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Geology and Geophysics  

Geology & Geophysics Program
Antarctica is not only one of the world's seven continents, but also comprises most of one of its dozen major crustal plates, accounting for about nine percent of the Earth's continental (lithospheric) crust. Very little of this land is visible however, covered as it is by the vast East Antarctic Ice Sheet and the smaller West Antarctic Ice Sheet. The ice sheets average some 3 kilometers deep – a virtual vault, 90 percent of the ice on Earth is here. And it is heavy, depressing the crust beneath it some 600 meters. These physical characteristics, while not static, are current. Yet thanks to the sciences of geology and geophysics, powered by modern instruments and informed by the paradigm of plate tectonics/continental drift, Antarctica is also a time machine.

Geologists have found evidence that there was once a forested supercontinent in the Southern Hemisphere, which they call Gondwanaland. Before the Earth's constantly shifting plate movement began to break it up 150 million years ago, Antarctica was a core piece of this assembly; its adjoining land has since become Africa, Madagascar, India, Australia and South America. The Antarctic Plate drifted south at little more than a centimeter each year, but geologic time eventually yields cataclysmic results: The journey moved it into ever colder, high-latitude climates, at a rate of about 4ºC for each million years; eventually life conditions had changed dramatically, and Antarctica arrived at a near polar position. This astounding history of rock and life on Earth has left a stratigraphic and fossil record, locked in and beneath the ice, the sea, and in the bedrock below both.

As the ice sheets developed, they assumed what has become a key role in modulating global climate, through their interaction with oceanic and atmospheric circulation. As a bonus, the South Pole also presents a strategic point to monitor the Earth's current seismic activity. Antarctica is the highest continent on Earth (about 2,150 m above sea level), with its fair share of mountains and volcanoes; thus many generic questions of interest to earth scientists worldwide also apply to this region. Some specific issues focused on by the Geology and Geophysics program include:

• determining the tectonic evolution of Antarctica and its relationship to the evolution of the continents from Precambrian time (600 million years ago) to the present;

• determining Antarctica's crustal structure;

• determining how the dispersal of antarctic continental fragments may have affected the paleocirculation of the world oceans, the evolution of life, and the global climate (from prehistoric times to the present);

• reconstructing a more detailed history of the ice sheets, identifying geological controls to ice sheet behavior, and defining geological responses to the ice sheets on regional and global scales; and

• determining the evolution of sedimentary basins within the continent and along continental margins.

All of these problems will be simplified as scientists improve their models of where, when, and how crustal plate movement wrought Antarctica and its surrounding ocean basins. The program funds investigation into the relationships between the geological evolution of the antarctic plate and the life and processes that can be deduced to accompany it: Paleocirculation of the world ocean, paleoclimate of the Earth, and the evolution of high-latitude biota. A current emphasis is the West Antarctic Ice Sheet Program (WAIS), research on the smaller of the continent's two ice sheets, conducted also under the aegis of the Glaciology program. Several important research support activities are also underway:

• Meteorites: In a partnership with NASA and the Smithsonian Institution, the program supports meteorite collection through ANSMET, the Antarctic Search for Meteorites, and chairs an interagency committee, responsible for curating and distributing samples of the antarctic meteorites.

• Mapping and geodesy: In partnership with the U.S. Geological Survey, the program supports mapping and geodetic activities as an investment for future research in earth sciences. The U.S. Antarctic Resources Center (US-ARC) constitutes the USAP contribution to the Scientific Committee on Antarctic Research (SCAR) library system for earth sciences information; housed here is the largest collection of antarctic aerial photographs in the world, as well as many maps, satellite images, and a storehouse of geodetic information.

• Marine sediment and geological drill cores: In a partnership with the Antarctic Marine Geology Research Facilty at Florida State University, the program manages and disseminates marine sediment and geological drill cores mined in Antarctica. The collection includes an array of sediment cores as well as geological drill cores from the Dry Valley Drilling Project, the CIROS drilling program, and the Cape Roberts Drilling Project. The facility fills requests for samples from researchers worldwide, and also accommodates visiting researchers working on site.

Air-ground study of tectonics at the boundary between the eastern Ross embayment and western Marie Byrd Land, Antarctica: Basement geology and structure.
Christine S. Siddoway, Colorado College.

The West Antarctice Ice Sheet (WAIS) is a complex, dynamic system. In Neogene time (between about 1.6 and 23.3 million years ago), movement and shifting of the Earth's crustal plates (tectonics) contributed to the formation of WAIS and associated ice streams of the region. A better understanding of what may have happened will contribute to a comprehensive model for the Cenozoic origins of the Ross Sea Rift, and should also help specify the extent of plume activity in Marie Byrd Land.

This project undertakes both geologic and geophysical investigations of these two regions. The goal is to determine the tectonic history of the region during the Cenozoic Era (the last 65 million years); specifically to learn whether Neogene structures that localized outlet glacier flow may have developed within the context of Cenozoic rifting on the eastern Ross embayment margin (on the one hand), or, rather, within the volcanic province in Marie Byrd Land; or perhaps due to a combination of both.

Faulting and volcanism, mountain uplift, and glacier down-cutting appear to be active now in western Marie Byrd Land, where generally east-to-west-flowing outlet glaciers incise much older (Paleozoic and Mesozoic) bedrock, and deglaciated summits indicate a previous north-south glacial flow direction.

Our study relies on Support Office for Aerogeophysical Research (SOAR) to collect data; this facility uses a high-precision differential global positioning system to support a laser altimeter, ice-penetrating radar, a towed proton magnetometer, and a Bell BGM-3 gravimeter.

