Laurent Millet

French National Centre for Scientific Research | CNRS · Laboratoire Chrono-Environnement UMR 6249

Since multi-site reconstructions are less affected by site-specific climatic effects and artefacts, regional palaeotemperature reconstructions based on a number of sites can provide more robust estimates of centennial- to millennial-scale temperature trends than individual, site-specific records. Furthermore, reconstructions based on multiple recor...

Freshwater lakes are important sources of methane (CH4) emissions, by organic matter degradation under anaerobic conditions (methanogenesis). Previous studies suggest that lakes contribute up to 16 % of natural emissions. About 60 % of the CH4 produced is used as an energy source by methane-oxidizing bacteria (MOB—methanotrophs), which could suppor...

Comparisons of climate model hindcasts with independent proxy data are essential for assessing model performance in non-analogue situations. However, standardized palaeoclimate data sets for assessing the spatial pattern of past climatic change across continents are lacking for some of the most dynamic episodes of Earth's recent past. Here we prese...

We performed a high-resolution study of chironomid assemblages in a sediment core retrieved from Lake Igaliku in southern Greenland. The well-dated core is located within the former Norse Eastern Settlement and covered the last 1500 yr. The comparison of chironomid stratigraphy (PCA axis scores) with instrumental temperature data, land use history...

Chironomid head capsules were extracted and identified in 52 samples along a sediment core retrieved from the Ech palaeololake, (710 m a.s.l. in the west-central Pyrenees). The pollen stratigraphy together with radiocarbon dates provided the basis for the chronology of the record. Since temperature changes were identified as one of the major determ...

The location of Bergsee (382 m a.s.l.), between the Black Forest and northern Alpine glaciers during their maximum extent of the Würm glaciation, makes the sediment record of this lake a unique palaeoenvironmental archive that probably recorded the entire Last Glacial Period. Here we present a chironomid record from Bergsee covering ca. 35 thousand...

Carbonate mud is crucial in the global carbon cycle and serves as a key sedimentary archive for paleoclimate reconstruction. Understanding the mechanisms behind its formation is crucial for explaining long-term carbon storage, including atmospheric carbon dioxide transfer to the hydrosphere and variations in mud production over geological timescale...

Lentic waters are biogeochemical reactors, producing and receiving carbon (C) originally fixed by the terrestrial and aquatic biosphere, which is then buried in sediments or respired back to the atmosphere in the forms of carbon dioxide (CO2) and one of the more potent greenhouse gas (GHG) methane (CH4). Additionally, lakes serve as archives of ter...

The Nain region including its archipelago (56• 3232N; 61•4134W) has been studied for long-term climate dynamics and human occupation. The human settlement on the coast of this part of North-Central Labrador (Nunatsiavut) is intricate, with the earliest archaeological sites of various cultures dating back to at least 5000 BC. The last migration wave...

Over the last decades, the intensification of anthropogenic activities and associated disturbances on lake watersheds have led to major changes in lakes trophic functioning through accelerated eutrophication. In many lakes, these changes are characterized by an unprecedented increase in organic carbon fluxes, potentially leading to a shift in bioge...

The eastern, maritime portion of the black spruce-moss bioclimatic domain in Québec (Canada) is characterized by large wildfires with low occurrence. However, it is still poorly understood how climate-fire interactions influenced long-term vegetation dynamics in the boreal forest of eastern Québec. The long-term historical climate-fire-vegetation i...

Despite covering only 3% of the global land surface, peatlands are an active part of the Critical Zone (CZ) exchanging large water and greenhouse gas (GHG) fluxes with the surrounding aquifers, surface waters, and the atmosphere. While ecosystem services of peatlands (carbon and water storage, buffering of local climate) are essential to address 21...

Previous studies suggested that past changes in carbon sources and pathways in pelagic food webs can be investigated using carbon isotopic analysis (δ13C) of ephippia, resting-eggs produced by Daphnia. However, ephippia are known to be produced by Daphnia within a relatively short time window. Moreover, δ13CDaphnia value may vary considerably over...

During the last glacial period, the North Atlantic region experienced pronounced, millennial-scale alternations in climate characterized by cold stadial and milder interstadial conditions. These rapid climatic changes are recorded in Greenland ice cores and various terrestrial proxy records of Europe. Nonetheless, the scarcity and age uncertainties...

