Background/Question/Methods Alpine and arctic lakes are sentinel ecosystems with respect to climate-change impacts, owing to the combination of low temperatures, nival hydrological regimes, and simplified trophic structures. Here we present detailed sedimentary evidence from a number of ‘pristine' lake ecosystems, for the transition into a geologic Stage delineated by human impacts on global biogeochemical cycles. The change in sediment source to these lakes with the recession of local glaciers, provides compelling sedimentological evidence of the Holocene – Anthropocene (H-A) transition. We synthesize geochemical evidence for the deposition of atmospheric nitrogen from lake sediments and tree rings. Finally, we compile 52 diatom-based paleolimnological records from North America and west Greenland, including alpine (n=15) and arctic (n=20) ecosystems, as well as a range of boreal-montane sites (n=17) as ‘controls'. For all of these sites, diatom compositional turnover (beta-diversity) was analyzed over the last ~450 years, as means to quantify the ecological change over the H-A boundary.

Results/Conclusions Collectively, our results show that many remote lakes, once considered pristine, have recorded the presence of the H-A transition. A more detailed analysis of the diatom paleoecology using a multivariate consideration of available environmental variables, reveals that climate and N deposition are acting synergistically and together exert a dominant influence on diatom assemblage turnover. Regional temperature increases during the 20^th century correspond to a latitudinal trend of increasing diatom beta-diversity in arctic lakes, whereas a southward increase of anthropogenic N deposition suggests that biogeochemical impacts are most pronounced in mid-latitude alpine lakes. We predict that these lakes will continue to shift towards new ecological states in the Anthropocene, as these two dominant forcings begin to intersect geographically.