SOAR should help us to glean glaciology data useful for studying driving stresses, glacial flow, and mass balance in the WAIS. The ground program, centered on the southern Ford Ranges, will map: Glaciated surfaces and deposits; small-scale brittle structures, to determine the regional kinematics; and datable volcanic rocks to establish the geochronology. We will also determine the relative significance of fault and joint sets, the timing relationships between them, and how they were probably formed; as well as exposure ages for erosion surfaces and moraines. To aid in the interpretation of potential field data and to aid paleomagnetic studies, we will sample magnetic properties and density as well as take ground-based gravity measurements and oriented samples.

By combining these airborne and ground investigations, we expect to gather the basic data to describe the geology and structure at the eastern boundary of the Ross embayment, in both outcrop and ice-covered areas. These data should help distinguish between Ross Sea Rift-related structural activity and tectonic uplift/faulting that may have occurred on the perimeter of the Marie Byrd Land dome and in the volcanic province. Ultimately, the aerogeophysical data and the outcrop geology should permit us to infer the geology that resides beneath the WAIS. (GF-088-O)

Global positioning system measurement of isostatic rebound and tectonic deformation in Marie Byrd Land, West Antarctica.
Bruce Luyendyk, University of California at Santa Barbara.

The Ross embayment and western Marie Byrd Land are part of the west antarctic rift system. Most scientists agree that this region is undergoing active deformation, but the rates and causes of deformation remain essentially unknown. Tectonic extension may be occurring in the Ross embayment as West and East Antarctica continue to separate. Crustal uplift could be occurring in western Marie Byrd Land due to isostatic rebound following the last glacial age.

If tectonic extension is occurring in the embayment – depending on its magnitude – it could greatly influence global plate circuit calculations. It could also constrain our understanding of the history of extension in the embayment and the consequent uplift history of the Transantarctic Mountains. Postglacial rebound in western Marie Byrd Land would depend on when and how the ice sheet was configured during the Last Glacial Maximum. The big question is whether the ice sheet collapsed in mid-Holocene time.

This study will install three continuous and autonomous global positioning system (GPS) stations on outcrops in western Marie Byrd Land, on baselines of around 100 kilometers. These stations will gather data over a 4-year period and operate in concert with GPS stations being installed in the Transantarctic Mountains in a separate project; the result will be a baseline array all across the Ross embayment. The array will also detect strain gradients in western Marie Byrd Land.

This system should determine crustal strain rates to an accuracy of 1 millimeter per year for horizontal, and 2 millimeters per year for vertical. The strain data from western Marie Byrd Land and the Transantarctic Mountains should enable us to construct both tectonic extension and glacial rebound models.

This joint project between Bruce Luyendyk of the University of California at Santa Barbara and a team at the Jet Propulsion Laboratory at the California Institute of Technology – Andrea Donnellan, Carol Raymond, and Erik Ivins – brings together experts in western Marie Byrd Land geology and tectonics, tectonic geodesy, and lithospheric deformation. (GF-121-O)

Structure and sedimentology of the Beardmore Group, Antarctica: Latest Neoproterozoic to Early Paleozoic tectonic evolution of the east antarctic margin.
John W. Goodge, Southern Methodist University.

The Neoproterozoic to early Paleozoic transition (700 to 500 million years ago) marked a critical period in the history of the Earth: A supercontinent known as Rodinia coalesced, then fragmented, and subsequently amalgamated as a second supercontinent, known as Gondwanaland. During these events, major mountain-building, continental erosion, species diversification, sea-level fluctuations, and changes in seawater composition took place. Scientists have built detailed records of sea-level fluctuations, faunal distributions, and post-depositional tectonism out of continental margin sedimentary sequences.

The Beardmore Group in Antarctica is one of these sequences, undoubtedly a significant element in the evolution of the east antarctic craton (the main, relatively stable nucleus of the continent). Yet very little is known about its depositional or tectonic history.

Previous workers have suggested that the entire Beardmore Group is Neoproterozoic in origin, and that it records two distinct deformations, the well-known Ross orogeny and an earlier, cryptic, "Beardmore" orogeny. Field reconnaissance and geochronologic studies reveal portions of the Beardmore Group to be significantly younger than previously believed, which would support the view that Ross deformation was the exclusive formative event. These preliminary data reflect uncertainty in the geologic relations of these rocks; we believe the Neoproterozoic tectonic history for the region must be revised.

To improve understanding of the Beardmore Group's depositional history and its role in orogenic events shaping the outer margin of Gondwanaland, we will conduct:

• detailed field study of the stratigraphy, sedimentology, and structure;

• provenance (source) study of graywackes to constrain the depositional setting of the turbidites;

• determination of carbonate isotopic compositions for comparison with the global carbon-isotopic refinement of depositional age(s) through uranium-lead dating of detrital and igneous zircons; and

• thermochronology to constrain the deformation ages with argon-isotopes.

During the 1999 – 2000 austral summer, we will continue to test a recently proposed model for Beardmore Group deposition and deformation; namely that Ross activity caused the latest Neoproterozoic/Early Paleozoic(?) rift-margin sedimentation and structural inversion. These explorations should help to resolve long-standing uncertainties in the geologic history of Antarctica, and improve our understanding of global paleogeographic and plate-tectonic events during the formation of the supercontinent at the close of the Proterozoic. (GO-014-O)

Antarctic mapping and geodesy.
Jerry L. Mullins and Richard E. Witmer, U.S. Geological Survey.

Geodetic surveying, aerial photography, remote sensing (principally using several varieties of satellite imagery), and mapping are all activities necessary for the successful operation of a multifaceted scientific and exploration effort in Antarctica. The U.S. Geological Survey provides these support activities to the U.S. Antarctic Research Program.

