Using peat bogs as palaeoenvironmental archives is a well-established
practice for reconstructing changing climate and anthropogenic activity in
the past. In this paper, we present multi-proxy analyses (element
geochemistry, pollen, non-pollen palynomorphs, stable Pb isotopes,
humification, ash content) of a 500 cm long,
Hochmoore eignen sich zur Rekonstruktion des Paläoklimas und anthropogener Aktivität in der Vergangenheit. In der vorliegenden Arbeit werden Multiproxy-Analysen (Elementchemie, Pollen, Mikrofossilien, stabile Pb-Isotope, Humifizierung, Aschegehalt) eines 500 cm langen und mittels der C-14-Methode datierten Torfkerns aus dem ombrotrophen Pürgschachener Moor im steirischen Ennstal (Österreichische Alpen) vorgelegt und diskutiert. Der Bohrkern umfasst einen Zeitraum von
In recent years, several multi-proxy studies of bogs, involving elemental, isotopic, and palynological and/or plant macrofossil analyses, have gained in significance, enabling relatively accurate interpretations and quantifications of human impact in the past (e.g. Bindler, 2003; De Vleeschouwer et al., 2007; van der Knaap et al., 2011; Segnana et al., 2019; von Scheffer et al., 2019). Ombrotrophic (rain-fed) peat bogs are solely dependent on precipitation and isolated from the direct influence of rivers, springs or groundwater. Consequently, such bogs provide excellent archives for reconstructing past atmospheric fluxes and environmental conditions (Martínez-Cortizas et al., 1999; De Vleeschouwer, 2010; Drexler et al., 2016). As the world faces future global climate change, a profound understanding of the interrelations of palaeoenvironmental and past human signals is of crucial importance for obtaining a more comprehensive picture of changes in the biosphere throughout the past. With respect to the Alpine region during the Holocene, palaeoenvironmental studies have mainly concentrated on the reconstruction of environmental conditions and past human impact in the Western Alps and western parts of the Eastern Alps (e.g. Büntgen et al., 2005; Nicolussi et al., 2005; Festi et al., 2014; Segnana et al., 2019). Only few data exist regarding areas in the eastern foothills of the Alpine main ridge in eastern Austria (e.g. Drescher-Schneider, 2003; Schmidt et al., 2006; Boch et al., 2009; Huber et al., 2010).
This present study seeks to close this research gap by combining (bio)geochemical, isotopic and palynological analyses of peat deposits of Pürgschachen Moor in the Austrian Alps. In particular, we provide further insights into palaeoenvironmental changes and the advent and chronology of prehistoric societies (from the Copper Age up to Roman times) and prehistoric copper mining in the Liezen area and the more eastern Eisenerz Alps. In addition, by also consulting the ash content and the humification degree, we aim to enhance the general knowledge regarding the interrelations between humification, mineralization, climate change and human impact in wetlands.
Pollen analysis is considered a reliable tool for inferring palaeoenvironmental changes and associated vegetational shifts (e.g. Finsinger et al., 2006; Herzschuh, 2007). Anthropogenic indicators in pollen records, such as crop plants and cereals, were first formalized by Behre (1981, 1988) and are nowadays commonly used to reconstruct agricultural and other human activities in the past (e.g. Bunting et al., 2001; Zhang et al., 2010; Rösch and Lechterbeck, 2016). The accuracy of assessments of past human activity by using geochemical methods depends, among other factors, on the mobility of relevant trace elements (e.g. Pb, Sb, Cu) in the peat environment.
Even though emissions of copper smelting processes are detectable in regular soils across tens of kilometres (Ettler, 2016) and the potential of Cu as an anthropogenic tracer in peat deposits has been highlighted by different authors (e.g. Ukonmaanaho et al., 2002; Rausch et al., 2005; Mighall et al., 2009; Novak et al., 2011; Mariet et al., 2016), other studies have questioned the reliability of Cu as a marker of past human impact in peat environments, due to its dependence on pH value, mineralogy (Rausch et al., 2005), vegetation (Shotyk et al., 2002; Ukonmaanaho et al., 2004) and hydrology (Tipping et al., 2003; Bobrov et al., 2011).
