Dr. Mareike Wolf

 

Dissertation Title

Reconstructing fire regimes from geochemical proxies in Late Quaternary soils

Supervisor: Prof. Dr. Wolf Amelung
Advisor: Dr. Eva Lehndorff

Abstract

Changes in ecosystems and land use frequently went along with regional burning events, but how fires accompanied human development, the onset of agriculture and changes in climate has eluded researchers. I hypothesised that information on past burning events may be reconstructed from the geochemical analyses of black carbon (BC), the residue of incomplete combustion. To test this hypothesis, it was the aim of my theses i) to elucidate the quantitative and qualitative characteristics of BC produced by different fire regimes, ii) to trace the properties of preserved BC remains in Pleistocene terrestrial archives (palaeosols), and iii) to reconstruct the Holocene fire history from limnic maar sediments in the Eifel, Germany.

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The analyses comprised the production of BC calibration materials in the lab, as well as the assessment of benzene polycarboxylic acids (BPCAs) as specific markers for BC, the organic carbon (Corg) and nitrogen content, oxygen index (OI: CO2/Corg) and hydrogen index (HI: HC/Corg), temperature of maximum heating (Tmax) and mid-infrared spectroscopy (MIRS). Methods were applied to artificial charcoals and environmental samples from terrestrial and limnic archives.

Alumni Wolf Pic3

The ratio of benzene polycarboxylic acids produced from BC calibration materials revealed a highly significant linear relation to combustion temperature (charcoals produced under laboratory conditions). Temperature ranges depicted for typical fire regimes were from the literature review. Test on natural charcoals was also positive (Wolf et al., 2013).

For i) the identification of different fire regimes a fire literature compilation was conducted. Three fire regimes were identified by fuel composition and temperature: grass and forest ground (285 ± 143 °C), shrub (503 ± 211 °C) and domestic fires (797 ± 165 °C). To evaluate the degree to which the chemical signatures of charcoal may serve as a fingerprint for fires, I analysed BC calibration materials, which comprised charcoals of wood and grass produced at 300-700 °C with varying combustion duration, as well as charcoals from natural fire events. The results showed that grass charcoal has consistently lower Corg contents and HI than wood. However, these values showed a bias towards the natural charcoals, probably because the latter contained higher amounts of mineral matter or because they were combusted under greater O2 supply. Nevertheless, the analyses of five-to six-times carboxylated BPCA (B5CA and B6CA, respectively) as well as analyses of OI and HI allowed a clear differentiation of natural charcoals, particularly of those stemming from forest ground fires (B5CA/B6CA 1.3-1.9; OI >20; intense CH2 stretching, Tmax <488 °C), grass fires (B5CA/B6CA 0.8-1.4; OI >20; weak CH2 stretching, Tmax <425 °C), and domestic fires, which, in contrast, revealed B5CA/B6CA values <0.8, OI values <20 and little MIR absorbance.

Applying these analyses to ii) three Pleistocene loess-palaeosol profiles in Western Germany (60  – 130,000 years before present (ka BP)) showed that these soils were able to archive BC amount and quality independent of differing soil formation and substrate properties, thus opening new perspectives for identifying past fire events.

The BC analyses of two palynological and geochemically well pre-described limnic maar lake sediment cores (14.7 and 11.5 ka BP to recent; objective iii) yielded reasonable fire history reconstruction: high BC inputs and BC quality in the Late Pleistocene revealed grass fires in a tundra-like ecosystem (average 4.1 g C m-2 a-1, B5CA/B6CA = 1.4), while with upcoming deciduous forests in the Early Holocene BC input decreased to 2.2 g C m-2 a-1 by stable quality. From 7.5 ka BP first agricultural land use impact was recorded by significantly higher BC inputs and low combustion temperatures (4.2 g C m-2 a-1; B5CA/B6CA increasing to 2). Later BC quality shifted to higher combustion temperatures with starting industrialisation from about 4 ka BP on (decreasing B5CA/B6CA ratio to 1). In modern times BC quality shifted further to higher combustion temperatures from the use of fossil fuels, which, intriguingly, do not go along with higher BC load. Overall, the BC amount and quality in the maar lakes was closer to the values from the BC calibration materials, suggesting that lake sediments preserve BC more completely than soils. The results showed that climatic changes only controlled the very early Pleistocene fire events; with onset of agriculture it was increasingly man who first set fires for the clearance of land, and finally controlled fires at later stages of industrialisation.

 

Education

2009 – 2013: Ph.D. Student at the Institute of Crop Science and Resource Conservations, University of Bonn

2009: Diploma in Geology, University of Cologne

 Diploma thesis: Flächenintegrierende Bestimmungen der PAK Konzentrationen in Straßenstäuben im Stadtgebiet Köln

 

Publications

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