Browsing by Author "Kassab, Christine M."
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Item Chemical weathering signatures at Mt. Achernar, Central Transantarctic Mountains II: Surface exposed sediments(Elsevier, 2022-10-01) Graly, Joseph A.; Licht, Kathy J.; Bader, Nicole A.; Kassab, Christine M.; Bish, David L.; Kaplan, Michael R.; Earth and Environmental Sciences, School of ScienceMt Achernar Moraine is a high altitude, high latitude blue ice moraine where typical conditions preclude the presence of liquid water. Cosmogenic and salt accumulation dating indicate that the moraine’s surface is progressively older away from the active ice margin, with surface exposure ages up to 1 Ma. We analyze the chemical and mineralogical transformations in the <63 µm fraction along transects across the moraine. Data include bulk chemical composition, crystalline mineralogy by X-ray diffraction (XRD), and the composition of amorphous or low abundance products of chemical weathering by sequential extraction. These data are analyzed by multiple regression as a function of exposure age and as a function of composition of the moraine’s cobble and pebble-sized clasts. Change with exposure age is defined by the development of salts and carbonate minerals along with the input of detrital material, principally from sedimentary rocks. Clay minerals and amorphous cements breakdown as detrital material in proportions far above their abundance in the rock clasts, whereas framework silicates (i.e. feldspars and quartz) break down in relatively small proportions. Both the carbonate minerals and some of the salts form from atmospheric acids (i.e. H2CO3) that in turn react with other minerals. Mass balance shows that the input of these atmospheric acids balances with gains in authigenic smectites, zeolites, and amorphous material. Many of these minerals also form in the subglacial environment, but are poorly represented in the underlying rock, suggesting a similar chemical weathering regime in both the subglacial and surface environments of this hyper cold and arid setting. The rate of CO2 drawdown into carbonate minerals increases as the moraine progressively thickens, from 3 mg·m2·a−1 in freshly emerging sediments to ∼50 mg·m2·a−1 after 500 ka of exposure. Weathering from acidic aerosols is proportional to atmospheric flux documented in ice cores and does not vary with moraine thickness. The carbonate mineral formation rates are more than an order of magnitude below those of the subglacial environment and as much as two orders of magnitude below those found in warm desert soils. Nevertheless, the drawdown of atmospheric CO2 into carbonate minerals occurs in a terrestrial setting where water exists only in vapor form.Item Exploring the use of zircon geochronology as an indicator of Laurentide Ice Sheet till provenance, Indiana, USA(Cambridge, 2017) Kassab, Christine M.; Brickles, Samantha L.; Licht, Kahty J.; Managhan, G. William; Earth Sciences, School of ScienceA pilot study was designed to evaluate the potential of zircon geochronology as a provenance indicator of till from the Lake Michigan, Saginaw, and Huron-Erie Lobes of the Laurentide Ice Sheet. Based on existing ice flow-path models, we hypothesized that till from each lobe would have different zircon age population distributions because the lobes originated from regions of the Canadian Shield with different bedrock ages. After correcting for zircon fertility, the majority of grains in all till samples are 1600–950 Ma, with ~30 % of ages >2500 Ma. This similarity means that till from the three lobes cannot be clearly differentiated based on their zircon populations. The dominant ages found and the homogeneity of distributions in the till indicates a non-Shield source and, instead, reflect an origin from some combination of underlying till and sedimentary bedrock in the Great Lakes region. Even though the datasets are small, the tills have similarities to zircon distributions in Michigan Basin rocks. This implies that a substantial fraction of zircon in till was not transported long distances from the Canadian Shield. Although zircon ages are not distinct between tills, the method provides a novel application to understand Laurentide Ice Sheet glacial erosion and transport.Item Formation and evolution of an extensive blue ice moraine in central Transantarctic Mountains, Antarctica(Cambridge UP, 2020-02) Kassab, Christine M.; Licht, Kathy J.; Petersson, Rickard; Lindbäck, Katrin; Graly, Joseph A.; Kaplan, Michael R.; Earth Sciences, School of ScienceMount Achernar