Sandra Passchier

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15u2009million years (Myr) ago, when planetary temperatures were 3–4u2009°C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7u2009Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450u2009kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40u2009kyr) and eccentricity (125u2009kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250u2009kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7u2009Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

Implications of quartz grain microtextures for onset Eocene/Oligocene glaciation in Prydz Bay, ODP Site 1166, Antarctica

Abstract This paper presents the results of the scanning electron microscopic (SEM) analysis of quartz grains from a selection of samples at Site 1166. Ocean Drilling Program Leg 188 drilled Site 1166 on the Prydz Bay continental shelf, Antarctica, to document onset and fluctuations of East-Antarctic glaciation. This site recovered Upper Pliocene–Holocene glacial sediments directly above Cretaceous through Lower Oligocene sediments recording the transition from preglacial to early glacial conditions. SEM analysis of quartz grains at Site 1166 was used to characterize the glacial and preglacial sediments by their diagnostic textures. Angular edges, edge abrasion as well as arcuate to straight steps, are the most frequent features in glacial deposits. The highest frequency of grains with round edges is present in Middle–Late Eocene fluvio–deltaic sands. However, angular outlines, fractured plates with subparallel linear fractures and edge abrasion indicating glacier influence are also present. Preglacial carbonaceous mudstone and laminated gray claystone show distinctive high relief quartz grains and some chemical weathering on grain surfaces. The results of the microtextural analysis of quartz grains are used to verify some critical periods of ice sheet evolution, such as the transition from the East Antarctic preglacial to glacial conditions on the continental shelf from Middle/Late Eocene to Late Eocene/Early Oligocene time.

Variability in Geochemical Provenance and Weathering History of Sirius Group Strata, Transantarctic Mountains: Implications for Antarctic Glacial History

ABSTRACT The major-element and trace-element geochemistry of 57 diamicts and muds of the Sirius Group is evaluated to determine the provenance and stratigraphic relations of geographically widespread outcrops in the Transantarctic Mountains. The origin and age of the Sirius Group are heavily debated because of the presence of marine diatoms inferred to indicate a reduced East Antarctic ice sheet as recent as the Pliocene. A key question is whether glaciers transported sediments and the enclosed diatoms from the East Antarctic interior to the Transantarctic Mountains. Major-element and trace-element ratios indicate that the greater part of the Sirius Group sediments have an East Antarctic provenance and that a few high-elevation deposits are locally derived. The degree of chemical weathering of the materials in the Sirius Group, expressed as the chemical index of alteration (CIA), has a wide range (41-70). On the basis of geochemical provenance, weathering intensity, and morphostratigraphic position, the Sirius Group can be subdivided into three subgroups, which probably represent multiple glacial phases: (1) sediments with a local provenance from high-elevation outcrops on spurs and mountain summits, > 2000 m, which contain abundant weathered materials; (2) sediments in high-elevated outcrops, > 2000 m, with intermediate CIAs, originating from an East Antarctic Ice Sheet, which was overriding parts of the Transantarctic Mountains and; (3) sediments recovered within, or on the margins of, the present glacial troughs at 1500-1800 m, with low CIAs and an East Antarctic provenance. Comparison with the weathering history inferred from drilling records in the nearby Victoria Land Basin reveals that glaciation in the Transantarctic Mountains probably commenced with small ice caps and alpine glaciers prior to the Eocene-Oligocene transition, and from the Oligocene onward continued with multiple phases of continental glaciation. These results and interpretations contribute considerably to the understanding of the landscape evolution of the Transantarctic Mountains, the Sirius Group debate, and the history of East Antarctic glaciation.

Provenance of the Sirius Group and related Upper Cenozoic glacial deposits from the Transantarctic Mountains, Antarctica: relation to landscape evolution and ice-sheet drainage

Abstract This study uses bulk X-ray diffraction (XRD), heavy mineral analysis and analysis of detrital modes of the Sirius Group and related Upper Cenozoic deposits to evaluate their provenance. The data suggest that deposition of the Sirius Group occurred as a result of long-term glacial denudation of the Transantarctic Mountains. Such a conclusion is in contrast with the traditional view of deposition on a pre-existing high relief glacial topography during one relatively short-term phase of glacial expansion by the East Antarctic Ice Sheet. Two petrofacies are distinguished, which have different geomorphological settings and paleo-ice flow directions. One petrofacies includes much material from the higher elevated sedimentary sequences of the Transantarctic Mountains and represents overriding of the mountains and deposition of part of the Sirius Group during or after the late Oligocene to early Miocene. The second petrofacies is characterized by a large input from crystalline basement sources and represents active erosion and deposition in glacial troughs during the Neogene. The second petrofacies of the Sirius Group shows similarities to the record from Dry Valley Drilling Project core 11 (DVDP-11), which indicates that East Antarctic ice was last flowing through the Dry Valleys in the late Miocene to early Pliocene.