Climate on the Blanca Massif, Sangre de Cristo Mountains, Colorado, USA, during the Last Glacial Maximum

Abstract

Temperature-index modeling is used to determine the magnitude of temperature depression on the Blanca Massif, Colorado, required to maintain steady-state mass balances of nine reconstructed glaciers at their extent during the Last Glacial Maximum (LGM). The mean temperature depression thus determined is ~8.6 +0.7/−0.9 °C where the uncertainties account for those inherent in the glacier reconstructions, in model parameters (e.g., melt factors), and possible modest changes in LGM precipitation. Associated equilibrium-line altitudes (ELAs) exhibit a statistically significant directional dependency being lower toward the north and east. Under the assumption that regional temperature change was uniform, required changes in precipitation vary systematically—also exhibiting a directional dependency coinciding with that in ELAs—and indicate increases (over modern) occurred on the eastern side of the massif while decreases occurred on the western side. This disparity represents a strengthening of a precipitation asymmetry, particularly winter precipitation, which exists today. The modern precipitation asymmetry may be a consequence of snow being blown over to the eastern side of the massif (advective transport) by southwesterly flow. Intensification of this flow during the LGM would have enhanced advection, and augmented snow accumulation on glaciers, thus explaining the lower ELAs and increased precipitation on that side of the massif.

Clumped Isotopes Record a Glacial‐Interglacial Shift in Seasonality of Soil Carbonate Accumulation in the San Luis Valley, Southern Rocky Mountains, USA

“…In the simulation designated “PR Low T,” we used daily air temperatures lowered uniformly by 7°C to simulate the effect of Last Glacial Maximum (LGM) temperature depression; however, for simplicity, we used measured precipitation as input for soil moisture recharge, as in the PR simulation. This amount of temperature depression was chosen because it is between the preferred temperature depressions derived from glacial mass balance modeling in the nearby Sangre de Cristo mountains (∼5°C: Leonard et al., 2017; ∼9°C: Brugger et al., 2021). In the simulation designated “MR Low T,” we used 7°C lower daily air temperature and modern precipitation but instead used the degree day snowmelt model to simulate snowpack and melt as input for soil moisture recharge in HYDRUS 1‐D.…”

“…Abundant paleoclimate evidence, reconstructed using mountain glacier, lake sediment, pollen and vegetation records, as well as paleoclimate modeling, indicate that the late Pleistocene was dramatically cooler than Holocene and present‐day conditions in western North America (Lora et al., 2017; Osman et al., 2021; Oster et al., 2015). For the southern Rocky Mountains just north of the study area, records of mountain glaciation coupled with glacier mass balance modeling indicate 5–9°C lower mean annual temperature during the LGM interval between ∼19 and 26 ka (Brugger et al., 2019, 2021; Dühnforth & Anderson, 2011; Leonard et al., 2017). This contrasts with the comparison of T (∆ 47 ) results between late Pleistocene samples and Holocene samples in our study sites, which show only ∼2°C lower glacial site average soil carbonate accumulation temperatures compared to Holocene sites, with largely overlapping ranges (Figures 5–7; Table 3).…”

“…Clumped isotope (∆ 47 ) thermometry has recently provided an additional novel paleotemperature indicator (Ghosh et al., 2006; Huntington & Petersen, 2023; Huntington et al., 2009), and has increasingly been applied to reconstructing late Pleistocene‐Holocene climate change from carbonate archives (e.g., mollusk shells, Eagle et al., 2013; lake tufas, Hudson et al., 2017; Santi et al., 2020; soil carbonates, Lechler et al., 2018; Lopez‐Maldonado et al., 2023). Recent work in the Rocky Mountains, comparing the magnitude of temperature shifts associated with the last glacial‐interglacial transition, highlights regional differences in temperature change reconstructed using numerical mass balance modeling of mountain glaciers at their glacial maximum extents, and disagreement between these glacial model‐based estimates and those from global and regional climate model simulations (Brugger et al., 2019, 2021; Leonard et al., 2017). Paleotemperature records based on ∆ 47 data from carbonates, like those presented in this work, have the potential to provide additional estimates from sites at lower elevations and/or latitudes that can further test data‐model agreement and improve understanding of past climate changes.…”

Latest Pleistocene glacial chronology and paleoclimate reconstruction for the East River watershed, Colorado, USA

“…In the Mosquito Range, temperature depressions of approximately −7.5 to −8°C were required to maintain maximum ice extents during the last glacial (Brugger et al, 2019a). In the Sangre de Cristo to the south, Leonard et al (2017) and Brugger et al (2021) found Pinedale maximum glaciers required approximately −5 and −8.5°C temperature changes relative to present, respectively-possibly underscoring the importance of 1). The data suggest variable initial deglaciation ages from maximum positions (i.e., normalized elevation and length = 1), followed by relatively consistent regional retreat to less than 20% maximum lengths and to high elevations by ca.…”

“…These studies broadly indicate that mountain glaciers in Colorado reached their maximum extents near the end of or shortly after the global last glacial maximum (LGM; 19.0–26.5 ka; Clark et al, 2009) followed by several thousand years of punctuated deglaciation, and, in some cases, minor re-advances, which is similar to results from other parts of the Rocky Mountains (e.g., Licciardi and Pierce, 2008, 2018; Quirk et al, 2020, 2022) and western North America (Laabs et al, 2020, and references therein). The glacier climate reconstructions available for the Rocky Mountains of Colorado (Brugger, 2010, 2019a, b; Leonard et al, 2017; Brugger et al, 2021) generally suggest temperature depressions of 7–9°C colder than present and only modest changes in precipitation. However, compared to other ranges in the Colorado Rocky Mountains, the Elk Range (Fig.…”