Delayed and rapid deglaciation of alpine valleys in the Sawatch Range, southern Rocky Mountains, USA

10 We quantify retreat rates for three alpine glaciers in the Sawatch Range of the southern Rocky Mountains following the Last Glacial Maximum using Be ages from ice-sculpted, valley-floor bedrock transects and statistical analysis via the BACON program in R. Glacier retreat in the Sawatch Range from at (100%) or near (~83%) Last Glacial Maximum extents initiated between 16.3 and 15.6 ka and was complete by 14.2 15 – 13.7 ka at rates ranging between 9.9 and 19.8 m a. Deglaciation in the Sawatch Range commenced ~2 – 3 kyr later than the onset of rising global CO2, but approximately in-step with rising temperatures observed in the North Atlantic region at the Heinrich Stadial 1/Bølling transition. Our results highlight a possible teleconnection between the North Atlantic sector and the southern Rocky Mountains. However, 20 deglaciation in the Sawatch Range also approximately aligns with the timing of Great Basin pluvial lake lowering. Recent data-modeling comparison efforts highlight the influence of the large North American ice sheets on climate in the western United https://doi.org/10.5194/gchron-2020-13 Preprint. Discussion started: 12 May 2020 c © Author(s) 2020. CC BY 4.0 License.

States, and we hypothesize that recession of the North American ice sheets may have influenced the timing and rate of deglaciation in the Sawatch Range. While we cannot 25 definitively argue for exclusively North Atlantic forcing or North American ice sheet forcing, our data demonstrate the importance of regional forcing mechanisms on past climate records.

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Alpine glaciers worldwide underwent substantial retreat in response to climate warming during the last deglaciation (Shakun et al., 2015;Palacios et al., 2020).
However, the general trend of warming through the last deglaciation was interrupted by internally forced and regionally heterogeneous climate changes such as the cool Heinrich Stadial 1 (17.5 -14.7 ka), abrupt warming into the Bølling-Allerød period (14.7 35 -12.9 ka), and the Younger Dryas cold period (12.9 -11.7 ka) all centered in the North Atlantic region (NGRIP members, 2004;Rasmussen et al., 2014). To thoroughly characterize the influence of these climatic oscillations, their expression throughout the Northern Hemisphere is often investigated using records of mountain glaciation (Ivy-Ochs et al., 2006;Schaefer et al., 2006;Young et al., 2011;Shakun et al., 2015; 40 Marcott et al., 2019;Young et al., 2019). Mountain glacier deposits serve as suitable archives since mountain glaciers are particularly sensitive to changes in climate (e.g. Oerlemans, 2005;Roe et al., 2017). Furthermore, where deposits are carefully mapped and dated, quantitative retreat or thinning rates of glaciers can be compared to records of climatic forcings. Using statistical approaches to quantify retreat and thinning rates 45 has been previously applied to ice sheets (e.g., Johnson et al., 2014;Koester et al., https://doi.org/10.5194/gchron-2020-13 Preprint. Discussion started: 12 May 2020 c Author(s) 2020. CC BY 4.0 License. 2017; Lesnek et al., 2020) but only for a few mountain glaciers (e.g., Hofmann et al, 2019).
In the western United States (US; Fig. 1), mountain glaciers expanded out of the high elevations of the Rocky Mountains, the Sierra Nevada, the Uinta Mountains, and 50 many other, smaller ranges during the Last Glacial Maximum (LGM; Porter et al., 1983;Pierce, 2003). During the last deglaciation, many glaciers retreated from their extended LGM positions and eventually melted from their cirques by the start of the Holocene (e.g., Marcott et al., 2019). Yet, the temporal and spatial patterns of retreat throughout the western US and their relationship to hemispheric and global forcing are still a 55 subject of debate. Glaciers in the western US may have retreated in response to rising global atmospheric CO2 concentrations, thus broadly synchronous with other mountain glaciers around the world (e.g. Shakun et al., 2015;Marcott et al., 2019). However, some evidence suggests a delay of deglaciation until the Bølling due to either persistent stadial conditions (e.g., Young et al., 2011) or as a response to increased local moisture 60 supply to some glaciers from nearby pluvial lakes (e.g. Laabs et al., 2009).
Over a decade of work has resulted in detailed moraine chronologies in three adjacent alpine valleys in the Sawatch Range of central Colorado (Fig. 2;Briner, 2009;Young et al., 2011;Shroba et al., 2014;Leonard et al., 2017b;Schweinsberg et al., 2020). While these studies primarily focused on mapping and dating the range-front 65 moraines and associated outwash terraces, a transect of ages from bedrock samples in Lake Creek valley (Fig. 2) documented rapid retreat between 15.6 ± 0.7 ka and 13.7 ± 0.2 ka (Leonard et al., 2017b;Schweinsberg et al., 2020). Schweinsberg et al. (2020) suggested a possible link between North Atlantic climate forcing and the rapid https://doi.org/10.5194/gchron-2020-13 Preprint. Discussion started: 12 May 2020 c Author(s) 2020. CC BY 4.0 License. deglaciation observed in Lake Creek valley, but similar transects from adjacent valleys 70 are lacking to bolster or refute this hypothesis. LGM ice limits from Dalton et al. (2020). Inset is of the western portion of North America. CIS = Cordilleran Ice Sheet, LIS = Laurentide Ice Sheet. https://doi.org/10.5194/gchron-2020-13 Preprint. Discussion started: 12 May 2020 c Author(s) 2020. CC BY 4.0 License.
Here, we combine 12 new cosmogenic 10 Be exposure ages with ten previously 80 published 10 Be ages from bedrock samples along transects in three adjacent alpine valleys in the Sawatch Range, southern Rocky Mountains (Fig. 2). By dating bedrock sites along valley transects, we characterize the timing and pace of glacier retreat during the last deglaciation. We calculate rates of deglaciation for each valley with bestfit time-distance plotting using the R program BACON (Fig. 4). Our results suggest that 85 glaciers in the Sawatch Range may have been influenced more heavily by regional forcing than by global CO2 concentrations.

