Palaeozoic and Mesozoic evolution of Antarctica

In an invited review paper, David Elliot provides a comprehensive and concise account of the geological and tectonic history of one of the world's great ranges, the Transantarctic Mountains. Although much of them is hidden from view under the present ice sheet, there are plentiful exposures, especially where they flank the Ross Ice Shelf and Ross Sea. The mountains were initiated in Neoproterozoic time with rifting of the supercontinent of Rodinia, followed by intrusion and metamorphism during the Cambrian Ross Orogeny. Subsequent erosion of the orogeny produced a major erosion surface, upon which Devonian to Jurassic sediments accumulated in an intra-cratonic basin, while magmatic arcs developed on the surrounding continental margins. Gondwana break-up in Early Jurassic time was associated with the emplacement of the extensive Ferrar Large Igneous Province, events that marked the reorganization of plates and uplift of the Transantarctic Mountains from Cretaceous time onwards.

A novel approach to deriving the history of faulting, uplift and unroofing of the Transantarctic Mountains is presented by Daniel Foley, Edmund Stump and others. Using a combination of apatite (U–Th)–He thermochronology and geomorphological analysis from satellite imagery, they show that the denudation rate of the mountains amounted to c. 0.04 mm per year from c. 140 to 40 Ma. They also infer differential movement of the order of 1 km across an inferred fault beneath one of Antarctica's largest outlet glaciers, the Byrd Glacier. The results support a model of relatively uniform cooling and unroofing of the region, with post-40 Ma fault-displacement that uplifted the south side of the glacier relative to the north.

In another tectonically focused paper, Adolfo Maestro and colleagues present the results of a study of intraplate stress fields under multiple remote stresses on the South Orkney micro-continent. Using a combination of fault, joint and tension gash population analyses in late Triassic to early Jurassic metamorphic rocks on Signy Island, they have determined compressive, strike-slip and extensional stress states. Overall, NNW–SSE horizontal compression was dominant, resulting from an inferred subduction-related tectonic regime, which then changed to north–south extension in Middle Jurassic to Early Cretaceous time. Complex additional stress changes took place during spreading of the Powell Basin from Eocene/Miocene time, and generated extensional structures across the micro-continent.

One of the most important stratigraphic units in Antarctica is the Beacon Supergroup, which is exposed throughout most of the length of the Transantarctic Mountains (separating the East and West Antarctic ice sheets) and in the Ellsworth Mountains in the middle of the West Antarctic Ice Sheet. These strata have a long history of investigation, starting with Scott's first expedition in 1901–1903. In an invited review, Margaret Bradshaw describes the Taylor Group, which is the lower division of the Beacon Supergroup and is of Devonian age. Resting on a major unconformity, the Kukri Erosion Surface, the Taylor Group is a cyclic series of coastal quartzose sandstones deposited in deserts, alluvial plain and shallow marine settings. Early Devonian marine faunas from the Ohio Range reveal links with other Southern Hemisphere continents, whereas mid to late Devonian terrestrial fish faunas from southern Victoria Land have global connections.

Two further papers concern palaeontological studies of specific Antarctic faunas. Firstly, Marcelo A. Reguero and colleagues describe Late Cretaceous terrestrial vertebrates from marine sediments of the James Ross Basin in the NW Weddell Sea. They focus on 10 occurrences of non-avian dinosaurs, notably on Snow Hill Island, and find that there is a Gondwanan ‘hallmark’ in the fauna that reflects climate-driven provinciality. Avian dinosaurs are also present in the region but are rare. The paper by Thomas Saucede et al. is one of the first to investigate quantitatively the palaeobiology of Antarctic echinoids. The paper explores this topic through four time intervals from Late Cretaceous time up to the present day, and provides palaeogeographical maps of the distribution of echinoids, all within the context of the evolution of Antarctic palaeogeography.

Cenozoic environments and glaciation

Cenozoic studies have gained precedence in Earth science research in the last two decades, stimulated by our desire to better understand the impact of the Antarctic cryosphere on global climate and sea-level. The Cenozoic Era embraces a pre-glacial phase, prior to c. 34 Ma (the Eocene/Oligocene boundary) followed by a phase of permanent and stable ice sheet-scale glaciations.

