MIS 3 WAS WETTER THAN YOU MIGHT THINK IN THE NORTHERN ROCKIES

The last glacial cycle (115-11 ka) featured several cool-wet episodes in the northern Rocky Mountains of the United States. While the best-preserved events reflected in CRN moraine chronologies date to MIS 2, chronologies from alluvial, lacustrine, and outburst-flood depositional systems reflect several cool-wet episodes preceding MIS 2. Each episode is documented by multiple landform ages and most appear in multiple geomorphic systems.

Alluvial fans dated with OSL aggraded during MIS 5-2 episodes and correlate with lacustrine and flood events dated via CRN, OSL, and 14C methods. In the Beaverhead Mountains, fans aggraded 110-115 ka, 84–94 ka, 66–74 ka, 43–61 ka, and 16–22 ka. In the Lost River Range, fans aggraded 35-60 ka, 20-35 ka, 10-20 ka, and 0-10 ka (Kenworthy et al., 2014). A Big Lost River alluvial terrace was dated to ca. 38 ka in the same study. The two most recent Big Lost River glacial lake outburst floods, sourced in the Pioneer Mountains, occurred 35 ka and 20 ka, as reflected in 3He ages of flood-transported boulders on the eastern Snake River Plain, despite the source-area moraines dating via 10Be to 20 ka (Warner, 2020). Pleistocene Lake Terreton, the terminus of that system, also displays OSL and 14C-dated highstands ca. 35 ka and 22 ka, and possibly ca. 43 ka (Amidon et al., 2016).

Does the prevalence of non-glacial MIS 5-3 alluvial, flood, and lacustrine events, relative to prevalent MIS 2 glacial events, reflect system sensitivity or preservation bias? We hypothesize that both factors are relevant. Alluvial fans may aggrade and lakes may rise when effective moisture is greater but temperature reduction is modest, while regional glacial systems may respond more strongly to pronounced cooling. Geomorphic overprinting in glacial systems also may favor the chronology of later events. Isolated moraine boulder ages and the Pioneer Mountains flood chronology suggest that glaciers during late-MIS 3 events were of near-similar extent to those during MIS 2 events.