Laurent Millet

Validation of climate model-inferred regional temperature change for late-glacial Europe

Comparisons of climate model hindcasts with independent proxy data are essential for assessing model performance in non-analogue situations. However, standardized paleoclimate datasets for assessing the spatial pattern of past climatic change across continents are lacking for some of the most dynamic episodes of Earths recent past. Here we present a new chironomid-based paleotemperature dataset designed to assess climate model hindcasts of regional summer temperature change in Europe during the late-glacial and early Holocene. Latitudinal and longitudinal patterns of inferred temperature change are in excellent agreement with simulations by the ECHAM-4 model, implying that atmospheric general circulation models like ECHAM-4 can successfully predict regionally diverging temperature trends in Europe, even when conditions differ significantly from present. However, ECHAM-4 infers larger amplitudes of change and higher temperatures during warm phases than our paleotemperature estimates, suggesting that this and similar models may overestimate past and potentially also future summer temperature changes in Europe.

High-resolution paleolimnology opens new management perspectives for lakes adaptation to climate warming

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 in Europe (Lakes Geneva, Annecy and Bourget). The dynamics of the dominant, local human pressures, as well as the ecological responses in the pelagic, benthic and littoral habitats were reconstructed using classical and newly developed paleo-proxies. Statistical modelling achieved the hierarchization of the drivers of their ecological trajectories. All three lakes underwent different levels of eutrophication in the first half of the XXth century, followed by re-oligotrophication. Climate warming came along with a 2°C increase in air temperature over the last century, to which lakes were unequally thermally vulnerable. Unsurprisingly, phosphorous concentration has been the dominant ecological driver over the last century. Yet, other human-influenced, local environmental drivers (fisheries management practices, river regulations) have also significantly inflected ecological trajectories. Climate change has been impacting all habitats at rates that, in some cases, exceeded those of local factors. The amplitude and ecological responses to similar climate change varied between lakes, but, at least for pelagic habitats, rather depended on the intensity of local human pressures than on the thermal effect of climate change. Deep habitats yet showed higher sensitivity to climate change but substantial influence of river flows. As a consequence, adapted local management strategies, fully integrating nutrient inputs, fisheries management and hydrological regulations, may enable mitigating the deleterious consequences of ongoing climate change on these ecosystems.

Lateglacial paleoenvironmental reconstruction using subfossil chironomid assemblages from Lake Lautrey (Jura, France)

Chironomid assemblages were studied in Lake Lautrey (Jura, France) along a 2.50 m long core spanning the whole Lateglacial period. A stratigraphical constraint cluster analysis showed 7 biozones and 13 sub-biozones within the chironomid biostratigraphy. Characteristic taxa were allocated to each of the 7 biozones by simple analysis of the relative abundance diagram whereas several Correspondence Analyses (C.A.) were needed for the remaining 13 sub-biozones. Characteristic taxon compositions were interpreted using available quantitative inference models for total phosphorus concentration ([T.P.]), chlorophyll-a concentration ([Ch1-a]), hypolimnetic oxygen conditions, and water temperature. Optimum ranks of the characteristic taxa against the environmental factors provided by these inference models were used to deduce an evaluation of environmental conditions. Paleoenvironmental interpretations of chironomid assemblage changes showed the Lateglacial climatic event succession (Oldest Dryas, Bolling/Allerod Interstadial, and Younger Dryas) commonly recognized in the area. Moreover, a bipartition of the Oldest Dryas, the occurrence of two minor oscillations during the Bolling/Allerod Interstadial and a trend in environmental changes (climatic improvement) during the Younger Dryas were also found.

