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the Mid-Atlantic Ridge (Original)

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Author Topic: the Mid-Atlantic Ridge (Original)  (Read 13314 times)
Carolyn Silver
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Posts: 4611

« Reply #30 on: July 28, 2008, 10:03:55 pm »


Sea-level changes and the Pleistocene Ice Age

Sea-level has been close to its present level for the past 6000 years, before which it was lower and fluctuating, last achieving its present position about 120,000 years ago. About 15,000-16,000 years ago, sea-level was 130-140 m below its present position. For the past 500,000 years it has been lower than today about 90% of the time.

These major changes coincide with the Ice Age. The last 1.65 million years of geological time -- the “Quaternary” -- is split into the Pleistocene and the Holocene epochs. The latter represents the last 10,000 years when most of the icesheets had melted.

Sea-level falls coincide with periods of glaciation whereas the rises occur during interglacials -- the warmer times between ice advances, like the present day and most of the Holocene.

The onset of the Ice Age began about 40 million years when surface waters in the southern oceans suddenly cooled and the deep ocean basins quickly filled with water ~10°C cooler than before that sank because of its increased density. By about 15 million years ago, the Antarctic Icecap had formed, accelerating production of cold waters. Consequently, siliceous diatom oozes became more abundant in the southern oceans because of increased upwelling that resulted from steeper temperature, and density, gradients.

About 6–5 million years ago, Miocene Epoch, sea-level fell by as much as 50 m, probably associated with expansion of the icecap in Antarctica. Termed the Messinian Event, this might have caused the Mediterranean Sea to dry up over ~1,000 years, producing vast salt deposits, preserved in the sediments of the sea floor.

About 5 million years ago there followed a brief warming trend and sea-level rose again leaving shallow marine sediments inland of modern coastlines around much of the world. Fossil floras and faunas show that climates were generally warmer than today -- Iceland had a temperate climate; southern England was subtropical.

Between 2 and 3 million years ago, ice caps began to form in the northern hemisphere.

During times of glacier growth, areas near glaciers experienced very cold conditions. Regions away from glaciers also experienced varied climates as climatic belts shifted. Because the world ocean temperatures became cooler, there was less evaporation; consequently, much of the world was drier than today. In contrast, some areas that are arid today were much wetter during times of glacial growth. For example, the temperate, sub-tropical and tropical zones were compressed toward the equator by the expanding cold belts – rain that now falls in the Mediterranean fell on the Sahara; the southwestern U.S. was wetter during glacial times because the high-pressure zone over the northern icecap deflected Pacific winter storms southward. Many salt lakes in the western US e.g., Great Salt Lake and Death Valley, were then flooded and greatly expanded.

Information on Quaternary climates comes from many sources – e.g., glacial features and deposits, pollen and sediments on the seafloor. The evidence from terrestrial deposits suggests that Pleistocene glaciation covered 27 million km2 or about three times the present area occupied by ice. The evidence from glacial deposits, mainly tills, in North America suggests at least four main glacial episodes, separated by warmer interglacials when the ice fronts melted back or retreated. In Europe, at least seven advances have been recognized. Part of the problem is that the advancing ice commonly erodes the deposits of earlier glaciations.

In the early 1960’s, Pleistocene ocean-floor sediments were examined for the first time for evidence of ice age climates. They showed that the glacial advances and retreats were far more complex than formerly believed. The evidence from the deep-sea oozes reflects changes in ocean temperatures and ocean water chemistry that can be related to climatic conditions.

The main evidence has come from fossil planktonic foraminifera on the ocean floor. Cores are recovered of the near surface sediments, which are then washed and sieved to concentrate the foraminifera shells = calcium carbonate. Some species are sensitive to warm or cold water, so by examining their distribution in a core, one can determine whether the overlying ocean water was warm or cool. Some species migrate to warmer waters when the ocean surface cools. For example, one species, Globorotalia menardii, is only found near the equator during cool glacial periods, but is found in higher latitudes during warmer interglacial periods.

Other planktonic foraminifera change their coiling direction in response to temperature fluctuations. The Pleistocene species, Globorotalia truncatulinoides coils to the right in water temperatures >10°C, while to the left in water <8-10°. Detailed climatic curves can be reconstructed from coiling ratios.

A widely used method is to measure the ratio of the isotopes 18O to 16O in the CaCO3 of planktonic foraminifera shells. Isotopes: all atoms of an element have the same number of protons in the nucleus, but may have different numbers of neutrons. Those having different numbers of neutrons are isotopes of the element: e.g., oxygen 16, oxygen 18.

The abundance of these two oxygen isotopes is related to the amount of oxygen in seawater when the shell is formed. The exact ratio of these two isotopes reflects the amount of ocean water stored in glacier ice. When water is evaporated from the oceans and precipitated on land to form glaciers, water containing the lighter 16O isotope is more easily evaporated than water containing the heavier 18O isotope. Consequently, Pleistocene glaciers contained more of the lighter isotope, while the oceans became enriched in the heavier isotope. These changes are recorded in the shells of planktonic foraminifera, which take up oxygen in their shells as calcium carbonate = CaCO3.

When the sediments have been dated by radiometric methods - radiocarbon dating and methods that data volcanic ash falls = the chronology of glacial and interglacial periods can be interpreted. Many more glacial/interglacial phases are suggested from the oceans than the records of glacial deposits on land. About 18 glacial expansions are recognized from deep-sea cores, increasing in intensity toward the latter part of the Pleistocene. Tying the sedimentary records from the oceans and land together, correlation, has proved difficult except for the youngest glacial periods . . .
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