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Report No 16

Appendix 5:

Report on Hobart Workshop published in EOS Transactions of the American Geophysical Union, 79(1), January 6th 1998
Antarctica's Role In Global Change Research Examined

APPENDICES

The greatest discoveries about Antarctica's late Mesozoic and Cenozoic geologic history have been made over the past four decades since the International Geophysical Year. These discoveries were by earth scientists that conduct research in many countries under their National Antarctic Programs, which in turn follow the broad science guidelines recommended by the Scientific Committee on Antarctic Research (SCAR). SCAR has many sub-groups that address thematic issues.

One of these groups, the Antarctic Offshore Stratigraphy Project (ANTOSTRAT) was tasked by SCAR to bring together a cross-section of Antarctic earth scientists, to outline the critical thematic topics related to the last 100 m.y. of Antarctic earth history, and to recommend objectives for the coming decade of Antarctic earth science studies--- to guide and promote Antarctic work. Such a group of discipline- and regional-specialists met at a recent workshop in Hobart, Tasmania (July 6-11), with participation of over forty scientists from eleven countries. A second SCAR workshop was held simultaneously by the ANTIME group, to focus on the last 20,000 years of earth history.

This report describes the ANTOSTRAT workshop. The principal outcome was the unanimous approval of a heavily debated recommendation that earth science studies in the coming decade should focus on acquiring rock samples via drilling, coring, and other techniques to help establish the onset and development of Antarctic glaciations, and link these glacial-interglacial periods to global climates of the past 100 m.y.

Many thousands of kilometers of remote-sensing geophysical data (e.g., high- and low-resolution seismic reflection, side-scan, magnetics, etc.) have been collected around the Antarctic margin over the past two decades. ANTOSTRAT scientists had compiled and analyzed these regional-survey data over the past decade, and their work was the foundation for workshop. The compilations focus on five regions of the Antarctic continental margin: Ross Sea, Wilkes Land, Prydz Bay, Weddell Sea, and Antarctic Peninsula (Cooper and Webb, 1994; Cooper et al., 1994).

Multichannel seismic-reflection (MCS) data are the principal imaging tool used to map geometries of sub-surface strata and geologic structures, and as such, have been central to the ANTOSTRAT studies. A key activity has been the implementation of the Antarctic Seismic Data Library System for Cooperative Research (SDLS) that now provides all researchers with open access to MCS data at 12 branches in 10 countries. The SDLS operates under the science aegis of SCAR and mandates of the Antarctic Treaty. The following summarizes some of the key discussion topics of the workshop.

Tectonics: Tectonics have played a key role in the evolution of Antarctica's ice sheets. Vertical tectonics in particular have formed topographic features that have dammed the movement of ice (e.g., Transantarctic Mountains), have created paleo-seaways in the Antarctic interior regions that ameliorated paleo climates (e.g., Wilkes sub-glacial basin), have provided pathways through the mountains where ice can drain seaward (e.g., Beardmore and Drygalski troughs), and have potentially controlled circum-Antarctic ocean-currents via bathymetric gateways (e.g., Scotia Arc platform).

Although the evolution of some of these features is still speculative, their influences on Antarctic isolation and segmentation, with eventual increased insolation and ice-sheet development are likely large. It was concluded that drilling into the sedimentary sections within and adjacent to these features is the only way to document their influences on Cenozoic glaciations.

Sedimentary basins: Only 2% of Antarctica's geology is exposed on the continent, with the rest covered by ice and water. Sedimentary sections are likely under the ice, but have not been seismically mapped and can only be inferred from geophysical data and limited onshore drilling in the McMurdo Sound region. Large sedimentary basins of late Mesozoic to Cenozoic age are known beneath all segments of the Antarctic continental margin, and hold up to 14 km of strata carried from the interior regions of Antarctica.

The basins' framework is reasonably well mapped seismically, but stratal ages are mostly unknown. The basinal sections are important, for documenting the different, but simultaneous events around Antarctica, such as variable movements of glaciers across the continental shelf, and coeval events in adjacent ocean basins, such as variable initiation of bottom-water currents. Because strata were deposited at different rates, basins with rapid sedimentation, such as the eastern Ross Sea, when drilled can give high-resolution records of the onshore and continental shelf events.

