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Reconstructing past climate seasonality is fundamental to understanding the nature of past climate changes. This is especially true in the Arctic, where climate is intensely seasonal and proxies that can distinguish climate conditions of multiple seasons in a single year are relatively rare. We propose that submerged aquatic mosses, which are abundant subfossils in some Arctic lake sediments and have distinctive seasonal growth morphologies, can be used to estimate past lake water oxygen isotope composition (δ18O lw) across multiple seasons. Aquatic mosses are abundant, well preserved, and grow continuously in Arctic lakes whenever light is available, with some species displaying unique seasonal morphologies influenced by water temperature. Although Greenland paleorecords suggest that aquatic moss oxygen isotope values (δ18O om) reflect the δ18O values of lake water, no modern calibration between δ18O om and δ18O lw exists in Greenland. We present a modern δ18O om vs. δ18O lw calibration using multiple moss species or morphotypes from eight lakes and ponds near Pituffik (Thule), northwest Greenland. We find strong linear relationships (r2 = 0.76-0.85) between the δ18O om and δ18O lw values of multiple species or morphotypes across the range of relatively low δ18O lw values at Pituffik, and our results indicate isotopic fractionations are similar to those found previously at lower latitudes. To assess the potential of mosses as archives of seasonal δ18O lw values, we analyzed δ18O om in season-specific segments of moss strands, with seasons identified based upon growth morphology. Moss inferred lake water δ18O values (δ18O lwom) are higher in autumn than spring or summer, likely due to increasing contributions of 18O enriched precipitation and the cumulative effects of lake water evaporation throughout the ice-free season. For moss subsampled throughout summer, δ18O lwom values generally increased through the season in parallel with observed δ18O lw values. Potential temperature dependent fractionation effects during biosynthesis, however, remain unconstrained and should be further addressed with future research. Overall, these findings suggest that aquatic mosses from lake sediments could be used to directly resolve climate seasonality of the past. • Aquatic moss strand δ18O values reflect lake water values through multiple seasons. • Summer subsampled moss strands show weekly to monthly evolution of lake water δ18O. • Aquatic moss vs. lake water δ18O calibrations are similar for species tested here. • Aquatic moss δ18O needs testing down core to evaluate past seasonal signals. • Temperature-dependent fractionation and pre-treatment effects need more research. [ABSTRACT FROM AUTHOR]

The Late-Quaternary climate of Beringia remains unresolved despite the region's role in modulating glacial-interglacial climate and as the likely conduit for human dispersal into the Americas. Here, we investigate Beringian temperature change using an ∼32,000-year lacustrine record of leaf wax hydrogen isotope ratios (δ2H wax) from Arctic Alaska. Based on Monte Carlo iterations accounting for multiple sources of uncertainty, the reconstructed summertime temperatures were ∼3 °C colder (range: −8 to +3 °C) during the Last Glacial Maximum (LGM; 21-25 ka) than the pre-industrial era (PI; 2–0.1 ka). This ice-age summer cooling is substantially smaller than in other parts of the Arctic, reflecting altered atmospheric circulation and increased continentality which weakened glacial cooling in the region. Deglacial warming was punctuated by abrupt events that are largely synchronous with events seen in Greenland ice cores that originate in the North Atlantic but which are also controlled locally, such as by the opening of the Bering Strait between 13.4 and 11 ka. Our reconstruction, together with climate modeling experiments, indicates that Beringia responds more strongly to North Atlantic freshwater forcing under modern-day, open-Bering Strait conditions than under glacial conditions. Furthermore, a 2 °C increase (Monte Carlo range: −1 to +5 °C) over the anthropogenic era reverses a 6 °C decline (Monte Carlo range: −10 to 0 °C) through the Holocene, indicating that recent warming in Arctic Alaska has not surpassed peak Holocene summer warmth. • Alaska's climate history is evaluated using leaf wax hydrogen isotope ratios in a 32,000 year-long lacustrine record. • The record reveals mild ice-age cooling, abrupt deglacial climate events, strong Holocene cooling, and recent warming. • Proxy-model comparison highlights the importance of the Bering Strait opening as a control of Beringian climate. [ABSTRACT FROM AUTHOR]

Human paleodemographic changes were related to environmental fluctuations for the North American Arctic and boreal region, including Greenland. Using the frequency of archaeological radiocarbon dates as a proxy for population size, past changes in population were estimated and quantitatively examined in relation to reconstructions of temperature and sea ice conditions. This analysis was conducted across three spatial scales in order to better identify potential climate impacts on population size: the entire area, four major cultural-environmental regions and sixteen sub-regions. The timing of initial settlement differed by region, and population size fluctuated through time, but there was nevertheless an overall, accelerating increase in most areas. There was a high correspondence between millennial and centennial-scale climate variability and paleodemographic changes across the region, with population size generally increasing during warmer periods and decreasing during cooling episodes. Late Holocene cooling (neoglaciation) triggered a nearly-synchronous population decline across the entire region at 3.9 ka, a time when significant societal disruptions have been identified around the world, and are here shown in the Arctic. Decreasing temperatures and increased sea ice coverage also influenced large-scale migration patterns of Paleo-Inuit peoples as well as their cultural evolution. The growth of Paleo-Inuit populations after 3 ka during continued climate cooling is consistent with archaeological evidence suggesting Paleo-Inuit technological change enabled adaptations to increased sea ice. • Multi-scale analysis of human-environment interactions discerns local and regional population dynamics. • Climate changes caused increases and decreases in population in the Arctic and Subarctic. • Neoglaciation triggered continental-scale population decline across the north. • Population density underwent long-term growth despite climate-induced regional decreases. [ABSTRACT FROM AUTHOR]