Stacking of discontinuous regional palaeoclimate records: Chironomid-based summer temperatures from the Alpine region

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 records are necessary for developing continuous climate records over time scales longer than covered by individual sequences. Here, we present a procedure for developing such reconstructions based on relatively short (centuries to millennia), discontinuously sampled records as are typically developed when using biotic proxies in lake sediments for temperature reconstruction. The approach includes an altitudinal correction of temperatures, an interpolation of individual records to equal time intervals, a stacking procedure for sections of the interval of interest that have the same records available, as well as a splicing procedure to link the individual stacked records into a continuous reconstruction. Variations in the final, stacked and spliced reconstruction are driven by variations in the individual records, whereas the absolute temperature values are determined by the stacked segment based on the largest number of records. With numerical simulations based on the NGRIP δ18O record, we demonstrate that the interpolation and stacking procedure provides an approximation of a smoothed palaeoclimate record if based on a sufficient number of discontinuously sampled records. Finally, we provide an example of a stacked and spliced palaeotemperature reconstruction 15000–90 calibrated 14C yr BP based on six chironomid records from the northern and central Swiss Alps and eastern France to discuss the potential and limitations of this approach.

Long-term dynamics in microbial eukaryotes communities: a palaeolimnological view based on sedimentary DNA

Assessing the extent to which changes in lacustrine biodiversity are affected by anthropogenic or climatic forces requires extensive palaeolimnological 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 two lakes (Lake Bourget in French Alps and Lake Igaliku in Greenland). From 176 samples, we sequenced a large diversity of microbial eukaryotes, with a total 16 386 operational taxonomic units distributed within 50 phylogenetic groups. Thus, microbial groups, such as Chlorophyta, Dinophyceae, Haptophyceae and Ciliophora, that were not previously considered in lacustrine sediment record analyses appeared to be potential biological markers of trophic status changes. Our data suggest that shifts in relative abundance of extant species, including shifts between rare and abundant taxa, drive ecosystem responses to local and global environmental changes. Community structure shift events were concomitant with major climate variations (more particularly in Lake Igaliku). However, this study shows that the impacts of climatic fluctuations may be overpassed by the high‐magnitude eutrophication impacts, as observed in the eutrophicated Lake Bourget. Overall, our data show that DNA preserved in sediment constitutes a precious archive of information on past biodiversity changes.

Tracking past mining activity using trace metals, lead isotopes and compositional data analysis of a sediment core from Longemer Lake, Vosges Mountains, France

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. Atmospheric contamination displays a pattern that is similar to that seen in peat bogs from the region, at least until the eighteenth century. Thereafter, the lake sediment record is more precise than peat records. Some regional mining activity, such as that in archaeologically identified eighteenth-century mining districts, was detected from the lead isotope composition of sediment samples. Compositional data analysis, using six trace metals (silver, arsenic, cadmium, copper, lead and zinc), enabled us to distinguish between background conditions, periods of mining, and of other anthropogenic trace metal emissions, such as the recent use of leaded gasoline.

Evolution des assemblages de Chironomidae (Insecta: Diptera) pendant le Tardiglaciaire dans le Lac du Lautrey (Jura, France): essai de reconstruction paléoenvironnementale

The study of subfossil Chironomidae assemblages from a 2.50 m long core spanning the entire Late-glacial period in Lake Lautrey (Jura, France) displays the main climatic events of this interval, i.e. Oldest Dryas, Bolling/Allerod Interstadial and Younger Dryas. Each climatic event was characterized by a particular chironomid community associated with a corresponding trophic state of the lake. Oligotrophic conditions prevailed during the Oldest Dryas, whereas a strong eutrophication characterized the Bolling/Allerod Interstadial. Moreover, the chironomid biostratigraphy appears not monotonous during this Interstadial. Two large developments of macrophyte belt were proposed at the beginning of the Bolling and the end of the Allerod, respectively. Furthermore, from their particular chironomid fauna two short intervals were identified displaying comparatively lower organic content. These intervals correspond to the Older Dryas and the Intra Allerod Cold Phase (I.A.C.P.). The beginning of the Younger Dryas is shown by a strong and rapid change in chironomid assemblages. The pelagic primary production appeared much reduced whereas the benthic production was rather favored. The progressive change in chironomid assemblages and specially the decline of C. ambigua (Zetterstedt) during the rest of the Younger Dryas, is interpreted as an improvement of the environmental conditions.