Dissertation Abstract
On the Late Saalian glaciation (160-140 ka): a climate modeling study
Colleoni, Florence 2009
Laboratoire de Glaciologie et Géophysique de l'Environnement (CNRS), Université Joseph Fourier (France), 206 pp.
This thesis focuses on the glaciation of the Late Saalian period (160 -140 ka) over Eurasia. The Quaternary Environment of the Eurasian North project evidenced that during this period, the Eurasian ice sheet was substantially larger than during the entire Weichselian cycle and especially than during the Last Glacial Maximum (21 ka, LGM). The Late Saalian astronomical forcing were different, especially at 140 ka considered as the Late Saalian glacial maximum, than during the LGM while greenhouse gas concentrations (CO2 and CH4) were similar. To understand how this ice sheet could have grown so large over Eurasia during the Late Saalian, we use an Atmospherical General Circulation Model (AGCM), an AGCM coupled to an oceanic mixed layer module and a vegetation model to explore the influence of regional parameters, sea surface temperatures (SST), vegetation and orbital parameters on the surface mass balance of the Late Saalian Eurasian ice sheet. Simulations were run for 21 years and focused on the Late Saalian glacial maximum (140 ka).
Results show that proglacial lakes cools the summer climate, reducing the ablation along the southern margins of the ice sheet while dust deposition on snow slightly warm the regional climate, inducing more ablation along these margins. At 140 ka, the impact of the substitution from conifera to tundra highly increases the regional albedo which causes an important regional cooling due to the development of a permanent snow cover along the entire ice sheet margins. Simulated 140 ka SST underestimate the available Late Saalian SST derived marine records and sea ice reached 40N in both North Atlantic and North Pacific. This leads to an excessively cold Northern Hemisphere climate, completely stopping the ablation process over the ice sheet and reducing the amount of moisture coming from the Atlantic.
According to geological evidences, the Late Saalian Eurasian ice sheet reached its maximum volume and extent before 160 ka but remains as large until Termination II (130 ka). Northern Hemisphere high latitudes summer insolation show a large insolation peak towards 150 ka and to understand how the Late Saalian Eurasian ice sheet could survive to this peak, we simulate the climate conditions (regional factors, SST and vegetation cover) for the three time-slices 160, 150 and 140 ka. The simulated 160 and 150 ka climate are milder than at the glacial maximum of 140 ka. Ablation is more important along the entire southern margin of the Eurasian ice sheet at 150 ka insolation, the Late Saalian Eurasian ice sheet, combined to proglacial lakes, to a vegetation cover and SST in agreement with the simulated climate, is large enough to generate its own cooling and to maintain over the continent. This thesis partly answers to the issue raised above. However, to understand how this ice sheet grew and to better estimate the interactions between ocean, atmosphere, continent and ice sheet, ice-ocean-atmosphere coupled simulations are necessary.
Results show that proglacial lakes cools the summer climate, reducing the ablation along the southern margins of the ice sheet while dust deposition on snow slightly warm the regional climate, inducing more ablation along these margins. At 140 ka, the impact of the substitution from conifera to tundra highly increases the regional albedo which causes an important regional cooling due to the development of a permanent snow cover along the entire ice sheet margins. Simulated 140 ka SST underestimate the available Late Saalian SST derived marine records and sea ice reached 40N in both North Atlantic and North Pacific. This leads to an excessively cold Northern Hemisphere climate, completely stopping the ablation process over the ice sheet and reducing the amount of moisture coming from the Atlantic.
According to geological evidences, the Late Saalian Eurasian ice sheet reached its maximum volume and extent before 160 ka but remains as large until Termination II (130 ka). Northern Hemisphere high latitudes summer insolation show a large insolation peak towards 150 ka and to understand how the Late Saalian Eurasian ice sheet could survive to this peak, we simulate the climate conditions (regional factors, SST and vegetation cover) for the three time-slices 160, 150 and 140 ka. The simulated 160 and 150 ka climate are milder than at the glacial maximum of 140 ka. Ablation is more important along the entire southern margin of the Eurasian ice sheet at 150 ka insolation, the Late Saalian Eurasian ice sheet, combined to proglacial lakes, to a vegetation cover and SST in agreement with the simulated climate, is large enough to generate its own cooling and to maintain over the continent. This thesis partly answers to the issue raised above. However, to understand how this ice sheet grew and to better estimate the interactions between ocean, atmosphere, continent and ice sheet, ice-ocean-atmosphere coupled simulations are necessary.