Geologists often claim that the past is the key to the future, and so understanding climate change that occurred a few million years ago may help us understand how the planet will respond to rapid future increases in carbon dioxide (CO2) in the atmosphere.
The main cause of climate change over the last 65 million years (the Cenozoic) was probably due to changing levels of CO2 in the atmosphere. We think an increase in the CO2 content of the atmosphere lead to the greenhouse effect which increased global temperatures (hothouse), and when the CO2 content decreased it reduced temperatures, ultimately leading to an icehouse (glacial) world. For the period in question the climate during the first 30 million years was a warming period, whilst the last 35 million years was a period of cooling.
There are three related questions that we need to answer in order to understand the possible causes of climate change during the Cenozoic:
- How do biological shells remove CO2 from the atmosphere?
- How was the hothouse world created in the past?
- How did the Earth recover from this state?
How does biological precipitation in the ocean deplete CO2?
Marine biogenic materials are generally made up of:
- organic matter
- calcium carbonate
Organic matter is produced during photosynthesis by small marine organisms called phytoplanktons.
Opal is produced by organisms such as diatoms (phytoplankton), sponges (animals), radiolarians (protozoa), and silicoflagellates (phytoplankton).
Calcium carbonate occurs in two forms:
- calcite e.g., foraminifera (protozoa), coccolithophorids (phytoplankton) etc.
- aragonite e.g., pteropods (pelagic mollusc), corals etc.
All calcium carbonate is formed from dissolved calcium and carbonate or bicarbonate present in the ocean by the following process.
CO2 from the atmosphere enters the surface of the oceans and forms aqueous carbonic acid. Aqueous carbonic acid is short lived and breaks down into hydrogen carbonate. The micro-organisms then use dissolved Calcium in seawater and hydrogen carbonate to form calcium carbonate as shell.
A calcium carbonate shell: Foraminifera: Subbotina
A calcium carbonate producing micro-organism uses twice as much CO2 in forming a shell but it then returns back one CO2 to the atmosphere, resulting into the net drawdown of CO2. In addition, if organic matter is preserved in the sediments it will remove six times as much CO2 .
How CO2 is released to create hothouse conditions during the Cenozoic?
The majority of biogenic materials are produced by micro-organisms in the surface waters of the ocean which sink to the seafloor to form sediments. As noted above, the calcium carbonate rich sediments capture CO2 in order to make shells. When these sediments are caught in the friction between tectonic plates (a cause of volcanoes and earthquakes) they heat up and release the CO2.
The carbon dioxide is released into the atmosphere at subduction zones, such as the Andean margin of South America, by volcanic activity.
According to the recently published article by Dennis V. Kent and Giovanni Muttoni, this process was responsible for the unusually warm conditions in the early part of the Cenozoic era when the Indian plate was moving northward towards the Eurasian plate by subduction, so heating up abundant carbonate-rich sediments on the northern margin of the Indian plate.
Why did the climate turn cold in the later part of Cenozoic?
When India collided with Eurasia about 50 million years ago, subduction ceased and so did the heating of the subducted sediments and the associated release of CO2 to the atmosphere. This removed a source of CO2 but there was an additional process at work which actively reduced CO2 in the atmosphere.
The weathering of continental silicate rocks leads to removal of CO2 from the atmosphere.
According to Kent and Muttoni’s story, when India entered into the equatorial humid belt 35 million years ago the hot and humid conditions enhanced the weathering of silicate rocks (as found in the volcanic materials of the Deccan traps of India and in the uplifted Himalayan mountains) and so shifted the delicate balance of the carbon cycle towards much cooler conditions. The ice house world lasted for tens of millions of years, setting the stage for our current climate.
This explanation for Cenozoic climate change appears to be simple, if one just considers the CO2 story related to northward movement of India and its collision with Asia. However, in reality the story is much more complex because of the wide range of sources and sinks for CO2 that come into play in the Earth system.
Many other factors must also have played a part in changing climate. These include North Atlantic rifting, the release of methane hydrate during Paleocene-Eocene thermal maximum, plate reorganisation and reduction of seafloor spreading rates, changes in the ocean circulation due to opening and closing of ocean gateways, and the burial of organic carbon by sediments eroded from the growing Himalayan mountains. Clearly all these variables contributed to the long term climate trend during the Cenozoic and ongoing research is unravelling which of these different processes provided a dominant control on climate change.
Find out more about this development in episode 2 of Breaking Science.
'Equatorial convergence of India and early Cenozoic climate trends'
by Dennis V Kent and Giovanni Muttoni
in PNAS (published ahead of print September 22, 2008)
'GEOCARB III: a revised model of atmospheric CO2 over Phanerozoic time'
by RA Berner and Z Kothavla
in American Journal of Science 301
'The role of the Himalaya and the Tibetan Plateau in climate control'
by NBW Harris
in Science Spectra 23