Neogene Climate Evolution and
the Development of Great Ice Sheets
Location of Major Northern Hemisphere Ice Sheets During the Neogene
Why did they form and When?
Two Factors were needed to bring about the development of these large ice sheets
1. Cool temperatures in high latitudes
2. Sufficient precipitation (moisture)
This figure shows the long term cooling starting at about 3.5 million years ago.
This figure also shows the beginning of northern hemisphere glaciers (glacial) starting about about 2.7 million years ago. Note the first blue spikes below the red.
So what caused the cooling trend that led to the formation of the glaciers in the Northern Hemisphere?
Tectonics contributed to the climate change
This figure shows the collision of the Indian and Asian plates. This collision (continent-continent collision) led to the uplift of the Himalayan mountain range.
The Himalayas contributed to the climate change in two ways.
1. The uplift of these rocks caused an increase in "weathering" of the rocks. Weathering reduced the amount of CO2 in the atmosphere.
2. The uplift of these very tall mountains (about 21,000 feet) caused the jet streams to be diverted northward around them. See the figure below.
The diversion of the Jet Stream caused moisture from the oceans to be transported into higher (cooler) latitudes where the moisture would fall as snow.
The second tectonic factor that lead to the major ice sheets in the Northern Hemisphere was the closure of the "Ocean Gateways" See the blue lines above. These are the locations of gateways that closed in the Neogene.
The closure of the gateways resulted in the type of surface currents we have today. One of the most important of these currents is the Gulf Stream. The closure of the central American gateway over Panama cut off the circulation between the Atlantic and Pacific. The currents near the equator were turned northward to form the Gulf Stream. This current transports great amounts of heat to the North Atlantic.
The closing of these gateways also opened up pathways for animal migrations. The figure above shows how animals mingled between North and South American when the Panamanian isthmus was uplifted above sea level.
But, if the gulf stream transports great amounts of heat the North Atlantic wouldn't this reduce the likelihood of ice sheets forming at high latitudes?
This is the North Atlantic Paradox
The North Atlantic Paradox
The reason that we think the warm Gulf stream contributed to the build up of ice is that the current brought a great amount of moisture via the atmosphere to northern Europe. See the figure above and remember; The gulf stream heats the air over the ocean which means the air can hold more moisture. The moisture in the atmosphere was transported over northern Europe where the air cools and the moisture condenses to fall as snow.
The second reason why the ice sheets formed was because of the cooling effect of the Earth's orbital changes. Remember that the changes in obliquity of (tilt) affect the summer temperatures in northern high latitudes. A reduced tilt results in cooler summers which allows the snow that fell during the winter to remain on the ground year-round. Overtime this builds up and build-up to form a large ice sheet.
Summary
Tectonic uplift of mountain ranges increased weathering and helped to reduce atmospheric CO2 and diverted the Jet Stream northward.
The closing of gateways changed the low latitude ocean currents.
The transport of moisture to high latitudes via the warm ocean currents provided necessary moisture to form ice sheets at high latitudes
The change in the Earth's orbit around the sun affected summer temperatures at high latitudes so winter snow did not melt during the summer months
Life During the Neogene
One of the most important evolutionary changes in the Neogene was in the Plant kingdom. The forests were significantly reduced in regions like North America when the Rocky Mountains were uplifted and the climate cooled. The drying out of the North American continent resulted in an expansion of bare lands. The herbaceous plants evolved to dominate many of these bare areas that had lost their forests.
This is a view of grasses which came to dominate the landscape of North American prairies. These types of plants can withstand cold temperatures (they die back and lay down seeds during the winter and then bloom in the spring). They prefer open land that is relatively dry. They could not compete well against the larger forest vegetation but once the cooler, dryer climate of the Neogene reduced the forests there was much open space for them to expand into.
Many of the mammals benefited greatly from the drying climate and the spreading of the grassland or savannas. Many of the most successful mammal groups evolved to have the ability to run fast and run long distances.