Late-Holocene summer temperature reconstruction from chironomid assemblages of Lake Anterne, northern French Alps

We present a chironomid-based reconstruction of late-Holocene temperature from Lake Anterne (2060 m a.s.l.) in the northern French Alps. Chironomid assemblages were studied in 49 samples along an g m long sediment core covering the last 1800 years. July air temperatures were inferred using an inference model based on the distribution of chironomid assemblages in 100 Swiss lakes. The transfer function has a leave-one-out cross-validated coefficient of determination (r(2)) of 0.88, a root mean square error of prediction 1.40 degrees C. Despite possible biases induced by methodological aspects and the ecological complexity of the chironomid response to both climate and environmental changes, the concordance of the Lake Anterne temperature reconstruction with other Alpine records suggests that the transfer function has successfully reconstructed past summer temperature during the last two millennia. The twentieth century is the only section of the record which shows a poor agreement with other climate reconstructions and the distinct warming found in most instrumental records for this period is not apparent in the Lake Anterne record. Stocking of the lake with fish from the early twentieth century onwards was found to be a possible cause of changes in the chironomid fauna and subsequent distortion in the inferred climate signal. Evidence was found of a cold phase at Lake Anterne between AD 400 and 680 a warm episode between AD 680 and 1350, and another cold phase between AD 1350 and 1900. These events were possibly correlated to the so-called 'Dark Age Cold Period' (DACP) the 'Mediaeval arm Period' (MWP) and the 'Little Ice Age' (LIA). The chironomid-based inference model reconstructed a July air temperature decrease of c. 0.7 degrees C for the DACP and 1.3 degrees C for the LIA compared with the temperature prevailing during the MWP.