Browsing by Author "Prothro, Lindsay O."

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Characteristics of the deforming bed: till properties on the deglaciated Antarctic continental shelf
    (Cambridge University Press, 2018) Halberstadt, Anna Ruth W.; Simkins, Lauren M.; Anderson, John B.; Prothro, Lindsay O.; Bart, Philip J.
    Contemporary ice stream flow is directly linked to conditions at the ice/bed interface, yet this environment is logistically difficult to access. Instead, we investigate subglacial processes important for ice stream flow by studying tills on the deglaciated Antarctic continental shelf. We test currently-accepted hypotheses surrounding subglacial processes and till properties with a Ross Sea dataset. Till shear strengths indicate a continuum of simultaneous processes acting at the bed, rather than discrete ‘deformation’ and ‘lodgement’ end-members. We identify a threshold water content representing saturated pore spaces, leading to basal sliding and meltwater channelization. Based on observations of till properties relative to glacial landforms, we challenge the assumption that low shear strength is linked to intense deformation. Spatial variability in landform morphology reflects variability in deforming processes at the sub-ice stream scale and suggests a maximum deforming bed thickness of 2 m at the grounding line. Regional till properties generally correlate with seafloor geology and deglacial history; the western Ross Sea is characterized by higher and more variable shear strengths and water contents, while lower-shear strength till was preserved in the Eastern Basin. These observations inform till interpretation and provide context for deforming beds beneath the modern ice sheet and on glaciated continental shelves.
  • Thumbnail Image
    Item
    Holocene reconfiguration and readvance of the East Antarctic Ice Sheet
    (Springer Nature, 2018) Greenwood, Sarah L.; Simkins, Lauren M.; Halberstadt, Anna Ruth W.; Prothro, Lindsay O.; Anderson, John B.
    How ice sheets respond to changes in their grounding line is important in understanding ice sheet vulnerability to climate and ocean changes. The interplay between regional grounding line change and potentially diverse ice flow behaviour of contributing catchments is relevant to an ice sheet’s stability and resilience to change. At the last glacial maximum, marine-based ice streams in the western Ross Sea were fed by numerous catchments draining the East Antarctic Ice Sheet. Here we present geomorphological and acoustic stratigraphic evidence of ice sheet reorganisation in the South Victoria Land (SVL) sector of the western Ross Sea. The opening of a grounding line embayment unzipped ice sheet sub-sectors, enabled an ice flow direction change and triggered enhanced flow from SVL outlet glaciers. These relatively small catchments behaved independently of regional grounding line retreat, instead driving an ice sheet readvance that delivered a significant volume of ice to the ocean and was sustained for centuries.
  • Thumbnail Image
    Item
    Insights into glacial processes from micromorphology of silt-sized sediment
    (Copernicus Publications, 2024) Lepp, Allison P.; Miller, Lauren E.; Anderson, John B.; O'Regan, Matt; Winsborrow, Monica C. M.; Smith, James A.; Hillenbrand, Claus-Dieter; Wellner, Julia S.; Prothro, Lindsay O.; Podolskiy, Evgeny A.
    Silt-rich meltwater plume deposits (MPDs) analyzed from marine sediment cores have elucidated relationships that are clearly connected, yet difficult to constrain, between subglacial hydrology, ice-marginal landforms, and grounding-zone retreat patterns for several glacial catchments. Few attempts have been made to infer details of subglacial hydrology, such as flow regime, geometry of drainage pathways, and mode(s) of sediment transport through time, from grain-scale characteristics of MPDs. Using sediment samples from MPD, till, and grounding-zone proximal diamicton collected offshore of six modern and relict glacial catchments in both hemispheres, we examine grain shape distributions and microtextures (collectively, grain micromorphology) of the silt fraction to explore whether grains are measurably altered from their subglacial sources via meltwater action. We find that 75 % of all imaged grains (n = 9400) can be described by 25 % of the full range of measured shape morphometrics, indicating grain shape homogenization through widespread and efficient abrasive processes in subglacial environments. Although silt grains from MPDs exhibit edge rounding more often than silt grains from tills, grain surface textures indicative of fluvial transport (e.g., v-shaped percussions) occur in only a modest number of grains. Furthermore, MPD grain surfaces retain several textures consistent with transport beneath glacial ice (e.g., straight or arcuate steps, (sub)linear fractures) in comparable abundances to till grains. Significant grain shape alteration in MPDs compared to their till sources is observed in sediments from glacial regions where (1) high-magnitude, potentially catastrophic meltwater drainage events are inferred from marine sediment records and (2) submarine landforms suggest supraglacial melt contributed to the subglacial hydrological budget. This implies that quantifiable grain shape alteration in MPDs could reflect a combination of high-energy flow of subglacial meltwater, persistent sediment entrainment, and/or long sediment transport distances through subglacial drainage pathways. Integrating grain micromorphology into analysis of MPDs in site-specific studies could therefore aid in distinguishing periods of persistent, well-connected subglacial discharge from periods of sluggish or disorganized drainage. In the wider context of deglacial marine sedimentary and bathymetric records, a grain micromorphological approach may bolster our ability to characterize ice response to subglacial meltwater transmission through time. This work additionally demonstrates that glacial and fluvial surface textures are retained on silt-sized quartz grains in adequate amounts for microtexture analysis, which has heretofore been conducted exclusively on the sand fraction. Therefore, grain microtextures can be examined on silt-rich glaciogenic deposits that contain little to no sand as a means to evaluate sediment transport processes.