Paleoclimate reconstructions based on biological proxies present methodological challenges, especially during non-analog conditions, such as the early Holocene. Here, two chironomid-based training sets from Finland were amalgamated to create a more accurate transfer function of summer air temperature. The aim was to reconstruct Holocene paleoclimate in northernmost Lapland, in an area that has been either too warm or too cold for reliable reconstructions using the original calibration models. The results showed that the combined calibration model had improved performance statistics. The temperature trends inferred from the downcore chironomid record using the original and combined models were very similar. However, there were major changes in their absolute values with the combined model showing greatly improved accuracy. The chironomid-based temperature reconstruction showed significant correlation with the previous pollen-based reconstructions from northwestern Finnish Lapland. However, differences were observed in the temperature trends of the early Holocene, when the chironomid-inferred temperatures rapidly increased, but the pollen-based reconstructions lagged behind suggesting that a cool climate continued for much longer. However, similar to the chironomid record, new plant macrofossil evidence from northwestern Finland also showed warmer-than-present early Holocene temperatures. Therefore, we conclude that the early Holocene was probably warm in northern Lapland.

Fossil remains of larval chironomids are one of the most useful paleolimnological proxies to reconstruct Holocene paleoclimate. We investigate the within-lake distribution of chironomids in relation to water temperature and test the usability of intralake calibration in chironomid-based temperature reconstructions. We compare our findings against the traditional multilake calibration technique in Holocene sediment cores from Finland. The results show that intralake water temperature optima of common taxa correspond closely with mean July air temperature optima in a regional multilake dataset, with exception of a few vegetation-associated taxa. A calibration model based on intralake water temperatures shows performance statistics that correspond to the general performance statistics level of multilake calibration models. However, the intralake model has somewhat increased prediction error. Despite the fact that the intralake model is sensitive to poor modern analogues, it shows correlation with the results using the multilake calibration approach in the Holocene reconstructions. Our results confirm that the within-lake distribution of chironomids along a water depth gradient is closely linked with water temperature gradient. This provides evidence for the close relationship between chironomids and temperature in general and suggests that, in addition to the regional scale, water temperature is a key variable in determining chironomid distribution at the site-specific scale. However, long-term changes in intralake gradients in other depth-related factors, such as hypolimnetic oxygen conditions, extent of photic zone and habitat distribution, may hamper the quantitative accuracy of chironomid-based temperature reconstructions, though the results also suggest that these changes are likely to have little general impact on the trends of chironomid-based air temperature reconstructions.

PAN-EURASIAN EXPERIMENT (PEEX) PROGRAM-TOWARDS ARCTIC-BOREAL SYSTEM UNDERSTANDING Pan-Eurasian Experiment (PEEX) program (https://www.atm.helsinki.fi/peex/) is an international, multidisciplinary, multiscale and multidimensional bottom up initiative established in 2012. The initiative has grown fast and currently it involves research communities from 25 different countries with a network of approximately 2000 researchers from Europe, Russia and China. The focus of the PEEX initiative is to solve interlinked global environmental challenges influencing societies in the Northern Eurasian region, specifically in the Arctic-boreal regions and the Arctic Ocean, which are located at latitudes higher than 45°N. These areas are expected to undergo substantial changes during the next decades (IPCC, 2014). The importance of the Northern regions even in a global point of view is foreseen to increase not only because of the climate change, but also due to globalization, shipping, demography and utilization of natural resources. The Arctic-boreal Northern Eurasian region, and especially the arctic coastal lines and Siberian region of the Russian territory, are extremely crucial for and sensitive to the global climate. Permafrost thawing together with the Arctic sea ice changes will have multiple environmental (greenhouse gas emissions, air quality), economic (energy production, use of mineral, traffic and shipping and infrastructures) and societal (urbanization, cultural changes) consequences, which are intricately interconnected with each other. In the PEEX approach the climate change is key driver in the dynamics of the land, atmosphere, aquatic and societal systems. The system-based structure of PEEX introduces altogether twelve thematic research areas. The approach will piece-by-piece develop into a holistic system understanding, which the PEEX community and stakeholders can quantify the most dominant feedbacks and interactions between the components within the system providing novel understanding in the dynamics of Arctic-boreal biogeochemical cycles of e.g. water, carbon, nitrogen