Human and livestock faecal biomarkers at the prehistorical encampment site of Ullafelsen in the Fotsch Valley, Stubai Alps, Austria – potential and limitations

The Ullafelsen at 1869 m a.s.l. in the Tyrolean Stubai Alps next to Innsbruck is an important (geo-)archaeological reference site for the Mesolithic period. Buried fireplaces on the Ullafelsen plateau were dated at 10.9–9.5 cal. kyrs BP and demonstrate together with thousands of flint stone artifacts the presence of hunter-gatherers during the Early Holocene. Most recently, we demonstrated the great potential of n-alkane and black carbon biomarkers for contributing to a better understanding of pedogenesis and landscape evolution. In order to study the importance of human and/or animals for occupation of this relevant geoarchaeological site, we carried out steroid and bile acid analyses on two modern faeces samples from cattle and sheep and on 37 soil samples from seven soil profiles at the Ullafelsen. The modern animal faeces show a dominance of 5β-stigmastanol and deoxycholic acid for ruminants (cattle and sheep), which is in agreement with literature data. The OAh horizons, which have accumulated and developed since the Mesolithic, revealed high contents of steroids and bile acids; the E (LL) horizon coinciding with the Mesolithic living floor is characterized by medium contents of steroids and bile acids. By contrast, the subsoil horizons Bh, Bs and BvCv contain low contents of faecal biomarkers indicating that leaching of steroids and bile acids into the podsolic subsoils is not an important factor. Deoxycholic acid is the most abundant bile acid in all soil samples and gives evidence for strong faeces input of ruminants. The steroid and bile acid patterns and ratios indicate a negligible input of human faeces on the Ullafelsen. β-Sitosterol as plant-derived steroid has also a strong influence on the faecal biomarker pattern in our soils. Root input into the subsoils is likely reflected by β-sitosterol contents. In conclusion, our results reflect a strong faecal input by livestock, rather than by humans as found for other Anthrosols such as Amazonian Dark Earths. Further studies need to focus on the question of the exact timing of faeces deposition.

Archaeology and ethnography demonstrate a human origin for Amazonian Dark Earths

Archaeological research provides clear evidence that the widespread formation of Amazonian Dark Earths (ADEs) in tropical lowland South America was concentrated in the Late Holocene, an outcome of sharp demographic growth that peaked towards 1000 BP. In their recent paper, however, Silva et al. propose that the high fertility of ADE is not of anthropic origin but instead the result of alluvial deposition starting in the Middle Holocene (8200-4200 cal BP). In order to support this argument, they marshal data and observations from a single expanse of ADE, the archaeological site of Caldeirão, and disregard or misread other studies of ADEs in the Central Amazon region. Silva et al.'s claim, an epilogue to ‘geogenic’ models laid to rest over 40 years ago, also dismisses research showing how long-term anthropic soil enrichment occurs as a result of daily practices at contemporary indigenous settlements. Here we critically review Silva et al.’s analysis and affirm that, like most ADEs, Caldeirão has anthropic soil horizons formed by burning, deposition, and reworking of refuse associated with indigenous settlement activities between 2500 and 500 BP.

A new hypothesis for the origin of Amazonian Dark Earths

Amazonian Dark Earths (ADEs) are unusually fertile soils characterised by elevated concentrations of microscopic charcoal particles, which confer their distinctive colouration. Frequent occurrences of pre-Columbian artefacts at ADE sites led to their ubiquitous classification as Anthrosols (soils of anthropic origin). However, it remains unclear how indigenous peoples created areas of high fertility in one of the most nutrient-impoverished environments on Earth. Here, we report new data from a well-studied ADE site in the Brazilian Amazon, which compel us to reconsider its anthropic origin. The amounts of phosphorus and calcium—two of the least abundant macronutrients in the region—are orders of magnitude higher in ADE profiles than in the surrounding soil. The elevated levels of phosphorus and calcium, which are often interpreted as evidence of human activity at other sites, correlate spatially with trace elements that indicate exogenous mineral sources rather than in situ deposition. Stable isotope ratios of neodymium, strontium, and radiocarbon activity of microcharcoal particles also indicate exogenous inputs from alluvial deposition of carbon and mineral elements to ADE profiles, beginning several thousands of years before the earliest evidence of soil management for plant cultivation in the region. Our data suggest that indigenous peoples harnessed natural processes of landscape formation, which led to the unique properties of ADEs, but were not responsible for their genesis. If corroborated elsewhere, this hypothesis would transform our understanding of human influence in Amazonia, opening new frontiers for the sustainable use of tropical landscapes going forward.

Amazonian dark earths in the fertile floodplains of the Amazon River, Brazil: an example of non-intentional formation of anthropic soils in the Central Amazon region

ABSTRACT Amazonian dark earths (ADEs) are fertile soils created by pre-Columbian Amerindian societies of the Amazon Basin. However, it is still not clear whether these soils were produced intentionally to improve infertile Amazonian upland soils or if they resulted from the accumulation of organic matter from sedentary settlements. This study characterizes the ADEs found in the naturally fertile alluvial floodplains of the Amazon River in the Central Brazilian Amazon according to total, exchangeable, and available contents of elements and organic carbon in soil profiles. ADEs contained higher levels of available elements and total P, Ca, Zn, and Cu. High total Cr, Ni, Co, and V content in these soils indicate that mafic minerals contributed to their composition, while higher contents of P, Zn, Ba, and Sr indicate anthropic enrichment. The presence of ADEs in floodplain areas strongly indicates non-intentional anthropic fertilization of the alluvial soils, which naturally contain levels of P, Ca, Zn, and Cu higher than those needed to cultivate common plants. The presence of archaeological sites in the floodplains also shows that pre-Columbian populations lived in these regions as well as on bluffs above the Amazon River.