Papers by Christine Kassab
Quaternary Science Reviews, 2017
Till in an extensive blue ice moraine in the central Transantarctic Mountains at Mt. Achernar sho... more Till in an extensive blue ice moraine in the central Transantarctic Mountains at Mt. Achernar shows relatively continuous deposition by East Antarctic derived ice throughout the last glacial cycle. The most recently exposed material along the active margin of the Law Glacier (Zone 1) has hummocky topography that transitions into to a relatively flat region (Zone 2), followed by a series of ~2 m high continuous, parallel/sub-parallel ridges and troughs (Zones 3-5). The entire moraine is ice-cored. Past surface changes of <40 m are indicated by a lateral moraine at the base of Mt. Achernar and substantial topographic relief across Zone 3. Pebble lithology and detrital zircon geochronology were analyzed on samples along a 6.5 km transect across the moraine which formed from sub-and englacial debris. Beacon and Ferrar Supergroup rocks comprise most rock types on the moraine surface. Overall, pebbles in Zones 1, 4 and 5 are dominated by igneous rocks of the Ferrar dolerite, whereas Zones 2 and 3 have ~40% more Beacon Supergroup sedimentary rocks. Zone 4 is characterized by distinctly colored lithologic bands, 5-20 m wide, that alternate between dominant Beacon and Ferrar rock types. When combined with surface exposure ages, we conclude that Zones 2 and 3 contain sediment accumulated through the last glacial maximum (LGM). In contrast to pebble data, the U-Pb zircon data from till across all zones show little variability and are consistent with a Beacon Supergroup source, as samples show significant populations from the Proterozoic, ~550-600 Ma and ~950-1270 Ma, as well as the late Archean ~2700-2770 Ma. The Mackellar, Fairchild, and lower Buckley Formations are interpreted as dominant sources of the detrital zircons. The zircon data lack the spatio-temporal variability indicated by the pebble fraction because the local Ferrar dolerite is not zircon bearing, highlighting the broader importance of using multiple techniques when interpreting provenance changes over time. Rather than reflecting major changes in ice flow path over time, the provenance changes are interpreted to indicate relative stability of the East Antarctic ice sheet, as the Law Glacier tapped into and eroded successively lower stratigraphic units of the Beacon Supergroup. This has important implications for interpreting offshore provenance records.
Journal of Glaciology, 2018
ABSTRACTStable isotopes of water (δ18O and δ2H) were measured in the debris-laden ice underlying ... more ABSTRACTStable 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 i...
Geology, 2017
Past behavior of outlet glaciers draining the East Antarctic Ice Sheet (EAIS) remains unresolved ... more Past 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-lowing, sublimating ice along the margin of Law Glacier. 10 Be, 26 Al, and 3 He 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. BACKGROUND Knowledge of East Antarctic Ice Sheet (EAIS) behavior prior to Marine Isotope Stage 2 (MIS2) is commonly obtained from ice cores or indirectly from marine records or modeling (e.g., Petit et al., 1999; DeConto and Pollard, 2016) because of the scarcity of well-dated terrestrial deposits. Exceptions include dating of lateral moraines and drift sheets in the Dry Valleys (e.g., Sugden et al., 1993; Staiger et al., 2006) and elsewhere in the Transarctic Mountains, where more widely spaced studies have tended to focus on a million-year time scale, or the history of glacier thinning since MIS2 (e.g.
Quaternary Research, 2017
A pilot study was designed to evaluate the potential of zircon geochronology as a provenance indi... more A 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 M...
