Microstratigraphic, lipid biomarker and stable isotope study of a middle Palaeolithic combustion feature from Axlor, Spain.

Archaeological research has increasingly focused on studying combustion features as valuable sources of information regarding past technological and cultural aspects. The use of microstratigraphic and biomolecular techniques enables the identification of combustion residues and substrate components, and infer about past fire-related activities and the environments. Our study conducted on a combustion feature (Level N, ∼100 Ka) at the Axlor cave, a Middle Paleolithic site in northern Iberia, exemplifies the interdisciplinary approach to combustion features. Micromorphological features revealed depositional activities associated with occupations such as hearth rake-out and trampling. Through molecular (n-alkanes, n-alcohols, and n-fatty acids) and isotopic analysis (δ13C16:0 and δ13C18:0), we infer the good preservation of organic matter, the contributions of non-ruminant fats, and the dead-wood gathering strategies by Neanderthal groups. By combining microstratigraphic and biomolecular approaches, our study significantly contributes to the advancement of our current understanding of Neanderthal pyrotechnology.

Compound-specific carbon isotope analysis of short-chain fatty acids from Pine tissues: characterizing paleo-fire residues and plant exudates.

Different types of plant tissues and resin can account for the wax lipids found in sedimentary contexts and archaeological samples. Consequently, there is increasing research to characterize the fatty acid carbon isotope ratios of different plant anatomical parts and their plant exudates (resin). With the aim to explore isotopic differences between plant tissues, state of the fine organic matter, effect of thermal degradation, and to identify plant residues we measured the δ13C values of short-chain fatty acids (δ13C16:0 and δ13C18:0) in: i) dead and fresh (collected and immediately dried) pine needles and branches (Pinus canariensis) and pine resin from laboratory-controlled heating experiments and ii) sediment and charred pine tissue samples from a wild pine forest fire. Our results are compared to previously published experimental open-air fire experiments and pine-fuelled archaeological combustion features. We found that for both fatty acid types, there are differences in δ13C signatures among anatomical parts and initial moisture content. These data allow us to characterize the isotopic signature of pine tissue and the effect of degradation on isotopic biomarkers, as well as to estimate combustion temperatures in pine-fuelled anthropogenic fires. Supplementary Information: The online version contains supplementary material available at 10.1007/s12520-023-01815-3.

Special Issue "Applications of Stable Isotope Analysis".

The isotopic composition of matter is controlled by different physical, chemical, and biological mechanisms [...].

Investigating Hydrogen Isotope Variation during Heating of n-Alkanes under Limited Oxygen Conditions: Implications for Palaeoclimate Reconstruction in Archaeological Settings.

This paper reports on a series of heating experiments that focus on n-alkanes extracted from leaf, bark, and xylem tissues of the Celtis australis plant. These lipid biomarkers were analysed for their compound-specific hydrogen isotopic composition (δ2Hwax) under limited oxygen conditions at 150, 250, 350, and 450 °C. Our results reveal isotopic variations in wax lipids of different plant organs during short-term low-temperature combustion. We conclude that, in the absence of a detailed characterisation of the depositional environment in advance of sampling, δ2Hwax values in archaeological or otherwise highly anthropogenic environments should be interpreted cautiously. In addition, we observed that variation in δ2Hwax of leaves is minimal at temperatures ≤ 350 °C, highlighting the potential for δ2Hwax in thermally altered combustion substrates to yield palaeoclimate information, which could allow researchers to investigate links between archaeological and climatic records at a high spatial and temporal resolution.

Characterisation of charred organic matter in micromorphological thin sections by means of Raman spectroscopy.

Burned or charred organic matter in anthropogenic combustion features may provide important clues about past human activities related to fire. To interpret archaeological hearths, a correct identification of the organic source material is key. In the present work, Raman spectroscopy is applied to characterise the structural properties of char produced in laboratory heating- and open-fire experiments. This reference data set is compared to analyses of three different archaeological sites with Middle Palaeolithic combustion contexts. The results show that it is possible to determine whether a charred fragment is the product of burning animal-derived matter (e.g. meat) or plant-derived matter (e.g. wood) by plotting a few Raman spectral parameters (i.e. position of G and D bands, and intensity ratios H D/H G and H V/H G) against one another. The most effective parameters for discriminating animal- from plant-derived matter are the position of the G band and the H V/H G intensity ratio. This method can be applied on raw sample material and on uncovered micromorphological thin sections. The latter greatly compliments micromorphology by providing information about char fragments without any clear morphological characteristics. This study is the first of its kind and may provide archaeologists with a robust new method to distinguish animal- from plant-derived char in thin sections. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s12520-020-01263-3.

Micro-contextual identification of archaeological lipid biomarkers using resin-impregnated sediment slabs.

