© 2016 Peter Free
Citation — to study
Oliver Jagoutz, Francis A. Macdonald, and Leigh Royden, Low-latitude arc–continent collision as a driver for global cooling, Proceedings of the National Academy of Sciences [PNAS], DOI: 10.1073/pnas.1523667113 (early edition, 18 April 2016)
Citation — to press release
Jennifer Chu, Study: Ancient tectonic activity was trigger for ice ages, MIT News (18 April 2016)
From Jennifer Chu’s well-constructed press release:
[The research team] found that 90 million years ago, the northeastern edge of the African plate collided and slid under an oceanic plate in the Neo-Tethys Ocean [see here], creating a chain of volcanoes.
At 80 million years ago, as Africa continued advancing north, the oceanic plate was pushed further up and over the continent, exposing ocean rock to the atmosphere, while simultaneously terminating the volcanoes.
Then, 50 million years ago, India merged with Eurasia in a second collision in which a different region of the oceanic plate was pushed up onto that continent.
Both collisions took place in the Intertropical Convergence Zone (ITCZ) [see here], an atmospheric region hovering over the Earth’s equator, in which trade winds come together to generate a region of intense temperatures and rainfall.
Certain types of rock, if exposed to high heat and heavy rain, undergo chemical reactions and effectively absorb carbon dioxide, a process known as silicate weathering. These rocks include basalts and “ultramafic” rocks, which are often found within oceanic plates. If these rocks are exposed to the atmosphere in a tropical region, they can act as very efficient carbon sinks.
The team hypothesized that the two collisions, involving Africa and then India, brought basaltic and ultramafic rocks up from the oceans and onto land, creating carbon sinks 80 and 50 million years ago.
Both collisions also effectively turned off carbon sources by burying volcanoes that had been emitting carbon dioxide and other gases into the atmosphere.
To know whether such a sequence of events directly reduced carbon dioxide in the atmosphere, the researchers looked to weathering rates of different rock types, including granites, basalts, and ultramafics. These rates, which have been calculated by other researchers, describe the way rocks erode and take up carbon dioxide, given exposure to a certain amount of rainfall.
They then applied these weathering rates to their model’s estimates of the amount of oceanic plate that was pushed up onto Africa and India, at 80 and 50 million years ago, respectively. After determining the amount of carbon dioxide sequestered by these rocks, they calculated the total amount of atmospheric carbon dioxide through time, from 100 million years ago to around 40 million years ago.
The team found that carbon dioxide dipped dramatically at precisely the time the two collisions occurred. The levels of carbon dioxide also mirrored the temperature of the oceans during this interval.
© 2016 Jennifer Chu, Study: Ancient tectonic activity was trigger for ice ages, MIT News (18 April 2016) (extracts)
How conveniently all these assumptions meshed:
our pretended ability to estimate the type and quantity of the tectonically surfaced ocean rock,
its chemical composition(s),
the rate and degree at and to which these chemically pertinent portions weathered,
to take up and sequester an allegedly quantifiable mass of carbon dioxide,
80 and 50 million years ago,
in a material, weather and climate world that we mostly cannot even crudely yet know.
I especially liked the fact that the volcano chain(s) simultaneously got (totally enough) squished, so as to turn off the problematic greenhouse gas faucet. And the Intertropical Convergence Zone apparently worked similarly to today, despite the distant past’s different distribution of land masses and (presumably) ocean and wind currents.
Now that’s some construct!
The moral? — There is no BS too large to be crammed into eagerly receptive models
I can imagine the mathematical computations (read “fudging”) that was necessary to make this hypothesis come out right.