For the pre-glacial era, Piotr Jadwiszczak describes the taxonomic diversity and palaeoecology of Eocene Antarctic penguins. He has identified at least 10 species of Eocene penguins preserved in the La Meseta Formation of Seymour Island, NW Weddell Sea. The earliest reported bones of the giant penguin (genus Anthropornis) date from Late Eocene times (c. 53 Ma); a range of smaller penguin species is also described.

In a different discipline, the central Scotia Sea was the site for the development of seven heat-flow stations in order to determine magnetic anomaly-based ages of the basement, as reported by Peter Barker and co-authors. Although results are ambiguous, they affirm that basin extension began in Eocene time, supporting the concept of a relatively young back-arc origin for the central Scotia Sea, but do not affect prior interpretations concerning the onset of the Antarctic Circumpolar Current.

The glacial record of Antarctica (post c. 34 Ma) is represented first by two papers focusing on the terrestrial record. Duanne White and colleagues present evidence from the Prince Charles Mountains for palaeotopographic changes in the Lambert Glacier–Amery Ice Shelf system. These authors identify up to 1–2 km of glacial incision into a pre-glacial palaeosurface. This incision by major Cenozoic outlet glaciers has driven uplift of up to 50 m per million years since mid-Miocene time. This landscape evolution was sufficient to focus and influence Cenozoic ice-drainage patterns.

Brenda Hall and co-workers summarize the behaviour of the Antarctic Ice Sheet during the late Quaternary Period by investigating ice dynamics in the Ross Sea sector during the Last Glacial Maximum. In this invited review, the authors found ice-sheet thicknesses of up to 1000 m along the coastline of the central and southern Transantarctic Mountains and Marie Byrd Land. In that region, the Late Glacial Maximum occurred around 18 000 years ago, but as late as 7000–10 000 years ago further inland. Significant thinning took place between 13 000 and 14 600 years ago, and Hall and colleagues conclude that the Ross Sea sector was unlikely to be a major contributor to a sharp eustatic rise in sea-level around 14 000 years ago, referred to as Meltwater Pulse 1A.

Several papers deal with the glacial record in the offshore realm, combining geophysical and core-sampling of sea-floor sediment, and reflecting the major investment in marine geological research in state-of-the-art ice-breakers by a number of nations. A paper by Kari Strand and colleagues focuses on a well-known core from the Ocean Drilling Program's Leg 188 to Prydz Bay in East Antarctica. This region is second only to the western Ross Sea for studies of the long-term glaciation of Antarctica, and is important because it lies at the mouth of a major conduit of ice from the heart of the continent, the Lambert Graben. By examining the lithofacies in the core using a variety of techniques to define provenance and transport history, the authors conclude that the first evidence of glaciers dates from late Eocene time. This signal is not direct, but preserved in 110 m of alluvial plain sediments that are believed to be glaciofluvial. Full-scale glaciation is indicated by 20 m of overlying glaciomarine sediments, confirming the results from earlier cores in Prydz Bay and elsewhere in Antarctica.

Stephen Pekar and co-workers present a comprehensive investigation of multichannel seismic data, collected offshore from New Harbour in the western Ross Sea, in order to investigate the Cenozoic stratigraphy and tectonic history of a region of extensional basins bordering the Transantarctic Mountains since late Eocene time. The seismic data are tied in to previously drilled sites, so are well constrained by stratigraphy and age-determinations. The work will provide a basis for possible future continental shelf drilling in the region.

In an invited review, John Anderson and colleagues assess marine geological evidence for the behaviour of the West Antarctic, East Antarctic and Antarctic Peninsula ice sheets over the last 30 000 years. These ice sheets had largely receded by early Holocene time, completing their contribution to eustatic sea-level rise. Ice-sheet thicknesses, based on grounding depths, range from 640 to 1640 m on the inner continental shelf. Retreat around the continent was asynchronous and stepwise, with factors specific to particular drainage basins controlling ice-sheet behaviour. The resulting episodic sea-level rise is inferred to come from ice-stream collapse.

