A 2600-year record of past polycyclic aromatic hydrocarbons (PAHs) deposition at Holzmaar (Eifel, Germany)

Highlights

• We reconstructed environmental and cultural changes of the last 2.6 millenia.

• The cold and wet migration period was clearly reflected in geochemical properties.

• The highest PAH concentrations occurred in the industrial modern period.

• The Roman era and the High Middle Ages experienced increased PAH deposition.

• PAHs are a valuable proxy to reconstruct natural and human combustion activity.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are a proxy for climate- and human-related historical fire activity which has rarely been used beyond 1800 AD. We explored the concentration and composition patterns of PAHs together with other proxies (charcoal, C, N, S, δ¹³C, δ¹⁵N, and δ³⁴S) in a sediment core of Holzmaar as indicators of variations in climate and anthropogenic activity over the past 2600 years. The concentrations of pyrogenic PAHs remained low (< 500 ng g^− 1) from the pre-Roman Iron Age (600 BC) until the first significant increases to ca. 1000–1800 ng g^− 1 between 1700 and 1750 AD related to regional iron production. The highest increases in pyrogenic PAH concentrations occurred with industrialization peaking in the 1960s. PAH concentrations in most recent sediments decreased to pre-industrial levels because of emission control measures and the switch from coal to oil and gas as major fuel sources. Fluxes of PAHs (mg km^− 2 yr^− 1) increased in the order Migration Period and Early Middle Ages < pre-Roman Iron Ages < Roman era < High Middle Ages and Renaissance < pre-industrial modern period < industrial modern period. The changes of PAHs fluxes in pre-industrial times parallel known changes in local, regional and continental anthropogenic activity and climatic variations or their interactions across these historical periods. Perylene, a mainly biologically produced compound, was the dominant PAH in pre-industrial times. The Migration Period and the Early Middle Ages witnessed the most profound and simultaneous changes to the sedimentary organic matter properties in the past 2600 years including the lowest PAH fluxes because of reduced human activity and more negative δ¹³C and δ¹⁵N values probably in response to colder and wetter conditions.

Introduction

Central Europe has witnessed several changes in environmental conditions and human activity over the past 2600 years. Driven by natural, social, economic and political events (or their combinations) these have influenced the sedimentary geochemical signatures. Sediments of the maar lakes in the Eifel Mountains of Germany (Fig. 1) are considered as excellent recorders of environmental and anthropogenic changes. The sediments of Holzmaar are deemed to be particularly well suited for reconstructions of past changes because their annually laminated sediment cores provide an accurate chronology. The latter is indispensable for reconstructions in historical times (Zolitschka et al., 2000, Brüchmann and Negendank, 2004, Kienel et al., 2005).

In the Eifel region and its larger surroundings, the major historical/cultural periods include the pre-Roman Iron Age, Roman Empire, Migration Period, Early Middle Ages, High Middle Ages, Renaissance, pre-industrial modern times and the industrial modern times beginning with the Industrial Revolution (Table 1; Zolitschka, 1990, Zolitschka et al., 2003, Sirocko, 2009, Büntgen et al., 2011, Kattel and Sirocko, 2011, Herbig and Sirocko, 2012). The most recent history has seen changes from predominant use of coal for energy production to oil in the 1960s and later increasing use of methane and the introduction of emission control measures in the 1970s (Lima et al., 2003, Bond et al., 2007, Elmquist et al., 2007). These historical periods can be differentiated by differences in intensity and type of anthropogenic activity related to population size, industrial and agricultural activity, fuel type, environmental protection measures, innovation, climatic forcing, natural disasters and political and socio-economic events.

Geochemical properties already studied in sediments of Holzmaar include total C and N concentrations, stable isotope ratios of C (δ¹³C values), and heavy metal concentrations which have been successfully used together with other biological and physical signatures of Holzmaar sediments as indicators of regional environmental and anthropogenic changes (Zolitschka, 1990, Lottermoser et al., 1993, Lücke et al., 2003, Zolitschka et al., 2003, Fuhrmann et al., 2004, Litt et al., 2009, Kattel and Sirocko, 2011, Herbig and Sirocko, 2012). Other geochemical proxies which have so far not been applied to Holzmaar sediments include the concentration and composition patterns of polycyclic aromatic hydrocarbons (PAHs), charcoal counts, total sulfur (TS) concentration and other stable isotope ratios (δ¹⁵N and δ³⁴S, Jiang et al., 1998, Talbot and Lærdal, 2000, Meyers, 2003, Elmquist et al., 2007).

PAHs are emitted from incomplete combustion of C-containing compounds such as fossil fuels and also as a result of forest and prairie fires (Baek et al., 1991, Wilcke, 2007). Because the main source of PAHs is combustion, the historical record of PAHs in sediment cores can be used in pre-industrial periods as proxies for the frequency and size of woodland fires related to both past environmental (mainly climatic) conditions and/or human activities (Venkatesan and Dahl, 1989, Jiang et al., 1998). In periods strongly influenced by anthropogenic activity, the historical record of PAHs reflects changes in emissions associated with population changes, forest clearings, domestic fires, industrial activities (from charcoal production to modern heavy industries), type of energy sources and most recently emission control regulations (Lima et al., 2003, Elmquist et al., 2007, Gabrieli et al., 2010). The qualitative pattern of PAHs in sediments is partly a reflection of combustion conditions and the type of fuel. The PAH pattern can therefore be used for identification of sources and combustion temperature (Masclet et al., 1987, Yunker et al., 2002). Some PAHs such as perylene, retene (1-methyl-7-iso-propylphenanthrene) and pimanthrene (1,7-dimethylphenanthrene) in sediments are, however, partly of biological origin. Under anoxic conditions, perylene is formed microbiologically from specific biological precursors of terrestrial and/or aquatic origin (Venkatesan, 1988, Silliman et al., 1998, Silliman et al., 2001). Retene and pimanthrene can be formed within the lake from biological transformation of precursor compounds that occur in needles of coniferous trees and also emissions from combustion of conifer tree biomass (Laflamme and Hites, 1978, Wakeham et al., 1980, Ramdahl, 1993, Benner et al., 1995, Yunker et al., 2002).

The PAH concentrations and composition patterns are a complimentary and perhaps even more differentiated fire marker than the number of charcoal pieces which is more frequently used as fire tracer, particularly for pre-industrial era (Conedera et al., 2009, Thevenon et al., 2010). A further fire marker, sulfur (S), found in lake sediments partly originates from anthropogenic combustion and natural fire sources.

In this paper, we focus on the use of concentrations and composition patterns of PAHs as a potential proxy to refine our understanding of past environmental and anthropogenic changes in the Eifel Mountains. We discuss the PAHs results together with the established proxies of charcoal counts, the elemental concentrations of C, N and S and the δ¹³C, δ¹⁵N and δ³⁴S ratios to support our interpretation of the PAHs results in a 400 cm-long sediment core from Holzmaar which corresponds to a record of about 2600 years.