Lacustrine Leaf Wax Hydrogen Isotopes Indicate Strong Regional Climate Feedbacks in Beringia Since the Last Ice Age
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 (δ2Hwax) 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.
A new record of Eastern Beringia paleoclimate provides evidence for a relatively mild LGM and a series of abrupt climate transitions during the deglacial period, with rapid warming intervals observed at 19 ka, 14.8 ka, 13.4, and 11.6 ka. Furthermore, data-model comparisons demonstrate that the magnitude of past Arctic amplification in this region has evolved from the last glacial period to today. In particular, whereas muted LGM cooling in Beringia indicates strong ameliorating effects of ice sheet orography and possibly enhanced continentality, the Holocene and modern changes are amplified relative to other Arctic sites, which we speculate is due to surface feedbacks including recent sea ice decline and shrub expansion in conjunction with the removal of factors such as the LIS which dampened regional temperature change during the deglacial. These results predict that Arctic Alaskan temperatures will continue to warm more rapidly in the future than other sectors of the Arctic.
Cold-season temperatures appear to be sensitive to winter insolation and greenhouse gas forcing, while summer temperatures show a stronger relationship with summer insolation. As mentioned previously, ice wedges and lacustrine alkenones are two approaches to investigate past winter conditions in Beringia. Currently, records of these proxies are found from the late glacial through the Holocene, and reconstructions extending through the LGM may help decipher how summer and winter climate drivers differed. Under deglacial boundary conditions, AMOC collapse results in a range of cooling of 1–3 °C in Alaska for a fully-open Bering strait and 0-2 °C when the strait is still closed. Under present-day (0 ka) boundary conditions, the temperature sensitivity to North Atlantic freshwater forcing is approximately double that of the deglacial simulations for both BSO and BSC scenarios, suggesting that a weakened AMOC has a much larger impact on Arctic Alaskan climate when the global climate is warmer. In addition, the prediction that the magnitude of cooling is ∼50% weaker when the Bering Strait is closed, may help explain the weak expression of HS1 in Lake E5 sediments. the isotope-inferred warming at Lake E5, in conjunction with the instrumental observations, indicates that recent temperature increase in northern Alaska exceeds the +1.4 °C pan-Arctic temperature change between the pre-industrial period and today. This amplified warming contrasts with the spatial pattern of temperature change across the deglaciation when Beringian warming was smaller than other parts of the Arctic. The contraction of winter and summer sea ice in the adjacent Chukchi Sea and the expansion of shrubs into low tundra biomes are likely feedbacks amplifying recent warming in northern Alaska.