Therefore, we also measured Pb and Sb, which have successfully been used to reconstruct anthropogenic activity in various studies as they show low mobility in peat environments (e.g. Cloy et al., 2009; De Vleeschouwer et al., 2009; Shotyk et al., 2017). However, post-depositional redistribution processes of Pb and Sb have also been demonstrated occasionally (Olid et al., 2010; Rothwell et al., 2010). Mobilization of heavy metals in peat, in general, has been attributed to long-term water table fluctuations (Tipping et al., 2003; Rothwell et al., 2010).
Stable Pb isotope measurements were conducted to improve the contextualization of the elemental results, attempting to differentiate between local, regional and non-regional anthropogenic and geogenic sources (e.g. Allan et al., 2018). Organic compounds, especially humic acids that are considered the main adsorption agents in peat, provide adsorption sites that enhance trace element accumulation (Yang et al., 2019). To improve the quality of the interpretation of past anthropogenic activity, heavy metal concentrations were contrasted with the humification degree, provided by Fourier-transformed infrared (FTIR) spectroscopy (see also Biester et al., 2012). FTIR spectroscopy can be used to identify changes in major peat chemical properties, such as changes in relative contributions of carbohydrates or aromatics (Niemeyer et al., 1992; Kalbitz et al., 1999). Due to preferential degradation of carbohydrates over aromatics, relative absorption indicative of these moieties can be used as an indicator of the degree of peat decomposition and/or humification (Artz et al., 2008; Broder et al., 2012; Biester et al., 2014).
Since the extent of decomposition of peat is mainly dependent on plant species (Drollinger et al., 2020), nutrient fluxes and hydrological conditions, a connection between climate and decomposition can be established (van der Linden and van Geel, 2006). An assessment of the humification degree can therefore also help to reconstruct climate changes in the past.
The Sr concentration in bogs, to a large extent controlled by groundwater inflow, is regularly used as an additional proxy for the trophic status of the bog (Shotyk et al., 2017). As humification is closely interlinked with mineralization (Tolonen, 1984), an interpretation of ash content variations contributes to the reconstruction of past climate.
Pürgschachen Moor is the largest intact valley peat bog in Austria with an areal extent
of
Overview map with the geographical location of Pürgschachen Moor in the Enns valley and
known prehistoric smelting sites and settlements (Klemm, 2003), embedded in
the regional geology (Geologische Bundesanstalt, 2019). Europe
Copper ores of the eastern Greywacke zone are typically characterized by
mineral phases of the tennantite–tetrahedrite solid-solution series
(
Palaeoenvironmental studies indicate a marked increase in metal pollution throughout Europe during the Bronze Age (e.g. Bränvall et al., 2001; Monna et al., 2004; Mighall et al., 2009; Longman et al., 2018; Wagreich and Draganits, 2018). This also applies to the Austrian Alps, where a gradual intensification of human activity is assumed for the middle Bronze Age and the early part of the late Bronze Age in different areas in Tyrol, Salzburg, Styria and Lower Austria (e.g. Röpke and Krause, 2013; Breitenlechner et al., 2014; Haubner et al., 2015; O'Brien, 2015; Pichler et al., 2018). Several studies have aimed to reconstruct the prehistoric human impact in the Mitterberg region in Salzburg, which was possibly the largest copper producer in Europe in the middle of the second millennium BCE (e.g. Stöllner, 2011; Breitenlechner et al., 2014; Pernicka et al., 2016). In contrast, relatively few detailed palaeoenvironmental studies exist regarding prehistoric settlement development and early mining in the Eisenerz Alps and the Gesäuse area. The earliest indications of large-scale settlement activity in the Palten valley, which forms the southernmost border of the Eisenerz region, date back to the late Neolithic period in the fourth millennium BCE (Drescher-Schneider, 2003; Preßlinger et al., 2009). Taking into account archaeological findings in the region, it is probable that a large number of non-specified prehistoric copper mining and smelting sites in the region can be attributed to the middle and/or late Bronze Age (Klemm, 2003; Preßlinger et al., 2009). The largest excavated bronze smelting site in the Eisenerz Alps, S1 in the Supplement (Fig. 1), has also been radiocarbon dated to the middle Bronze Age (1600–1350 cal BCE; Kraus, 2014).
