Hydrological changes and chemical weathering through time in the southwestern Alps using isotopes from siliceous microalgae

Informations projet

Hydro-ALPS

N° de convention de subvention: 101065751

DOI

10.3030/101065751

Projet clôturé

Date de signature de la CE 22 Juin 2022

Date de début 1 Juillet 2022

Date de fin 30 Juin 2024

Financé au titre de

• Marie Skłodowska-Curie Actions (MSCA)

Coût total

Aucune donnée

Contribution de l’UE

€ 195 914,88

Coordonné par

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 1 - Hydro-ALPS (Hydrological changes and chemical weathering through time in the southwestern Alps using isotopes from siliceous microalgae)

Période du rapport: 2022-07-01 au 2024-06-30

Résumé du contexte et des objectifs généraux du projet

Across middle Europe, the Alps cover four sub-climatic regions and play a major role in accumulating and supplying water to the continent. Famously called as the “water towers of Europe”, they host most of the headwaters of the rivers Danube, Rhine, Po and Rhone. Located in the Mediterranean basin, the southwestern Alps are known to be particularly vulnerable to flood, landslide events and summer droughts. Precipitation is expected to decrease by 20-30 % by 2100 threatening water resources. However, large uncertainties remain on future precipitation regimes at a local scale. Climate models spatial resolution do not integrate regional climate heterogeneity and models are calibrated on a short temporal “observations” window (the last century mainly). The Hydro-Alps project aims at providing new perspectives on the history of past local hydrological changes that needs to be considered in order to improve climate projections and strategies to water resources issues. Hydrological changes and chemical weathering will be studied in the southwestern Alps for the pre- industrial period and over several millennia. Valuable information can be provided by microalgae called diatoms which are accumulating through time in the sediments of alpine lakes. They build a cell wall in silica (SiO2) which records lake water chemistry during the shell formation. The project will rely on measurements of oxygen and silicon isotopes from diatoms extracted from lacustrine sediments used as tracers of local hydrological conditions and chemical weathering of the crystalline bedrock. This innovative approach will provide new information on mountainous past climate variability by combining isotopic tracers on same sample and new perspective about long-term changes in the Mediterranean climate. Altogether, the Hydro-Alps project will improve our understanding of climate changes at local scale and will help stakeholders to adapt water resource strategies.
Major accomplishments expected include the coring and water sampling of alpine lakes in the Southwestern Alps, measurements of oxygen and silicon isotopes in diatoms since the last glaciation, integration of results in open access databases, several publications and communication in national and international conferences.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

The activities performed during the action were organized in 5 work packages including 4 focusing on the technical and scientific activities. The work carried out to complete the objectives includes a successful coring of an alpine lake (Lake Petit) at 2200 m a.s.l located in the Mercantour national park. The 14 sediment cores retrieved from the lake were correlated with each other to produce a unique master core. Core description included colours, granulometry, high resolution images. Sediment samples were sieved every three centimetres in order to pick terrestrial macroremains for radiocarbon dating. After sample treatments and diatom purification for isotope analysis, a total of 46 samples (23 data points with two replicates added to 20 data points already published) were measured on a manual fluorination line for δ18O analysis and 109 samples were measured on a Neptune MC-ICP-MS for δ30Si analysis (43 data points with three replicates). Oxygen and silicon isotope measurements on diatom samples allowed to reconstruct the evolution of hydrological parameters linked to changes in the lake water budget since the lake formation. Additionally, δ30Si measurements led to a high-resolution record of changes in chemical weathering of the silicate substrate during key periods of climate change.
A total of 62 diatom samples have been counted with a minimum of 400 diatom valves counted in each sample in order to reconstruct the evolution of the lake ecosystem. Additional samples from the same depths have been treated for pollen analysis to follow the effects of vegetation cover changes. In addition, modern isotope variability has been studied by sampling water and diatoms from the modern lake ecosystem in collaboration with the National Park of Mercantour. Water samples, snow, diatoms and soil samples have been taken during two seasons of fieldwork in 2022 and 2023. These samples have allowed to follow changes in dissolved silica inputs between the different seasons and the different sources of water. Measurements of silicon isotope in waters and soils allowed to trace silicon isotope signature between water sources and the different remaining types of soils in the watershed (podzol/cambisol) as well as diatom utilization in the lake. Measurements of oxygen/deuterium isotopes in waters and snow made it possible to study the effect of evaporation processes in the lake ecosystem and changes of the source of water inputs between seasons. Finally, a collaborative work consisting in a comparison of diatom assemblages and isotope measurements with other proxies and studies from the same area highlighted common trends and extreme events (correlation matrix, PCR). These analyses included X-Ray Fluorescence scanning of sediment cores and ICP-MS measurements of major elements and measurements of biogenic silica, carbon and nitrogen content.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

The coring and building of a master core led to a total sequence of 7 meters of sediment accumulation covering the all period from the end of deglaciation to present day. The age-depth model built based on a total of 21 datations available at the study site is covering the last 14 500 years. The multiproxy comparison of results has highlighted the evolution of detrital inputs to the lake and diatom production since the last glaciation as well as the effects of changes in vegetation cover on lake nutrient status and diversity. For the first time in high alpine altitude environments, a multiproxy lacustrine record is able to study precisely climate events under natural variability and anthropogenic pressure using both silicon, oxygen isotopes and more traditional measurements of sediment composition. This synthesis of data highlighted the presence of several climate pulses at the beginning of the Holocene not described previously. The record has also shown the importance of the impacts of the 4.2 ka climate event in comparison to the total environmental variability during the interglacial period. The results have been included into a regional synthesis and the dataset available at the site has been partly published in a synthesis of oxygen isotope records in lacustrine environments. Measurements of dissolved silica content, oxygen/deuterium and silicon isotopes have been made in triplicates on 21 water samples sampled in June, August and September 2022 and May, August and October 2023. The results have allowed to follow changes in dissolved silica inputs between the different seasons and the different sources of water improving highly our knowledge on modern isotope geochemistry in these remote systems for further studies. Overall, the occurrence of century-scale climate events in the Holocene needs further research, as well as the climate forcings triggering them as climate model projections will be more accurate if they include the full range of natural climate variability. The study of their timing and environmental impacts at a local scale is also of societal importance to plan and adapt management actions for rapid climate change.