Open Problems and Potential Research Themes for Future Geoscience Investigations in Antarctica

In contrast to all other continents, 99% of Antarctica (including its rocks and geological structures) is covered by a major ice sheet. Less than 1% of the continent provides our geological knowledge and even this has not been investigated thoroughly in all places. On the other hand, about half of the covered area has been surveyed geophysically, through mainly aeromagnetic and gravimetric techniques. Therefore, further investigations are needed and the image of the Antarctic geological structure and its history will likely change, improve and be completed in the future. Several mountain ranges in the Antarctic are difficult to access and therefore have been seldom visited in the past. However, their study would decidedly improve the understanding of geologic and geotectonic connections. It is evident that there are still conspicuous gaps in our knowledge of Marie Byrd Land, the Pensacola Mountains, eastern Dronning Maud Land and East Antarctica between 60° and 120°E. Moreover, the unknown ice-covered interior needs targeted studies starting from well-known areas, for example, tracing known geological rock complexes from their exposed areas under the ice with the help of suitable geophysical methods. The confidence we can have in geophysical interpretations declines with increasing distance from directly accessible rock complexes. Even a few spot checks of rock samples from isolated deep drill-holes would provide a better reliability of interpretations, i.e. calibration of airborne geophysical data.

The concept of the formation of supercontinents represents an important step in higher level global cycles in order to unravel the history of development of continents and oceans, and to improve our knowledge on global plate tectonic and geodynamic processes. Since Antarctica, and particularly East Antarctica, had a central position in both Rodinia, between 1,300 and 700 million years ago, and Gondwana, between 550 and 200 million years ago (today's southern continents), geological information archived in this ''keystone'' region is therefore important not only for a well-founded analysis of local conditions, but also in relation to our understanding of Earth system processes in general. Abundant evidence exists for the former connection of both Antarctic cratonic areas (the East Antarctic and Grunehogna cratons) to geologically similar provinces on neighbouring continents. The study of East Antarctica is a prerequisite for the reconstruction of the assembly and break-up processes of both supercontinents, Rodinia and Gondwana. This pertains to each of the plate configurations, including the orogens as well as the processes in general, which have led to the assembly of the continental lithosphere. Geological and palaeomagnetic data show that East Antarctica includes at least two older cratonic fragments, and Grenvillian and Pan-African structures in coastal exposures support this notion. The larger part (East Antarctic Craton s.s.) continues into India and Australia, while a small fragment (Grunehogna Craton) connects to the Kalahari Craton of Africa. Newest results show that even the East Antarctic Craton s.s. consists of a number of cratonic nuclei (Fitzsimons, 2000a; Boger and Miller, 2004). These results confirm the importance of Antarctica for the reconstruction of continental distribution in early Earth history.

Significant contributions to the understanding of global plate tectonic and geodynamic processes are also stored in the orogenic belts of Pan-African age (600-500 million years ago) which contain important information about the juxtaposition of West and East Gondwana. In Antarctica, the Shackleton Range and parts of East Antarctica between Dronning Maud Land, Lutzow Holm Bukta and Prydz Bay belong to these belts (Buggisch et al., 1990; Jacobs et al., 1998; Tessensohn et al., 1999; Fitzsimons, 2000b; Boger et al.. 2002; Paech, 2005; Buggisch and Kleinschmidt, 2007). Equivalent rocks have been found in the African Mozambique Belt (Paech, 1985; Jacobs et al., 1998; Paech et al., 2005). So far, nothing is known about how these fragments continue under the ice and how they can be connected with the better researched mountains in Dronning Maud Land and the Transantarc-tic Mountains. Equally unknown is the formation, age and relation of subglacial mountains in the East Antarctic interior, e.g. the Gamburtsev Subglacial Mountains. At approximately the same time as the development of internal Pan-African orogenic belts, the geographic domain corresponding to the present Transantarctic Mountains experienced the Ross orogeny, the result of dominant accretionary tectonic processes produced by the subduction of the palaeo-Pacific plate under the palaeo-Pacific Gondwana margin.

There is widespread consensus in considering the end of the Ross orogeny as a nearly synchronous event along the different sectors of the belt (e.g. Stump, 1995; Encarnacion and Grunow, 1996). Voluminous granitoids (Granite Harbour Intrusive Complex) intruded as batholiths at c. 530480 Ma (Encarnacion and Grunow, 1996) represent a unifying feature throughout the length of the Transantarctic Mountains (Borg et al., 1990; Stump, 1995). But although the general tectonic history of the Ross Orogen is fairly well known within each of the major segments of the Transantarctic Mountains, significant variations in lithostratigraphic, structural and metamorphic patterns, as well as in granitoid geochemical affinity, are evident between the different segments. Considerable uncertainty still remains about the onset of the subduction, the tectonic setting of the early granitoids with variable chemical affinities (from calc-alkaline to alkaline and carbonatite) of southern Victoria Land, the nature of the contact between the orogenic belt and the East Antarctic Craton and the relations with the Pan-African structures of the Shackleton Range. Until our knowledge of the relationships of the tectono-metamorphic histories and of the detailed chronology of the magmatic episodes between the segments is improved a comprehensive tectonic model of the development of the Ross improved remains to be formulated.