Setting
The high peaks of south-central Colorado and northern New Mexico compose 90 the southern end of the Rocky Mountain Range in North America and were home to many alpine glaciers during multiple glaciations throughout the Pleistocene ( Fig. 1; Pierce, 2003;Leonard et al., 2017b;Marcott et al., 2019). Transects of 10 Be ages from bedrock along valley axes exist for a few valleys in the upper Boulder Creek drainage in the Front Range, Colorado (Benson et al., 2004;Ward et al., 2009;Dühnforth and 95 Anderson, 2011). While some evidence from the Boulder Creek drainages may suggest delayed deglaciation, chronologic scatter in the ages makes it difficult to determine the exact timing and how quickly glaciers retreated to their cirques. Existing ages from one valley the Sangre de Cristo Range, south-central Colorado, suggest that a glacier there remained near its LGM terminus until ~16 ka, but then retreated to its cirque in a period 100 of ~2 kyr (Leonard et al., 2017a). In the Animas River valley of the San Juan Mountains, southwest Colorado, existing 10 Be ages indicate glacier retreat began as early as ~19 https://doi.org/10.5194/gchron-2020-13 Preprint. Discussion started: 12 May 2020 c Author(s) 2020. CC BY 4.0 License. ka, with retreat of half of the total valley length occurring from ~16 -13.5 ka (Guido et al., 2007). Relatively early initial retreat of the glacier in the Animas River valley is contingent on dating at a single site. Near Baldy Peak in Northern New Mexico, LGM 105 moraines and what appear to be cirque moraines have been surveyed in the Winsor Creek valley (Armour et al., 2002;Marcott et al., 2019). 10 Be ages from the cirque, ~4 km up-valley from the LGM moraines, range from 15.8 -14.3 ka, suggesting that the glacier retreated to near its cirque within that interval. The recessional and LGM moraines remain undated so it is difficult to know when the glacier began retreating. In 110 summary, while there is some chronologic scatter in ages from these sites, there is evidence to suggest that some glaciers in the southern Rocky Mountains remained relatively expanded through the beginning of the last deglaciation and were delayed in their retreat. However, once retreat was underway, all sites observed thus far reveal that glaciers completely retreated at least up to their cirques prior to the Younger Dryas 115 cold period with no evidence for subsequent moraine deposition.
Prominent moraines originally mapped as part of the surficial geologic map of the Granite 7.5' quadrangle (updated by Shroba et al., 2014) exist at the mouths of multiple glacially sculpted valleys within the Sawatch Range. Of these, moraines deposited at the mouths of three adjacent valleys, Lake Creek, Clear Creek and Pine Creek, have 120 been thoroughly surveyed and dated (Fig. 2;Briner, 2009;Young et al., 2011;Schweinsberg et al., 2020). The moraine chronologies reveal that following the LGM, which culminated between ~22 and 19 ka, glaciers remained at (100%) or near (82 -83%) their LGM lengths until ~16 -15 ka, after which the moraine record stops; in all

