Lecture 22  Cenozoic Climate:  From Hot House to Ice House

Chapter 17

Focus Question:  How do we know when and how the ice ages began?

1.        The Pleistocene Epoch of the Cenozoic began 2.6 million years ago, and is marked by numerous “ice ages” or glacial-interglacial cycles.  When we look at how the Cenozoic began, 65 million years ago, and compare the Paleocene and Eocene to the Pleistocene, we see remarkable differences.  Look at Ron Blakey’s map of the Eocene here http://jan.ucc.nau.edu/~rcb7/50moll.jpg

And compare it with his map of the Pleistocene here http://jan.ucc.nau.edu/~rcb7/Pleistmoll.jpg  and note

a.        The Eocene was warmer, with high sea level and ice free (look at Antarctica). 

b.      The Pleistocene was colder, with lower sea level and lots of ice sheets (look at North America, Eurasia, and Antarctica)

c.       So, what happened to cause such a remarkable change?

2.        Three major tectonic events occurred in the mid and late Cenozoic that started global cooling.  These events were

a.        Mid Cenozoic time-Antarctica was finally isolated as South America pulled away from Antarctica, and the Southern Ocean completely surrounded the continent at the South Pole. 

b.      Mid-Cenozoic time-India docked with Eurasia, the Himalayan Mountains and Tibetan Plateau formed, so that an enormous area was elevated -at the same time, the Alpine Orogeny was underway, and this contributed to the cooling, too.

c.       Late Cenozoic time-the Panama Seaway closed as Central America emerged, isolating most of the Atlantic Ocean basin and preventing mixing between the Atlantic and Pacific at the Panama Seaway.

3.        Each of these events drove global cooling.

a.        Isolating Antarctica-Relatively cold continent vs not so cold ocean around it-moisture from the surrounding ocean supplied the snow/ice that started building the ice sheet.  Big ice sheets reflect a lot of light and heat back into space (albedo effect), creating a cooling effect.  Big ice sheet leads to cooling, which leads to a bigger ice sheet, etc. etc.

b.      Himalayan Mountains and Tibetan Plateau uplift (and Alpine Orogeny, too)-lots of erosion used CO2 from the atmosphere in weathering events-at such a large scale, this caused a drop in CO2, and cooling

c.       Panama Seaway closing-mixing no longer taking place between warm tropical ocean water of the Atlantic with the Pacific.  Now, warm, salty water flowed north (Gulf Stream) to cool and sink in the North Atlantic, and setting in motion the ocean conveyer belt of circulation, which brings cool water down from polar areas an warm water towards the polar areas. 

4.        These tectonic events were combined with the general effect of three orbital cycles (known as Milankovitch cycles) that are always going on.  Orbital cycles are repeating patterns observed in the motion of the Earth as it travels around the Sun.  Each planet has its own orbital cycles.  Three are important for our consideration here:

a.        Eccentricity-the shape of the orbit.  Earth follows a path around the Sun that ranges from being close to a circle to slightly elliptical -  100,000 year cycle. 

Circle pattern-the amount of incoming solar radiation is about the same whenver the Earth is on the orbit

Ellipse – the amount of solar radiation varies, depending on where the Earth is on the orbit

b.      Obliquity (also known as Tilt) – variation in the angle of the tilt of the Earth’s axis (strongly influences the seasons in mid to high latitudes).  This varies slightly in a repeating pattern – 41,000 year cycle.

More tilt-more incoming solar radiation in the mid to high latitudes in summer, less in winter

c.        Precession-the rotation of the orbit, seen in the rotation of the equinoxes and the changes through time, - 21, 000 year cycle

Example-today, northern hemisphere experiences summer when Earth is tilted towards the Sun but is actually the farthest it will be from the Sun during the year.

About 11,000 years ago, northern hemisphere experienced summer when Earth was tilted towards the Sun but was the closest it could be to the Sun during the year.

5.        So, the tectonic events triggered the cooling during Cenozoic time, but the pattern of cooling, in the glacial-interglacial cycle, was shaped by the orbital cycles, which govern how much incoming solar radiation is received by the Earth.  So, once this got going then, how many glacial-interglacial cycles have we had?

a.        Continental records of glacial advances and retreats are not very good, because each advance of a glacier tends to wipe out the record that was previously there. 

b.      The best place to go is in the ocean floor, where sediments quietly accumulate in large basins.

c.       The method to track these glacial interglacial patterns is to core ocean floor sediments, and measure the oxygen isotope ratios in the accumulating plankton fossils.

6.        The ratio of oxygen-18 to oxygen-16 is used to track the glacial-interglacial cycle.  This works because these two isotopes are stable, and are present in water (H2O) and in the shells of tiny plankton called foraminifera (forams, for short).  Forams make their shells of calcite (CaCO3) so they are going to take up oxygen to make their shells.

a.        Oxygen 16 is more common than oxygen-18, but weighs less.  Its always easier to lift in evaporative processes.

b.      During interglacials (like today) water evaporates off the surface ocean, rains out on the continents, and then re-enters the oceans by way of the rivers.

a.        So oxygen-16 that moves from the ocean to the continents comes back again, and there is a lot of oxygen-16 in the ocean, and in the forams.

7.       During glacials, however, the water evaporating off the surface ocean is locked up in big continental glaciers, and does not return to the sea. 

a.        This results in relatively more oxygen 18 in the ocean, and more oxygen 18 in those little forams.

b.      This also results in sea level drop.

8.        So, when we see a lot of oxygen-18 in the foram shells, we are in glacial conditions, and sea level is lower.  When we see a lot of oxygen-16 in the foram shells, we are in an interglacial condition, and sea level is higher.

a.        The oxygen isotope records allow us to track these glacial-interglacial events, and we can see that there are 104 glacial-interglacial stages in the past 2.6 million years.  The current interglacial has its own Epoch name-The Holocene.

9.       So we can track the beginning of the cooling, in mid-Cenozoic time in the Southern Hemisphere with the isolation of Antarctica by mid-Miocene time, and late Cenozoic time (around 3 million years ago) in the Northern Hemisphere, with the closing of the Panama Seaway.  The details of the cycles are in the ocean sediments, and the oxygen-18/oxygen-16 ratio is used to identify the sequence.

10.  Major landscape features in the U.S. have been shaped by the glaciers or are tied to the glacial climatic events.  For example

a.  The southwestern US was the site of huge lakes that developed during the last full glacial-their remnants today are much smaller, and include Great Salt Lake, UT and Pyramid Lake, NV.

b.  The Great Lakes formed as the last ice sheet retreated, and their drainages switched around numerous times since the lakes started filling.

c.  Cape Cod and the islands of Nantucket and Martha's Vineyard are glacial features-they are end moraines-enormous heaps of sand that were abandoned by the retreating ice sheet-when the glacier pause for a while in its retreat, it left these "end moraines" .  These formed from about 23,000 to 14,000 years ago as the last ice sheet retreated.

 Next and Last Lecture:  The modern configuration of land, ocean and climate:  The Holocene Epoch