Part of the Antarctic continental shelf that has seen little attention until now has been the Amundsen Sea Embayment, which is the subject of a paper by Katharina Hochmuth and Karsten Gohl. Using a combination of geophysical and core-sampling techniques, they found that the embayment is characterized by large bathymetric depressions, formed by former ice streams of the West Antarctic Ice Sheet. By focusing on one of the largest linear depressions, the Abbot glacial trough, and analysing its glacial depositional and erosional history, they found that the delivery of sediment by grounded ice was influenced by several basement highs. The main Abbott glacier system was influenced by the extensions and convergence of several ice streams we see today, including Pine Island, Thwaites, Cosgrove and Abbott glaciers.

Continuing with the marine geological theme, Daniela Sprenk and co-workers used marine sediment cores from two deep-sea sites in the Scotia Sea to record bioproductivity changes in the Southern Ocean through the last glacial cycle. These cores, resolved on a decadal-scale, have a biogenic opal flux that records the influences of sea ice distribution and summer sea surface temperatures. South of the Polar Front, the lowest bioproductivity occurred during the Last Glacial Maximum, when the upwelling of mid-depth water was reduced and sea-ice growth intensified. Bioproductivity increased abruptly around 17 000 years ago, in concert with decreasing seasonal sea ice coverage.

Maryline Vautravers and co-workers provide data and interpretation of a marine sediment core dating back to 75 ka that was recovered from a depth of 2300 m off the west coast of the Antarctic Peninsula. The core is dominated by glaciomarine sediment, but there are sufficient planktonic foraminifera to provide a stable-isotope stratigraphy and relative palaeomagnetic intensity record. They show that, during Marine Isotope Stage 3, foraminiferal abundance peaks correlate with warming in Antarctica, as expressed in the oxygen-isotope record of an ice core from eastern Dronning Maud Land. Ice-rafted debris is most abundant in sediment from the last deglaciation (c. 19–11 ka), as sea-level rise destabilized the West Antarctic and Antarctic Peninsula ice sheets that had previously advanced over the shelf during the Late Glacial Maximum low-stand.

In another core analysis, Naresh Pant and colleagues examine the much longer U1359 core from Expedition 318 of the Integrated Ocean Drilling Program, to determine the glacial history of the East Antarctic Ice Sheet off the coast of Wilkes Land. They established a record of multiple sourcing of iceberg debris from the East Antarctic craton, the Ross Orogen and the Ferrar Large Igneous Province, the last two of which lie a considerable distance to the SE. From 60 m of core, these authors used a mix of heavy mineral analysis, X-ray diffraction and micro-beam techniques to characterize the sediment, for which they assigned a Pleistocene age.

Earth-surface processes

Glacial and periglacial processes in Antarctica have become increasingly recognized as important for interpreting the behaviour of former ice sheets and the periglacial zones beyond them. Several papers deal with these topics, emphasizing the processes occurring in cold-arid and semi-arid areas of Antarctica. Other topics covered in this section include investigations of snow-cover and atmosphere–ocean–solid Earth interaction.

Cold-based glaciers are an important part of the cryospheric system in Antarctica and, until recently, it was argued that they had little geomorphological significance. However, a new body of evidence suggests that cold-based glaciers can erode, transport and deposit debris, leading to a distinctive sediment–landform assemblage for cold-based glaciers. Cliff Atkins reviews the evidence for cold-based glacier activity in South Victoria Land, emphasizing the dual ability of these glaciers both to preserve landscapes and to erode and modify them. In this invited review, Atkins explores recent improvements in our understanding of cold-based glacier structure, process and interaction with various substrates. In the light of these advances in understanding, the landscape exposure record can be better interpreted, and previously unknown glacial events can be recognized.

South Victoria Land is also the scene for research by David Marchant and co-workers, who analyse geomorphological processes and landforms in the Dry Valleys. The Dry Valleys have three microclimates (a coastal thaw zone, an inland mixed zone, and an upland stable zone), resulting in considerably different landscapes, with unique landforms in each climatic zone. This invited review of landscape development has implications for our understanding of long-term climate change and ice-sheet stability.

