supercontinents: Laurasia in the northern hemisphere and Gondwana in the southern hemisphere (Fig. 6.3). Twenty million years later, Gondwana itself began fracturing into two smaller continental masses: East Gondwana (Antarctica, Australia, India, Madagascar) and West Gondwana (South America and Africa).

Detachment of South America and Africa followed with the formation of the

South Atlantic Ocean during the late Jurassic (Fig. 6.3). By 135 million years ago, Madagascar and India began moving northward from Australia and Antarctica. Eventually, India slammed into Asia and created the Himalayas. Like a giant jigsaw puzzle being pulled apart, the individual continental pieces of the modern Earth system began to appear.

At the end of the ''age of dinosaurs,'' 65 million years ago, Antarctica already was located over the South Pole where—as Captain Cook remarked—it had been ''fixed since creation.'' This polar position of Antarctica was like a key that finally unlocked the remaining sub-continents of the Gondwana complex during the Ce-nozoic (Table 6.1).

Around 55 million years ago, as the South Tasman Rise and Australia separated from the Gondwana Province of East Antarctica (30° west to 160° east longitude), a shallow-water connection between the southern Indian and Pacific Oceans was created. By the start of the Oligocene, around 38 million years ago, an ocean

TABLE 6.1 Antarctic Environmental Changes and Events during the Cenozoic Epochs "

(million Cenozoic years) epoch Antarctic environmental change or climatic event

55 Paleocene Australia began to separate from Antarctica; complete cir cumpolar current blocked by South Tasman Rise 39 Eocene South Tasman Rise separated and allowed circumpolar cur rent; shallow water connection between Southern Indian and Pacific Oceans

38 Oligocene Climate-glacial threshold at the Eocene-Oligocene bound ary; sea ice began to form; rapid temperature decrease to nearly 5°C; crisis in deep sea faunas; increased bottom water formation and thermohaline circulation initiated; major deepening of the calcium carbonate compensation depth 38-25 Oligocene Antarctic glaciation, but no ice cap; cool temperate vegetation disappearing from Antarctica 30-25 Oligocene Opening of the Drake Passage; deeper and unrestricted cir cumpolar flow; change in deep-sea sediment distribution patterns

14-11 Miocene Antarctic ice cap formed; closure of Australian-Indonesian deep sea passage; calcareous biogenic sediments displaced northward and replaced by siliceous sediments from diatoms with higher sedimentation rates; development of Antarctic Polar Front at the Antarctic Convergence 5 Pliocene Ice volume increased beyond present; global climate cooling

3 Pliocene Closure of the Isthmus of Panama and Northern Hemisphere ice-sheet development

1.8 Pleistocene Increased upwelling at the Antarctic Divergence and in creased biogenic productivity around Antarctica; glacial-interglacial cycles a See Figure 6.5.

already had formed southward of Australia that enabled the development of an incomplete circumpolar current system around Antarctica.

The Andean Province of West Antarctica, in the Antarctic Peninsula and Ross Sea regions, began separating from South America around 30 million years ago. With the full opening of the Drake Passage between South America and the Antarctic Peninsula, around 25 million years ago, Antarctica finally was surrounded completely be a circumpolar ocean—geographically and thermally isolating the entire continent from the other land masses on Earth.

Another major oceanographic event occurred around 3 million years ago, when the Isthmus of Panama between North and South America closed, altering oceanographic and atmospheric exchanges between the equatorial Pacific and Atlantic Oceans—coincident with glacial development of the Arctic. Driven by plate tectonics over geologic time scales (Figs. 6.2 and 6.3), such changes in the configurations of the continents and ocean basins effectively establish boundary conditions that constrain the environmental dynamics of the Earth system.

antarctic glaciation

With its progressive isolation during the Cenozoic (Fig. 6.3), Antarctica became the coldest region on Earth. Moreover, Antarctic cooling has persisted and intensified over millions of years with temperature decreases propagating across the planet. This impact of Antarctica—as the global heat sink—is recorded over geological time scales in the sediments of the surrounding Southern Ocean.

How did plate tectonics influence the progressive glaciation of Antarctica?

Profiles of sedimentary deposits commonly are generated remotely, across hundreds of square kilometers, by the reflection of sound waves traveling through the sea floor. This acoustic technique is based on the principle that sound velocities vary with the density of the medium (i.e., sound propagates fastest through solids, slower through liquids, and slowest through gases), such that different density or compositional layers can be contrasted. One of the most powerful acoustic techniques in marine research—multichannel seismic reflection—utilizes streamers of hydrophones (often extending several kilometers behind a vessel) that record the two-way travel time of sound waves emitted from the ship through the sediments and reflected back to the ship.

Based on the velocity of the sound waves from the multichannel seismic surveys (around 2 kilometers per second), two-way travel times indicate that the underlying sediments in the Ross Sea are thousands of meters thick (Fig. 6.4). The acoustic profiles also reveal well-stratified sedimentary deposits as well as ero-sional surfaces. Aside from the spatial features of the sediments, however, the acoustic profiles lack information on sediment compositions and ages.

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