Antarctica and the Southern ocean: paleoclimatology of the deep freeze

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1 Indian Journal of Marine Sciences Vol. 37(4), December 2008, pp Antarctica and the Southern ocean: paleoclimatology of the deep freeze S Rajan 1 & N Khare 2 National Centre for Antarctic and Ocean Research, Vascoda-gama, Goa , India 1 *(E.mail: rajan@ncaor.org) The present study consists of a review of the climatic evolution of the Antarctic and Southern Ocean realms since the first massive ice sheet build-up in Antarctica during the Cenozoic. It elucidates the strong link between the cryosphere, oceans and atmosphere. The above aspects are built on a wealth of proxy data from different archives and from across the globe. However, as the studies indicate, there are major limitations as well that hamper a proper understanding of the forcing mechanisms behind the long-term as well as abrupt climate changes. The most serious handicap is the lack of synchronization of records from various archives. Some critical areas such as the Southern Ocean remain grossly under sampled. [Keywords: Southern Ocean, Cenozic, climate change, Neogene, glaciation] Introduction The first massive ice sheet build-up occurred in East Antarctica during the Cenozoic. Polar Regions and the surrounding oceans have been the drivers as well as sensitive indictors of global climate change since than. While the Southern Ocean girdling Antarctica has been a critical component in the development, persistence and decay of the Antarctic cryosphere, the oceanographic processes in the north east Atlantic Ocean have been suggested to be a key element not only in modulating the climate of the Northern Hemisphere but also in the deep circulation on a global scale. Taken together, the cryospheric and oceanographic processes can be regarded as a coupled system in generating a pattern of glacial-interglacial cycles that have dominated the global climate since through much of the Cenozoic. Superimposed on the long-period glacialinterglacial climate cycles are certain abrupt climatic variations or events at decadal, centennial or millennial-scale frequencies 1-2. Proxy records of climate from the ice cores and marine sediments shows a synchronicity of these abrupt climate changes in both the hemispheres on millennial time-scales 3-5. Cooling in the North Atlantic region is generally associated with warming in Antarctica, giving rise to the concept of a bipolar seesaw 6. Although elegant in concept, this model however, fails to explain some of the major issues such as the evidence for large air 2 Present Address: Ministry of Earth Sciences, Block # 12, C G O Complex, New Delhi , India temperature shifts of North Atlantic and Greenland, or the temporal relationship between the abrupt shifts in Greenland temperature as opposed to relatively slow changes in the Antarctic. These difficulties can be explained to a great extent by changes in the oceanic heat transport related to melt water discharge into the North Atlantic and a reduction of the thermohaline circulation 7. This coupled ocean-sea ice model also invokes atmospheric circulation and precipitation as essential ingredients to transmit the abrupt climate signals to other regions. This reveals the strong interhemispheric coupling. As can be seen from the foregoing discussion, despite our fairly comprehensive knowledge of the climatic perturbations, a number of questions remain unanswered. An assessment of the sensitivity of the global climate and its non-linearity necessitates a proper understanding of several of the key issues related to climate change on different time scales. In particular, the following questions and issues are significant: The role of the Southern Ocean seaways in the initiation of Antarctic glaciation Growth, stability and decay of the Antarctic ice sheets over the Neogene The role of Southern Ocean in the global climate variabilities The role of tropical and mid-latitude atmospheric and oceanic conditions/ circulations in modulating the Antarctic climate, and vice-versa

2 RAJAN & KHARE: ANTARCTICA & THE SOUTHERN OCEAN: PALEOCLIMATOLOGY OF DEEP FREEZE 387 What role does coupling between the atmosphere, cryosphere and ocean play in southern hemisphere climate variability and change? The present study summarizes our current understanding of the changing climate of the polar realm since the first major ice sheet build-up. It also highlights the dominant role of the oceans, and in particular, the Southern Ocean in modulating the climatic variabilities throughout the Cenozoic. Initiation of Cenozoic Glaciation The transition from a relatively warm and ice-free Paleocene to the build up of massive Antarctic ice sheets by Oligocene has been more spasmodic than gradual. Ice-rafted detritus in the deep ocean sediment cores (e.g. from ODP sites 738 and 744 on the southern end of Kerguelen Plateau) indicate that ephemeral ice-sheets apparently appeared in east Antarctica during mid to late Eocene. However, the first significant Cenozoic East Antarctic ice sheet developed not before the earliest Oligocene. In contrast, the earliest glacial event on the Antarctic Peninsula post-dates the East Antarctic glacial event by at least 4 Ma. Similarly, the West Antarctic ice sheet that advanced onto the continental