Shelf records can in turn be compared with those of adjacent ocean basins to help link and calibrate proxy deep-ocean-basin records. The offshore regions, where thick sedimentary sequences exist, offer the best potential sites for drilling and coring along several transects across different segments of the continental margin to decipher Antarctica's glacial history.

Seismic stratigraphy: Seismic stratigraphic studies provide the strongest evidence to date for local and regional expansions of glaciers onto the Antarctic continental shelves since Paleogene time. Thick sedimentary wedges at the continental shelf edge and strongly varied shelf sequences imaged by acoustic data attest to nearby glaciers. Numerous seismic surveys have been conducted and glacial-geology models created, yet the geologic samples collected are inadequate to "ground-truth" the surveys and to verify the models.

It is not yet possible to develop a circum-Antarctic sequence stratigraphy like that commonly applied to low-latitude continental margins (e.g., Vail model). Future Antarctic seismic studies should focus on systematically characterizing glacial seismo-stratigraphic facies and features on all segments of the Antarctic continental margin. These would be used, in conjunction with drill-core and down-hole logging data, to establish an Antarctic sequence stratigraphic model for the Cenozoic glacial era. Emphasis should be on very high-resolution seismic data, closely gridded surveys, and accurate ties of seismic horizons to drill core and logging data.

Glacial sedimentologic processes: Few comprehensive process studies have been done to precisely relate modern Antarctic environments to the varied depositional processes that may occur there, to help guide interpretation of older geological successions. The absolute definition of glacial lithofacies is still strongly debated, and reflects the large variablility in environments and processes in time and space. It was recommended that urgent attention be directed to three subjects: characterizing the sedimentology of seismic-stratigraphic units, to establish offshore "type localities"; characterizing sub-glacial and grounding-line depositional systems; and developing quantitative models as to how glacial-sedimentary sequences accumulate on the continental margin.

Linkages: The linkages between the Antarctic continental shelf and abyssal ocean basin are weak, and hinder global correlations for climates of the past 100 m.y. and longer. Significant gaps exist in circum-Antarctic paleoenvironmental histories for Late-Jurassic to Early Cretaceous, Late Cretaceous, and the Cretaceous-Tertiary boundary. Only few cores exist for these intervals, and are inadequate to derive, for example: the extent and impact of the Early Cretaceous anoxic event recorded by the "black shales" recovered in the Weddell Sea; the pan-Antarctic Late Cretaceous paleoclimates as Antarctica moved into its polar position; and the patterns of faunal changes and mass extinctions, if any, across the Cretaceous-Tertiary boundary.

Significant benthic faunal restructuring occurred in association with the oceanic Paleocene-Eocene thermal event, and there is uncertainty about the influence of this event on watermass evolution, onshore-climates and initial ice-sheet formation. Drilling in the Southern Ocean has compiled comprehensive calcareous and siliceous microfossil-based biostratigraphies, yet the impacts of onland Antarctic climatic changes on watermass characteristics and circulation patterns on these microfossil groups is poorly known. Further core samples in specific regions of the Southern Ocean are needed to resolve these uncertainties.

Onshore basins: The extensive sub-ice basins and the mountain drainage systems of onshore Antarctica may hold Mesozoic and Cenozoic strata. These areas often occur near volcanic regions, thereby providing dateable volcanic rocks. Sampling these onshore strata via drilling through the ice, would give the most direct paleoclimate record for Antarctica, for comparison with continental shelf and deep-ocean strata, but has mostly not been attempted due to difficult logistics.

Priorities outlined for onland investigations of marine-seaway and terrestrial rocks include: greater recovery of sub-ice rocks; improved chronostratigraphic control from marine biostratigraphy and volcanic debris; and mapping of sub-ice features to delineate sub-sea-level basins with likely stratigraphic sections.

Climate models: Global Climate Models (GCM) have increasingly higher temporal- and spatial-resolutions than previously, and may now help derive regional long-term paleoenvironments in and around Antarctica. In applying GCM models to Antarctica, it was noted that model accuracy is highly dependent on the quality of boundary condition data. For useful regional GCMs of Antarctica, geologists must provide detailed information on such things as topography, vegetation cover, geological data on local climates, and accurate times for these conditions.

