A 20,000-yr-old record of vegetation and climate from Lower Red Rock Lake, Centennial Valley, Southwestern Montana
Mumma, Stephanie Ann
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A ca. 20,000-yr-old sediment core from Lower Red Rock Lake (LRRL) in the Centennial Valley of southwest Montana was obtained to reconstruct the vegetation, climate, and environmental changes during and following the late-Pinedale Glaciation in the northern Rocky Mountains. The base of core LRRL 06P49 consisted of inorganic silt and clay, deposited when a large glacial lake occupied the eastern Centennial Valley. The glacial lake receded during the late-glacial period when its western outlet stabilized. Prior to 17,000 cal yr BP, high pollen percentages of Juniperus, Poaceae, Asteraceae, and other herbs as well as low pollen accumulation rates suggest cold dry conditions. The sparsely vegetated landscape at LRRL is consistent with paleoclimate model simulations that show southward displacement of the jet stream and a strong glacial anticyclone during the full-glacial period. Between 17,000 and 10,500 cal yr BP, increases in Picea and Abies pollen percentages suggest a shift to subalpine parkland and warmer conditions than before as result of the northward shift of the jet stream and increasing summer insolation. From 10,500 to 7100 cal yr BP, pollen percentages of Picea and Abies decreased and those of xerophytic taxa (e.g., Chenopodiaceae and Pseudotsuga) increased, suggesting development of steppe and open forest. Warmer drier conditions in the early Holocene were likely a response to increased summer insolation and a strengthened Pacific subtropical high-pressure system. From 7100 to 2400 cal yr BP, cooler and moister conditions at LRRL, driven by decreasing summer insolation, led to the expansion of high-elevation conifers, steppe, and wetlands. Increases in Picea and Abies pollen percentages after 2400 cal yr BP indicate further closing of forests at high elevations and even cooler and wetter conditions than before. Examination of the first arrival of Pseudotsuga in pollen records throughout the region shows that its arrival was later at sites on the Atlantic side of the Continental Divide as compared to sites on the Pacific side. The geographic pattern suggests that the Continental Divide posed a topographic or climate barrier in the late-glacial period, delaying the migration of Pseudotsuga menziesii from glacial refugia.