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Influence of sea ice and subglacial hydrology on the dynamics of outlet glaciers flowing into Lützow-Holm Bay, East Antarctica
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| Title: | Influence of sea ice and subglacial hydrology on the dynamics of outlet glaciers flowing into Lützow-Holm Bay, East Antarctica |
| Other Titles: | 東南極リュツォ・ホルム湾に流入する溢流氷河の変動に海氷と底面水文環境が与える影響 |
| Authors: | 近藤, 研 Browse this author |
| Issue Date: | 25-Dec-2023 |
| Publisher: | Hokkaido University |
| Abstract: | In this thesis, satellite data analysis and field observations were conducted with the aim of understanding the mechanisms of flow changes of outlet glaciers in the Antarctic ice sheet. In Chapter 3, the relationship between flow changes and sea ice variations over the past two decades in five outlet glaciers (Shirase, Skallen, Telen, Honnor and Langhovde glaciers) flow- ing into Lutzow-Holm Bay, East Antarctica, was analyzed using satellite data. In Chapter 4, short-term flow changes of Langhovde Glacier were observed by glacier surface measure- ments, and its mechanisms were investigated by direct observations at the glacier base using hot-water drilling. Satellite data were analyzed for the five studied glaciers to measure terminus positions, flow velocities, surface elevations, and sea ice extent in front of the glaciers for the period 2000?2020. The terminus positions were measured by manually delineating the front using geographic information system (GIS) software on satellite imagery (JERS-1, ASTER, Land- sat 4/7/8). Flow velocities were obtained using the annual mean flow velocity dataset of the Antarctic ice sheet distributed by NASA MEaSUREs (ITS_LIVE). I also analyzed Landsat 7/8 imagery to obtain the flow fields with high spatiotemporal resolution by applying the feature- tracking method. Furthermore, the strain distribution of the glacier was analyzed by processing the obtained glacier flow field. Changes in surface elevations were measured using digital el- evation models from Reference Elevation Model for Antarctica (REMA) and laser altimetry data (ICESat-1/2). To measure sea ice variations near the glacier fronts, the reflectance of visible satellite imagery was analyzed within the regions located in front of the glacier. The pixels within the regions were classified into sea ice, open water, or clouds. The analysis was performed with Google Earth Engine to efficiently process the large amount of data acquired during 2000?2020. Satellite data analysis revealed that the terminal positions of the glaciers advanced by 0.6?38.7 km from 2000?2016 and then retreated by 0.4?6.0 km by 2018. The flow speeds of the largest three glaciers (Shirase, Skarlen and Thelen glaciers) showed decel- eration during 2010?2015 and acceleration during 2015?2018. Elevation increased by 6.5 ± 0.2 m and 2.4 ± 0.2 m near the grounding line of Shirase and Skarlen glaciers in 2012?2016, followed by decrease by ?0.8 ± 0.1 m and ?3.03 ± 0.07 m. These results indicate that the glaciers retreated, accelerated, and thinned after the sea ice broke up in 2016. These results suggest that the land-fast sea ice in front of the glacier termini suppressed iceberg calving and glacier flow. Furthermore, the longitudinal strain rate of the glaciers decreased during 2010?2015 and increased during 2015?2018, indicating that the elevation changes observed near the grounding line were due to changes in strain rate caused by flow changes. These re- sults indicate that relatively small changes in the stress balance due to sea ice breakup caused glacier acceleration and calving, which in turn caused changes in ice thickness. Field observations were carried out on Langhovde Glacier from December 2021 to Febru- ary 2022 to observe short-term flow changes that are difficult to detect by satellite data. In order to directly observe the relationship between glacier basal sliding and subglacial water pressure variations, a device equipped with a three-axis accelerometer and a water pressure sensor was developed. The device was installed at the base of Langhovde Glacier, using a 550-m long borehole drilled approximately 1 km upstream from the grounding line during a hot-water drilling campaign in the austral summer 2021/22. On the glacier surface, global navigation satellite system (GNSS) and an automatic weather station were operated to observe glacier flow changes and meteorological conditions. As a result of the observations, flow ac- celeration of up to 20% was measured when the subglacial water pressure rapidly increased between 2 and 4 January. During the same period, impulsive and strong horizontal vibration events were observed at the subglacial accelerometer. The occurrence of the events was twice greater than the background value, suggesting active basal sliding during the speedup. The increase in the subglacial water pressure occurred a day after the rainfall of 30 mm d?1 over the glacier. The correspondence strongly suggests that the drainage of the surface water into the glacier bed enhanced basal sliding and caused short-term glacier flow acceleration. These results are the first observation showing that the surface-to-bed hydrological connections in Antarctica caused pressurization of the drainage system and speedup, which have traditionally been considered to have limited or no impacts on the subglacial conditions. The study revealed the variations of the five outlet glaciers flowing into Lutzow-Holm Bay, East Antarctica, during 2000?2020 for the first time. Satellite observations revealed that the stress changes due to sea ice variations caused glacier flow changes. Field observa- tions on Langhovde Glacier captured short-term summer flow acceleration for the first time in East Antarctica. Direct observations at the glacier bed revealed that the flow acceleration was caused by the enhanced sliding due to the subglacial water pressure elevated by the drainage of surface water to the glacier bed. The results of the satellite and field observations indicate that small changes in the glacier stress balance at the ice front or the base resulted in significant flow changes. In other words, outlet glaciers of the Antarctic ice sheet are highly sensitive to changes in the surrounding environment such as the atmosphere and the ocean, which results in glacier flow changes. These results contribute to an improved understanding of glacier vari- ations in Lutzow-Holm Bay and provide new insights into the mechanisms of outlet glacier changes in the Antarctic ice sheet. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第15670号 |
| Degree Level: | 博士 |
| Degree Discipline: | 環境科学 |
| Examination Committee Members: | (主査) 教授 杉山 慎, 教授 Ralf Greve, 准教授 青木 茂, 教授 澤柿 教伸 （法政大学 社会学部） |
| Degree Affiliation: | 環境科学院（地球圏科学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/91170 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 環境科学院(Graduate School of Environmental Science)
学位論文 (Theses) > 博士 （環境科学）
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