shelves did not develop until late Miocene 8-9. Concomitant with the appearance of the first icesheet on the Antarctic mainland, the Earth s climate in general, underwent a major change about Eocene- Oligocene times. However, the exact reasons for this change are far from clear. The pole-ward drift of Antarctica after the break-up of the Gondwanaland, the opening of the circum-antarctic seaways leading to the development of a vigorous circum-antarctic current and the ultimate thermal isolation of Antarctica have been suggested to be of significance in the development of the Antarctic cryosphere 10. However, simple opening of a circum-antarctic seaway could not have been the sole factor that led to formation of the East Antarctic ice-sheet. If the opening of the seaways were the only factors, then there should have been enhanced cooling with increasing opening of the gateways. This would have resulted a step or ramp effect on the paleoclimate 11. Instead, after the early Oligocene build-up, the East Antarctic ice sheet retreated, and from late Oligocene upto the middle Miocene, the East Antarctica was either intermittently ice-free or only partially ice-free. Recent studies also indicate that many other variables related to the ocean-atmosphere interaction such as, changes in atmospheric pco 2 concentrations are also required to explain the Cenozoic climate variability Zachos 14 however, discounts this possibility and suggests that a variation in moisture supply was the critical element controlling the growth and retreat of the ice-sheet. Miocene Cryosphere Development The massive ice sheets that had developed during early Oligocene started retreating and disintegrating by late Oligocene. Distinct warm surface-water events associated with the ice sheet retreat, substantial reduction of sea ice and reduced temperature gradients across the Polar Front Zone are characteristic of the Southern Ocean during this phase of warmth 13,15. The marine fossil record also supports the persistence of increasingly warm sub-tropical conditions throughout the Early Miocene. Peak warmth is supported by maximum Miocene diversity amongst planktic foraminifera 15. Following the Neogene climatic optimum, there was a long-term step-wise expansion of the East Antarctic Ice sheet in the middle to earliest Late Middle Miocene, followed by the development of a permanent ice sheet on West Antarctica in the latest Miocene 10. The records of global sea level fluctuations and oxygen isotope excursions corresponding to this phase of ice sheet growth point to a major reorganization of the Earth s climate system. However, the global cooling indicated by a ~1 increase in the benthic foraminifer δ 18 O values at around 14 Ma lags the initiation of Antarctic ice growth by ~1 million years Interestingly, the initiation of middle Miocene East Antarctic Ice sheet expansion coincides with an interval of warm Southern Ocean waters 16. The climatic disintegration which commenced at the end of the Neogene climatic optimum culminated at around 10.8 Ma in the maximum extension of the Antarctic ice sheet as far north as the Agulhas Ridge region in the present Sub Antarctic zone 18. Concurrently, there was an increased expansion of the continental ice and massive production of cold deep water, related to the establishment of the West Antarctic sheet. The ice-house conditions were soon replaced by warmer conditions and a weakened Southern Ocean thermal isolation at around 10.4 Ma 18.

3 388 INDIAN J. MAR. SCI., VOL. 37, NO. 4, DECEMBER 2008 Plio - Pleistocene Climatic Perturbations By early Pliocene (5.0 Ma), environmental conditions in the Southern Ocean began to approach closely those of the Quaternary. Global climatic conditions were by and large warmer during the early and middle Pliocene compared with the late Miocene and the succeeding late Pliocene. The late Pliocene interval from 3.0 to 2.5 Ma marks one of the most significant climatic transitions in the Earth s Cenozoic history (Fig. 1). It was marked by the development and progressive expansion of the Northern Hemisphere ice sheets and attendant global cooling. Distinct surface-water mass cooling in the Southern Hemisphere is also associated with this ice build-up 19. The climatic perturbations which followed the global cooling event at about 2.6 Ma, initiated a pattern of glacial-interglacial cycles controlled by orbital obliquity and later eccentricity, which have dominated the global climate to date. Evidence for these cyclic events is fairly widespread, ranging from polar to equatorial sedimentary records from the deep ocean to the continental and the Antarctic realm The Last 1,00,000 Years During the last two decades, a wealth of information related to the climate changes during the past 1,00,000 years (straddling the last Glacial period) has been obtained. This has been made possible by the availability of high-resolution marine and polar ice-core records from across the globe, better-defined proxy indicators and lesser analytical uncertainties. The most significant observation from the climate database pertains to the presence of several highamplitude, high-frequency abrupt climatic variations or events at millennial-scale ( years) Fig. 1 Cenozoic benthic foraminifer oxygen isotope compilation showing the major ice-sheet build up and