講演情報
[ACC45-P08]Grain-size and mineralogical characteristics of glacial meltwater-derived sediments in Eyre Fjord, Patagonia
*粕谷 拓人1,2、波多 俊太郎3,4、佐藤 洋太2,5、Marius Schaefer6、杉山 慎3,7 (1.九州大学 大学院理学府 地球惑星科学専攻、2.国立研究開発法人 海洋研究開発機構、3.北海道大学 低温科学研究所、4.国立極地研究所、5.オーストリア科学技術研究所、6.アウストラル・デ・チリ大学、7.北海道大学 北極域研究センター)
キーワード：
南パタゴニア氷原、ピオ・オンセ氷河、懸濁粒子、X線回折分析、粒度分析
While most outlet glaciers of the Patagonian Icefields are currently retreating and thinning, Glaciar Pío XI (49°S) advanced during the 20th century. This marine-terminating glacier drains into Eyre fjord, which connects to the southeast Pacific Ocean through several channels. The sedimentary environments in front of the glacier have evolved with its advance: prior to ~2010, the terminus formed a calving front, whereas after ~2010, an ice-contact delta developed at the fjord head. A previous study, based on sediment cores from the fjord floor, suggested a major shift in submarine sedimentation from plume-dominated to turbidity current-dominated processes in response to the delta formation. However, satellite observations revealed a large-scale collapse of the ice-contact delta between November and December 2022, and sedimentological data after this event have been lacking.
Here we present detrital grain-size and mineralogical data from sediment samples collected in Eyre fjord during a field campaign in December 2024. At that time, the glacier terminus exhibited two contrasting settings: a marine-terminating margin in the west, where meltwater was discharged directly into the fjord, and an ice-contact delta in the east, where meltwater flowed across the sediment mound. We conducted CTD measurements and collected (1) suspended particles from surface waters using a bucket, (2) fjord bottom sediments using an Ekman-birge grab sampler, and (3) settling particles using a handmade sediment trap deployed near the ice front. Surface and bottom samples were obtained from sites proximal, intermediate, and distal to both the western and eastern margins. The sediment trap was moored ~3 m above the fjord floor at ~50 m water depth, ~500 m from the western margin.
Grain-size and X-ray diffraction analyses indicate a uniform detrital composition in all surface-water suspensions, characterized by glacial rock flour with a single grain-size mode at ~5 μm. In contrast, bottom sediments and settling particles from proximal and intermediate sites contain additional coarser fractions and higher proportions of non-clay minerals such as quartz and plagioclase. Bottom sediments near the western marine-terminating margin exhibit a distinct secondary mode at ~40 μm, suggesting bedload-dominated sedimentation, possibly related to turbidity currents. Although settling particles near the western marine-terminating margin and bottom sediments in front of the eastern ice-contact delta show similar grain-size distributions with a shoulder at ~25 μm, the coarser fraction is more pronounced at the western site. This contrast suggests that the eastern ice-contact delta acts as a geographic filter, trapping coarser grains on the delta mound as sediment-laden meltwater is discharged as river flow. Distal sites exhibit grain-size patterns and mineralogical compositions identical to those of surface-water suspensions, consistent with deposition by suspension settling (rain-out). These results demonstrate that the present sedimentary system in front of Glaciar Pío XI comprises both direct meltwater plume-dominated sedimentation near the western ice front and fluvially mediated, delta-controlled sedimentation near the eastern ice front.
Here we present detrital grain-size and mineralogical data from sediment samples collected in Eyre fjord during a field campaign in December 2024. At that time, the glacier terminus exhibited two contrasting settings: a marine-terminating margin in the west, where meltwater was discharged directly into the fjord, and an ice-contact delta in the east, where meltwater flowed across the sediment mound. We conducted CTD measurements and collected (1) suspended particles from surface waters using a bucket, (2) fjord bottom sediments using an Ekman-birge grab sampler, and (3) settling particles using a handmade sediment trap deployed near the ice front. Surface and bottom samples were obtained from sites proximal, intermediate, and distal to both the western and eastern margins. The sediment trap was moored ~3 m above the fjord floor at ~50 m water depth, ~500 m from the western margin.
Grain-size and X-ray diffraction analyses indicate a uniform detrital composition in all surface-water suspensions, characterized by glacial rock flour with a single grain-size mode at ~5 μm. In contrast, bottom sediments and settling particles from proximal and intermediate sites contain additional coarser fractions and higher proportions of non-clay minerals such as quartz and plagioclase. Bottom sediments near the western marine-terminating margin exhibit a distinct secondary mode at ~40 μm, suggesting bedload-dominated sedimentation, possibly related to turbidity currents. Although settling particles near the western marine-terminating margin and bottom sediments in front of the eastern ice-contact delta show similar grain-size distributions with a shoulder at ~25 μm, the coarser fraction is more pronounced at the western site. This contrast suggests that the eastern ice-contact delta acts as a geographic filter, trapping coarser grains on the delta mound as sediment-laden meltwater is discharged as river flow. Distal sites exhibit grain-size patterns and mineralogical compositions identical to those of surface-water suspensions, consistent with deposition by suspension settling (rain-out). These results demonstrate that the present sedimentary system in front of Glaciar Pío XI comprises both direct meltwater plume-dominated sedimentation near the western ice front and fluvially mediated, delta-controlled sedimentation near the eastern ice front.