Lake ecosystems contribute significantly to atmospheric methane and are likely to become even bigger methane emitters with the global spread of hypoxia/anoxia in freshwater ecosystems. Here we characterized the spatial heterogeneity of methane production potential, methane concentration, archeal and bacterial communities across Lake Remoray sedimen...

Over the past decades, the abundance and area of macrocharcoal (i.e. ≥ 150 μm in diameter) fragments from sedimentary sequences have been quantified using visual or semi-automated methods to reconstruct fire histories. However, the lack of uniformity between counting methods used in each study could introduce methodological biases influencing fire...

Representing 30% of the global forest, boreal biomes play a key role for human activities and climate regulation. Their dynamics are intrinsically linked to fire, developing mainly in mature stands of black spruce (Picea mariana (Mill.) B.S.P.). Recent studies project an increase of fire regimes in Canadian forests under climate change; however, ea...

Intensification of anthropogenic activities in many lake catchments during the twentieth century led to increased autochthonous organic matter sedimentation and degradation of hypolimnetic oxygen conditions due to the intensification of heterotrophic processes. These processes can be amplified by the effect of climate warming on thermal stratificat...

Boreal ecosystems provide numerous goods and services essential to human activities, such as wood and paper supply or the regulation of natural phenomena (floods, diseases) (Hassan et al., 2005). They also play a major role in the global climate balance, storing ~32% of the world's biogenic carbon (Pan et al., 2011; Bradshaw, 2015). Their dynamics...

Ciliates are unicellular heterotrophic organisms that play a key role in aquatic planktonic and benthic food webs. Advances in sedimentary DNA (sed-DNA) analysis offer the possibility to integrate these bioindicators in paleoenvironmental reconstructions. In this study, we used the top–bottom paleolimnological approach and metabarcoding techniques...

The Lateglacial period is marked by strong changes in environment and climate in Europe, associated to the different steps of the deglaciation. This paper aims to reconstruct the vegetation and climate changes from the deglaciation to the early Holocene (15.2–10.8 ka cal BP) in Western Europe. Pollen investigations are performed at a centennial res...

1. Carbon isotope analysis (δ 13 C) of Daphnia resting-eggs (ephippia) in sedimen-tary records can be used to reconstruct past carbon transfers in pelagic food webs in lakes. However, there may be seasonal variability of cladoceran δ 13 C and ephippia production that could affect their use as palaeoecological indicators of pelagic carbon transfers....

Ciliates are unicellular heterotrophic organisms that play a key role in the planktonic and benthic food webs of lakes, and represent a great potential as bioindicator. In this study, we used the top-bottom paleolimnological approach to compare the recent and past (i.e. prior to major anthropogenic impacts) ciliate communities of 48 lakes located a...

Lake ecosystems contribute significantly to atmospheric methane and are likely to become even bigger methane emitters with the global spread of hypoxia/anoxia in freshwater ecosystems. Here we characterized the spatial heterogeneity of methane production potential, methane concentration, archaeal and bacterial and methanogen communities across Lake...

Covering ~30% of the global forest area, boreal forests provide numerous goods and services essential to human activities (wood and paper supply, cultural activities, natural resource extraction...) (Gauthier et al. 2015). They also provide a key role in climate regulation, representing ~20% of the global forest carbon sink (Pan et al. 2011). The d...

Bottom waters hypoxia spreads in many lakes worldwide causing severe consequences on whole lakes trophic network. Here, we aimed at understanding the origin of organic matter stored in the sediment compartment and the related diversity of sediment microbial communities in a lake with deoxygenated deep water layers. We used a geostatistical approach...

Within the scope of an interdisciplinary project funded by the German Research Foundation (DFG – Deutsche Forschungsgemeinschaft), lake sediments from the centers of the large Black Forest lakes (Titisee, Schluchsee, Feldsee and Bergsee) were analysed archaeobotanically and geochemically for the first time. Complementary archaeological (surveys, ex...

Long-term time series have provided evidence that anthropogenic pressures can threaten lakes. Yet it remains unclear how and the extent to which lake biodiversity has changed during the Anthropocene, in particular for microbes. Here, we used DNA preserved in sediments to compare modern micro-eukaryotic communities with those from the end of the 19t...