Methods and materials
Sample collection for 10 Be dating from Clear Creek and Pine Creek valleys was conducted in the summers of 2017 and 2018. Twelve samples were collected from exposed, glacially sculpted bedrock surfaces along the Clear Creek (n=8) and Pine

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Creek (n=4) valley floors, spanning from just within range-front moraines up to each respective cirque (Figs. 2 and 3). Samples were processed at the University at Buffalo Cosmogenic Isotope Laboratory following slightly modified versions of quartz purification and beryllium extraction procedures refined at the University of Vermont (Corbett et al., 2016). After quartz purification, samples were dissolved in acid along 155 with a 9 Be carrier spike. Beryllium was then purified and extracted, oxidized, and packed into targets for measurement at the Center for Accelerated Mass Spectrometry at Lawrence Livermore National Laboratory. 10 Be/ 9 Be ratios were measured and https://doi.org/10.5194/gchron-2020-13 Preprint. Discussion started: 12 May 2020 c Author(s) 2020. CC BY 4.0 License. standardized to the reported 07KNSTD3110 ratio of 2.85 x 10 -12 (Nishiizumi et al., 2007). Our 12 ages and 10 previously published ages were calculated using the Cronus 160 Earth online calculator (developmental version 3; https://hess.ess.washington.edu/math/index_dev.html; Balco et al., 2008). We calculate ages using the Promontory Point production rate  and the LSDn scaling model (Lifton et al., 2014)a combination used extensively throughout the western US (e.g., Licciardi and Pierce, 2018;Quirk et al., 2018;Brugger et al., 2019;165 Schweinsberg et al., 2020). Below, we discuss in more detail how different production rate calibrations and scaling schemes impact our results. is the end point (e.g., 0% or minimum length). The retreat rates presented here are net retreat rates, although it is possible there may have been short-lived re-advances that did not lead to significant moraine deposition. BACON outputs a time series of age-185 length points and 95% confidence intervals.

