Cosmogenic chlorine-36 constraints on Holocene glacier change in Iceland

Understanding temperature variability during the Holocene is critical for constraining baselines of natural climate variability. Temperate mountain glacier extent is limited most significantly by summer air temperature, thus geological records of past glacier length changes represent a useful proxy for this climatic variable. Iceland’s maritime glaciers with their high sensitivity to temperature and precipitation changes serve as robust indicators of climate variability in the North Atlantic region. Previous reconstructions of Iceland’s Holocene glacier and climate history have relied primarily on marine sediment cores, terrestrial geomorphological evidence, and glaciological modelling. These proxies highlight a correlation between glacier fluctuations and regional climate variability and suggest notable glacier retreats during early and mid-Holocene warm periods.

Here, we present cosmogenic chlorine-36 measurements from four outlet glaciers of the Vatnajöküll ice cap in Iceland that test and further constrain the occurrence of past glacier minima during the Holocene. Unlike the more commonly used method of cosmogenic surface exposure dating of moraines, which constrains the timing of past glacier advances, our application targets the remnant cosmogenic signals of prehistoric exposure events preserved in freshly exposed proglacial surfaces. Our data thus tests for the occurrence and constrains the duration of past glacier retreat events and, thereby, warmer times during the Holocene. Our results support the hypothesis that Icelandic glaciers were smaller than present for several millennia during the Holocene and when combined with existing datasets of Icelandic climate, our new results allow us to reconstruct both glacier advance and retreat through the Holocene.