It was proposed that GCM models be constructed for ten critical periods ("time slices") in the evolution of the Antarctic Ice Sheet, to improve the understanding of Antarctica's long-term role in the global climate system. "Time slices" of 1-2 my duration will be undertaken for periods from Late Cretaceous time to the Present, and include mid-Campanian cooling (~75 m.y.), late Paleocene-Eocene thermal boundary (~52 m.y.), early to mid-Pliocene warmings (~4.3 and 2.5 m.y.) and others.

Future drilling and coring: Like the past, the greatest future advances in deciphering Antarctic glacial history are likely to come from drilling and coring of the extensive sedimentary deposits that underlie Antarctica and its continental margin (Webb, 1990; Barrett, 1996). Prior drilling by the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Program (ODP) provided the information on which many prevailing models are built. Yet the drilling information is fragmentary and incomplete, and was last done a decade ago.

Over the past three years, ANTOSTRAT has laid the groundwork in planning and developing proposals to ODP for future circum-Antarctic drilling in areas where extensive prior surveys were done and thick sedimentary sections are known. ODP Leg 178 is scheduled to be drilled in one of these regions, the Antarctic Peninsula, in February 1998. Hopefully, other ODP drilling legs will follow on other parts of the margin.

Efforts by other groups are in progress to develop offshore high-speed diamond drilling systems that can be mounted on ships of opportunity, or on sea-ice, to drill up to 200 m into strata of the continental shelf. Such systems may "come on line" within the next decade, and must be capable of penetrating the ubiquitous meters-thick hard diamictite that lies near the sea floor around Antarctica, and has defended underlying strata from nearly all ship-board free-fall coring devices. Deeper drilling than 200 m sub-surface, will only be possible from dedicated drilling ships like ODP and sea-ice-rigs like that be undertaken at Cape Roberts in the southwest Ross Sea. Use of these systems requires years of planning and large budgets &endash; but is the only way to get the needed information.

Reality check: The dreams of Antarctic geoscientists must be balanced by the reality of logistics and costs, as the workshop was sagely admonished by a 40-yr Antarctic veteran in field work and program development, and assigned as workshop contrarian. Yet, he noted, we should strive for several critical objectives: to improve fundamental definitions; seek temporal- and spatial-resolutions at the scale of the processes and events being studied (resolution is seemingly inversely proportional to cost); cherish ODP and other drilling; seek to explain paleoenvironmental impacts of non-uniform periodicities in the geologic record; and seek solutions to seemingly impossible "logistic" problems in Antarctica.

Workshop recommendations: The consensus of the workshop, after long debate, was that Antarctic earth science research of the next decade (1998-2008) should focus on acquiring geologic samples via drilling and coring, to provide "ground truth" information for our existing remote-sensing surveys and geologic models.

Also was decided that SCAR should establish an advisory group (Group of Specialists) to promote and coordinate a range of programs directed toward investigating the onset and development of glaciation in Antarctica since late Mesozoic times. Specific tasks would include gathering geoscience data for working with climate and ice sheet modellers to prepare ice- and climate-scenario maps of Antarctica at selected time intervals; promoting and coordinating ODP proposals; and promoting the use of existing sampling systems and development of new shallow-drilling systems.

It was clear that future major advances in Antarctic earth science will require access to geologic samples from many "time slices" to adequately "ground-truth" the extensive remote-sensing data and the multifarious paleoenvironmental models.

References

Barrett, P. J., Antarctic paleoenvironment through Cenozoic times - A review, Terra Antarctica, 3, 2, 103-119, 1996.

Cooper, A.K. and Webb, P.-N., The ANTOSTRAT Project: An International Effort To Investigate Cenozoic Antarctic Glacial History, Climates, and Sea-level Changes, Terra Antarctica, 1, 239-242, 1994.

Cooper, A.K., Barker, P.F., Webb, P.-N., Brancolini, G. (Eds.), The Antarctic Continental Margin: Geophysical And Geological Stratigraphic Records Of Cenozoic Glaciation, Paleoenvironments, And Sea-Level Change, Terra Antarctica, 1, 2, 236-480, 1994

Webb, P.-N., The Cenozoic history of Antarctica and its global impact, Antarctic Science, 2, 1,3-21, 1990.

Peter-Noel Webb, Department of Geological Sciences and Byrd Polar Research Center, The Ohio State University, Columbus, Ohio 43210, and Alan K. Cooper, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025