Results
All 22 sculpted-bedrock 10 Be ages, which span from immediately inboard of the innermost moraine to the cirque floors, range between 16.0 ± 0.4 and 13.5 ± 0.3 ka (Fig.   190 2, Table 1). In Lake Creek valley, seven ages span from 67. Ages calculated using the Promontory Point production rate calibration  and LSDn scaling (Lifton et al., 2014) b Ages calculated using the Northeast North America production rate clibration (Balco et al., 2009) and Lm scaling (Lal, 1991;Stone, 2000) https://doi.org/10.5194/gchron-2020-13 Preprint. the LGM moraine suggest that the glacier re-advanced to or remained at its LGM extent until nearly the same time when glaciers in the other two valleys deposited recessional moraines (Briner, 2009;Young et al., 2011).
Most ages in each valley are in stratigraphic order and fall within the 95% confidence interval calculated in BACON, except for four ages (Fig. 4). Ages from Lake
Results from BACON analysis suggest the net retreat rate for the glacier in Lake Creek valley between 15.6 ± 0.7 ka (Schweinsberg et al., 2020) and 13.7 ± 0.2 ka averages 19.8 ± 10.0 m a -1 (Fig. 4). The net retreat rate calculated from BACON for the glacier in Clear Creek valley between 15.6 ± 0.7 ka and 13.7 ± 0.3 ka averages 11.1 ± 215 3.7 m a -1 . Finally, the net retreat rate for the glacier in Pine Creek valley from the LGM position at 16.3 ± 0.4 ka (Young et al., 2011) to 14.2 ± 0.3 averages 9.9 ± 5.7 m a -1 .   Ages in solid black fill at the bottom of each transect are from recessional moraine ages (Young et al., 2011;Schweinsberg et al., 2020). BACON results are mean (color lines) and 95% confidence intervals (gray shading). Left y-axes are total valley floor distances from the LGM moraine to the base of each respective cirque headwall (note that scales are different because valley lengths are different). Right y-axes are the same, but 225 normalized values, where 1 = LGM moraine position and 0 = base of cirque headwall. D) Distribution of all ages using both PPT  and LSDn (Lifton et al., https://doi.org/10.5194/gchron-2020-13 Preprint. Discussion started: 12 May 2020 c Author(s) 2020. CC BY 4.0 License.

2014)
, and NENA (Balco et al., 2009) and Lm (Lal, 1991;Stone, 2000) production rate calibration and scaling scheme combinations discussed in the text. Creek cirque of 15.3 ± 0.3 ka may be an outlier because it is older than the next age downvalley (14.6 ± 0.3 ka) as well as a second sample from the cirque of 14.2 ± 0.3 ka.

Reliability of bedrock ages
Although we interpret our results using the Promontory Point production rate calibration site  and the LSDn scaling scheme (Lifton et al. 2014), we calculate exposure ages using another commonly used calibration site that is from 245 northeastern North America (NENA; Balco et al., 2009) and another commonly used scaling scheme (Lal/Stone-Lm;Lal, 1991;Stone, 2000). Samples used for the NENA production rate calibration range in elevation between ~50 to 400 m asl and are located ~3000 km northeast of the Sawatch Range. This combination produces ages between 9 to 12% older ( Fig. 4; Table 1). We do not feel confident in calculating exposure ages 250 using other production rate calibration sites since the sites in closest proximity likely https://doi.org/10.5194/gchron-2020-13 Preprint. Discussion started: 12 May 2020 c Author(s) 2020. CC BY 4.0 License.
shared the most similar exposure histories. Ultimately, we favor the Promontory Point production rate calibration site  because the site is closest in both location (site is ~600 km from the Sawatch Range) and elevation (sample elevations are ~1600 m asl) to our study area. influences were minimal. We find that all three valley glaciers did not begin significantly retreating until ~5 -6 kyr after the culmination of the LGM in the Sawatch Range; however, once glacier retreat initiated, deglaciation was completed within ~2 kyr.
From the existing records in the southern Rocky Mountains synthesized above, we find that the pattern of deglaciation observed in the Sawatch Range was consistent 275 in a few but not all sites across the region. Collecting more records of alpine deglaciation in the southern Rocky Mountains may be necessary to further test which pattern, if any, is the dominant pattern of deglaciation in the region.

