Eileen Eckmeier

Prehistoric alteration of soil in the Lower Rhine Basin, Northwest Germany—archaeological, 14C and geochemical evidence

In Early Holocene, Chernozems were assumed to have covered the entire loess landscape of the Lower Rhine basin—today mirrored by the distribution of Luvic Phaeozems. These Luvic Phaeozems have characteristic dark brown (Bht) horizons accumulating clay and humus, inherited and translocated from their precursors Chernozem black humic A horizons. We examined Luvic Phaeozems along a 33-km-long and 2.0–2.5-m-deep gas pipeline trench in the Lower Rhine Basin, west of Cologne. Along this transect we discovered clusters of hundreds of regularly shaped pits. These pits were always connected to the Bht horizons of adjacent Luvic Phaeozems. The Luvic Phaeozem horizons and the pits were investigated by combining methods from (geo-) archaeology (geographical distribution within the landscape, shape of the pits, soil texture), geochemistry (content of carbon, nitrogen and black carbon), palaeobotany (species determination of charcoals) and AMS 14 C measurements. We found that the Luvic Phaeozems occurred not only in the loess-covered landscape but also in the sandy Holocene floodplain, and their distribution could not be limited to certain slope positions or parent material. Carbon and nitrogen concentrations in the Luvic Phaeozem horizons and pits were larger than in the surrounding Luvisols, whereas the C/N ratios were small (<10). Material found in the Luvic Phaeozem pits was clearly different from material found in prehistoric settlements. The pits investigated here never contained artifacts, and carbon and nitrogen concentrations and C/N ratios were smaller. We found charcoal particles, and black carbon contributed up to 46% of the total organic carbon. The AMS 14 C ages of charcoals and black carbon indicated that fire occurred from Mesolithic (9500–5500 BC) to the Medieval Ages (500–1500 AD), and mainly in the Late-/End Neolithic period (4400–2200 BC). We conclude that (i) the Luvic Phaeozem pits and horizons are man-made, formed during several archaeological epochs between Mesolithic and Middle Ages, (ii) these pits must have been formed outside the actual prehistoric settlements (offsite) and may serve as a novel archaeological feature, (iii) the purpose of these pits at present is not clear and (iv) human activity has altered and ultimately formed the investigated soils of the Lower Rhine basin in prehistoric time.

Conversion of biomass to charcoal and the carbon mass balance from a slash-and-burn experiment in a temperate deciduous forest:

Anthropogenic burning, including slash-and-burn, was deliberately used in (pre)historic Central Europe. Biomass burning has affected the global carbon cycle since, presumably, the early Holocene. The understanding of processes and rates of charcoal formation in temperate deciduous forests is limited, as is the extent of prehistoric human impact on the environment. We took advantage of an experimental burning to simulate Neolithic slash-and-burn, and we quantified the biomass fuel and charcoal produced, determined the resulting distribution of the charcoal size fractions and calculated the carbon mass balance. Two-thirds of the charcoal particles (6.71 t/ha) were larger than 2000 μm and the spatial distribution of charcoal was highly variable (15—90% per m2). The conversion rate of the biomass fuel to charcoal mass was 4.8%, or 8.1% for the conversion of biomass carbon to charcoal carbon, and 58.4 t C/ha was lost during the fire, presumably as a component of aerosols or gases.

DATING CHARRED SOIL ORGANIC MATTER: COMPARISON OF RADIOCARBON AGES FROM MACROCHARCOALS AND CHEMICALLY SEPARATED CHARCOAL CARBON

Radiocarbon dating of charcoal in soils is commonly used to reconstruct past environmental processes. Also microcharcoal that is chemically isolated from soil organic matter by high-energy UV photo-oxidation can be dated with 14C accelerator mass spectrometry (AMS). We compared the 14C AMS ages of 13 pairs of hand-picked macrocharcoals and microcharcoal samples separated via the UV oxidation method; both charcoal fractions were taken from the same soil samples (prehistoric pit fillings). We found that in most cases, the microcharcoal fraction yielded older ages than the single macro- charcoal pieces, and that the differences between the ages are not systematic. A reason for these age differences might be that the microcharcoal fraction consists of more stable components than macrocharcoals and thus yields older ages. Dating of microcharcoal would give a mean age of charred organic matter in soil material and the ages of the more stable compounds. Thus, 14C data obtained from the microcharcoal fraction in soils is not comparable to macrocharcoal ages and should not be used to complement existing macrocharcoal data sets.

Comparison of solid-state 13C NMR spectra of soil organic matter from an experimental burning site acquired at two field strengths

Solid-state 13C cross polarisation (CP) nuclear magnetic resonance spectra were acquired for 15 soil organic matter samples on 2 different spectrometers (200u2009MHz and 400u2009MHz). Distributions of broad functional group classes—carboxyl, aryl, O-alkyl, and alkyl—were determined by integration across broad chemical shift regions. The distributions derived from the 2 spectrometers were closely correlated (r2u2009=u20090.77–0.93). Only slight biases were identified; carboxyl C contents were on average 8% lower and alkyl C contents 5% higher for spectra acquired on the 400u2009MHz spectrometer. These results indicate that valid 13C CP spectra can be acquired at field strengths up to 400u2009MHz, and that spectra acquired at different field strengths can be directly comparable.