Browsing by Author "Kaplan, Michael R."
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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 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 Polar desert chronologies through quantitative measurements of salt accumulation(Geological Society of America, 2018-02) Graly, Joseph A.; Licht, Kathy J.; Druschel, Gregory K.; Kaplan, Michael R.; Earth Sciences, School of ScienceWe measured salt concentration and speciation in the top horizons of moraine sediments from the Transantarctic Mountains (Antarctica) and compared the salt data to cosmogenic-nuclide exposure ages on the same moraine. Because the salts are primarily of atmospheric origin, and their delivery to the sediment is constant over relevant time scales, a linear rate of accumulation is expected. When salts are measured in a consistent grain-size fraction and at a consistent position within the soil column, a linear correlation between salt concentration and exposure age is evident. This correlation is strongest for boron-containing salts (R2 > 0.99), but is also strong (R2 ≈ 0.9) for most other water-extracted salt species. The relative mobility of salts in the soil column does not correspond to species solubility (borate is highly soluble). Instead, the highly consistent behavior of boron within the soil column is best explained by the extremely low vapor pressure of boric acid at cold temperatures. The environment is sufficiently dry that mobility of salt species within the soil column is controlled by vapor phase effects. In other cold desert settings, topsoil salts, specifically boron, may be employed as a proxy for relative sediment exposure age.Item Provenance of the ice-cored moraine at Mt. Achernar, Law Glacier, Antarctica(2014) Bader, Nicole Ann; Licht, Kathy J.; Kaplan, Michael R.; Swope, R. Jeffery; Flood, TimothyGlacial till from the Mt. Achernar moraine (MAM) records pre- and post- last glacial maximum (LGM) compositional variability of an East Antarctic moraine sequence through time and space. Pebble lithology, detrital zircon geochronology, and till geochemistry were analyzed on samples from a 6.5 km transect. Hummocky topography occurs with the most recently exposed material along the active ice margin (Zone 1), followed by a relatively flat and low region (Zone 2), and then a series of ~2 m high parallel/sub-parallel ridges and troughs accompanied by distinct color changes that are directly related to the dominant lithology of the region (Zones 3–5). Zone 3 is dominated by ~38% more sedimentary rocks than adjacent zones and has an overall shape of a broad arch superimposed with smaller ridges. Zone 4 is composed of distinct colored bands that alternate between dominant sedimentary and mafic igneous lithologies. These dominant sedimentary and intermediate/mafic igneous rocks for all Zones are interpreted to be primarily the Beacon and Ferrar Supergroup rocks respectively. The U/Pb data from the till is consistent with a Beacon Supergroup source as samples consistently show significant populations from the Permian ~250-260 Ma, the Proterozoic ~565–600 Ma, ~950–1270 Ma, and ~2300-2320 Ma, as well as (and) the late Archean ~2700-2770 Ma. The Pagoda, Mackellar, Buckley, and Fremouw Formations are potential sources of the detrital zircons. When paired with surface exposure ages, the U/Pb data indicates that the debris source has been consistent over the past ~555 ka, implying relatively stable ice sheet behavior. However, ice sheet change is indicated by a trim line present on Mt. Achernar that can be traced back to the boundary between Zones 3 and 4, as well as a change in pebble lithology, geochemistry, and morphology of Zone 3. Zone 3 records a time of ice sheet thickening and a change in provenance during the LGM. Zone 4 is pre-LGM, Zone 2 records deglaciation, and Zone 1 is still actively connected to the Law Glacier. This study reveals the broader importance of using multiple provenance techniques when interpreting provenance changes in till over time.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.