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Modern Bivalve example.

January 30, 2011 Comments off

Found this surprisingly helpful putting into perspective how bivalves live today. What we’ve learnt about relates to what the guy’s talking about which is cool.x

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Oxygen Isotope Level: Second Round.

November 22, 2010 1 comment

summary These can help tell geologists temperature changes through time.

Oxygen has 3 stable isotopes (16O,17O,18O), but the majority(99.76%) of the worlds oxygen is 16O. There is also a noticeable percentage of 18O(0.2%)

When the water from the sea evaporates the lighter of the 3 isotopes 160 is removed easier than the others so there is more 180.

The ratio (relative amount) of these two types of oxygen in water changes with the climate. By determining how the ratio of heavy and light oxygen in marine sediments, ice cores, or fossils is different from a universally accepted standard, scientists can learn something about climate changes that have occurred in the past. The standard scientists use for comparison is based on the ratio of oxygen isotopes in ocean water at a depth of 200-500 meters.

Oxygen becomes trapped in the ice caps, this alters the balance of 160 and 18 This is noticeable when the global temperatures drops. More 180 is present during these periods.

Graph of oxygen 18 depletion as a function of temperature

As air cools by rising into the atmosphere or moving toward the poles, moisture begins to condense and fall as precipitation. At first, the rain contains a higher ratio of water made of heavy oxygen, since those molecules condense more easily than water vapor containing light oxygen. The remaining moisture in the air becomes depleted of heavy oxygen as the air continues to move poleward into colder regions. As the moisture reaches the upper latitudes, the falling rain or snow is made up of more and more water molecules containing light oxygen. So the oxygen levels go back to there original ways. This also happens when the ice melts as the amount of 160 returns to normal.

If there is a lot of 180 this will indicate glaciation and also less water if there is more ice. So sea levels are lower to than usual.

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The Vail Sea-Level Curve.

November 14, 2010 4 comments

Dr Peter R. Vail has been honored by the SEG in recognition of his “pioneering work in seismic stratigraphy and continuing work in unifying geophysical and geological concepts in interpretation.” Pete Vail graduated from Dartmouth College in 1952 with an A.B. degree. He attended Northwestern University from 1952 to 1956 where he received his M.S. and Ph.D. degrees. He began his career with Exxon in 1956 as a research geologist with the Carter Oil Company, an Exxon affiliate in Tulsa, Oklahoma.

Sequence stratigraphy arrived on the geologic “stage” in 1977 when Vail and his co-workers published on techniques they had developed at Esso Production Research to interpret seismic cross-sections. They assumed that continuous seismic reflectors on acoustic geophysical cross-sections are close matches to the chronostratigraphic surfaces, or time boundaries like bedding planes and unconformities. (we need to know what this previous sentence means in plain English!)

They had established that unconformities were clearly recognizable on marine seismic sections and assumed that like the unconformities of the Paleozoic were the products of worldwide changes in sea level or eustasy (OK, can anybody come up with a definition of eustasy?) The term “eustasy” or “eustatic” refers to changes in the amount of water in the oceans, usually due to global climatic changes. The melting of glaciers at the end of ice ages is an example of eustatic sea level rise. When the Earth’s climate cools, water is evaporated from the oceans and is precipitated on land masses as permanent ice and snow. This causes sea levels to fall relative to a stable land mass.

They noted that the unconformities enveloped packages of reflectors and called these seismic sequences. They demarked these with seismic reflectors onlapping and terminating either against the lower unconformity surface or against each other. (What does that mean?)

The Vail curve (and the monograph itself) was the subject of debate among geologists, because it was based on undisclosed commercially confidential stratigraphic data, and hence not independently verifiable. Because of this, there were later efforts to establish a sea-level curve based on non-commercial data.

Since then, in 1987-1988 a revised eustatic sea-level curve (what does eustatic mean?) Euastatic means there is a world wide of sea level due to the icrease in volume of water in the ocean basins. Volume changes are due too mass increases from melted of grounded ice and thermal expansion or contraction of the ocerans as the warm or cool . Over geological time the shape and volume of the ocean basins themselves also envole.   for the Mesozoic and Cenozoic eras was published, now known as the Haq sea-level curve, in reference to the Pakistani-American Oceanographer Dr. Bilal Haq.

Isostatic effects of ice-sheets-The formation of ice-sheets can cause the Earth’s surface to sink, this results in the sea level to rise. Also mountain ranges will also result in sea level rise.

The formation of new volcanoes on an MOR will result in the sea level to rise  as the sea floor rises as the volume of water stays the same.

If the earth becomes cooler then the two poles will expand resulting in more ice and less sea water so the  sea level will fall.

Alex Buller

James Edmondson.

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