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SCAR Report No 16,

Appendix 7

THE LATE PHANEROZOIC TERRESTRIAL REALM
Peter-N. Webb Department of Geological Sciences & Byrd Polar Research Center
The Ohio State University, Columbus, Ohio, 43210

Introductory Remarks

The terrestrial record of late Phanerozoic (the last 100 m.y., or Late Cretaceous-Cenozoic) Antarctica is the most poorly understood of all major continental landmasses, even after forty years of intensive investigation since the International Geophysical Year (1957-58) . It is sobering that we have probably documented no more than about 15 percent of late Phanerozoic time in the the Antarctic terrestrial realm. Conclusions and hypotheses based on proxy data rom Deep Sea Drilling Project and Ocean Drilling Program investigations provide generalized views of events on Antarctica at different points in the Cretaceous and Cenozoic but these are unable to contribute high resolution detail or portray events in specific regions of the continent.
It is often observed that Gondwana fragmented and dispersed during the late Phanerozoic, leaving Antarctica isolated in a high latitude location and surrounded by a major ocean system; that a hothouse (greenhouse) world evolved into an icehouse world during the same interval of time; and that geosphere-hydrosphere-atmosphere events in the southern high latitudes were major factors in global evolution from a non-glacial to a bi-polar (cryospheric) Earth. This scenario seems reasonable, but Antarctic data sensu stricto has played little direct role in arriving at currently accepted doctrine.
The presence of two large ice sheets are the principal reasons why we are at an impasse as far as the terrestrial record is concerned. We must now ask ourselves two major questions. First, should we accept the fact that long-time-scale global change can and should be resolved, at all temporal levels, via a bigger and better proxy (usually paleoceanographic) records. Second, if the answer to this question is NO, then what actions do we propose to to take to improve the proximal (continental) record.
If the decision is made to redouble our efforts in building a more complete late Phanerozoic terrestrial data base, then we must urgently reassess our science strategies and also count the cost of the major technological and logisitic efforts that will be required.

Available Terrestrial Data

Most existing terrestrial data are documented at points along the margins of the continent. Late Cretaceous and Paleogene data are best developed along the Pacific and Atlantic margins of the Antarctic Peninsula; and most Neogene data derive from the Amery Graben-Prince Charles Mountains and Transantarctic Mountains regions of East Antarctica, and Marie Byrd Land. Little or no information is available from outcrops in the interior of West or East Antarctica, although some indirect information comes from material transported to the contininent margins. A rough estimate suggests the total terrestrial data base is representative of no more than 15 percent of the area of West and East Antarctica. No one will seriously suggest that late Phanerozoic terrestrial history and its impact at the global level can be argued from such a restricted geographic sample. It is fortuitous, however, that in this relatively small area there is a close association of sedimentary and datable volcanic rocks and so what we have had to work with is quite well time-constrained.

Priorities

I now suggest and comment on priorities one might include in a future long-time-scale terrestrial science plan. This discussion assumes that new programs of scientific drilling are necessary and will lead to the successful recovery of geological materials which provide significantly improved temporal and geographic data point coverage for the last 100 m.y. It is suggested that each topic discussed below be categorized in terms of macro-temporal (5 to10 m.y.), meso-temporal (0 .1 m.y. to 1 m.y. ), and micro-temporal (1 yr to 10,000 yr) scales.
Standard chronostratigraphy, biochronostratigraphy, & magneto-chronostratigraphy - The best known terrestrial localities occur at the continental margins, usually in passive and compressional tectonic environments, and with sedimentary successions having a close association with volcanic and marine sediments. These areas will continue to be essential in refining terrestrial histories in marginal environments, and in providing linkages to high resolution deep sea data bases. Standard isotope dating procedures and marine biostratigraphy will continue to provide temporal resolutions at the million year scale. However, it is unlikely that the amount of time accounted for in these areas will be increased substantially.
Terrestrial successions from the tectonically stable continental interior will be more difficult to date by conventional methods. Opportunities for refined time control will arise if interdigitating terrestrial and marine successions and infra-cratonic volcanic provinces are encountered. Ash shower penetration across the interior montane and basinal regions might provide additional time control for pre-glacial or deglacial episodes on the continent. Wherever possible these and other methods should be used to provide time-constraints of major erosional, weathering and depositional phases; and in assessing the duration of high latitude geosphere processes.
Paleotopography and geomorphology - Present knowledge of sub-ice sheet topography relies on a combination of geophysical techniques, glacio-isostatic adjustment estimations, and hypsometric contouring. Time-slice physiographic contouring procedures should be devised at macro-temporal scales (i.e. 10 m.y. intervals of time). More refined paleotopographic controls might be possible for some intervals of time by the use of accurately dated elevational datums provided by paleontological, eustatic and other data. Improved paleotopographic analysis will allow a pinpointing of both upland topography and the lowland drainage patterns which acted as major sediment transport conduits to infra and peri-cratonic freshwater and marine deltas. Factors to be considered here include: evolving deep crustal tectonic history, glacial and non-glacial erosion phases and associated crustal adjustments, ice volume changes and associated glacio-isostacy, characterization of the terrestrial hydrosphere and cryosphere through time, environmental diversity (i.e. fluvial, lacustrine, cold and warm deserts, etc), weathering and pedogenic development, and landscape evolution.