Given the lack of archaeological evidence assignable to the Iron Age, a
decrease in anthropogenic activity during the following centuries is
suggested (Klemm, 2003). More intense human impact during the Roman period is
indicated by different archaeological finds and evidence for Roman trade
routes in the region (Herbert, 2015). A decline in archaeological findings
corresponding to the Migration Period points towards a decrease in human
activity during this interval (Klemm, 2003). Siderite mining (
Even though no detailed palaeoclimate studies regarding the region of Eisenerz Alps have been conducted so far, we expect that the regional palaeoclimate variations show consistencies with palaeoclimate models of other parts of the Alps (e.g. Heiri, 2003; Nicolussi et al., 2005) and other climate records in the Northern Hemisphere (Vinther et al., 2006). Accordingly, we assume a general decrease in the average temperatures around 3000 cal BCE and an impact of recurring cold cyclical oscillations (Rotmoos 2, Löbben, Göschenen 1, Göschenen 2) in the late Holocene for this region (Ivy-Ochs et al., 2009; Kutschera et al., 2017).
The sample location is in the centre of Pürgschachen Moor (47
Location of the coring site in the centre of the ombrotrophic
Pürgschachen Moor (
The peat body at the central treeless peatland area of Pürgschachen Moor reaches depths
of 7–8 m; the transition from sedge to
A
Six samples, comprised of various
A total of 65 pollen samples at sub-sampling intervals varying between 2.5 and 10 cm were analysed. In order to obtain pollen concentration data,
The pollen diagram is divided into local pollen zones (LPZ P1–P13) that reflect the vegetation development (Berglund and Ralska-Jasiewiczowa, 1986). Periods of human impact (K1–K7) are displayed separately.
Quantitative analyses of Cu, Sr, Zr, Sb, and Pb were carried out by using
an iCAP Qc ICP-MS system (Thermo Fisher Scientific, Bremen, Germany) at the
Institute of Chemical Technologies and Analytics (TU Wien, Vienna). Different
research groups demonstrated accurate elemental analysis of peat using
quadrupole-based ICP-MS instruments (e.g. Shotyk et al., 2001; Krachler et
al., 2002; Mihaljevič et al., 2006; Mighall et al., 2014). Peat aliquots
of 200 mg were digested using an acid mixture of 4 ml
Calibration standards from 1 to 100
Due to the focus on the reconstruction of prehistoric anthropogenic
activity, no elemental measurements were performed for the first 75 cm.
Operation settings are given in the Supplement, Table S1. Due to the assumed
absence of unambiguous non-anthropogenic reference sections in the core, no
enrichment factors (see e.g. Weiss et al., 1999) were calculated. Ash
contents were determined as the percentage of the dry weight (see Tolonen,
1984), with a precision balance after burning the samples at 550
Stable Pb isotope measurements were carried out by using the iCAP Qc ICP-MS
system described in Sect. 2.5. The isobaric interference between
FTIR spectra of peat samples were recorded by means of a Cary 660 FTIR
spectrometer (Agilent, Santa Clara, CA, USA) at the Institute of Landscape
Ecology in Münster, Germany. On the one hand, the selection of samples
for FTIR analyses was based on prominent trends in the ash content curve; on
the other hand it was oriented at the sample selection for ICP-MS analysis.
For FTIR analyses, 2 mg of powdered peat sample was mixed with 200 mg KBr
(FTIR grade, Sigma Aldrich, St. Louis, MO, USA) and pressed to a 13 mm pellet. To reduce analytical noise and to obtain comparable spectra for all
samples, 32 scans per sample and subsequent baseline correction of data were
conducted. The humification index was approximated by calculating the ratio
between the peak intensity at 1630 cm
Generally, the retrieved core is dark brown to black; contains a lot of
amorphous material; and appears to be moderately to highly decomposed,
according to the von Post humification scale (von Post, 1924). The section
Radiocarbon dates from Pürgschachen Moor.