Year-round data acquisition, cataloging, and data dissemination activities will continue in the U.S. Antarctic Resource Center for geospatial information. Field surveys will be conducted in support of specific research projects, and as part of a continuing program to collect the ground-control data necessary to transform existing geodetic data to an earth-centered system suitable for future satellite mapping programs.

LandSat data will be collected as part of satellite image mapping activities; this will permit continued publication of additional 1:50,000 scale topographic maps in the McMurdo Dry Valleys region. Such topographic maps provide a uniform base map on which to portray scientific information (from geology, glaciology, biology and other studies) such that it is spatially accurate. These, as well as the satellite image maps, are used by scientists to plan and execute future research work. Spatially-referenced, digital cartographic data will be produced in tandem with the published maps. (GO-052-M, GO-052-P and GO-052-S)

Response of the East Antarctic Ice Sheet to Middle Miocene global change.
David R Marchant, Boston University.

As evidence of global climate change continues to accumulate, scientists concentrate on models that might indicate what impacts such change could have. For example what could happen to the East Antarctic Ice Sheet?

One of the largest known global climate shifts occurred in Middle Miocene time (between about 15.6 and 12.5 million years ago). As the isotopic composition of oxygen in the oceans shifted, dramatic global cooling and reorganization of ocean circulation patterns can be seen. This dramatic and irreversible shift set the stage for modern oceanic and atmospheric circulation, and for the bipolar ice ages that have dominated climate records for the last 12.5 million years. How did Antarctica respond to this great climate shift? Could growth of the antarctic ice sheet have initiated this shift? If so, how might future fluctuations in the volume of ice on East Antarctica influence atmospheric and oceanic circulation?

Recently there was an unexpected breakthrough in antarctic geology, discovery of Miocene-age volcanic ashes interbedded with surficial sediments in southern Victoria Land. These terrestrial deposits provide unambiguous data from which to generate precise climatic and glaciological reconstructions of how the global climate changed and the ice sheet evolved. This site appears to be the only place in Antarctica where pristine, Miocene-age, unconsolidated deposits are preserved at the ground surface. These data are further permitting scientists to address key questions, such as:

• What contributing factors in Antarctica led to the abrupt global cooling about 14 million years ago?

• Does the middle Miocene shift in the isotopic composition of the oceans signify a major expansion of east antarctic ice?

• Or rather, does this isotopic shift instead reflect a change in ocean temperature or circulation?

• And a related question – When did cold, hyper-arid, polar-desert conditions (signifying the development of the polar East Antarctic Ice Sheet) first evolve in Antarctica?

In analyzing these deposits, we expect to obtain precise chronological control, based on 50 laser-fusion isotopic analyses of in-situ volcanic ashes and 20 cosmogenic, exposure-age analyses of ancient deposits. We also expect to develop a coeval record of the Miocene paleoclimate, based on textural changes in alpine drifts, the areal distribution of ice-marginal lakes, the abundance of dated, patterned ground and ventifact pavements, and the geochemistry of buried soils and volcanic-ash deposits. (GO-054-O)

The Ferrar magmatic mush column system, Dry Valleys, Antarctica.
Bruce D. Marsh, Johns Hopkins University.

Earth's basic structure was formed by processes involving the crystallization of magma (molten rock). Operating on billion-year time scales, these processes have produced a wide diversity of rock types. In turn, these different elements comprise the continents and the ocean basins – the basic surface features of Earth. Yet many of the details of these physical and chemical processes remain obscure.

Present day volcanism exemplifies this overall process of differentiation – so many different varieties of lava erupt – yet scientists have not been able to relate this diversity to the prolonged and detailed deep-Earth processes that undoubtedly generate it. Solidified bodies of magma (plutons) that were once deeply buried and are now exposed through erosion also furnish evidence, but most often how these plutons relate to the magmatic-volcanic system is not clear.

This research is pointed at this fundamental problem: We will examine magma crystallization processes by studies of sills from the Ferrar Group in Antarctica. These studies should expose the relationship of plutonism to volcanism and may provide some important insights on planetary magmatism. The Ferrar magmatic system of the McMurdo Dry Valleys, Antarctica (Ferrar-DV) exemplifies the emerging global paradigm. Magmatic sheets or sills occur in stacks, connected below to a deep-seated magmatic source and above to a volcanic center.

The world's major magmatic systems reveal this pattern, as they tend to occur at ocean ridges (e.g., Kilauea, Mt. Etna, Stillwater, and Rum, among many others). Only the Ferrar-DV, however, clearly reveals the critical physical and chemical connections between the deep, mush-dominated system and the near-surface, pre-eruptive sill system.

This project seeks to ascertain the full physical and chemical nature of the Ferrar-DV magmatic system, by:

• fully delineating its vertical and horizontal extent and explaining how it was established

• explaining the mechanics of formation of the Dais layered intrusion;

• producing a map of Ferrar rocks throughout the Dry Valleys; and

• producing a 3-D model of the opx tongue and feeder system.

The central science goal is to elucidate a rarely seen transition between plutonic and volcanic systems, one which may have implications fundamental to planetary magmatism. (GO-056-O)

Antarctic search for meteorites.
Ralph Harvey, Case Western Reserve University.

Since 1976, ANSMET (the Antarctic Search for Meteorites program) has recovered more than 9,000 meteorite specimens from locations along the Transantarctic Mountains. Antarctica is the world's premier meteorite hunting ground for two reasons:

• Although meteorites fall all over the globe at random, the likelihood of finding a meteorite is enhanced if the background material is plain and the accumulation rate of terrestrial sediment is low; this makes the East Antarctic Ice Sheet the perfect medium.

• Along the margins of the sheet, ice flow is sometimes blocked by mountains, nunataks, and other obstructions; this exposes slow-moving or stagnant ice to the fierce katabatic winds which can diminish the ice and expose what is known as a "lag deposit" of meteorites (a representative portion of those that were sprinkled throughout the volume of ice lost to the wind). When such a process continues for millenia, the concentration of meteorites unveiled can be spectacular.