The importance of Antarctica is also well recognized with regard to the destructive processes of plate tectonics, including the fragmentation of supercontinents. The young continental margins and rift structures of Antarctica, as well as their development, document the break-up of Gondwana and its meaning for the present-day habitats and environmental conditions. Aside from a small section along the Antarctic Peninsula, the continental margins of the Antarctic depict the fault structures of the breakup of Gondwana leading to the formation of the present southern continents and oceans. Antarctica has lain in its south polar position since at least the Late Cretaceous (approximately 130 Ma). Its isolation from the neighbouring continents through the opening of the Southern Ocean and the formation of the Antarctic Circumpolar Current began at this time. The further break-up process led to the formation of the Southern Ocean. The exact opening processes and their effects on palaeoceanography have not yet been satisfactorily reconstructed because high-quality magnetic data are lacking in many key areas of the oceans. This is especially the case for the South Pacific and areas between Antarctica and Africa and Australia, respectively.

For an understanding of the plate tectonic development of the Antarctic continent, as well as climatic and biological developments, several events must be considered:

• 130-100 million years ago: the opening of the Weddell, Lazarev and Riiser-Larsen Seas and thus the opening of the southern Atlantic and Indian Oceans;

• 110-80 million years ago: the genesis of the southern Kerguelen Plateau;

• 80-40 million years ago: the separation of Tasmania/Australia from Antarctica;

• approximately 40 million years ago: the main phase of development of the Ross Sea Rift;

• 30-20 million years ago: the separation of South America from Antarctica, and thus opening of the Drake Passage and Scotia Sea with the formation of the Circumpolar Current.

Aside from the basic questions about the mechanisms and the consequences of these global processes, e.g. of climatic nature, the breakup of Gondwana also led to the current mosaic of continental plates. It was the starting point for many current processes and patterns, for example, the glaciation of the polar regions, the present-day oceanic and atmospheric circulation, the distribution of climatic zones and biota, and finally the global conditions for our existence. The present state of knowledge indicates that the distinct steps of the break-up of Gondwana must be determined more precisely. In particular, the phase since the total isolation of Antarctica is globally relevant for the development of ocean circulation and climate patterns and it must therefore be further deciphered.

More specifically, an increased research effort would be essential to provide new and more complete on- and off-shore (mainly from drill-holes) data to fill the numerous knowledge gaps on the Cenozoic Antarctic glacial history and to allow a better timing of the Antarctic Circumpolar Current onset (Barker and Thomas, 2004; Barker et al., 2007).

Indeed, the important role of the polar regions is emphasized by general circulation models (Sloan et al., 1996; Oglesby, 1999). The presence of thick ice sheets in the continents influences latitudinal gradients and sea-ice formation, and it drives the formation of cold Antarctic bottom water which through deep ocean currents reaches the lower latitudes. The oxygen isotope record from deep-sea cores, and eustatic changes inferred from sequence stratigraphic records on passive continental margins, have led our knowledge of the long-term and broad history of the Antarctic Ice Sheet. However, interpretations based on these proxy records of glacio-eustasy have little direct confirmation from geologic records in Antarctica, and in numerous cases have led to conflicting interpretations (Harwood et al., 1991, 1993; Moriwaki et al., 1992; Wilson, 1995; Miller and Mabin, 1998). In this context, since direct data from the Antarctic region are essential to validate these models, a series of drilling projects have targeted the Antarctic continental margin to retrieve high-resolution stratigraphic records of Antarctica's glacial and climatic history (Cooper and Webb, 1992; Barker et al., 1998; Hambrey et al., 1998; Barrett et al., 2000, 2001; Hambrey, 2002) and more work in this area is planned for the next decade e.g. (ANDRILL, Harwood et al., 2002).

Palaeoclimatic reconstructions based on the results of the Cape Roberts drilling project in the Ross Sea region for the early Oligocene to early Miocene time show two major climatic phases both documenting a warmer climate in the Antarctic than that which prevails today (Hambrey et al., 1998; Barrett et al., 2000, 2001; Naish et al., 2001). Also in the early Quaternary, which was characterized especially in the northern hemisphere by cool climatic conditions, the Cape Roberts core proved that there were times when the Antarctic experienced higher temperatures than today (Hambrey et al.. 1998). However, despite the successes of drilling to date, there remain major unresolved questions concerning Cenozoic tectonic and palaeoenvironmental evolution of the Antarctic region. Chief among these is the still controversial problem of how stable the Antarctic Ice Sheets were during the last 20 My and the timing of the onset of glaciation. The former has been one of the main scientific targets of the first ANDRILL drilling seasons in 2006 and 2007 austral summers (Harwood et al., 2002) and the ongoing investigations are attempting to determine the responses of past ice sheets and shelfs to climate forcing, including variability at a range of time-scales over the last 20 My. The latter has remained an elusive target for both CIROS and Cape Roberts drilling. Indeed, in CRP-3, Oligocene strata passed via an unconformity into the pre-glacial Devonian-Triassic Beacon Supergroup rocks. The search goes on for sites which will recover an Eocene and earlier Cenozoic, or even a Cretaceous record. This issue, along with others, will likely be addressed by future drilling programs that will also complement data from the Ross Sea with data from other coastal regions of Antarctica, and will provide a deeper insight into the climatic relevance of this region.

Was this article helpful?

0 0

Post a comment