Landscape evolution is increasingly seen as an important part of interpreting past glacial events, and Bethan Davies and co-workers present new modern and palaeoprocess–landform data from the northern Antarctic Peninsula. This paper investigates the behaviour of the Antarctic Peninsula Ice Sheet from the Last Glacial Maximum and through the Holocene, culminating with the modern periglacial environment. The authors emphasize the interplay between glacial, periglacial and paraglacial processes, and the importance in recognizing this when interpreting the palaeo-geomorphological record.

A little-studied aspect of earth-surface processes in Antarctica is the development of soils. In a paper by Megan Balks, a wide range of soils in different topographic settings are considered. The regional coverage includes the Transantarctic Mountains (including the Dry Valleys), coastal areas of East Antarctica and the Antarctic Peninsula. The authors explore how soils develop under different regional climates and in different topographic settings. They argue that organisms, time and parent material are the dominant influences on soil properties only in localized situations. The authors conclude that Antarctic soils are not well categorized, and there is a need to introduce new categories.

Schirmacher Oasis and Larsemann Hills are relatively little-studied regions of Antarctic as far as Quaternary glacial history and contemporary processes are concerned, and they receive welcome attention in a paper by Rajesh Asthana and colleagues. These authors document a wide range of glacial and periglacial phenomena, and use scanning-electron microscopy of quartz grains and granulometry to pinpoint the transfer mechanisms of sediment, particularly through glaciers and the East Antarctic Ice Sheet.

Kevin Hall provides a thorough and thought-provoking review of periglacial processes and landforms around the Antarctic. This invited review takes into account a wide range of environments, from the relatively wet and mild sub-Antarctic oceanic islands through to the cold dry continent. These modern environments offer insights into conditions in the Northern Hemisphere during the last glacial, and also provide analogues for extraterrestrial periglacial environments, such as those on Mars. Periglacial processes and products can also be used to monitor long-term climate change. This review focuses on new research directions, including linkages between biotic components and abiotic responses, periglacial synergies and resulting landform development.

Measuring the response of the Earth's crust to ice sheet unloading is the subject of a paper by Stephanie Konfal and co-authors. Using the first values of crustal tilt resulting from the differential uplift of lacustrine strandlines obtained in the McMurdo Dry Valleys, they link these results with age-data to provide a history of solid earth deformation since deglaciation. They found that the magnitude of gradients increases exponentially with age, indicating an ongoing response to deglaciation since the Late Glacial Maximum. This tilt pattern contradicts that predicted by models of glacial isostatic adjustment for Antarctica.

Masaki Kanao and colleagues address a topic of rapidly developing interest – the physical interaction between atmosphere, ocean and the solid Earth (including the cryosphere). At Syowa Station, East Antarctica, they installed an infrasound sensor that measures microseism-baroms, thereby recording the impact of storms and sea-ice fracturing. These attributes are a useful proxy for characterizing ocean-wave climate, and the measurements here form part of a network of such studies in the southern high-latitudes.

Measurement of annual snow is also important for long-term monitoring. This is time-consuming and best done using remote imaging techniques. Carla Mora and co-workers investigated the possibility of using Envisat scenes for snow cover classification. However, they found a high degree of noise, which meant that high-resolution snow mapping was not possible.

Conclusions

The papers in this volume represent only a fraction of those presented at the International Symposium on Antarctic Earth Sciences in Edinburgh in July 2011. However, combined with several invited reviews, they demonstrate a remarkable diversity of Earth science interests in the Antarctic. Most noteworthy is that the last decade demonstrates a marked shift in emphasis from pre-Cenozoic geology to topics related to the 34 or so million years of Antarctic glacial history. This trend reflects the recognition, documentation and understanding of the palaeorecord of ice-sheet growth and decay. Furthermore, Antarctic Earth history is critically important for informing us about potential future trends, as the impact of global warming is increasingly felt on the continent and its surroundings.