It is important to continue recovering antarctic meteorites because they are the only currently available source of new, non-microscopic extraterrestrial material. As such, they provide essential "ground truth" (existence proof) about the composition of asteroids, planets, and other bodies of our solar system. This connection brings The National Aeronautics and Space Administration (NASA) into ANSMET as a collaborator; their robotics development project will aid the search in the Allen Hills region.

During the 1999-2000 field season, ANSMET will visit the Walcott Névé region (just north of the headwaters of the Beardmore Glacier, and just south of the Law Glacier). This has been a prolific source of meteorites for ANSMET field parties. Our primary target will be an area informally called "Foggy Bottom" at the south end of the Walcott Névé, where 5 previous seasons of work have recovered over 3,400 meteorites. Other targets this season are the Goodwin Nunataks icefields (near Foggy Bottom) and the MacAlpine Hills icefield. Another significant effort will involve reconnaissance of icefields further north, in the Miller and Geologist ranges surrounding the headwaters of the Nimrod Glacier.

After the main party returns to McMurdo in early January, a two-person ANSMET field party will visit the Elephant Moraine icefield northwest of the Allan Hills. In addition to trying to re-establish several key survey points to specify meteorite find locations across the region, ANSMET personnel will participate in a robotics experiment in meteorite collection. The National Aeronautics and Space Administration and investigator Dimi Apostolopoulos will stage a number of experiments here that could yield data useful for future collection efforts elsewhere in the solar system. (GO-058-O and GO-059-O)

A test for Tertiary-age deep fluvial incision and strongly melting valley-glaciers in the McMurdo Dry Valleys using ground-penetrating radar: A pilot project.
Michael Prentice, University of New Hampshire.

The study of geology, as with most sciences, is closely linked to technological advancements in measurement and exploration. This research deploys ground-penetrating radar (GPR) – a technique not previously used on antarctic sediments – to study two types of large morphologic features in the McMurdo Dry Valleys in order to address controversy in the geology of Antarctica.

• First, spurlike aprons. Situated below tributary valleys that project into trunk valleys, spurlike aprons have been interpreted to be remnants of bedrock fluvial spurs; as such, they provide a record of preglacial fluvial incision of trunk-valley floors to sea level. If this interpretation is correct, it follows that a major sector of the Transantarctic Mountains landscape is largely fluvial and not glacial in origin. A corollary to this thesis requires the antarctic ice/climate system to have been quite stable throughout the Tertiary ice ages, that is, restricted to its current high-polar state.

We believe the bedrock/spur interpretation to be unsupported and – considering form and structure analyses – unlikely. We will conduct a GPR survey of the best example (below Denton Glacier in eastern Wright Valley) in search of relevant data.

• Second, large moraines. Using form and structure analyses, we postulate that large moraines are stratified. If so, these features reflect deposition from valley glaciers that possessed strong melting margins. Deposition of this type implies that at least the regional antarctic ice/climate system was in a subpolar state – that is, was warmer, wetter, and more dynamic than the high-polar state.

We will use GPR to study three moraines whose internal composition is unknown, and which might provide evidence on the issue. Two such moraines are in central McKelvey Valley, possibly associated with glaciolacustrine sediment; the other is in central Wright Valley, and associated with glaciomarine sediment.

In addition, we will examine a few distinctive drift patches that crop out asymmetrically on trunk-valley walls, using conventional surficial geologic techniques. These patches appear to be traceable into high alpine valleys and thus may have been deposited from expanded alpine glaciers that merged with trunk-valley glaciers. If true, these patches record the synchronous advance of alpine and trunk-valley glaciers, which would imply that the coeval climate was significantly wetter and warmer than it would have been in the high-polar state. (GO-063-O)

Late Cretaceous and Cenozoic reconstructions of the southwest Pacific.
Steven C Cande, Scripps Institution of Oceanography.

Crustal plate motion is never as predictable as earth scientists would like – witness the devastation wrought by unpredicted earthquakes. In Antarctica, there is controversy regarding a possible missing plate boundary, as well as tectonic uncertainties in the motion between East and West Antarctica; in particular, questions about the relative drift between major hotspot groups. The plates of the Southwest Pacific region are rotational, so that earthquakes are relatively rare. Still, the models that describe the motion of the Pacific, Antarctic, and Australian plates – and the continental fragments of New Zealand, West Antarctica, Iselin Bank, East Antarctica, and Australia – could be improved. This is the object of this research.

Previous work has documented mid-Tertiary seafloor spreading in a NNW-striking direction, producing magnetic anomalies between East and West Antarctica. This would explain the approximate 150 km-opening of the Adare Trough, north of the Ross Sea. The hypothesized motion, however, is insufficient to resolve the apparent discrepancy between the actual plate motions and those that would follow from the assumption that the hotspots were fixed.

The motion between East and West Antarctica indicates a very small rotation. Thus, scientists would like to develop models of finite plate rotation in this area to a high degree of accuracy, particularly for older times. This goal is now attainable, based on the analysis and interpretation of data that should result from this project, in conjunction with other data sets compiled by Japanese and Italian scientists on recent cruises in the region.

Our new marine geophysical data will be collected on selected transits of the Nathaniel B. Palmer. We hope to:

• improve the rotation model for mid-Tertiary extension between East and West Antarctica by directly considering the plate boundary between the Pacific and Australia plates in the calculation of Australia-West Antarctica motion;

• improve the reconstructions for Late Cretaceous and Early Tertiary times by including new constraints on several boundaries not previously used in the reconstructions;

• address the issue of the fixed position of global hotspots through the implications of new rotation models; and,

• re-examine the geophysical data from the western Ross Sea embayment in light of a model for substantial mid-Cenozoic extension. (GO-071-O)

Vertebrate paleontology of the Triassic to Jurassic sedimentary sequences in southern Victoria Land and the Beardmore Glacier area, Antarctica.
William R Hammer, Augustana College.

