Surges of the Black Rapids Glacier tracked climate over the last 600 years

Deposits of surge-type glaciers are widespread in the glacial geologic record; however, it is unclear how climate changes occurring at time scales of decades to centuries affect surge-type glaciers. Here we reconstruct the history of the Black Rapids Glacier (BRG) in the eastern Alaska Range since AD 1400 using a combination of geomorphology, stratigraphy, lichenometry, radiocarbon dating, and dendrochronology. Moraines in the glacier's foreland record four advances, all of which left deposits typical of surging glaciers. A surge in the AD 1600s dammed a lake which drained in an outburst flood ca. AD 1703-04. Another outburst flood from a larger glacier-dammed lake occurred in the AD 1400s. Based on the BRG's observed glaciology and its history over the last several centuries, its surge cycles have varied between 80 and 120 years. Between AD 1400 and 1900, the most extensive surges of the BRG coincided with minima in the Seuss / de Vries solar cycle when non-surging glaciers in the region also advanced. Synchroneity between the BRG, solar minima, and non-surging glaciers is surprising given that the terminus of the BRG was largely unresponsive to climate for 80–120 years between surges. One explanation is that the BRG's surge cycle shortened during the Little Ice Age (LIA, ca. AD 1300–1900) to the point that its climate-response lag resembled that of neighboring, non-surging glaciers. Although the reconstructed chronology of the BRG shows no indication of the surge cycle decreasing during the LIA, fading of the record with time makes it difficult to exclude this possibility. Another explanation is that the BRG's 80- to 120-year cycle is the result of tuning by the solar cycle over the course of millennia. Tuning occurred when quiescent phases that coincided with solar minima were shortened because of faster replenishment of the glacier's reservoir zone. The opposite occurred when the glacier's quiescent phases coincided with solar maxima. The net result was to align the surge cycle of the BRG with solar minima. Some combination of shortened surge cycles during the LIA and tuning by the solar cycle may be why the glacial-geologic record of this particular surge-type glacier provides a surprisingly dependable record of regional climate over the past 600 years.