A 157-cm-long sediment core from Longemer Lake in the Vosges Mountains of France spans the past two millennia and was analyzed for trace metal content and lead isotope composition. Trace metal accumulation rates highlight three main input phases: Roman Times (cal. 100 BC–AD 400), the Middle Ages (cal. AD 1000–1500), and the twentieth century. Atmos...

Paleolimnological approaches offer one of the best opportunity for studying the long-term response of lakes to local and global pressures. However, classical biological proxies used in paleolimnology are so far restricted to specific groups that have diagnostic features preserved upon fossilisation. Recent applications of DNA-based have shown their...

The overarching aim of this paper is to investigate the lake ecosystem response to different drivers over a long term period by a paleolimnological study in Lake Narlay (46°64N, 5°91E) located in the Jura Mountains of France. It is a small, hard-water lake with a maximum water depth of 40 m and extended anoxic condition of the bottom water. Previou...

We aim at understanding the influence of climate change on the functioning of two freshwater lakes in South Greenland by combining limnological and paleolimnological methods. The results offer a means of testing the potential use of subfossil chironomid for paleoclimate reconstruction. First, the current ecological status of Lake Igaliku and Lake 9...

Between 45 and 14.7k cal a BP, the North Atlantic climate was characterized by a succession of short-lived Greenland Stadials (GS)/Interstadials (GI), which impacted terrestrial environments. We present a new pollen record from the Bergsee (47°34′20″N, 7°56′11″E), which was ice free during the Last Glacial. We constructed a robust chronology for th...

Assessing the extent to which changes in lacustrine biodiversity are affected by anthropogenic or climatic forces requires extensive paleolimnological data. We used high-throughput sequencing to generate time-series data encompassing over 2200 years of microbial eukaryotes (protists and Fungi) diversity changes from the sedimentary DNA record of 2...

1. Lakes play a key role in the regulation of the global carbon cycle. However, their functioning can be strongly impacted by anthropogenic pressures and climate variability. Understanding the response of the carbon cycle to environmental changes remains a crucial, elusive goal for both ecosystem managers and aquatic ecologists. In particular, the...

This study aims at reconstructing the limnological conditions of Lake Narlay, a lake of which a previous study has shown major changes in the trophic reliance on methane of the benthic food web. The evolution of environmental and limnological conditions in and around Lake Narlay is reconstructed using combined analyses of subfossil chironomid remai...

Palaeoenvironmentalists from the Chrono-environment laboratory in Besançon have been working in Greenland since 2006. The Study area is located in southern Greenland, within the Norse Eastern settlement occupied from 986 to 1450 cal. BC. The study aims at characterizing the impact of this first colonization on a pristine environment. The lacustrine...

Varved lake sediments provide opportunities for high-resolution paleolimnological investigations that may extend monitoring surveys in order to target priority management actions under climate warming. This paper provides the synthesis of an international research program relying on >150 years-long, varved records for three managed perialpine lakes...

Varved lake sediments provide opportunities for high-resolution paleolimnological investigations that may extend monitoring surveys in order to target priority management actions under climate warming. This paper provides the synthesis of an international research program relying on >150 years-long, varved records for three managed perialpine lakes...

This study aims at estimating the potential of the hydrogen stable isotope (δ2H) analysis of chironomid remains (HC) to reconstruct past changes in the methane (CH4) cycle in lakes. A rearing experiment was first designed to assess the contribution of hydrogen derived from diet to the chironomid biomass and the offset between larvae and their HC. R...

The heterogeneity of the subfossil chironomid deposition (in terms of assemblages and paleo-diet) has to be studied to assess the reliability of paleolimnological interpretations (i.e., spatial representativeness). We collected 31 cores in the deepest part of Lake Grand Maclu (Jura, France) with the aim of assessing the spatial structures at two sp...

Stable C isotope ratio (δ(13)C) values of chironomid remains (head capsules; HC) were used to infer changes in benthic C sources over the last 150 years for two French sub-Alpine lakes. The HCs were retrieved from a series of sediment cores from different depths. The HC δ(13)C values started to decrease with the onset of eutrophication. The HC δ(13...

