Deglacial Warming and Wetting of Northern Alaska

Abstract:

Aeolian sand dunes swept across northern Alaska during the last glacial maximum. Today, summer temperatures are moderate and soils can remain waterlogged all summer long. How did the transition from a cold and dry glacial to a warm and wet interglacial take place? To answer this question we reconstructed temperature and precipitation changes during the last deglaciation using biomarker hydrogen isotopes from a new 28,000 year-long sediment core from Lake E5, located in the central Brooks Range of Alaska. We use terrestrial leaf waxes (dD_terr, C28-acid), informed by dD measurements of modern vegetation, to infer dD of precipitation, an indicator of relative temperature change. Biomarkers from aquatic organisms (dD_aq, C18-acid) are used as a proxy for lake water isotopes. The offset between the two (e_terr-aq) is used to infer relative changes in evaporative enrichment of lake water, and by extension, moisture balance.

dD_terr during the last glacial period was -282‰ compared to -258‰ during the Holocene, suggesting a 5.6 ± 2.7 °C increase in summer temperature using the modern local temperature-dD relationship. Gradual warming began at ~18.5 ka, and temperature increased abruptly at 11.5 ka, at the end of the Younger Dryas. Warming peaked in the early Holocene from 11.5 to 9.1 ka, indicating a Holocene thermal maximum associated with peak summer insolation.

The e_terr-aq supports a dry LGM and moist Holocene. Other sediment proxies (TIC, TOC, redox-sensitive elements) support the e_terr-aq, and reveal a shift to more positive P-E beginning around 17 ka, suggesting rising temperature led increases in precipitation during the last deglaciation. Moreover, differing patterns of dD_terr and e_terr-aq during the deglaciation suggest that the relationship between temperature and precipitation changed through time. Such decoupling, likely due to regional atmospheric reorganization as the Laurentide ice sheet waned, illustrates the importance of atmospheric dynamics in controlling Alaskan climate.