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SCAR Report 16

Appendix 7

LATE PHANEROZOIC ANTARCTICA: GLOBAL CHANGE CHALLENGES
AT MACRO-, MESO- AND MICRO-TEMPORAL SCALES
Peter-N. Webb, Department of Geological Sciences & Byrd Polar Research Center,
The Ohio State University, Columbus, Ohio, 43210, USA

The last 100 m.y of the Phanerozoic Eon (Cretaceous-Cenozoic) was marked by major latitudinal and longitudinal transport of continents, significant interactions between plates, active sea floor spreading and formation of the modern oceans, ever-evolving current circulation systems, major biotic appearances and terminations, rapid organic evolution, dynamic adjustments of biogeographic province demarcation, and a transition from a generally warm hothouse Earth to a bipolar icehouse Earth. At the global level, Cretaceous and Cenozoic earth sciences are reasonably well understood and integrated, and major events well constrained in time. While it is generally agreed that the Paleoaustral Region (late Phanerozoic terrestrial Antarctica and the surrounding oceans) played a significant role in late Phanerozoic events, particularly in the last ~50 m.y., most data bases are at the reconnaissance level, and the southern high latitudes are still omitted from many global climate modeling experiments. In other words, we still can only speculate on the role of many of the earth system linkages between the Paleoaustral Region and other parts of Earth.

Late Phanerozoic studies in the Paleoaustral Region over the last four decades exhibit several interesting trends. Chronostratigraphy (or the record of time represented by a sedimentary or igneous rock record) has improved steadily, but remains inadequate by modern standards. As the complexity of Paleoaustral geological history and processes has become more fully understood, the number of discrete events, episodes, transitions, etc., recognized, has multiplied, and the need to invoke more geological time to accommodate these events has been accepted. Take an extreme example. Geological histories for post-Jurassic Antarctica were often compacted into the Quaternary in the 1960's, but now, forty years later, are thought to span long intervals of Cenozoic and even late Mesozoic time. In some instances our control of the geological time factor is reasonably accurate. More often than not it is crudely relative, highly subjective, and possibly in error by ten or more million years.

It is time, then, to reassess our understanding of all Paleoaustral/polar earth system processes, the rates of processes, the relationships between processes, the changing role and priority of the major forcing factors over time, and our mastery of geological time itself. Having taken care of infra-Paleoaustral affairs we will better positioned to deal with complex extra-Antarctic earth system linkages.

Late Phanerozoic Global Earth Systems

The transition from so-called global hothouse to icehouse worlds during the late Phanerozoic has been explained in terms of extra-terrestrial and terrestrial (Earth) factors or phenomena, operating at a variety of temporal scales (Crowley and North, 1991, Paleoclimatology, Oxford University Press). Marine and terrestrial geosphere/biosphere data bases from the low and middle latitudes have been successfully used to recognise late Phanerozoic global paleoclimate and paleoenvironment periodicity, trends, phases, thresholds, and events, of various frequencies and amplitude. Some low-middle latitude data have even been employed to argue proxy interpretations of paleoclimate/cryosphere history and trends in the Paleoaustral Region, including East and West Antarctica.

If the low-middle and high latitude hydrosphere, cryosphere, and atmosphere formed a closely coupled interactive system during the late Phanerozoic, as has been proposed, useful isochronous, diachronous and other datums of various temporal resolution should be decipherable in tropical/temperate and polar data sets. It is, then, simply a matter of identifying these interactive systems and interpreting global change patterns apparent in the collective data bases. For example, it should be possible to couple third order global eustatic cycles, polar glacial-deglacial cycles, and high resolution stable isotope oscillation patterns in deep sea data bases in a well constrained global framework. We are well aware that this is not the situation and this and other equally vexing problems remain unresolved.

Characterization Of Late Phanerozoic Paleoaustral Phenomena And Events

Our immediate charge is to prepare Paleoaustral data bases for eventual integration into a variety of global studies, including time-series and time-slice paleoclimate modeling. Are Paleoaustral geosphere-biosphere data sets complete, understood, and organized in ways that facilitate comparisons, correlations, modeling of data, and earth system analysis and synthesis? I suspect not.

The formulation of earth system circuitry at various scales is a highly subjective undertaking. That is, we all apply varying weights to the importance of time, processes, events, periodicity, etc, in the geosphere, hydropshere, cryosphere, atmosphere and biosphere. In many instances our views of Phanerozoic history and the hypotheses and interpretations we erect are strongly influenced by individual attitudes to uniformitarianism or actualism.

The Paleoaustral earth system scientist might be advised to return to basics, focus on the details of observational data, consider relationships between diverse data bases, and rank types of data and the problems they might be used to solve. In other words, there should be a clearer understanding as to how different types of data might be applied to solving different types of problems, and problems of different magnitude. If we do not characterize our enormous polar data bases in some logical way we cannot hope to participate in future long-time-scale global change experiments and syntheses.

One of my primary recommendations for this workshop is that we compile an inventory of Paleoaustral topics, themes, processes, events, phenomena, etc, and organize these against the best time schemes we can muster. This should provide us with some understanding of what we have to work with, where our data are strong, weak, or totally lacking, and importantly, where we should invest future field, laboratory, technical and logistic effort and resources. To initiate this proposed phase of (re)evaluation, I propose that we characterize all existing and new geological and geophysical data at three levels of temporal resolution. Here is an example of what I have in mind.

The Ideal Paleoaustral Region Data Base

For late Phanerozoic Paleoaustral Region data and datums to be useful in a variety of global exercises they should have a temporal resolution value of at least 2 million years, and the many significant datums should be distributed through all 100 million years. Let’s examine the status of our data. Documented Paleoaustral processes, phenomena, and events, characterized at the three temporal levels suggested above, should now plotted against the 100 m.y. late Phanerozoic time scale. I would be surprised if the existing Paleoaustral data based survived the 2 million year resolution test of applicability, although some short spans of the record might.

What do we learn from this exercise? The terrestrial record is very incomplete. The marine record is significantly better, but one has to develop a composite record from widely scattered areas of the Paleoaustral Region. It is almost impossible to relate late Phanerozoic terrestrial and marine histories within the Paleoaustral Region at a level of accuracy found in other parts of Earth.

Specific Late Phanerozoic Global Change Thematic and Topic Objectives: A major goal for the next decade should be an improvement in our understanding of earth system linkages within the Paleoaustral Region; and between the Paleoaustral Region and lower latitude deep ocean, continental shelves and terrestrial environments. Examples of global, hemispheric and regional themes and topics will be introduced and discussed at the workshop, and include macro-, meso- and micro-temporal phenomena. A few examples of these include: