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FROM GREENHOUSE TO ICEHOUSE: ARCTICS ROLE IN THE DEVELOPMENT OF CENOZOIC CLIMATIC EXTREMES AND RAPID CLIMATE CHANGE
Cenozoic climatic extremes
A major element in the evolution of Cenozoic environments has been the transformation from warm Eocene oceans with low latitudinal and bathymetric thermal gradients into the more recent modes of circulation characterized by strong thermal gradients, oceanic fronts, cold deep oceans and cold high-latitude surface waters. About 92% of all water in today's oceans are colder than ~10°C. In the Eocene, 50 million years ago, all water in the oceans was warmer than 10°C. Bottom temperatures in the early Eocene, the time of maximum Cenozoic warmth, were of the order of 12°C, and large-scale continental ice sheets did not exist because Earths warm climate inhibited the growth of continental ice-sheets (Miller et al., 1987; Zachos et al., 2001).
The transition to today's world, Antarctica covered by a continental ice-cap and seasonally variable but persistent sea-ice cover in the Arctic, is linked to both the change in climate that increased latitudinal gradients and to oceanographic changes that connected surface and deep-sea circulation between high- and low-latitude oceans. Thus, throughout the course of the Cenozoic, the climate on Earth has changed from one extreme (Paleogene greenhouse lacking ice) to another (Neogene icehouse with bipolar glaciation).
It has long been recognized that our lack of knowledge about the role the Arctic played in the maintenance and development of these climatic extremes is a major gap in our ability to understand and model global environmental change (e.g., COSOD I, 1981; COSOD II, 1987; ODP Long Range Plan, 1996; COMPLEX, 1999; IODP Science Plan, 2001).
The recovery of a 450 m thick, continuous Cenozoic stratigraphic section, encompassing 50 My, from the central part of the Lomonosov Ridge would fill that gap and represent a fundamental step to a quantitative description of global change that incorporates the influence of the Arctic Ocean. Key among our climate objectives is to determine when the Arctic became ice-covered, and to study the variability of sea ice in terms of frequency, extent and magnitude. In this context, the Miocene uplift of the Himalayan-Tibetan region is of particular interest as it may have triggered enhanced flow of Siberian rivers and changed the fresh-water balance of the Arctic's surface waters, considered to be a key factor in the formation of Arctic sea-ice (Driscoll and Haug, 1998).
Rapid climate change
Cenozoic sedimentation rates on the central parts of the Lomonosov Ridge are probably too low to allow ultra-high resolution (sub-annual to decadal) studies of climate change. Late Neogene and Pleistocene sediments on the huge and shallow Siberian shelves were deposited at rates which could permit ultra-high resolution, but problems pertaining to jurisdiction, hydrocarbons and permafrost indicate that higher-resolution sites must be located elsewhere. The sediment section draping the crest on the Lomonosov Ridge becomes progressively thicker when approaching the Siberian (Laptev Sea) margin (Jokat, 1999) and the Lena River. The total sediment thickness above the unconformity is two- to three-fold compared to that occurring on the central parts of the Lomonosov Ridge. The southernmost sites proposed (at ca. 81°N to 82°N and 800 m to 1400 m water depth) would avoid the jurisdiction, permafrost and hydrocarbon problems of the shelf environment but still permit sub-millennial scale resolution and studies of Arctic rapid climate change in the Pleistocene and Neogene.
These two topics, Earths change from extreme warmth (lack of glaciation) to extreme cold (bipolar glaciation), and rapid climate change, are key elements in the IODP Science Plan. Scientific drilling in the Arctic is the only means available to collect the data necessary to decipher the history of the Arctic Ocean and to connect to the history of the Greenland ice sheet and the North Atlantic. This proposed drilling program would be the first controlled sampling of the Arctic seafloor, with the potential to provide much more detailed, continuous information than has come from short (<10 m), opportunistically sited cores. These data would open a new chapter in the study of Northern Hemisphere climatic behaviour.
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