Drivers of southern Rocky Mountain deglaciation 280
Records of global climate change over the last deglaciation suggest a link between rising CO2 concentrations and global temperature (Denton et al., 2010;Shakun et al., 2012;Putnam et al., 2013). However, there is noticeable spatial heterogeneity in both the timing and magnitude of warming through the last deglaciation that cannot be attributed to global CO2 forcing alone (e.g., Clark et al., 2012). We find that the initiation 285 of significant deglaciation in some locations across the southern Rocky Mountains lagged rising CO2 concentrations by as much as ~2 -3 kyr (Fig. 5), which suggests these glaciers were more likely influenced by regional forcings rather than global CO2.
Ice core records-among other records-reveal a complex pattern of abrupt warming and cooling events that occurred in the North Atlantic region during the last 290 deglaciation ( Fig. 5; Buizert et al., 2014). Despite rising CO2 concentrations beginning ~18 ka, North Atlantic records reveal that cold conditions persisted until 14.7 ka, known as Heinrich Stadial 1 (HS-1). Following these sustained cold conditions, an abrupt transition to warmer conditions is marked by the HS-1/Bølling boundary at 14.7 ka (Buizert et al., 2014). We find that the timing of abrupt warming documented in the 295 North Atlantic at the HS-1/Bølling transition aligns somewhat closely with the timing of https://doi.org/10.5194/gchron-2020-13 Preprint. from cold stadial conditions to significant warming. The similarity between alpine glacier records in the southern Rocky Mountains and North Atlantic climate history indicates a possible teleconnection between the two regions.
In addition to the alpine glaciers that existed in the mountainous regions of the 335 western US during the late Pleistocene, large pluvial lakes such as Lake Lahontan and Lake Bonneville existed across the Great Basin ( Fig. 1; Gilbert, 1890;Russell, 1885;Orme, 2008). These lakes could have been sustained by increased precipitation delivery to the southwestern US (e.g., Munroe and Laabs, 2013;Oster et al., 2015;Lora and Ibarra, 2019) or were maintained simply by colder temperatures persisting 340 throughout the region (e.g., Benson et al., 2013). Recent syntheses of past Great Basin lake levels reveal that Lahontan and Bonneville lakes resided at relative high stands between 15.5 and 14.5 ka (Benson et al., 2013;Reheis et al., 2014;Oviatt, 2015). After this time, each lake experienced notable declines in lake level (Fig. 5), which could have been the result of reduced precipitation due to re-arranging storm tracks, warming 345 temperature or a combination of both (Benson et al., 2013;Oster et al., 2015;Lora and Ibarra, 2019 to conclude what the primary driver of deglaciation in the Sawatch Range was; it may be a combination of both forcings. We find that the approximate timing and rate of deglaciation observed in the Sawatch Range points to abrupt warming and/or drying, and is supported by pluvial lake level records in the western US, which have also been 365 tied to both North Atlantic forcing and North American ice sheet forcing (Munroe and Laabs, 2013;Benson et al., 2013;Lora and Ibarra, 2019). Regardless, the data synthesized here underscore the dominance of regional forcing mechanisms over global forcing mechanisms on some climate records in the western US.

Conclusions
We constrain the timing and rate of deglaciation in three alpine valleys in the Sawatch Range, southern Rocky Mountains. Beryllium-10 ages from ice-sculpted bedrock in each valley reveal the significant retreat of glaciers from their LGM extents (100%) or near (82 -83%) their LGM extents was initiated shortly after 16.3 -15.6 ka, 375 despite ~2 -3 kyr of prior global warming forced by rising atmospheric CO2. Glaciers in three adjacent valleys retreated rapidly to their cirques within ~2 kyr, culminating at ~14.2 -13.7 ka, at rates ranging between 19.8 to 9.9 m a -1 . We recognize that using the NENA production rate and Lm scaling produces ages 9 -12% older than the ages reported herein, which would change the interpretation of the dataset. However, we 380 favor the PPT/LSDn combination because the PPT calibration site is closest in proximity and elevation to the Sawatch Range.
We hypothesize that one of two possible regional mechanisms were responsible for driving the pattern of deglaciation for some glaciers in the southern Rocky Mountains. First, we find that some alpine glaciers in the region began retreating around Basin pluvial lake regression to warming and the migration of prevailing storm tracks due to atmospheric re-organization that may have been forced by separation of North American ice sheets. Thus, warming and drying induced by abrupt atmospheric re-395 organization at the time of LIS and CIS separation may have driven both Great Basin lake level lowering and rapid alpine glacier retreat in some valleys in the southern Rocky Mountains. While we cannot conclude that either one of the aforementioned forcing mechanisms was solely responsible for deglaciation of the Sawatch Range, we suggest that either one or both were stronger controls than global CO2 forcing.