Vertebrates of several different ages spanning the Triassic to Jurassic (roughly from 250 to 150 million years ago) have been discovered in recent years in Antarctica. These fossils provide paleontologists a unique opportunity to study the evolution of high latitude faunas, which in turn contribute to our understanding of high latitude climates during the Mesozoic.

This project undertakes antarctic field work in the Transantarctic Mountains, to search for and collect new terrestrial vertebrates of Triassic and Jurassic age. During the austral summer of 1999-2000, with the support of helicopters operating from McMurdo Station, we plan to explore exposures of the Feather and Lashly Formations in southern Victoria Land. These field efforts will concentrate on finding exposures equivalent to the Hanson Formation – where the only Jurassic dinosaur fauna from Antarctica was found – as well as searching for new Middle to Upper Triassic fossil vertebrates. We will also construct interpretations of the taphonomic/depositional settings for each of these localities.

During the project's second year, we will prepare and study the vertebrate finds, including investigations of the evolutionary significance of the member taxa of these high latitude Mesozoic faunas. (GO-074-O)

Dry valleys seismograph project.
Kent Anderson, U.S. Geological Survey.

One recurrent issue in seismography is "noise" – that is background phenomena that can interfere with clear and precise readings. The Dry Valleys Seismograph Project – a cooperative undertaking with the New Zealand Antarctic Program – was established to record broadband, high-dynamic-range, digital seismic data from the remote Wright Valley, a site removed from the environmental and anthropogenic noise ubiquitous on Ross Island.

The Wright Valley site provides one of the few locations on the continent with direct access to bedrock. The station there consists of a triaxial broadband borehole seismometer [100 meters (m) deep] and a vertical short-period instrument at 30 m. The seismological data are digitized at the remote location, telemetered by repeaters on Mount Newell and Crater Hill, and received eventually by the recording computer at the Hatherton Laboratory at Scott Base, where a backup archive is created.

These data will eventually reach the international seismological community; from Hatheton they pass via a point-to-point protocol link to the Internet at McMurdo Station and thence to the Albuquerque Seismological Laboratory for general distribution. This data set has beautifully complemented the data from other seismic stations operated by the Albuquerque Seismological Laboratory at Amundsen-Scott South Pole Station, Palmer Station, and Casey, the Australian station. GO-078-A

Mount Erebus volcano observatory: Gas emissions and seismic studies.
Philip R. Kyle and Richard C. Aster, New Mexico Institute of Mining & Technology.

Mount Erebus on Ross Island is Antarctica's most active volcano; also the only one with a persistent convecting lake of molten silica and alkali-rich photolitic magma in its summit crater. This makes Erebus one of the few volcanoes on Earth with nearly continuous, small explosive activity and internal earthquake (seismic) activity. As such, it provides the ideal natural laboratory to study these phenomena: How gas is given off by magma, and the seismic activity that results from a convecting magma conduit.

This project entails a combination of seismic studies and gas emission rate measurements, designed to elucidate the nature and dynamics of the magmatic plumbing system, as well as eruptions and degassing from the lava lake.

The gas studies will provide some of the first data available on carbon dioxide degassing from a highly alkalic magma system. They should also help to evaluate how much lead from Mount Erebus (relative to lead released by marine aerosols) gets into the snow on the East Antarctic Ice Sheet, and thus shed light on hypotheses about the anthropogenic origins of lead. Further goals of the gas studies are to:

• examine the role of Erebus as a source of gas and aerosols to the antarctic environment;

• understand the role of volcanism as a source of carbon dioxide emissions to the atmosphere, especially for a highly alkalic magma;

• understand the evolution of the main volatile substances (water vapor, carbon dioxide, total sulfur, fluorine and chlorine) in the Erebus magmatic system, as well as their role in the eruptive behavior of Erebus; and

• correlate the nature of the gas emissions with the observed seismic activity.

The seismic studies of the volcano will:

• add a permanent broadband seismic station to the array and update the present data acquisition system;

• deploy a temporary network of broadband stations to augment the results of a previous pilot study; and

• expand development of current software to allow automatic and precise timing of earthquake occurrences and thus allow precise locations to be determined.

The resultant data should enhance the collection of earthquakes that we are using in a computer model of the interior of the volcano, as well as provide a tool scientists can use for volcano surveillance, eruption monitoring, and for detecting subtle changes in the internal behavior of volcanoes. The broadband data will support a detailed study of the explosion mechanism, especially the very-long-period signals they emit. It should also help us detect temporal and spatial variability in earthquake mechanisms, which in turn might provide more insights into how variations in gas emissions may be implicated. (GO-081-O)

Global positioning system measurements of crustal motion in Antarctica.
Barclay Kamb, California Institute of Technology, and Carol Raymond, Jet Propulsion Laboratory.

Geodesy is the mathematically-grounded science of using measurements to map the positions and shapes of the Earth's surface features. The satellite-based global positioning system (GPS) has revolutionized the techniques and enhanced the accuracy of geodetic work. This project has established a geodetic network in the Transantarctic Mountains of Antarctica to measure both vertical and horizontal crustal velocities:

• The vertical crustal velocities measured by GPS reflect the viscoelastic response of the solid Earth to antarctic deglaciation. That data will enable us to evaluate discrepancies between models that describe when antarctic deglaciation probably occurred: Either in Late Pleistocene-Early Holocene (about 10,000 years ago) or later, in Late Holocene time. These data will also constrain the length of time over which the antarctic ice sheet disintegrated, and should provide a reconstruction of its changes. If antarcitc deglaciation occurred in the mid-Holocene, we would expect a specific pattern of uplift, near the Transantarctic Mountains. This new, high-precision GPS geodetic system should be able to pin that event within a time span of 4 years.

