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GEOPHYSICAL INVESTIGATION OF GLACIER GEOMETRY AND SUBGLACIAL GEOLOGY, AND THEIR INFLUENCE ON GLACIER DYNAMICS: CASE STUDIES OF TAKU GLACIER, ALASKA AND THWAITES GLACIER, WEST ANTARCTICA
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2025-05
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Geoscience
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https://doi.org/10.34944/wjz5-a622
Abstract
Mass loss from glaciers and ice sheets is a significant contributor to the rise of global mean sea level (GMSL) and will continue to be in the future. Rising GMSL will cause displacement of coastal populations, loss of ecosystems and rising food in- security. To plan for and mitigate these impacts, it is important to improve estimates of the rate of GMSL rise in the future. Large uncertainties remain in the future evolution of glaciers in the Greenland and Antarctic Ice Sheets due to poorly con- strained processes such as Marine Ice Sheet Instability (MISI). Additionally, glaciers outwith ice sheets lack observations of many model input parameters, which lead to uncertainties in predictions of their rate of mass loss. Key parameters affecting marine-terminating glaciers both within and outwith ice sheets are the bed elevation and the material at the bed. The bed elevation is well known for glaciers within ice sheets due to extensive data from radio-echo sounding (radar) surveys. But in glaciers with temperate ice, high attenuation due to high meltwater content within and at the base of ice prevents radar from surveying the bed. Therefore, many glaciers out- with ice sheets lack accurate observations of their bed elevation. For glaciers within ice sheets, although their bed elevation is well known, obtaining measurements of the material beneath the bed relies on intensive ground-based techniques and hence observations are limited in spatial extent. In this work, we focus on two glaciers, Taku Glacier in Alaska and Thwaites Glacier in West Antarctica, and improve on observations of their subglacial environments that are currently poorly constrained. We employ a multi-modal geophysical technique, utilizing a combination of seismic and gravity- and magnetic-anomaly data.
Taku Glacier is a tidewater glacier located in the Juneau Icefield, Alaska. Until recently, the Taku had been advancing or stable in contrast to most of other glaciers in the Juneau Icefield. In 2018, the Taku began retreating and limited observations of the bed geometry leaves uncertainty on how its retreat will proceed. We collect new ground-based gravity measurements and perform a 3D inversion with a novel methodology to estimate the bed elevation along a 12-km segment of the glacier ∼30 km upstream of the current terminus. We derive the first along-flow bed elevation profile on the Taku and find the deepest bed at 445 ± 166 m below sea level with a maximum ice thickness of 1556 ± 143 m. We additionally find two bedrock bumps in the along-flow profile, which would be important in stabilizing a potentially rapid retreat due to marine influence.
Thwaites Glacier, located in the Amundsen Sea Embayment in West Antarctica, has been identified as the key component in the future evolution of the West Antarctic Ice Sheet. Its retrograde slope makes it susceptible to rapid retreat from MISI but uncertainty remains on when and how fast that retreat may occur. Modeling has shown the bed types under Thwaites will influence its rate of retreat but currently the bed-type distribution is only known within two small areas. We model the crustal structures under Thwaites to identify the origin of the currently identified bed-type distribution and inform how the bed type might vary over the whole glacier. We perform this modeling at two sites on Thwaites using a combination of long-offset seismic and gravity- and magnetic-anomaly data.
The first is at the lower region of Thwaites, close to the coast. Here we model the crust along two lines extending ∼120 km inland from the coast. We find a 40-km- long sedimentary basin with a maximum thickness of 1651 ± 218 m and two mafic intrusions 5 - 10-km deep with maximum thickness of ∼9 km. The sedimentary basin coincides with an area of continuous soft bed, indicating the sedimentary rock is the likely source of weak, easily deformable sediment at the bed. The sedimentary basin and mafic intrusions are indicative of a rift origin, indicating the dynamics of Thwaites are being influenced by subglacial geologic structures resulting from the development of the West Antarctic Rift System.
The second study site on Thwaites is further inland. Here we model the crustal structures along a ∼180 km line that extends inland from the first study site across the central region of Thwaites. Previous studies across the central region of Thwaites have found thinned crust across a narrow region. We find sedimentary rocks with the maximum thickness of ∼2000 m coinciding with this area of thinned crust and estimate that the crust has low flexural rigidity across this region. These results suggest that the central region of Thwaites has undergone a relatively recent rifting. We additionally model more mafic intrusions but find they decrease in size under the central region, compared to the first study closer to the coast. This suggests the intrusions may have been formed by a different mechanism than the rifting in the near- coastal region. We find sedimentary rock under ∼75% of the central-region profile, indicating there is a plentiful supply of deformable sediment at the bed of Thwaites. The possible extensive sediment cover and thinned crust are likely influencing the ice dynamics on Thwaites by facilitating fast ice flow.
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