How the Earth works lecture 9: Tectonics and Climate Flashcards
Describe the main variables which control and determine climate change
most significant geological
controls are:
Astronomical variations in Earth’s orbit
Volcanic activity
Evolution of mountain belts
Configuration of the continents
Supercontinent cycle
What is the main driver of short term climate change?
Changes in Earth’s
orbit and tilt:
Milankovitch cycles
Affects amount of
solar insolation – this
drives oceanic and
atmospheric
circulation
What proxies are used to evaluate climatic change in the recent geological past?
Greenhouse gases (CO2, methane) absorb solar radiation and warm the atmosphere.
Air bubbles in ice cores in Antarctica show variations in CO2 and methane
for example the Vostok core, a well known ice core over 2 kilometres thick was from Vostok in East Antarctica. We can roughly tell the age of different parts of the ice core by various means including counting growth layers.
Describe the role of oxygen isotopes in determining Palaeoclimates
Oxygen isotopes—two isotopes: 16O and 18O.
16O water evaporates faster than 18O water.
During ice ages:
16O in seawater evaporates faster.
Seas become 16O-depleted, 18O- enriched.
18O/16O increases in remaining seawater.
Shells grown in this sea will reflect the altered 18O/16O ratio.
Ice cores reveal the ratio of 16O and 18O over time.
What is the model that uses the Oxygen isotopes to support inter glacial and glacial periods?
The Milankovitch model
Describe the effects of volcanic eruptions on the climate
Local ‘rain-shadow’ effect
Global effect due to eruption of gas and particles
Ash and SO2 blasted into the stratosphere
SO2 + H2O = droplets of sulphuric acid
These absorb and reflect sunlight
Krakatoa-size (1883) eruption can lead to a global fall of 0.3-1°C
Describe the role of mountain ranges in affecting the climate
Rising mountains can act as barriers to winds, therefore affecting the climate.
For example the Andes Uplift raised the Andes to an altitude of >4000 m by the end of the Neogene, establishing a barrier for southeast trade winds in the sub-tropics and for westerly winds in the mid-latitude regions of South America.
Blocking of the westerly winds results in enhanced precipitation on the western side of the mountain range (Chilean, Patagonian glaciers) and causes a strong rain shadow on the eastern side (Patagonian desert).
How does mountain building affect the carbon cycle
Calcium carbonate is present within sediment on the seafloor
This sediment may well be subducted together with the underlying oceanic lithosphere.
The sediment may be metamorphosed deep within the subduction zone and the chemical reactions involved will form new metamorphic minerals as well as CO2 which may be released to the atmosphere via volcanoes.
Describe the level of atmospheric Co2 in the Phanerozoic
Significantly higher concentrations of CO2 during the Palaeozoic Era
levels were low during the Carboniferous ice-house climate but then rose again through the Mesozoic
There was a steady decrease since the Tertiary
Describe how weathering affects the climate
Weathering of silicate minerals by acid hydrolysis consumes CO2 from the atmosphere. This occurs because bicarbonate ion is produced by the disassociation of CO2 in water to form carbonic acid.
Bicarbonate ion then reacts with Ca2+ in seawater to produce carbonate mineral.
For every 2 molecules of CO2 consumed, 1 becomes sequestered in limestone, with a net loss to the atmosphere.
Reduction in atmospheric CO2 reduces the greenhouse warming of the surface and leads to global cooling.
What young mountain chain is guilty of accelerating the atmospheric co2 draw down in the Tertiary?
atmospheric CO2 draw-down during the Tertiary accelerates about 40 myr ago, about the time that India collides with Eurasia to commence the formation of the Himalayas.
What is one example of mountain building and weathering affecting the climate
The uplift of the Tibet plateau intensified the Asian monsoon and the large amount of rainfall combined with the steep relief and high mechanical erosion rates has resulted in some of the highest chemical weathering rates observed for any region.
These chemical weathering reactions, over 40 million years, have consumed atmospheric CO2, thus weakening the global “greenhouse” effect and causing the growth of ice sheets
Increase in weathering = reduction in greenhouse gases
Describe Snowball Earth
A period between 750 and 580 million years ago when palaeomagnetic evidence shows that many of the Earth’s continents were grouped around the equator.
Despite this, glacial deposits of this age are common on many of these continents.
Implies much greater ice coverage than in any recent ice age
Ocean ice in the tropics?
Hence ‘Snowball Earth’ but controversial, could be ‘Slushball Earth’.
What is the supposed cause of Snowball earth?
Major mountain building as Rodinia was formed
Fast weathering of silicate rocks in tropical landmasses takes CO2 out of atmosphere
Cooling and expansion of polar ice-caps
Ice-caps reflect solar radiation leading to more cooling
Further cooling and feedback cycle, Earth mostly freezes over
Describe the major long term controls of tectonics on the climate
The supercontinent cycle:
Plate tectonics drives continental movement.
Ocean basins open and close.
Continental landmasses collide and rift apart.
Supercontinents (like Pangaea) have formed several times.
Continents approach each other, abundant volcanic activity pumps CO2 into atmosphere, global ‘greenhouse’ warming
Supercontinent forms – volcanic activity lessens, ice caps develop on mountains and reflect solar radiation, erosion takes CO2 out of atmosphere, ‘ice-house’ climate
Supercontinent breakup, numerous rifts form and volcanism increases (generating more CO2), continents disperse and subside, thus reducing area available for erosion, back to ‘greenhouse’ effect