Terrestrial stratigraphic record - Formal lithostratigraphy (i.e. groups, formations, and members) should be proposed, disconformities highlighted, with both constrained by age control wherever possible. Sub-ice sheet drillhole data should be coupled with acoustic and other geophysical surveys in attempts to understand the thickness and extent of the probable very extensive sub-ice sheet sedimentary basins. Physical property measurements would be undertaken on all core material and drillholes subjected to geophysical logging.

Terrestrial-marine stratigraphic interfaces and relationships - Because of the very extensive coastal zone around Antarctica and the probability of marine advances and retreats across the continental margin and into the continental interior (produced by glacial-deglacial cycles, eustatic oscillations, and tectonic emergence and subsidence), there exists the probability of very complex but meaningful terrestrial-marine succession relationships. This provides a unique opportunity to examine a number of significant regional and global problems. These include ice history-sea level history, mass transport of sediment to the continental shelf basins, maritime paleoclimate, and evolving littoral distribution patterns.

Biosphere - Plate tectonic events, involving the fragmentation of the Gondwana supercontinent, the northward flight of the component fragments, and the geographic isolation of Antarctica in a polar setting, make for a unique late Phanerozoic terrestrial biospheric history. The fate of the greenhouse biological isolates and their icehouse (cryosphere) successors is of central importance in assessing global change in the southern high latitudes. Current terrestrial data bases provide a wealth of information on the paleobotanical record for West and East Antarctica, but the invertebate and vertebrate paleozoological record is surprisingly meager. A concerted effort should be maintained to build on current paleontological data bases through systematic and assemblage studies. Equally important are contributions these floras and faunas make to understanding biogeographic range (provinces) across the continent, and the significance this holds in the tracing paleoenvironment and paleoclimate shifts. Attempts should be made to portray provincial range oscillation at the macro-temporal (5-10 m.y.) and meso-temporal (1 m.y.) scales. Such studies will provide indicators, by geographic and topographic region, of the magnitude and rapidity of climate change, events surrounding the greenhouse to icehouse transition, patterns of evolution, migration, biotic thresholds, refugia, extinctions, and the emergence of the modern Antarctic biota after the demise of the Paleoaustral elements. Studies of taxa at the micro-temporal scale (1 y. to 10,000 yrs), particularly floral elements from the Neogene, provide essential data on survivorship, adaptation, propagation, seasonality, and decadal macro- and microclimate.

Paleoclimate - The documentation of paleoclimate over the past 100 m.yrs., at several levels of temporal resolution, demands synthesis of all available data sets. Firstly, specific data sets are not universally applicable through the entire late Phanerozoic. Secondly, specific data sets are not equally well preserved through all 100 m.y. We must then, use any and all available information that can be gleaned from geosphere, hydrosphere, cryosphere, atmosphere and biosphere sources. The following factors consitute high priority objectives: variability, periodicity, frequency and amplitude in climate; seasonal and multi-season records including temperature extremes and averages, duration of major climate phases, thermal thresholds, lower atmosphere temperature gradients between terrestrial and marine environments, recognition of the active phases of water (ice, water, rain, snow, cloud, vapor, etc), the recognition of catastrophic change (floods), and comparisons between different terrestrial Paleoaustral regions and between the Paleoaustral region and lower latitudes.

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