moraine is a terrestrial sediment archive that preserves a record of ice-sheet dynamics and climate over multiple glacial cycles. Similar records exist in other blue ice moraines elsewhere on the continent, but an understanding of how these moraines form is limited. We propose a model to explain the formation of extensive, coherent blue ice moraine sequences based on the integration of ground-penetrating radar (GPR) data with ice velocity and surface exposure ages. GPR transects (100 and 25 MHz) both perpendicular and parallel to moraine ridges at Mount Achernar reveal an internal structure defined by alternating relatively clean ice and steeply dipping debris bands extending to depth, and where visible, to the underlying bedrock surface. Sediment is carried to the surface from depth along these debris bands, and sublimates out of the ice, accumulating over time (>300 ka). The internal pattern of dipping reflectors, combined with increasing surface exposure ages, suggest sequential exposure of the sediment where ice and debris accretes laterally to form the moraine. Subsurface structure varies across the moraine and can be linked to changes in basal entrainment conditions. We speculate that higher concentrations of debris may have been entrained in the ice during colder glacial periods or entrained more proximal to the moraine sequence.Item Middle to Late Pleistocene stability of the central East Antarctic Ice Sheet at the head of Law Glacier(GSA, 2017-08) Kaplan, M. R.; Licht, Kathy J.; Winckler, G.; Schaefer, J. M.; Bader, Nicole; Mathieson, C.; Roberts, M.; Kassab, Christine M.; Schwartz, R.; Graly, Joseph A.; Earth Sciences, School of SciencePast behavior of outlet glaciers draining the East Antarctic Ice Sheet (EAIS) remains unresolved prior to Marine Isotope Stage 2 (MIS2). Study of blue ice moraines provides a relatively untapped approach to understand former EAIS activity. We focus on a blue ice moraine near Mount Achernar in the central Transantarctic Mountains, at the edge of the polar plateau. The well-preserved moraine consists of quasi-continuous or hummocky sediment ridges that form on top of upward-flowing, sublimating ice along the margin of Law Glacier. 10Be, 26Al, and 3He cosmogenic nuclide ages on boulders from the ridges are coherent and in general are progressively older with distance from the relatively clean ice of the Law Glacier margin. Moraines closest to the Law Glacier margin postdate MIS2; farther away, they date to the last glacial cycle, and with more distance they are hundreds of thousands of years old. We conclude that cosmogenic dating of some blue ice moraines can provide age limits for changes at the heads of outlet glaciers that drain the central East Antarctic Ice Sheet, including prior to MIS2. Furthermore, the geomorphological, cosmogenic nuclide, and sedimentological evidence imply that the East Antarctic polar plateau adjacent to the central Transantarctic Mountains has been relatively stable for at least 200 k.y.Item Warm-based basal sediment entrainment and far-field Pleistocene origin evidenced in central Transantarctic blue ice through stable isotopes and internal structures(Cambridge University Press, 2018-04) Graly, Joseph A.; Licht, Kathy J.; Kassab, Christine M.; Bird, Broxton W.; Kaplan, Michael R.; Earth Sciences, School of ScienceStable isotopes of water (δ18O and δ2H) were measured in the debris-laden ice underlying an Antarctic blue ice moraine, and in adjoining Law Glacier in the central Transantarctic Mountains. Air bubble content and morphology were assessed in shallow ice core samples. Stable isotope measurements plot either on the meteoric waterline or are enriched from it. The data cluster in two groups: the ice underlying the moraine has a δ2H:δ18O slope of 5.35 ± 0.92; ice from adjoining portions of Law Glacier has a slope of 6.69 ± 1.39. This enrichment pattern suggests the moraine's underlying blue ice entrained sediment through refreezing processes acting in an open system. Glaciological conditions favorable to warm-based sediment entrainment occur 30–50 km upstream. Basal melting and refreezing are further evidenced by abundant vapor figures formed from internal melting of the ice crystals. Both the moraine ice and Law Glacier are sufficiently depleted of heavy isotopes that their ice cannot be sourced locally, but instead must be derived from far-field interior regions of the higher polar plateau. Modeled ice flow speeds suggest the ice must be at least 80 ka old, with Law Glacier's ice possibly dating to OIS 5 and moraine ice older still.