Paleoglaciology of the Dalijia Shan, northeastern Tibetan Plateau
Understanding past variations in the extent of glaciers is important for reconstructing past clim... more Understanding past variations in the extent of glaciers is important for reconstructing past climates and predicting future climate change, and mountain glaciers provide a particularly sensitivity record of paleoclimate. Investigating the paleoglacial history of the Dalijia Shan, a small mountain range on the northeastern margin of the Tibetan Plateau, is important both regionally and as a contribution to the growing knowledge of paleoglaciation on the Tibetan Plateau; the Dalijia Shan is one of the northeasternmost mountain ranges of the Tibetan Plateau that has a glacial record and it is located near the transition zone between the two major climate systems influencing the Plateau. A variety of techniques are used here to investigate the record of paleoglaciation of the Dalijia Shan: (1) glacial landform mapping from a combination of remote sensing imagery (Digital Elevation Models, Landsat 7 ETM+ satellite imagery, and Google Earth™), (2) calculating valley morphometry to determine valley type (fluvial or glacial), and (3) 10Be terrestrial cosmogenic nuclide (TCN) exposure age dating to determine the timing of glacial events. Paleoglaciation in the Dalijia Shan was relatively limited in extent. Most of the glacial landforms (moraines and U-shaped valleys) are located around Dalijia Peak, covering ∼1.3% of the study area (10,730 km2). Quantifying valley morphometry confirms the classification of U-shaped versus V-shaped valleys in this region. The extent of glaciation in valleys that have experienced both glacial and fluvial erosion is identified from the exponent values of the power law function, confirming the mapping results. Four glacial events are identified based on the TCN ages from moraines within the Deheisui Valley: ∼38.6 +32.4/-6.85 ka (MIS3), ∼23.5 +3.38/-1.43 ka to 21.8 +3.24/-1.42 ka (MIS2), ∼17.3 +1.05/-0.61 ka (MIS2), and ∼11.7 +0.24/-0.22 ka (Younger Dryas). Based on the locations of the associated moraines, glaciation appears to have been more extensive during the MIS3 event, which is similar to observations made in western, central, and southern regions of the Tibetan Plateau and less extensive during the global Last Glacial Maximum (MIS2). This study provides insights on the influence of climate systems and the timing of paleo-glaciations along the northeastern margin of the Tibetan Plateau, as it shows that the glacial chronology of this region is consistent with that of other regions on the Plateau and is asynchronous with the Northern Hemisphere ice sheets during the Late Quaternary
Spatial and Temporal Patterns of Past Mountain Glaciation in the Tian Shan, Central Asia
Paleoglaciology of the central East Antarctic Ice Sheet as revealed by blue-ice sediment
Quaternary Science Reviews
Geochimica et Cosmochimica Acta
Mt Achernar Moraine is a high altitude, high latitude blue ice moraine where typical conditions p... more Mt 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 mm 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. H 2 CO 3) 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 CO 2 drawdown into carbonate minerals increases as the moraine progressively thickens, from 3 mgÁm 2 Áa À1 in freshly emerging sediments to $50 mgÁm 2 Áa À1 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 CO 2 into carbonate minerals occurs in a terrestrial setting where water exists only in vapor form.
The development of blue ice moraines from englacial debris bands as detected by GPR, Mt Achernar, central Transantarctic Mountains, Antarctica
AGU Fall Meeting Abstracts, Dec 1, 2016
East Antarctic Ice Sheet Stability Since the Mid-Pleistocene Recorded in a High-Elevation Ice-Cored Moraine
Blue ice moraine development and evolution, Mt Achernar, central Transantarctic Mountains, Antarctica
Approximately 23,000 yrs. ago, during the Wisconsin glacial advance, three ice lobes from the Lau... more Approximately 23,000 yrs. ago, during the Wisconsin glacial advance, three ice lobes from the Laurentide ice sheet originating in Canada flowed into Indiana. The Lake Michigan lobe entered from the north, the Erie lobe from the northeast, and the Saginaw lobe in between. The distribution of deposits from the lobes is easily discernable in some locations where the land has been molded into moraines, but in other locations, the ice sheet history is uncertain. We investigate whether the age spectra of sand-sized zircon minerals in the glacial deposits can be used as a reliable tracer of glacial ice lobe movement when the direction of ice flow is ambiguous. Glacial till samples from Lake Michigan, Saginaw, and Erie lobe tills and one sample from a till of unknown origin were analyzed using zircon geochronology. Because the upstream bedrock has distinctly different ages it was hypothesized that the samples would reflect that by having different ages. A total of 450 grains were analyzed v...