The pollen zone (PZ) P1 (484–450 cm;
Reduced pollen diagram of the peat core, comprising trees, shrubs,
cultivated plants, other indicators of human impact and typical wetland
flora. For reasons of clarity,
Diagram of non-pollen palynomorphs (NPPs), showing changes in
wetness and animal content. NPP types with a constant presence of less than
0.5 % are shown as dotted lines. For reasons of clarity,
In PZ P2 (450–425 cm), a fir- and beech-rich spruce forest still
predominates in the vicinity of the bog. In the pollen record, a slight
decrease in
Apart from an expansion of
In PZ P4 (387–378 cm;
A continuous increase in
PZ P6b/K1b (1500–1255 cal BCE) is characterized by frequent finds of
Cerealia and Cannabaceae and an increase in
PZ P7 (305–275 cm) covers the early and a part of the late Urnfield
period (1250–1010 cal BCE). The pollen record of PZ P7 show lower levels
of Cannabaceae and
PZ P9/K3 (252–225 cm) is characterized by a decrease in Cannabaceae and Poaceae. Decreases in
charcoal, spruce and beech and a marked increase in hazel are associated
with PZ P10/K4 (225–172 cm).
The pollen record of PZ P11/K5 (172–135 cm) still implies a dense forest
stand and displays pronounced increases in Poaceae,
The first occurrences of rye, walnut and sweet chestnut stretch back to PZ P12/K6 (135–95 cm), accompanied by a slight decrease in beech, fir and spruce. Charcoal is barely detectable.
In PZ P13/K7 (95–70 cm) the number of most forest trees, with the exception of pines and birches, is strongly reduced. In addition to a more frequent occurrence of rye and other cereals, an increase in charcoal and walnut was found. Furthermore, the presence of heather and an increase in pine as well as a decrease in wetness indicators were determined.
Sr concentrations are rather invariable throughout the core, ranging from 3 to 10 mg kg
Element concentrations, as obtained from ICP-MS analyses, of Cu,
Sr, Zr, Sb and Pb (mg kg
The results of our study demonstrate climate-driven interrelations between the pollen record, the ash content and peat decomposition. Cultural activity, in contrast, is mainly reflected in the pollen record and by enrichments of trace elements (see Sect. 4.2). Varying ash contents in peat bogs have been linked to climate changes in two ways: on the one hand, the ash content is affected by a varying dust flux due to climate variations and human land-use change, which directly influence the amount of mineral matter in the bog (Pratte et al., 2017). On the other hand, the amount of mineral matter in peat is strongly interrelated with humification processes (Tolonen, 1984). Due to mass loss of organic matter in peat bogs in the course of proceeding decomposition during drier periods (see also Huang et al., 2013; Xiao et al., 2017), an enrichment of mineral matter per volume unit takes place, resulting in an increase in the ash content (Boelter, 1969). This, in turn, might improve nutrient availability and could lead to a positive feedback on microbial decomposition of peat (Kempter, 1996).
On Pürgschachen Moor, the strongest positive correlation between humification and ash content was generally found in sections with higher ash contents. Positive correlations between decomposition rates and the ash content in peat are emphasized by different authors (e.g. Tolonen, 1984; Kempter, 1996; Martínez Cortizas et al., 2007; Engel et al., 2010; Leifeld et al., 2011; Drzymulska, 2016). However, a direct linkage between higher ash contents and warmer climate conditions causing slower growth rates and higher mineralization rates (see also Sîre et al., 2008; Huang et al., 2013; Xiao et al., 2017) does not always hold up to scrutiny, as Stivrins et al. (2017) imply by underlining the capacity of peat bogs to buffer climate variations. This is further aggravated by the fact that, from a climatological perspective, no stable relationship between temperature and precipitation exists (Bell et al., 2018).
Despite that, with regard to the present study, the erratic increase in the
ash content (up to
A pronounced Zr peak and a decrease in
Similar interrelations (high ash content, marked changes in the wetness indicators, increasing humification) are apparent in the section 80–70 cm that corresponds to the initial phase of the Medieval Warm Period (Kutschera et al., 2017) and in the section 500–490 cm, corresponding to the warm period around 3000 cal BCE, following the Rotmoos II oscillation (Ivy-Ochs et al., 2009). Strongly increased ash contents in this lowermost section are, however, at least partly linked to the more frequent presence of roots and tree trunks that precluded deeper drilling.
