Browsing by Author "Hemming, Sidney R."

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    Analysis of Antarctic glacigenic sediment provenance through geochemical and petrologic applications
    (Elsevier, 2017-05) Licht, Kathy J.; Hemming, Sidney R.; Department of Earth Sciences, School of Science
    The number of provenance studies of glacigenic sediments in Antarctica has increased dramatically over the past decade, providing an enhanced understanding of ice sheet history and dynamics, along with the broader geologic history. Such data have been used to assess glacial erosion patterns at the catchment scale, flow path reconstructions over a wide range of scales, and ice sheet fluctuations indicated by iceberg rafted debris in circumantarctic glacial marine sediments. It is notable that even though most of the bedrock of the continent is ice covered and inaccessible, provenance data can provide such valuable information about Antarctic ice and can even be used to infer buried rock types along with their geo- and thermochronologic history. Glacigenic sediments provide a broader array of provenance analysis opportunities than any other sediment type because of their wide range of grain sizes, and in this paper we review methods and examples from all size fractions that have been applied to the Antarctic glacigenic sedimentary record. Interpretations of these records must take careful consideration of the choice of analytical methods, uneven patterns of erosion, and spatial variability in sediment transport and rock types, which all may lead to a preferential identification of different elements of sources in the provenance analyses. Because of this, we advocate a multi-proxy approach and highlight studies that demonstrate the value of selecting complementary provenance methods.
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    Double dating detrital zircons in till from the Ross Embayment, Antarctica
    (2014-05-21) Welke, Bethany Marie; Licht, Kathy J.; Hemming, Sidney R.; Martin, Pamela
    U/Pb and (U-Th)/He (ZHe) dating of detrital zircons from glacial till samples in the Ross Embayment, Antarctica records cooling after the Ross/Pan-African orogeny (450-625 Ma) followed by a mid-Jurassic to mid-Cretaceous heating event in the Beacon basin. Zircons were extracted from till samples from heads of major outlet glaciers in East Antarctica, one sample at the mouth of Scott Glacier, and from beneath three West Antarctic ice streams. The Ross/Pan-African U/Pb population is ubiquitous in these Antarctic tills and many Beacon Supergroup sandstones, thus 83 grains were analyzed for ZHe to subdivide this population. Two ZHe age populations are evident in East Antarctic tills, with 64% of grains 115-200 Ma and 35% between 200-650 Ma. The older population is interpreted to be associated with the Ross/Pan-African orogeny including cooling of the Granite Harbour Intrusives and/or exhumation of the older basement rocks to form the Kukri Peneplain. The lag time between zircon U/Pb, ZHe and 40Ar/39Ar ages from K-bearing minerals show cooling over 200 My. Grains in East Antarctic tills with a ZHe age of 115-200 Ma likely reflects regional heating following the breakup of Gondwana from the Ferrar dolerite intrusions, subsidence within the rift basin, and a higher geothermal gradient. Subsequent cooling and/or exhumation of the Transantarctic Mountains brought grains below the closure temperature over a span of 80 My. This population may also provide a Beacon Supergroup signature as most of the tills with this age are adjacent to nunataks mapped as Beacon Supergroup and contain an abundance of vi Beacon pebbles within the moraine. Nine zircons grains from three Beacon Supergroup sandstones collected from moraines across the Transantarctic Mountains yield ages from 125-180 Ma. West Antarctic tills contain a range of ZHe ages from 75-450 Ma reflecting the diverse provenance of basin fill from East Antarctica and Marie Byrd Land. ZHe and U/Pb ages <105 Ma appear to be distinctive of West Antarctic tills. The combination of U/Pb, ZHe and 40Ar/39Ar analyses demonstrates that these techniques can be used to better constrain the tectonic evolution and cooling of the inaccessible subglacial source terrains beneath the Antarctic Ice Sheet.
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    Rapid erosion of the central Transantarctic Mountains at the Eocene-Oligocene transition: Evidence from skewed (U-Th)/He date distributions near Beardmore Glacier
    (Elsevier, 2021) He, John; Thomson, Stuart N.; Reiners, Peter W.; Hemming, Sidney R.; Licht, Kathy J.; Earth and Environmental Sciences, School of Science
    Apatite (U-Th)/He thermochronology has the potential to reconstruct records of erosional exhumation that are critical to understanding interactions between climate, tectonics, and the cryosphere at high latitudes on million-year timescales. However this approach is often hindered by the problem of intrasample single-grain date dispersion. Here we present an extensive new apatite (U-Th)/He dataset (n = 361) from the central Transantarctic Mountains of East Antarctica between 160°E to 170°W and 84 to 86°S, and show that apparently uninterpretable data in most samples are a reflection of inadequate sampling of skewed date distributions. We outline a workflow for interpreting such dispersed data and demonstrate that geologically meaningful age interpretations are possible in the case of rapidly cooled samples, despite the wide array of potential causes for date dispersion. We show that for samples and compilations with a large number of single-grain analyses (n > ∼25), the youngest probability distribution peak represents the most likely time of fast cooling through the apatite (U-Th)/He closure temperature. When fewer grains are analyzed, the youngest peak is represented best by the minimum date or first quartile date, depending on sample size. Using this workflow, we show that since the latest Eocene, up to 8.8 km of exhumation occurred to incise the deepest point of the Beardmore Glacier trough. Rapid incision began at c. 37-34 Ma (at the latest by 34±3 Ma), coinciding with or slightly preceding the initiation of Antarctic glaciation at the Eocene-Oligocene transition, and contributed to at least 2.6 km of exhumation within the first 3-6 million years, at an apparent exhumation rate of no less than 0.4 mm/a.