cooling episodes 24 frequencies. For instance, continuous profiles of the oxygen isotope (δ 18 O) measurements and variations in the dust content from the two ice cores drilled from Greenland point to 24 such abrupt events during the period spanning kyr BP These short-term climate changes, which have come to be known as Dansgaard-Oeschger (D-O) events are marked by abrupt warmings, each of kyr duration. The intervening coolings in contrast are more gradual and evolve over a timescale of a few hundred to thousand years, punctuated by some shorter cold events. The wide spread nature of the D-O events has been confirmed in many paleoclimatic records ranging from Antarctic cores, to the sediment archives from the North and South Atlantic, north eastern Pacific and northern Arabian Sea Despite its near-global imprint, the physical processes responsible for these events continue to be enigmatic. These sudden events may have been caused by abrupt slow-down or breakdown of the North Atlantic Thermohaline circulation (THC) consequent on the changes in the freshwater fluxes. It is possible that melting of the icebergs may have provided the freshwater for the disruption of the THC The climate change events from outside the North Atlantic contribute to a better understanding of the underlying mechanism behind the THC. For instance, in the South Atlantic, abrupt cold events in the North Atlantic appear as warming This synchronicity is only a natural corollary to a THC collapse in the North Atlantic. Because the Atlantic THC is associated with a cross-hemispheric heat transport, a THC collapse would result in excess heat in the South. Observations such as this gave rise to the concept of the bipolar seesaw, described earlier. There are major limitations that preclude a proper understanding of the forcing functions behind the abrupt climatic perturbations 35. The most important is a relatively poor dataset outside the North Atlantic region. Practically nothing is known about the longtem climate variability in the tropics. The Southern Ocean still remains a gray area, although many proxy records point to its significance in pacing the climate variabilities. The regionality of the isotopic signals in Antarctica is as yet to be assessed. Correlation of the abrupt climate changes decipherable in the ice cores and marine records in Antarctica with those on land are hampered by the paucity of chronologically well constrained land data.

4 RAJAN & KHARE: ANTARCTICA & THE SOUTHERN OCEAN: PALEOCLIMATOLOGY OF DEEP FREEZE 389 The Holocene Finale Compared to the dramatic climate changes which characterized the last Glacial period (LGM), the Holocene has generally been assumed to be a stable interglacial period. The best-known event which marks the commencement of the Holocene is the Younger Dryas (YD), a distinct, brief (~103 years), cool climate episode straddling the transition from the last glacial period to the Holocene. YD event is most pronounced in the terrestrial records of northwest Europe, northeastern North America and in the marine sediments of North Atlantic. The event has also been picked up in the ice-core records of Greenland and Antarctica Oceanic records of the Holocene from the Southern Ocean in general, display early warmth followed by a middle Holocene cooling 38. High-resolution records from the ice cores of Antarctica display little climate variability. The only major exception is the Taylor Dome record which shows an abrupt cooling event at ~ ka 60. Conclusions The most striking aspect of the paleoclimatological study of the polar and Southern Ocean realms is the wealth of the data available from different archives to infer the sequence of Holocene environmental and climatic change. The mid-holocene was a dynamic period of global climate change. Nevertheless, information on East Antarctic coastal environments during the Holocene is relatively sparse. This is surprising as sedimentary records from the interface between land and sea can provide chronologies of climatic change, isostatic uplift, relative sea level and the colonization of newly formed biomes. Despite this, our knowledge of the global climate variabilities since the first ice sheet developed in the East Antarctica is far from adequate. The cryospheric changes around Antarctica during the mid-holocene may have been influenced by extratropical transport of water vapor. Decreasing CO 2 concentrations since the mid-holocene have indeed been identified from the ice core at Dome C in Antarctica. It is suggested that this tropical-high latitude feedback has influenced the dynamics of the climate system and global temperatures at least since the mid-holocene. However, we are still unable to determine whether the coupled ocean-atmosphere-cryosphere system or an external forcing mechanism was responsible for the observed changes. Perhaps of critical importance are i) the need for a synchronisation of the records from different archives, and, ii) a dire need to have adequate records from such areas as the Southern Ocean. Acknowledgements Authors thank Shri Rasik Ravindra, Director, National Centre for Antarctic and Ocean Research, Goa, for encouragement and the three reviewers for their constructive suggestions that helped to improve the manuscript. 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