• Horizontal deformation induced by rebound can also be measured; these data help constrain models of present-day changes in antarctic ice mass by monitoring how the lithosphere is deformed by ongoing glacial loading and unloading. We predict that horizontal component (the lithospheric response to ongoing ice-mass changes) to be an order of magnitude smaller than the vertical (the viscoelastic response to late Pleistocene-Holocene deglaciation). Conversely, that predicted rebound signal should be much larger than the associated tectonic uplift rates. Our baseline measurements will cross known fault lines in the Transantarctic Mountains, and may capture co-seismic motion, in the event of an aseismic slip or an earthquake.

This autonomous GPS station (AGS) network sends daily data reports to McMurdo Station, and has been designed as a permanent installation that will continue to monitor motion in the region. Advanced processing techniques – such as orbit modeling, troposphere correction, ionosphere correction, and extraction of annual and seasonal solid Earth and ocean tidal signals – have been developed to refine the accuracy of these crustal velocity measurements, especially for the vertical component. (GO-082-O)

GPS measurements of rock and ice motions in South Victoria Land.
Ian M. Whillans, Terry J. Wilson, and Clyde C. Goad, Ohio State University.

Southern Victoria Land and its environs appear to contain invaluable evidence about the tectonics of Antarctica and how the ice sheet may have changed. Leading models differ on one vital question: Is the continent rebounding due to reduced ice load from East and/or West Antarctica, or is the current tectonic motion better explained by Terror Rift or uplift of the Transantarctic Mountains? This project uses Global Positioning System (GPS) measurements of this region to probe for data relevant to this controversy.

We will also measure ice motion, hoping to develop data that would test models for ice flow in the Allan Hills meteorite-concentration region, and help determine whether small glaciers in the McMurdo Dry Valleys are thickening or thinning. This aspect of the work requires setting monuments into rock and ice, and establishing their current position with GPS receivers; any motion will be detected as the data set is compounded in subsequent years.

Our field activities involve close cooperation with the U.S. Geological Survey. (GO-084-O)

Logistics support for global seismic station at the South Pole.
Rhett Butler, Incorporated Research Institution for Seismology.

Seismology, perhaps as much as any other science, is a global enterprise: The waves resulting from the Earth's interior motion can only be interpreted through simultaneous measurements at strategic points all over the planet. To help establish the facilities required to accomplish this crucial scientific mission, IRIS (the Incorporated Research Institution for Seismology) was created in 1985.

IRIS is a consortium of institutions that run research programs in seismology. Fifty-seven universities are currently members, including nearly all U.S. universities that run seismological research programs. IRIS has developed a 10-year plan to implement the global seismographic network (GSN). In addition to portable stations, the GSN will comprise about 100 broadband, digital, wide-dynamic-range stations, designed to broadcast such that any site on Earth will have real-time access to the complete global data set.

We will install, maintain, and operate IRIS seismic equipment at Amundsen-Scott South Pole Station and at Palmer Station, Antarctica. Such capability is essential for seismic studies of Antarctica. The resultant state-of-the-art, broadband, digital seismic capability continues the National Science Foundation effort to improve seismic instrumentation globally, and will provide a crucial link in the GSN. (GO-090-O and GO-091-O)

Paleohistory of the Larsen Ice Shelf: Evidence from the marine record.
Eugene W. Domack and Patrick D. Reynolds, Hamilton College.

Over the last 10 years, scientists have observed a dramatic decay and disintegration of floating ice shelves along the northern end of the Antarctic Peninsula. Meteorological records and satellite observations attribute this catastrophic decay to regional warming, measured at nearly 3ºC during the last 50 years. (The links between this warming and the global change attributable to the greenhouse effect, however, are not fully understood.) Such a retreat of floating ice shelves is unprecedented in historic records, but scientists do not know enough about the longer context, and thus are interested in the natural variability of ice shelf systems over the last few thousand years.

The Larsen Ice Shelf provides an excellent point of reference for such studies: Located in the northwestern Weddell Sea along the eastern side of the Antarctic Peninsula, the shelf (according to satellite images taken over the past five years) is undergoing a rapid, catastrophic retreat that is unprecedented in historic time, leaving vast areas of the seafloor uncovered and in an open marine setting.

This project of marine geologic research will collect a series of short sediment cores (within the Larsen Inlet and in areas that were previously covered by the Larsen Ice Shelf). By applying established sediment and fossil criteria to the cores, we hope to discover and demonstrate whether the Larsen Ice Shelf has experienced periods of retreat and subsequent advance (similar to that of the last half century) within the Holocene (the last 10,000 years).

Previous work throughout the Antarctic (in areas of the Ross Sea, Antarctic Peninsula, and Prydz Bay) has used ice shelf depositional models to discern the timing of ice shelf retreat/advance. This research should build on that data and enhance understanding of the dynamics of ice shelf systems and their role in climate oscillations – past and future. (GO-096-O)

Acquisition and operation of broadband seismograph equipment at chilean bases in the Antarctic Peninsula region.
Douglas Wiens and Gideon P. Smith, Washington University.

The present-day tectonics and seismological structure of the Antarctic Peninsula and Scotia Plate region are among the most poorly understood of any location in the world. This region offers a unique and complex geodynamic setting, as illustrated by recent changes in the pattern of volcanism and other tectonic activity. We constitute the U.S. component of an international effort – using a large-scale deployment of broadband seismographs – to study the seismotectonics and seismic structure of these regions.

