about 10 Ma) and wide area.

Paleoclimatic context

Since Tripunctisporis has not been recorded later than the early Eocene, the occurrence in the Schöningen Formation may be with the youngest records known thus far at least in Europe. Reasons for its apparent extinction are likely to be connected with some changes in climate. The wet/dry climate of the Schöningen Formation with high fire frequency in the later stages, under which the proposed restiad-like peat bogs spread and flourished, was succeeded during the middle Eocene by a perhumid climate supporting a lush paratropical rain forest (Riegel et al. 2015; Riegel and Wilde 2016; Wilde and Riegel 2022; Mai 1995) after a severe drop of fire activity (Bond 2015). This, perhaps, left no room or suitable habitat for Sphagnum peat growth. Later, the rapid decline of Restionaceae in the northern hemisphere is initiated by the climatic cooling during the Oligocene and particularly forced by the growing competition from the Gramineae and Cyperaceae (Hochuli 1979).

The Paleogene greenhouse peaked with the long-term-warming trend of the EECO between c. 53.3 and 49.1 Ma before present (Zachos et al. 2001; Westerhold et al. 2018). The most intriguing aspect of the change from topogenous to ombrogenous mires in the Schöningen Formation is that it coincides with the onset of the EECO. During pre-EECO times, the Paleogene greenhouse world was characterized by short-term but extreme thermal events. They showed striking perturbations in the global carbon cycle as represented by distinct negative carbon isotope excursions (CIE), which were caused by the rapid and massive input of 13C-depleted carbon into the atmosphere–ocean system (e.g., Kennett and Stott 1991; Dickens 2001; Cramer et al. 2003; Lourens et al. 2005; Zachos et al. 2003, 2008, 2010; Kirtland Turner et al. 2014). The strongest CIE, which indicates the Paleocene/Eocene Thermal Maximum (PETM, e.g., Kennett and Stott 1991; Bains et al. 2000; Röhl et al. 2000; Westerhold et al. 2017), has recently been identified to occur in the middle of the Main Seam, at which point a distinct change from a Nyssa-Taxodium type mire forest to a Myrica shrub forest can be observed (Lenz et al. 2022). However, similar but less prominent changes can be observed in Seams 1 and 2 independent of thermal events and may be considered as typical responses to nutrient deficiency in topogenous mires (Lenz et al. 2021). Thus, sudden warming in the wake of the PETM probably intensified the regular change in vegetation in the middle of the Main Seam and caused the appearance of a few thermophilic pollen (e.g., palm pollen) in moderate numbers.

A cooling following the PETM is indicated by a marked rise in temperate elements (e.g., Alnipollenites) in Seam 1. Between the top of Seam 1 and the middle of Seam 2 a further thermal event is indicated by a CIE and may be correlated with another short-term thermal event such as the Eocene Thermal Maximum 2 (ETM 2; Methner et al. 2019; Lenz et al. 2022).

Between Seam 3 and Seam 6 of the Schöningen Formation, at least four minor CIEs have been observed (Lenz et al. 2022). Although they cannot be correlated to any specific thermal event known from the marine record, they clearly correspond to the gradual warming trend of the EECO (Lenz et al. 2022). However, strongly fluctuating δ13CTOC values point to high amplitude climate fluctuations and unstable climate conditions which are characteristic for the EECO (Westerhold et al. 2018). Therefore, the bog facies interval with the occurrence of ombrogenous mires covers at least onset and peak of the EECO. The change from forest mire peatlands to restiad type peat bogs may be explained by increased environmental stress due to the cumulative and lasting effect of quickly recurring thermal instability combined with increased precipitation that promoted the growth of ombrogenous peat bogs. A change in temperature during the EECO is indicated by the advance of palm pollen in Seam 4 and early in Seam 6.

Since all seams except the Main Seam are sandwiched between interbeds showing varying degrees of marine influence, changes in the position relative to the open sea or in sedimentary environments around can be ruled out as causes for the shift from forest mire to peat bog conditions.

Conclusions

We traced the abundance and distribution of sphagnoid spores through 3 Myr of the latest Paleocene to early Eocene in a nearly 100 m thick section including several lignites with mostly estuarine interbeds. Sphagnum played an increasingly important role in the formation of seven lignite seams which were exposed until recently in the Schöningen Southfield opencast mine. Three distinct genera of sphagnoid spores, Tripunctisporis, Distancoraesporis, and Sphagnumsporites, have been distinguished and their proportion and abundance in each of the seams determined. It was possible to show that the importance of Sphagnum-type spores increased within each seam from early to late stages of seam formation as well as from one seam to the next up-section. Tripunctisporis plays a particularly important role by almost entirely replacing the other two types of sphagnoid spores higher up in the section in combination with a significant change in floral composition and prominent petrographic characteristics. The three lowermost seams including the PETM and ETM 2 are rich in woody remains and made up of a swamp forest vegetation. The seams higher up in the section, which formed during the EECO, are thin-bedded to laminated and originated from a shrubby and herbaceous heath-like vegetation in which the parent moss of Tripunctisporis flourished. The change in facies from an arboreal swamp to a low growing shrub and peat moss community is considered to have taken place under the constraints of increased temperatures and precipitation on the coastal plain vegetation due to the cumulative and lasting effect of rapidly recurring thermal events during the EECO rather than a single event.