Birth Of The Polar Ice Sheets

Excepting the massive blips associated with episodes of global or near-global ice cover, the Earth has been icefree for much of its history. During the Paleozoic and

Mesozoic eras, from about 550 to 65 Ma, life expanded dramatically on the planet, and there is little evidence of glaciations through this time. The Cretaceous period, from 144 to 65 Ma, was the apex of the Mesozoic, characterized by sea levels 200 m higher than present, deep ocean temperatures of up to 15°C, and tropical flora and fauna in the polar regions. The cryosphere most certainly had no presence at this time, with the likely exception of occasional snowfalls in polar regions and on the world's highest mountains.

The Cenozoic era, which followed the Cretaceous, marked a turnaround for planetary temperatures and the cryosphere. The Earth began to cool at about 50 Ma (figure 9.1), most likely driven by reduced carbon dioxide levels in the atmosphere and associated water vapor feedbacks. Reduced tectonic outgasing and increased weathering sinks for C02 as a result of major mountainbuilding events have been invoked to explain this, although there are still many questions as to what has driven Earth's long-lerm climate evolution. Increased photosynthetic activity and carbon sequestration as the global oceans cooled probably played a role in Cenozoic climate change. Cryosphere-climate feedbacks also had an influence, through the slow return of snow and ice to the landscape and the birth of the Antarctic and Greenland ice sheets.

Antarctica began to accumulate ice by 35 Ma, most likely through glacial inception from mountain glaciers and icefields that spread to lower elevations as climate cooled. By 23 Ma, the Drake Passage and Tasmanian

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Figure 9.1. (a) Marine benthic á18O records from the Southern Ocean for the past 65 million years, chronicling the Cenozoic cooling and several major cryospheric events, including the period of Antarctic ice sheet growth ca. 15 Ma, and the onset of the Quaternary glacial cycles at about 2.5 Ma (Zachos et al., 2008). (b) Zoomin on data for the past 4.4 Ma (Lisiecki and Raymo, 2005). Higher values of á18O in marine sediments indicate both cooler temperatures and greater ice volume on the continents.

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Figure 9.1. (a) Marine benthic á18O records from the Southern Ocean for the past 65 million years, chronicling the Cenozoic cooling and several major cryospheric events, including the period of Antarctic ice sheet growth ca. 15 Ma, and the onset of the Quaternary glacial cycles at about 2.5 Ma (Zachos et al., 2008). (b) Zoomin on data for the past 4.4 Ma (Lisiecki and Raymo, 2005). Higher values of á18O in marine sediments indicate both cooler temperatures and greater ice volume on the continents.

Passage were opened, as divergent motion of the tectonic plates separating Antarctica from South America and Australia progressed to allow a continuous oceano-graphic passage. This allowed the Antarctic Circumpolar Current to develop, largely isolating Antarctica from midlatitude air and water masses. Ocean sediments document the transition that took place in Antarctica at this time, as ice displaced coniferous forests and something resembling the modern-day ice sheet took root on the continent. East Antarctica is believed to have been continuously ice-covered since about 15 Ma. The West Antarctic ice sheet may have developed more recently, during the past 10 million years (Myr). Modeling studies indicate that the extent of West Antarctic ice probably fluctuated during the past 5 Myr, partially collapsing and regrowing in response to cyclical variations in Earth's axial tilt (the same orbital forcing that drives Northern Hemisphere glacial cycles).

Once established, the Antarctic ice sheet grew to a thickness of several kilometers, further cooling its own climate, strengthening the Antarctic vortex, and promoting cold air outflows that help to cool the Southern ocean and drive increased Southern Hemisphere sea-ice extent. The associated albedo feedbacks contributed to large-scale cenozoic cooling. Expanded sea-ice production and colder surface ocean waters are necessary ingredients for the formation of Antarctic bottom water, which circulates through most of the world's deep ocean. The growth of the Antarctic ice sheet was therefore a major climate event in Earth's recent history.

As global cooling continued through the late Ceno-zoic, the Arctic also began to support seasonal snow and sea ice during the past several million years. Ice-rafted debris indicates that mountain glaciers and icefields in eastern Greenland must have flowed down to the coast at times during the past 10 Myr, and by 3 million years ago this ice cover had expanded to most of the island. The Greenland ice sheet has waxed and waned with the Quaternary glacial cycles of the past 2.5 Myr, but a central core of ice may have persisted throughout this time.

The closure of the Panama seaway and the development of the Isthmus of Panama isolated the North Atlantic basin between 4 and 5 Ma. It has been argued that this increased the moisture transport to high northern latitudes, promoting the development of the Greenland ice sheet and the Quaternary glacial cycles in the Northern Hemisphere. This undoubtedly had an impact on the North Atlantic gyre, and the North Atlantic deep-water formation that we know today may have had its onset at this time. It is unclear, however, whether this had a major role to play in Northern Hemisphere glaciations. The timing is too early, and substantial high-latitude warming accompanied the increased moisture, which generally has a stronger influence than precipitation on the viability of ice sheets. It is more likely that the cooling trend of the past 5 Myr allowed temperatures to transgress a threshold, beyond which it became cold enough to allow the Greenland ice sheet to expand and develop a permanent presence. Ice-climate feedbacks would have supported its survival once established. The flickering ice sheets in North America and Europe soon followed.

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