During 1997 and 1998, a network of 11 broadband seismographs in the Antarctic Peninsula region and southernmost Chilean Patagonia were installed and have since been maintained. Data return from this project has been excellent, producing some interesting initial results. We need to extend these observations, however, because there have been few larger magnitude regional earthquakes and thus internal strain can be presumed to be increasing, a longer time frame should elicit data on more revealing events.

However, instruments from this project are borrowed from the IRIS-PASSCAL instrument pool and must be returned to PASSCAL in April 1999. This project undertakes the transformation of these three temporary seismometers into semi-permanent stations, and the continued seismological investigation of the region for 4 years. Washington University equipment will also be used to operate a fourth station, in Chilean Patagonia.

Continued operation of these stations should enhance understanding of the seismicity of the South Shetland Trench, an unusual subduction zone where young lithosphere is subducting very slowly. The continuing collaboration between Washington University and the Universidad de Chile will contribute important seismological data to the IRIS data center, as well as to other international seismological collaborators.

Such mutual exchanges with other national antarctic seismology research programs will accumulate data from a variety of other proprietary broadband stations in the region. These data will support seismic studies of the upper mantle velocity structure of several complicated tectonic regions in the area, including the South Shetland subduction zone, the Bransfield backarc rift, and diffuse plate boundaries in the areas around Patagonia, the Drake Passage, and along the South Scotia Ridge. Such studies should provide important constraints on the crustal structure beneath the stations; in turn improved structural models will help to pinpoint better locations for future instruments. (GO-097-O)

Antarctic network of unattended broadband integrated seismometers (ANUBIS).
Sridhar Anandakrishnan, Pennsylvania State University.

Despite much attention in recent years, the structure and dynamics of the antarctic crust and the composition and geometry of the mantle are still poorly understood. Seismology remains the primary method for studying these structures, as well as processes in the Earth's deeper asthenosphere, but Antarctica lags far behind in the effort to improve global seismic imaging and tomography. On this huge continent, there are only eight broadband seismic observatories. Except for the installation at South Pole, those stations are along the margins of the continent and none are in West Antarctica. By contrast, there are 200 permanent stations worldwide in the Federation of Digital Seismograph Networks (FDSN), and some 1,000 more, in national networks not yet integrated into the FDSN.

This project is developing a passive seismic network for the antarctic interior. We will develop and deploy 11 long-term broadband seismic stations on the continent itself. Because 98 percent of the continent is ice covered, these stations will be installed at the surface of the ice sheet. The body-wave data thus recorded from regional and teleseismic earthquakes can be analyzed at each station for local crustal thickness, lamination, Poisson's ratio (a measure of crustal composition), crust and mantle anisotropy (a measure of current and former stress regimes), and identification of rift zones and crustal block boundaries. In addition, the data from all stations (including the existing peripheral ones) can be used for seismic tomographic analysis to detail lateral variations in these properties.

Six of the stations will be installed at existing automatic geophysical observatory sites (in East Antarctica), which will provide heat and power for the data loggers. The remaining five stations will be established in West Antarctica, and will be powered and heated by wind turbines during the austral winter. (GO-180-O)

UNAVCO's global positioning system receiver.
Bjorn Johns, University NAVSTAR Consortium.

UNAVCO, a consortium of 100 international universities and laboratories, joined to promote the use of global positioning system (GPS) for geosciences research. As part of this activity, UNAVCO manages a satellite facility at McMurdo Station and provides equipment and logistical assistance to earth scientists making use of the (GPS) for research in crustal dynamics, earthquake, volcano, and global change research. In addition to supporting researchers in the U.S. Antarctic Program, UNAVCO also supports operations at McMurdo Station and Taylor Valley. The UNAVCO facility in Boulder, Colorado, offers a complete support program for university-based investigators including

• preseason planning,

• GPS equipment,

• training,

• field support and technical consultation, and

• data processing and archiving.

UNAVCO also represents the academic research community, acting as a central clearinghouse for information on the scientific applications of GPS. As a part of its service to investigators, UNAVCO works to improve GPS surveying accuracy by testing GPS equipment and techniques.

Using GPS, vector baselines between receivers separated by 100 kilometers or more are routinely measured to within 1 centimeter (that is, 100 parts per billion). UNAVCO is also able to support researchers in investigations of global, regional, and local crustal motions where maximum accuracy (in the millimeter range) of baseline measurement is required. GPS measurements using portable equipment can be completed in a few hours or less. Such expediency lends itself to research applications in global plate tectonics, earthquake mechanics, volcano monitoring, and regional tectonics. (GO-295-O)

The young marginal basin as a key to understanding the rift-drift transition and andean orogenesis: OBS refraction profiling for crustal structure in Bransfield Strait.
James A. Austin, Gail L. Christeson, Ian W. Dalziel, and Yosio Nakamura, University of Texas, Austin.

Bransfield Strait (in the northern Antarctic Peninsula) is one of a small number of modern basins that may be critical for understanding ancient mountain-building processes. The Strait is an actively-extending marginal basin in the far southeast Pacific, situated in an inactive volcanic arc between the Antarctic Peninsula and the South Shetland Islands. Widespread crustal extension, accompanied by volcanism along the Strait's axis, may be associated with slow underthrusting of oceanic crust at the South Shetland Trench. Some 140 million years ago (during the Jurassic-Early Cretaceous), similar "back-arc" extension is known to have occurred along the entire Pacific margin of the supercontinent known as Gondwanaland. This area has become western South America and West Antarctica.

Then, some 100 million years ago, mid-Cretaceous deformation of these basins initiated the uplift that produced the Andes. Thus, scientists believe that elucidating the deep structure and evolution of Bransfield rift could provide a model of crustal precursor activity that will more fully describe the early evolution of this globally important mountain chain.