Characterizing organic matter preserved in archaeological sediment is crucial to behavioral and paleoenvironmental investigations. This task becomes particularly challenging when considering microstratigraphic complexity. Most of the current analytical methods rely on loose sediment samples lacking spatial and temporal resolution at a microstratigraphic scale, adding uncertainty to the results. Here, we explore the potential of targeted molecular and isotopic biomarker analysis on polyester resin-impregnated sediment slabs from archaeological micromorphology, a technique that provides microstratigraphic control. We performed gas chromatography-mass spectrometry (GC-MS) and gas chromatography-isotope ratio mass spectromety (GC-IRMS) analyses on a set of samples including drill dust from resin-impregnated experimental and archaeological samples, loose samples from the same locations and resin control samples to assess the degree of interference of polyester resin in the GC-MS and Carbon-IRMS signals of different lipid fractions (n-alkanes, aromatics, n-ketones, alcohols, fatty acids and other high polarity lipids). The results show that biomarkers within the n-alkane, aromatic, n-ketone, and alcohol fractions can be identified. Further work is needed to expand the range of identifiable lipid biomarkers. This study represents the first micro-contextual approach to archaeological lipid biomarkers and contributes to the advance of archaeological science by adding a new method to obtain behavioral or paleoenvironmental proxies.

Insights into the timing, intensity and natural setting of Neanderthal occupation from the geoarchaeological study of combustion structures: A micromorphological and biomarker investigation of El Salt, unit Xb, Alcoy, Spain.

Middle Paleolithic lithic and faunal assemblages throughout Eurasia reflect short-term Neanderthal occupations, which suggest high group mobility. However, the timing of these short-term occupations, a key factor to assess group mobility and territorial range, remains unresolved. Anthropogenic combustion structures are prominent in the Middle Paleolithic record and conceal information on the timing and intensity and natural setting of their associated human occupations. This paper examines a concentration of eleven combustion structures from unit Xb of El Salt, a Middle Paleolithic site in Spain through a geoarchaeological approach, in search of temporal, human impact and paleoenvironmental indicators to assess the timing, intensity and natural setting of the associated human occupations. The study was conducted using micromorphology, lipid biomarker analysis and compound specific isotope analysis. Results show in situ hearths built on different diachronic topsoils rich in herbivore excrements and angiosperm plant residues with rare anthropogenic remains. These data are suggestive of low impact, short-term human occupations separated by relatively long periods of time, with possible indicators of seasonality. Results also show an absence of conifer biomarkers in the mentioned topsoils and presence of conifer charcoal among the fuel residues (ash), indicating that fire wood was brought to the site from elsewhere. A microscopic and molecular approach in the study of combustion structures allows us to narrow down the timescale of archaeological analysis and contributes valuable information towards an understanding of Neanderthal group mobility and settlement patterns.

Tracking climate change in oligotrophic mountain lakes: Recent hydrology and productivity synergies in Lago de Sanabria (NW Iberian Peninsula).

Mountain lakes are particularly sensitive to global change as their oligotrophic conditions may be rapidly altered after reaching an ecological threshold, due to increasing human impact and climate change. Sanabria Lake, the largest mountain lake in the Iberian Peninsula and with a recent history of increased human impact in its watershed, provides an opportunity to investigate recent trends in an oligotrophic, hydrologically-open mountain lake, and their relationship with climate, hydrological variability and human pressure. We conducted the first systematic and detailed survey of stable isotope compositions of Sanabria Lake and Tera River together with limnological analyses during 2009-2011. δ18Olakewater and δDlakewater seasonal fluctuations are strongly linked to river discharges, and follow the monthly mean isotopic composition of precipitation, which is controlled by NAO dynamics. δ13CPOM and δ13CDIC revealed higher contribution of allochthonous organic matter in winter and spring due to higher river inflow and lower primary productivity. Increased phytoplankton biomass in late summer correlated significantly with higher pH and Chl-a, and higher nutrient input and lower river inflow. However, the small δ13CPOM seasonal amplitude underlines the stability of the oligotrophic conditions and the isotopic variation in POM and DIC reflect small seasonal fluctuations mostly as a consequence of strong throughflow. The stability of hydrology and productivity patterns is consistent with Holocene and last millennium reconstructions of past limnological changes in Sanabria Lake. The results of this study indicate that trophic state in this hydrologically-open mountain lake is strongly controlled by climate variability, but recent changes in human-land uses have increased sediment delivery and nutrients supply to the lake and have to be considered for management policies. Monitoring surveys including isotope techniques provide snapshots of modern isotope variability, and serve as a benchmark for assessing the environmental impacts of future developments and long-term climate changes in mountain lakes.