skip to main content


Title: Modeling glacier extents and equilibrium line altitudes in the Rwenzori Mountains, Uganda, over the last 31,000 yr
Mountain glacier moraine sequences and their chronologies allow us to evaluate the timing and climate conditions that underpin changes in the equilibrium line altitudes (ELAs), which can provide valuable information on the paleoclimatology of understudied regions such as tropical East Africa. However, moraine sequences are inherently discontinuous, and the precise climate conditions that they represent can be ambiguous due to the sensitivity of mountain glaciers to temperature, precipitation, and other environmental variables. Here, we used a two-dimensional (2-D) iceflow and mass-balance model to simulate glacier extents and ELAs in the Rwenzori Mountains in East Africa over the past 31,000 yr (31 k.y.), including the Last Glacial Maximum (LGM), late glacial period, and the Holocene Epoch. We drove the glacier model with two independent, continuous temperature reconstructions to simulate possible glacier length changes through time. Model input paleoclimate values came from branched glycerol dialkyl glycerol tetraether (brGDGT) temperature reconstructions from alpine lakes on Mount Kenya for the last ~31 k.y., and precipitation reconstructions for the LGM came from various East African locations. We then compared the simulated fluctuations with the positions and ages (where known) of the Rwenzori moraines. The simulated glacier extents reached within 1.1 km of the dated LGM moraines in one valley (93% of the full LGM extent) when forced by the brGDGT temperature reconstructions (maximum cooling of 6.1 °C) and a decrease in precipitation (-10% than modern amounts). These simulations suggest that the Rwenzori glaciers required a cooling of at least 6.1 °C to reach the dated LGM moraines. Based on the model output, we predict an age of 12–11 ka for moraines located halfway between the LGM and modern glacier extents. We also predict ice-free conditions in the Rwenzori Mountains for most of the early to middle Holocene, followed by a late Holocene glacier readvance within the last 2000 yr.  more » « less
Award ID(s):
1702319
PAR ID:
10390043
Author(s) / Creator(s):
Editor(s):
Waitt RB, Thackray GD
Date Published:
Journal Name:
Special paper Geological Society of America
Volume:
548
ISSN:
0072-1077
Page Range / eLocation ID:
171–188
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. null (Ed.)
    Important information about past climates can be determined from reconstructed equilibrium line altitudes (ELA) of mountain paleoglaciers, specifically the temperature and precipitation accompanying a glacier in equilibrium. Previous reconstructions of Late Pleistocene ELAs of mountain glaciers across the western United States have been used to infer the pattern of temperature and precipitation change across the region, although most of the work was based on presumed ages and limited mapping of glacial deposits and landforms. Cosmogenic nuclide exposure dating of moraines combined with updated mapping and aerial imagery afford an opportunity to revisit the pattern of regional ELAs during multiple episodes of the last Pleistocene glaciation. The goal of this research is to reconstruct ELAs in the same region of previous reconstructions based on glacial sediments that have been dated using cosmogenic nuclide exposure ages. We focus on the large number of glacial valleys with moraines corresponding to the Last Glacial Maximum (LGM; 26.5-19.0 ka). Paleo-ELAs are estimated using the toe to headwall altitude ratio and the accumulation area ratio determined from published glacier reconstructions and existing glacial mapping. Cosmogenic-exposure ages of moraines are compiled from the informal cosmogenic nuclide exposure age database for alpine glacial features (ICE-D Alpine) and represented in a geographic information system along with ELAs for each glacial valley. A reconstructed ELA surface spanning the conterminous western United States is produced using existing algorithms in ArcGIS. Results show reconstructed ELAs generally lower than initially estimated and a larger range of ELAs across the region. In the Sierra Nevada, ELAs increase southeastward, which is consistent with previous estimates, spanning a range from 1800 to 2800 m asl. ELAs rise eastward across the Basin and Range toward the western shore of the area covered by Lake Bonneville, and then decrease eastward toward the Wasatch Mountains. This pattern is inconsistent with previous estimates and may reflect a west-to-east precipitation gradient that differs from modern climate. We discuss this pattern and broader features of the ELA surface of the LGM and later episodes of the last Pleistocene glaciation. 
    more » « less
  2. Abstract

    The magnitude of tropical cooling during the Last Glacial Maximum (LGM; ∼19–26.5 ka) remains controversial, with sea‐surface temperatures cooling by several degrees less than most temperatures reconstructed at high elevations. To explain this discrepancy, past studies proposed a steeper (increased) lapse rate—the temperature decrease with elevation—during the LGM relative to today. For instance, LGM temperatures in East Africa reconstructed from branched GDGTs from multiple elevations support an ∼0.9°C/km increase in the lapse rate during the LGM relative to present day. Lapse rates are a critical part of the Earth's climate sensitivity and atmospheric energy transfer, and it is vital to know whether and by how much the tropical lapse rate steepened during the LGM. Here, we simulate LGM glacier extents in the Rwenzori Mountains of Uganda with and without a change in lapse rate using a range of temperature and precipitation estimates. We find that the lapse rate must have been steeper than present for glaciers to reach their LGM positions using available sea‐level temperature and precipitation estimates for East Africa.