In general, low average Sr concentrations below 10.5 ppm and low ash
contents (
Pb isotopic signatures (
Strong positive correlations between Pb and Sb in peat soils affected by anthropogenic activity such as mining, smelting and industrialization have been highlighted by different authors (e.g. Steinnes, 1997; Shotyk et al., 2004; Cloy et al., 2005; Galuszka et al., 2014). Yet, anthropogenic activities are not necessarily the only reason for higher concentrations of Pb and Sb in ombrotrophic peat, as a positive relationship between Pb and Sb on the one hand and the humification degree on the other is demonstrated in various studies (e.g. Pilarski et al., 1995; Ho et al., 2000; Gao and Huang, 2009; Biester et al., 2012). However, with regard to this study, humification-normalized Pb and Sb values do not change the interpretation of past anthropogenic activity in the area of Pürgschachen Moor.
Though positive correlations between Cu and humification are known from the literature (e.g. Yang and Hodson, 2018), no such correlation was determined for peat of Pürgschachen Moor. This correlation might have been overwritten by occasional input of anthropogenic copper smelting dust. The weak positive correlation between Sb and Cu might be linked to the smelting of antimony-rich copper ores.
A comparative analysis of anthropogenic proxies in the pollen record (i.e. cultivated plants, herbs shrubs, charcoal) and in the geochemical record (Pb, Sb, Cu) reveals interrelations. However, since no continuous concordance between the palynological and geochemical anthropogenic proxies were found over time or depth, it is reasonable to assume that prehistoric regional settlement activity in the Liezen area was only partly connected to mining.
Taking into account the low average Pb concentrations (
A weak forest opening combined with higher levels of cultivated plants at 470 cm, corresponding to
A slight decrease in
Increasing human impact in the region in the middle Bronze Age
(
Distinct enrichments of Pb and Sb in one sample, corresponding to
By combining palynological and geochemical analyses, we were able to provide
a substantial reconstruction of prehistoric human and metallurgical activity
as well as the palaeoclimate in the region of Pürgschachen Moor that is largely coherent
with previous palaeoenvironmental and archaeological studies. After first
indications of low settlement (
The palaeoenvironmental assessment of the pollen data was further substantiated and refined by interrelating it with the humification degree and ash content. Owing to this fact, we were able to correlate climate optima in the late Holocene with changes in the geochemical and pollen and NPP records. Even though considered negligible for the interpretation of this present study, the close interrelationship between humification and tracer elements such as Pb and Sb demonstrates the necessity for a stronger analytical consideration of the humification index in the context of interpreting anthropogenic pollutants from peat bog archives. In order to improve the explanatory power of palaeoenvironmental assessments, future studies may try to further disentangle humification-related trace element sorption from mineralization processes in peat soils.
All underlying chemical data have been submitted to PANGAEA
The supplement related to this article is available online at:
WK (corresponding author) designed the study, prepared sample material for subsequent analysis, conducted geochemical and laboratory analyses, and wrote the manuscript. The peat core was retrieved by WK and RDS. Pollen analysis was performed by RDS. KHK conducted the FTIR analyses; SD provided FTIR data normalization. AL, LB and FH were involved in planning and supervision of the geochemical analyses. MW contributed to the design and the structure of the manuscript. DF established the Bayesian age–depth model.
The authors declare that they have no conflict of interest.
This present work was funded by the uni:docs fellowship programme for doctoral candidates (University of Vienna). We would like to thank Mathias Steinbichler, Susanne Klemm, Stephan Glatzel, Andreas Maier, Tomasz Goslar, Susanne Gier, Sabine Hruby-Nichtenberger and Lars Scharfenberg for technical and logistical support. We are also grateful to Benjamin Schmid for proofreading the manuscript.
This research has been supported by the University of Vienna uni:docs fellowship programme for doctoral candidates (grant no. 6395). The article processing charges were partly funded by the Quaternary scientific community, as represented by the host institution of EGQSJ, the German Quaternary Association (DEUQUA).
This paper was edited by Christopher Lüthgens and reviewed by two anonymous referees.