Elodie Brisset

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.

5000 years of lacustrine ecosystem changes from Lake Petit (Southern Alps, 2200 m a.s.l.): Regime shift and resilience of algal communities

Sediments from Lake Petit (2200 m a.s.l., Southern Alps) are particularly relevant for analysis of coupled landscape palaeoecology and palaeolimnology. Diatom assemblages, organic matter composition of sediments (total nitrogen and organic carbon) and Pediastrum boryanum concentrations were obtained from a 144-cm-long core, enabling the reconstruction of the aquatic ecosystem over nearly the last 5000 cal. BP. From 4800 to 4300 cal. BP, Lake Petit was a stable diatom-productive water body dominated by alkaliphilous diatoms (Staurosirella pinnata). During this period, nutrients and cations were supplied by the chemical weathering of podzols that developed under conifer woodlands. This overall stability was suddenly interrupted at 4200 cal. BP by a major detrital pulse that was probably climate linked (4200 cal. BP event) and that triggered a drop in diatom productivity and diversity. From 4100 to 2400 cal. BP, diatom productivity progressively decreased, whereas Pediastrum developed. Diatom assemblages were more diversified (predominance of Pseudostaurosira robusta, P. brevistriata and P. pseudoconstruens) and reflected a regime of continuous erosion, whereas slopes were colonised by grazed grasslands. Finally, from 2400 cal. BP to the present day, diatom assemblages reveal a slight acidification and nutrient enrichment of waters concomitant with increasing human pressure in the catchment. These results demonstrate the close links between ecosystems and the ready propagation of disturbances throughout watersheds that might lead to abrupt regime shifts in such alpine environments.

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.