Northern Hemisphere Ice sheet forcing on the Holocene climate of Eurasia
Tuesday, 16 December 2014
The Early to Middle Holocene retreat of the Laurentide Ice Sheet has scarcely been explored in the attribution of climate dynamics to explain Eurasian paleoclimate records, despite that areal ice extent remained significant until after ~7 ka. Climate models that incorporate these ice-sheet dynamics predict relatively cold winters in the continental interior of Eurasia prior to 6 ka, in contrast to much warmer summers, which are paced directly by solar insolation and paleoecological feedbacks. With respect to sea-level pressure, these models also predict a weakening of subpolar lows and reduced zonal wind transport in the Subarctic region during winter, characteristic of enhanced meridional circulation and Rossby-wave amplification over the midlatitudes. Utilizing a high-resolution record of winter climate (11.9 ka–Present) from the southern Ural mountains, we test the hypothesis that retreat of the Laurentide Ice Sheet paced Eurasian winter climate into the Middle Holocene. Stable-isotope analysis of two U-Th-dated speleothems from Kinderlinskaya Cave, Russia revealed a long-term increase in δ18O that cannot be explained by direct orbital forcing, solar irradiance, or North Atlantic temperature and circulation dynamics. Instead, we interpret the trend in δ18O to reflect a poleward shift in climate zones and a transition from predominantly meridional to zonal circulation over the continental interior, associated with the downstream impact of continental ice sheets in North America. This interpretation is supported by quantitative comparison of our record to western Eurasian proxies of winter climate and δ18O in precipitation, through which we attempt to describe synoptic-scale shifts in dominant modes of atmospheric circulation. These findings confirm that the climatic impact of continental ice sheets, especially during winter, may persist several millennia into interglacial periods on a hemispheric scale.