Years of international earth sciences research in Bransfield Strait have produced consensus on important aspects of its geologic environment. It is a young (probably ~4 million years old) rift in preexisting Antarctic Peninsula crust; continued stretching of this crust results in complex fault patterns and associated volcanism. The volcanism, high heat flow, and mapped crustal trends are all consistent with the basin's continuing evolution as a rift.

It is also agreed the volcanism is recent and continues to occur along a "neovolcanic" zone centralized along the basin's axis. Multichannel seismic data collected aboard Maurice Ewing in 1991 illustrate the following basin-wide characteristics of Bransfield Strait:

• widespread extension and faulting,

• the rise of crustal diapirs or domes associated with flower-shaped normal- fault structures, and

• a complicated system of fault-bounded segments across strike.

The geophysical evidence also suggests northeast-to-southwest propagation of the rift, with initial crustal inflation/doming followed by deflation/subsidence, volcanism, and extension along normal faults.

Although Bransfield Strait exhibits geophysical and geologic evidence for extension and volcanism, continental crust fragmentation in this "back-arc" basin is still underway, and has yet to generate ocean crust. Instead, the Bransfield rift lies near the critical area that marks the transition from intracontinental rifting to seafloor-spreading. Because the basin is asymmetric and appears to manifest shallow intracrustal detachment faulting, it is near the edge of the spectrum of models to explain how continents break up. As such, the region is an excellent "natural lab" for studying the diverse processes that form continental margins.

Thus some significant questions arise: What stage of evolution is the Bransfield rift experiencing? Does it provide a basis for constructing models of Andean mountain-building? The answers are to be found by defining Bransfield rift's deep crustal structure. This research work will begin this task by collecting and analyzing high-quality, high-density ocean bottom seismometer (OBS) profiles both along and across the Strait's strike. The scientific objectives are to:

• develop a detailed seismic velocity model for this rift;

• calibrate velocity structure and crustal thickness changes associated with presumed northeast-to-southwest rift propagation, as deduced from the multichannel seismic interpretations;

• document the degree to which deep velocity structure corresponds to along- and across-strike crustal segmentation; and

• assess structural relationships between the South Shetland Islands "arc" and Bransfield rift.

The OBS data we expect to collect will also be integrated with other geophysical explorations of the region (such as both Ewing profiles). This piece of the puzzle will complement ongoing deep seismic analysis of Antarctic Peninsula crust to the southwest, as well as additional OBS monitoring for deep earthquakes. Ultimately, the complex plate tectonic evolution of this region may be clarified. (GO-306-O)

Glacial history of the Amundsen Sea shelf.
Thomas Kellogg, Davida Kellogg, and David Belknap, University of Maine.

The West Antarctic Ice Sheet (WAIS) is the last marine-based ice sheet in the world, and may be inherently unstable. The limited marine geological and geophysical data available from the Amundsen Sea suggest that grounded ice or an ice shelf occupied the inner Amundsen Sea embayment until perhaps 1,000 to 2,000 years ago, and this ice may have retreated rapidly since then. Organized inquiry has been formalized with the WAIS science plan. Their Priority Goal H2 asks:

"What is the deglaciation history in the eastern Ross, the Bellingshausen and Amundsen Seas?"

This project responds to the last of these targets by undertaking a study of the marine geology and geophysics of the Amundsen Sea continental shelf from 100º W to 130º W.

Unlike other embayments in West Antarctica, major ice streams draining into the Amundsen Sea from the interior of the WAIS lack buttressing ice shelves. Analysis of the distal portions of these ice streams (Pine Island and Thwaites Glaciers) indicate glacial mass is either in balance or may be dissipating. Because both ice streams have beds that slope downward toward the center of the ice sheet, grounding-line recession resulting from either continued thinning or sea-level rise could trigger irreversible grounding-line retreat, leading to ice-sheet disintegration. If this ice sheet were to melt, the rise of sea level around the globe would follow.

This project will examine bathymetric data of the Amundsen Sea continental shelf to determine the positions of former ice-steam channels, and to help select optimal sites from which to obtain sediment cores. Single-channel seismic reflection studies will also help identify sites for coring, and will locate and identify morphologic features indicative of former grounded ice (e.g., moraines, scours, flutes, striations, till wedges and deltas). The coring will be concentrated along former ice flow-lines. Core samples will be analyzed in the laboratory for sedimentology, to look for basal tills (indicating former grounded ice and its former extent), and for calcareous and siliceous microfossils. The chronology of grounding-line and ice-shelf retreat from a presumed position during the Last Glacial Maximum near the shelf break will be established, using accelerator mass spectrometry (AMS) carbon-14 dates of acid-insoluble particulate organic carbon.

This project shares ship time in the Amundsen Sea with a project in physical oceanography. Our marine geologic data and collected samples – together with the findings of other investigators – should provide another important step toward a comprehensive interpretation of the history of the WAIS. (GO-307-O)

Support Office for Aerogeophysical Research (SOAR).
Donald D. Blankenship, University of Texas at Austin.

The Support Office for Aerogeophysical Research (SOAR) was established to facilitate aerogeophysical research over continental Antarctica, by abetting research efforts to understand the dynamic behavior of the ice sheet and the nature of the underlying lithosphere. SOAR uses high-precision laser altimetry to develop data on gravity, magnetics, and navigation. SOAR also provides information essential to selecting sites for ice coring in West Antarcta, as well as locations for over-snow seismic transects.

This type of interdisciplinary yet focused research and support represents a unique part of the U.S. Antarctic Program, and could revolutionize how earth science and glaciology research is conducted in Antarctica. (GS-098-O)

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