Abstract
Well-dated records of alpine glacier fluctuations provide important insights into the temporal and spatial structure of climate variability. Cirque moraine records from the western United States have historically been interpreted as a resurgence of alpine glaciation in the middle-to-late Holocene (i.e., Neoglaciation), but these moraines remain poorly dated because of limited numerical age constraints at most locations. Here we present 130 10Be ages on 19 moraines deposited by 14 cirque glaciers across this region that have been interpreted as recording these Neoglacial advances. Our 10Be chronology indicates instead that these moraines were deposited during the latest Pleistocene to earliest Holocene, with several as old as 14–15ka. Our results thus show that glaciers retreated from their Last Glacial Maximum (LGM) extent into cirques relatively early during the last deglaciation, experienced small fluctuations during the Bølling–Allerød–Younger Dryas interval, and remained within the maximum limit of the Little Ice Age (LIA) advance of the last several centuries throughout most of the Holocene. Climate modeling suggests that increasing local summer insolation and greenhouse gases were the primary controls on early glacier retreat from their LGM positions. We then infer that subsequent intrinsic climate variability and Younger Dryas cooling caused minor fluctuations during the latest Pleistocene, while the LIA advance represents the culmination of a cooling trend through the Holocene in response to decreasing boreal summer insolation.
Highlights
Glaciers are among the most sensitive responders to climate change, and their well-documented retreat over the past century is a clear signal of global warming.[1,2,3] Long records of glacier fluctuations provide an important context for assessing the role of natural variability on this recent glacier retreat, helping to isolate an anthropogenic signal.[4]
Moraine chronologies Moraines sampled for 10Be dating are 15–30 km up-valley from moraines marking the Last Glacial Maximum (LGM) and
Glacial events from ~15 to 13 ka occurred when simulated interglacial summer temperatures in the western U.S were largely established (Fig. 3f) and atmospheric carbon dioxide levels were relatively stable (Fig. 3b). Such glacier variability may reflect forcing by natural modes of multidecadal climate variability that influences the western U.S glaciers.[35]
Summary
Glaciers are among the most sensitive responders to climate change, and their well-documented retreat over the past century is a clear signal of global warming.[1,2,3] Long records of glacier fluctuations provide an important context for assessing the role of natural variability on this recent glacier retreat, helping to isolate an anthropogenic signal.[4]. The numerical ages of these type moraines, were not well constrained, and in the few instances where numerical dating was subsequently applied,[15,16] some of these moraines have instead been attributed to the late-Pleistocene Younger Dryas cold event (12.9–11.7 ka).[17,18,19,20] The ages of most of these particular cirque moraines remain highly uncertain,[21] which has important implications for our general understanding of Holocene climate,[22] glacier change,[23] proposed rapid climate change events during the Holocene,[24] and silicate weathering estimates.[25,26] the presence of Neoglacial moraines beyond the LIA extent would imply a cooler pre-LIA climate, in contrast to regional and hemispheric temperature reconstructions[22] that show a cooling trend through the middle-to-late Holocene culminating with maximum cooling during the LIA
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