Reconstructing fire regimes from geochemical proxies in Late Quaternary soils

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. 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.
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; Ol >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 tundralike
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.

Resources

Bibliography

Wolf, M. (2014): Reconstructing fire regimes from geochemical proxies in Late Quaternary soils. University of Bonn, University of Bonn