Interest in stream restoration has increased over the last two decades, leading to a growth in th... more Interest in stream restoration has increased over the last two decades, leading to a growth in the industry to the point that it has become a 1 billion dollar per year enterprise (Bernhart et al 2005, Thompson 2002). In northcentral Pennsylvania alone, over $9 million has been spent since 1999 on stream restoration projects, from the designing stage to actual construction and reconstruction of the sites. Even though a extensive amount of money is being spent on the construction of these projects, very little to no post-monitoring is taking place. Without post-monitoring of the projects, it is unknown if they actually work. After a successful statewide stream restoration assessment was completed in North Carolina by the advisor of this project, it was discussed to undertake a similar one for the state of Pennsylvania starting with the northcentral region. This includes Bradford, Cambria, Cameron, Centre, Columbia, Lycoming, McKean, Montour, Northumberland, Potter, Sullivan, Tioga, an...
Alternative coordinate systems for analyzing cross-section shapes of glaciated valleys: A case study from the Dalijia Mountains, China
Physical Geography, 2013
ABSTRACT The morphometry of glaciated valleys has been of considerable significance in evaluating... more ABSTRACT The morphometry of glaciated valleys has been of considerable significance in evaluating models and processes of valley form development, and is typically evaluated using a power law function fit to topographic data. Results of the power law method are sensitive to the location of the coordinate system origin, but most previous research has used a coordinate system centered on the valley midpoint and has not systematically assessed alternative approaches. Here, we test four options for selecting the coordinate system origin, using cross-section profiles from glacial, fluvial, and transitional valleys in the Dalijia Mountains, China: (A) the lowest elevation point, (B) the valley midpoint, (C) the lowest point on the valley side wall, and (D) the highest point on the valley side. Option A produces results that are most consistent with expected values and trends for shape parameters in the Dalijia Mountains. Over half of the values calculated using Option B overestimate key valley-shape parameters compared to Option A. Options C and D produce similar values to each other, but these values are very different from Option A and less useful for landform inter-pretation. In morphometric studies designed to evaluate impacts of glacial erosion using the power law approach, we recommend that alternate coordinate system origins be evaluated, including the lowest elevation point on a cross-profile. Introduction In areas that have been glaciated, valleys that have a U-shaped cross-section are often described as an end member of valley-form transformation from fluvial (or pre-glacial) to glacial morphologies, and the degree of shape transformation is used as a measure of glacial modification (e.g. Benn. In a simple evolution-ary model, valleys that are subjected to glacial erosion will change from some pre-glacial configuration, often a V-shaped fluvial form, and become more U-shaped as glaciers erode the valley bottom and sides Montgomery, 2002). In this type of model, the stage of valley evolution, or degree of glacial modification, can be assessed based on quantification of the valley&#39;s morphology in terms of cross-valley shape (Doornkamp &amp; King, 1971; Graf, 1970; Hirano &amp; Aniya, 1988). In reality, pre-glacial valleys may have a wide range of cross-sectional forms. Over time, phases of
Quaternary Science Reviews, 2014
Paleoclimatic constraints from regions at the confluence of major climate systems are particularl... more Paleoclimatic constraints from regions at the confluence of major climate systems are particularly important in understanding past climate change. Using geomorphic mapping based on remote sensing and field investigations, combined with in situ cosmogenic 10 Be and 26 Al dating of boulders associated with glacial landforms, we investigate the chronology of past glaciation in the Inylchek and Sary-Dzaz valleys in the eastern Kyrgyz Tian Shan, a tectonically active area with some of the highest peaks in the world outside of the Himalayas. Cosmogenic 10 Be and 26 Al exposure ages of boulders on moraines record up to five glacial advances including: Lateglacial age lateral moraine remnants and meltwater channels in the upper Inylchek Valley; Last Glacial Maximum (LGM, Marine Oxygen Isotope Stage [MIS] 2) moraines in the Sary-Dzaz Valley and in a terminal moraine complex at the west end of the Inylchek Valley, overriding older moraines; an MIS 4 or 5 moraine remnant above the Inylchek terminal moraine complex; and an older high moraine remnant down-valley from the confluence of the Inylchek and Sary-Dzaz valleys. The evidence for glacial extent in this study is consistent with a limited ice expansion hypothesis for Tian Shan glaciation. Published results from the western and central Kyrgyz Tian Shan do not show evidence for significant LGM glacier expansion, which in combination with the results presented here, indicate a spatial variation in glacier records along the Tian Shan. This may reflect either paleoclimatic gradients or the impact of local physiographic conditions on responses to regional climate change, or both.