Quel intérêt pourrait présenter l'étude de l'impact humain sur l'environnement quand on sait que le Groenland fut tardivement peuplé (vers 2500 av. J.-C) par des populations de chasseurs-cueilleurs originaires d'Amérique du Nord ? Des agriculteurs se sont pourtant aventurés dans ces contrées a priori peu hospitalières, pendant les quelques siècles...

The main objective of this study is to document paleofire activity during the late-glacial/early-Holocene transition in temperate ecosystems. For this purpose, we cored lakes Paladru and Moras (Rhone valley, France) and quantified sedimentary charcoal accumulation rate and fire frequency. To assess the role of climate and vegetation in paleofire ac...

We present radiometric, palaeoclimatological, palaeoenvironmental and archaeological data for the period 40 000–8000 cal BP in the Jura Mountains (eastern France). These mountains culminate at ∼1700 m a.s.l. and are today characterised by a semi-continental climate. During the Last Glacial Maximum, the range supported a local ice cap. While recent...

Au sud-ouest du Groenland, le développement de l’agriculture à la faveur du réchauffement climatique en cours induit une mutation sociétale et des conséquences environnementales nouvelles. La rétro-observation d’une situation quasi analogue autour de l’an mil et du développement de l’agro-pastoralisme viking sur les mêmes territoires entre le XIe e...

We present radiometric, palaeoclimatological, palaeoenvironmental and archaeological data for the period 40 000e8000 cal BP in the Jura Mountains (eastern France). These mountains culminate at ~1700 m a.s.l. and are today characterised by a semi-continental climate. During the Last Glacial Maximum, the range supported a local ice cap. While recent...

High-altitude lakes are vulnerable ecosystems that require protection and sustainability management, although their overallfunctioning is still poorly understood. In France protected area managers and scientists are cooperating to address this problem.Their results show the huge diversity of these altitude lakes and imply specificities in their fun...

Long‐term trends, spanning about 150 years, in δ ¹³ C values in the head capsules ( HC s) of three chironomid taxa and pelagic cladoceran exoskeletons are reported for a reoligotrophicated, deep, clear‐water lake using a multidepth approach. The trends were taxon specific, and Bayesian change point analyses defined three homogenous temporal sequenc...

The main drivers of fire regimes in southern Europe are climate, vegetation and land-use changes that interact at different spatio-temporal scales. These complex interplays between “natural” and anthropogenic forcings hinder the identification of fire-climate linkages on the long time scale. In this paper, we focused on the Last Glacial–Holocene tr...

We conducted a rearing experiment with the chironomid species Chironomus riparius to assess the relationship between the δ13C values of chironomid larvae and the δ13C values of their exuvial head capsules. Our experiment was also designed to study the extent of the trophic fractionation factor (Δ13C) under different dietary conditions. Three food s...

1. The specific effects of three types of anthropogenic forcing (nutrient concentrations, climate change and fisheries management practices) on the benthic invertebrate community over the last 150 years in a re-oligotrophicated large, deep subalpine lake were investigated using chironomid remains. The structural changes in the chironomid assemblage...

On the basis of a multi-proxy approach and a strategy combining lacustrine and marine records along a north– south transect, data collected in the central Mediterranean within the framework of a collaborative project have led to reconstruction of high-resolution and well-dated palaeohydrological records and to assessment of their spatial and tempor...

We sampled modern chironomids at multi- ple water depths in Lake Annecy, France, before reconstructing changes in chironomid assemblages at sub-decadal resolution in sediment cores spanning the last 150 years. The lake is a large, deep (zmax = 65 m), subalpine waterbody that has recently returned to an oligotrophic state. Comparison between the wat...

The spatiotemporal distribution of biochemical varves spanning the last 150 yr was investigated using 40 cores collected over a depth gradient in a large subalpine lake—Lake Bourget—in the French Alps. Four-dimensional sedimentological, biological, and geochemical analyses show that varve preservation can be used as a reliable proxy to reconstruct...

High-altitude lakes are vulnerable ecosystems that require protection and sustainability management, although their overall functioning is still poorly understood. In France protected area managers and scientists are cooperating to address this problem. Their results show the huge diversity of these altitude lakes and imply specificities in their f...

On the basis of a multi-proxy approach and a strategy combining lacustrine and marine records along a north–south transect, data collected in the Central Mediterranean within the framework of a collaborative project have led to reconstruction of high-resolution and well-dated palaeohydrological records and to assessment of their spatial and tempora...