     
    more » « less
  3. Abstract

    Well-dated records of alpine glacier fluctuations provide important insights into the temporal and spatial structure of climate variability. Cirque moraine records from the western United States have historically been interpreted as a resurgence of alpine glaciation in the middle-to-late Holocene (i.e., Neoglaciation), but these moraines remain poorly dated because of limited numerical age constraints at most locations. Here we present 13010Be ages on 19 moraines deposited by 14 cirque glaciers across this region that have been interpreted as recording these Neoglacial advances. Our10Be chronology indicates instead that these moraines were deposited during the latest Pleistocene to earliest Holocene, with several as old as 14–15ka. Our results thus show that glaciers retreated from their Last Glacial Maximum (LGM) extent into cirques relatively early during the last deglaciation, experienced small fluctuations during the Bølling–Allerød–Younger Dryas interval, and remained within the maximum limit of the Little Ice Age (LIA) advance of the last several centuries throughout most of the Holocene. Climate modeling suggests that increasing local summer insolation and greenhouse gases were the primary controls on early glacier retreat from their LGM positions. We then infer that subsequent intrinsic climate variability and Younger Dryas cooling caused minor fluctuations during the latest Pleistocene, while the LIA advance represents the culmination of a cooling trend through the Holocene in response to decreasing boreal summer insolation.

     
    more » « less
  4. Abstract. Over the last century, northwestern Canada experienced some of the highest rates of tropospheric warming globally, which caused glaciers in the region to rapidly retreat. Our study seeks to extend the record of glacier fluctuations and assess climate drivers prior to the instrumental record in the Mackenzie and Selwyn mountains of northwestern Canada. We collected 27 10Be surface exposure ages across nine cirque and valley glacier moraines to constrain the timing of their emplacement. Cirque and valley glaciers in this region reached their greatest Holocene extents in the latter half of the Little Ice Age (1600–1850 CE). Four erratic boulders, 10–250 m distal from late Holocene moraines, yielded 10Be exposure ages of 10.9–11.6 ka, demonstrating that by ca. 11 ka, alpine glaciers were no more extensive than during the last several hundred years. Estimated temperature change obtained through reconstruction of equilibrium line altitudes shows that since ca. 1850 CE, mean annual temperatures have risen 0.2–2.3 ∘C. We use our glacier chronology and the Open Global Glacier Model (OGGM) to estimate that from 1000 CE, glaciers in this region reached a maximum total volume of 34–38 km3 between 1765 and 1855 CE and had lost nearly half their ice volume by 2019 CE. OGGM was unable to produce modeled glacier lengths that match the timing or magnitude of the maximum glacier extent indicated by the 10Be chronology. However, when applied to the entire Mackenzie and Selwyn mountain region, past millennium OGGM simulations using the Max Planck Institute Earth System Model (MPI-ESM) and the Community Climate System Model 4 (CCSM4) yield late Holocene glacier volume change temporally consistent with our moraine and remote sensing record, while the Meteorological Research Institute Earth System Model 2 (MRI-ESM2) and the Model for Interdisciplinary Research on Climate (MIROC) fail to produce modeled glacier change consistent with our glacier chronology. Finally, OGGM forced by future climate projections under varying greenhouse gas emission scenarios predicts 85 % to over 97 % glacier volume loss by the end of the 21st century. The loss of glaciers from this region will have profound impacts on local ecosystems and communities that rely on meltwater from glacierized catchments.

     
    more » « less
  5. null (Ed.)
    Abstract Tropical glaciers have retreated alongside warming temperatures over the past century, yet the way in which these trends fit into a long-term geological context is largely unclear. Here, we present reconstructions of Holocene glacier extents relative to today from the Quelccaya ice cap (Peru) and the Rwenzori Mountains (Uganda) based on measurements of in situ14C and 10Be from recently exposed bedrock. Ice-extent histories are similar at both sites and suggest that ice was generally smaller than today during the first half of the Holocene and larger than today for most, if not all, of the past several millennia. The similar glaciation history in South America and Africa suggests that large-scale warming followed by cooling of the tropics during the late Holocene primarily drove ice extent, rather than regional changes in precipitation. Our results also imply that recent tropical ice retreat is anomalous in a multimillennial context. 
    more » « less