Claire Delhon

Changes in erosion patterns during the Holocene in a currently treeless subalpine catchment inferred from lake sediment geochemistry (Lake Anterne, 2063 m a.s.l., NW French Alps): The role of climate and human activities

A high-resolution sedimentological and geochemical study was performed on a 20 m long core from the alpine Lake Anterne (2063 m a.s.l., NW French Alps) spanning the last 10 ka. Sedimentation is mainly of minerogenic origin. The organic matter quantity (TOC%) as well as its quality (hydrogen (HI) and oxygen (OI) indices) both indicate the progressive onset and subsequent stabilization of vegetation cover in the catchment from 9950 to 5550 cal. BP. During this phase, the pedogenic process of carbonate dissolution is marked by a decrease in the calcium content in the sediment record. Between 7850 and 5550 cal. BP, very low manganese concentrations suggest anoxic conditions in the bottom-water of Lake Anterne. These are caused by a relatively high organic matter (terrestrial and lacustrine) content, a low flood frequency and longer summer stratification triggered by warmer conditions. From 5550 cal. BP, a decrease in TOC, stabilization of HI and higher sedimentation rates together reflect increased erosion rates of leptosols and developed soils, probably due to a colder and wetter climate. Then, three periods of important soil destabilization are marked by an increased frequency and thickness of flood deposits during the Bronze Age and by increases in topsoil erosion relative to leptosols (HI increases) during the late Iron Age/Roman period and the Medieval periods. These periods are also characterized by higher sedimentation rates. According to palynological data, human impact (deforestation and/or pasturing activity) probably triggered these periods of increased soil erosion.

Phytolith assemblages as a promising tool for reconstructing Mediterranean Holocene vegetation

The reliability of phytolith assemblage analysis for characterizing Mediterranean vegetation is investigated in this study. Phytolith assemblages are extracted from modern and buried Holocene soils from the middle Rhone valley (France). The relation between modern phytolith assemblages and the surrounding vegetation, as well as between fossil assemblages and contemporaneous vegetation, already reconstructed through other proxies, is discussed. We demonstrate that the main northwestern Mediterranean biomes are well distinguished by soil phytolith assemblage analysis. In particular, the density of pine and nonconiferous trees (densities expressed relatively to the grass cover) and the overall degree of opening of the vegetation appear well recorded by three phytolith indexes. North Mediterranean vegetation changes during the Holocene period, mainly tree line shifts, pine wood development and deforestation are poorly documented, due to the scarcity of proxy-preserving sites. Phytolith assemblage analysis of soils, buried soils, and sediments appears to be a promising technique to fill this gap.

Non-reversible geosystem destabilisation at 4200 cal. BP: Sedimentological, geochemical and botanical markers of soil erosion recorded in a Mediterranean alpine lake

A 144-cm-long core was obtained in Lake Petit (2200 m a.s.l., Mediterranean French Alps) in order to reconstruct past interactions between humans, the environment and the climate over the last five millennia using a multidisciplinary approach involving sedimentological, geochemical and botanical analyses. We show a complex pattern of environmental transformation. From 4800 to 4200 cal. BP, podzol-type soils progressively developed under forest cover. This stable situation was interrupted by a major detrital pulse at 4200 cal. BP that we consider as a tipping point in the environmental history. At this point, pedogenetic processes drastically regressed, leading to the development of moderately weathered soils. More frequent detrital inputs are recorded since 3000 cal. BP (ad 1050) as the human impact significantly increased in the catchment area. We conclude that destabilisation of the environment was triggered by climate and exacerbated by human activities to a stage beyond resilience.

In pursuit of a missing transition: the Mesolithic and Neolithic radiocarbon chronology at La Font-aux-Pigeons rockshelter

Abstract The 1950s excavations at Châteauneuf-lès-Martigues—type site of the Late Mesolithic Castelnovian phase—played a significant role in shaping theories about the nature of the Neolithic transition in the western Mediterranean. Results of new AMS dating and Bayesian modelling of extant short life samples now date the Late Mesolithic deposits to c. 6460–6200 cal BC, and the Cardial deposits to c. 5260–4860 cal BC. The long gap within the stratigraphic sequence is interpreted as a consequence of erosion during the mid sixth millennium BC. These results overturn the older argument for Mesolithic–Neolithic continuity at this key site.

Is Choice Acceptable? How the Anthracological Paradigm May Hinder the Consideration of Fuel Gathering as a Cultural Behaviour

Charcoal analysis is a powerful and well-established means of documenting past environments and the impact of ancient societies on vegetation. Nowadays, the palaeoecological accuracy of anthracological spectra built from scattered charcoal derived from long-term use of domestic fuel is widely acknowledged. This broadly accepted paradigm relies on the hypothesis that firewood gathering is done following the Least Effort Principle, in areas frequented for routine activities, and may include any species encountered therein. Nevertheless, ethnographic studies, despite the fact that they often underline the opportunistic aspects of wood gathering, always reveal that choice comes into play at some stage of the process. Moreover, most anthracologists think in terms of ‘fuel management’, admitting that firewood gathering is subject to technical, economic and cultural prescriptions. For historical times and specialised activities, we easily accept that fuel may be chosen according to various properties, including species. However, we still deny that prehistoric societies potentially preferred or rejected certain wood species for feeding their domestic fires. Through concrete ethnographic and archaeobotanical examples and a case-study, this paper aims to question our capacity to consider that fuel procurement could be affected by cultural traits, to identify the behaviours that could make the charcoal record differ from the vegetation and, finally, to consider charcoal assemblages from an anthropological angle

The Holocene deposits of Lake Petit (2200 m asl, Southern French Alps): climatic and anthropogenic controls on mountain sediment dynamics?

Numerous archaeological studies carried out in high-elevation areas of the Southern French Alps have documented ancient human occupation of mountains, mainly characterized by pastoral and mining activities. Besides, the area was affected by both continental and Mediterranean climate instability throughout the Holocene. Lake Petit, located in the Mercantour massif, was investigated in order to reconstruct past interactions between humans, the environment and climate, using a multidisciplinary approach. This study was carried out on a 144 cm-long gravity core providing a continuous record spanning the last 5000 years. Our multi-proxy investigation (sedimentological, geochemical and palynological analyses), allowed us to reconstruct three main phases, evidencing a progressive destabilization of the local environment. The first phase (ca. 4800 to 4300 cal. BP) was characterized by relatively stable environmental conditions, as attested by a high lake productivity (deposition of pure diatomite) and by the local presence of trees (presence of conifer stomata). The second phase (ca. 4300 to 1500 cal. BP) could correspond to a drastic cleaning of landscapes since the lake record highlights an abrupt switch in sediment source marked by an input of terrestrial organic matter. Regular occurrences of anthropogenic pollen assemblages might indicate early pastoral activity in the lake vicinity, while wetter conditions characteristic of the Neoglacial period may have trigger hillslopes destabilization. The most recent phase (since ca. 1500 cal. BP) is marked by a final degradation of the environment: the lake productivity dramatically decreased contemporaneously with a fall of the AP/NAP pollen curve. This phase also exhibits the highest values of anthropogenic pollen taxa and concentrations of lead. It undoubtedly corresponded to one of an intensive agro-pastoral pressure as well to local mining activities.