Quaternary Research, 2013
Cosmogenic nuclide (CN) apparent exposure dating has become a widely used method for determining ... more Cosmogenic nuclide (CN) apparent exposure dating has become a widely used method for determining the age of glacial landforms on the Tibetan Plateau with > 1200 published ages. We present the first 10Be exposure ages from the Dalijia Shan, the most northeastern formerly glaciated mountain range on the Tibetan Plateau. The moraine groups identified from field and remote sensing imagery mapping record four glacial events at 37.07 ± 3.70 to 52.96 ± 4.70 ka (MIS 3), 20.17 ± 1.79 to 26.99 ± 2.47 ka (MIS 2), 16.92 ± 1.49 to 18.76 ± 1.88 ka (MIS 2), and 11.56 ± 1.03 to 11.89 ± 1.06 ka (Younger Dryas). These ages indicate that glaciation in the northeastern Tibetan Plateau is much younger than previously thought. In addition, this record is consistent with many other regions on the Tibetan Plateau, with a local last glacial maximum during MIS 3 asynchronous with Northern Hemisphere last glacial maximum during MIS 2. The Dalijia Shan might also include an event of Younger Dryas age, but t...
Glacial geomorphology of the Dalijia Shan region, northeastern Tibetan Plateau
Journal of Maps, 2013
Geochemistry, Geophysics, Geosystems, 2016
Till from moraines at the heads of six major outlet glaciers in the Transantarctic Mountains (TAM... more Till from moraines at the heads of six major outlet glaciers in the Transantarctic Mountains (TAM) and from till beneath three West Antarctic ice streams have a ubiquitous zircon U-Pb age population spanning the time of the Ross/Pan-African Orogenies (610-475 Ma). Geochronology and thermochronology of detrital minerals in these Antarctic glacial tills reveal two different thermal histories for the central and southern TAM. Double-dating of the zircons reveals a geographically widespread (U-Th)/He (ZHe) population of 180-130 Ma in most of the till samples. Sandstone outcrops at Shackleton Glacier, and three Beacon Supergroup sandstone clasts from three moraines, have ZHe ages that fall entirely within this range. The similar population and proximity of many of the till samples to Beacon outcrops lead us to suggest that this extensive ZHe population in the tills is derived from Beacon Supergroup rocks and reflects the thermal response of the Beacon Basin to the breakup of Gondwana. A second population of older (>200 Ma) ZHe ages in tills at the head of Byrd, Nimrod, and Reedy Glaciers. For the tills at the head of the Nimrod and Byrd Glaciers, integrating the double-dated zircon results with 40 Ar/ 39 Ar of hornblende, muscovite and biotite, and U-Pb and (U-Th-Sm)/He double-dates on apatite yields a typical pattern of early rapid orogenic cooling (4-108C/Myr) 590-475 Ma after the emplacement of the Granite Harbour Intrusives. Low temperature thermochronometers at these sites yield variable but quite old ages (ZHe 480-70 Ma and AHe 200-70 Ma) that require a long history at low temperature.
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Papers by Christine Kassab