ALPINE AND SUBALPINE LANDSCAPE RESPONSE TO POST-GLACIAL CLIMATE CHANGE IN THE SAN JUAN MOUNTAINS: A COMPARISON OF NEW LANDSCAPE AND CLIMATE RECORDS
Abstract
Post-glacial interactions between climate, landscapes, and soils remain poorly understood, especially in alpine and sub-alpine areas. Here, I aim to increase understanding of the dynamic interactions between climate, landscape evolution, and soil development by compiling detailed records of all three. First, pollen assemblages, diatom assemblages, and sedimentology from Cumbres Bog in the southern San Juan Mountains of Colorado provide a record of climate change since the end of the Last Glacial Maximum (LGM, 16-22 ka regionally). Next, geomorphic mapping in the upper Conejos River Valley of the San Juan Mountains provides evidence of incision and aggradation that has occurred since the end of the LGM. Lastly, nineteen soils, examined for particle size, Fe extractions, and organic carbon, provide a chronosequence across multiple parent materials.The Cumbres Bog record provides strong evidence of: cooling during the Younger Dryas (~12.8-11.5 ka), generally warm, stable climate until 6 ka, and cooler, more variable climate after 6 ka. Additionally, pollen ratios and fossil diatoms indicate that cold periods generally match with previously identified periods of rapid climate change and occurred at 10.6, 8.7-7.9, 7.0-6.9, 5.4-5.2, 3.3-3.0, 2.3, 2.0 and 1.5 ka. This record also adds resolution to previous regional records and indicates that the periodicity of climate change changed from 2,000-3,000 years during the interval from 11.5-6 ka to 700-1,100 years for the interval from 6-3.5 ka, then to <500 years after 3.5 ka. These changes correspond with increased El Niño-Southern Oscillation (ENSO) activity after the mid-Holocene (~6 ka).The upper Conejos River Valley appears to have undergone three distinct periods of aggradation. The first occurred during the Pleistocene-Holocene transition (~12.5 - 9.5 ka) and is interpreted as paraglacial landscape response to deglaciation after the LGM. Evidence of the second period of aggradation is limited but indicates a small pulse of sedimentation at ~ 6 ka. A third, more broadly identifiable period of sedimentation occurred in the Late Holocene (~2.2 - 1ka). The latest two periods of aggradation are concurrent with the ENSO related increases to the frequency of climate change. This suggests that Holocene alpine and sub-alpine landscapes respond more to rapid ENSO-driven changes in climate than to large singular climatic swings. More specifically, it is likely that landscapes respond to the strengthened ENSO indicated by increased frequency of climate change. Soil development and radiocarbon dating indicate that hillslopes were stable during the Holocene even while aggradation was occurring in valley bottoms. Thus, we can conclude that erosion does not occur equally throughout the landscape but is focused above headwater streams, along tributary channels, or on ridgetops.Lastly, the soil chronosequences indicate that ratios of oxalate/dithionite Fe extractions exhibit a robust trend with age for all soils. The relationship between extractable iron and time is in contrast with other soil properties, such as reddening, profile thickness, and clay content, which are not good indicators of age. Variation in eolian deposition and parent material sedimentologylikely led to the observed variability in soils of similar age.