Australia: The Land Where Time Began

A biography of the Australian continent 

Cenozoic carbon cycle

According to the authors¹ solid Earth sources and sinks are not generally in balance at any given time [30, 47]. The are several mechanisms that remove CO2 from surface reservoirs:

Metamorphism and volcanic outgassing where carbonate-rich oceanic crust is being subducted beneath moving continental plates are the main mechanisms by which CO2 is returned to the atmosphere.

Outgassing and burial of CO2 are each typically at levels of 1012-1013 C/year [30, 47-48]. When there has been unusual levels of plate tectonic activity, as when carbon-rich oceanic crust is being subducted or there are high levels of orogeny, the imbalance between outgassing and burial can be a significant portion of the 1-way carbon flux. A net imbalance of about 1012 mol C/year can be maintained for thousands of years, though in the geochemical carbon cycle negative feedbacks reduce the perturbation rate of the surface reservoir [49]. If it is confined to the atmosphere such an imbalance would be about 0.005 ppm/year, but is only about 0.0001 ppm/year, as CO2 is distributed among the surface reservoirs. Compared with the increase of about 2 ppm/year of human increase of atmospheric CO2 this rate is negligible, though such a crustal imbalance alters the concentration of CO2 by 100 ppm over a period of 1 million years.

India moved north rapidly between 60 and 50 Ma at 18-20 cm/year [50], through a region that had been a depocentre for carbonate and organic sediment for a very long time. The authors¹ suggest the subduction of this crust that was rich in carbon would have made it a source of large volumes of CO2, the outgassing of which would have made it a prime cause of global warming, peaking with the Indo-Asian collision 50 Ma. After the closing of the intervening ocean, and the end of subduction and the uplift of the Himalayas/Tibetan Plateau the amount of CO2 in the atmosphere must have decreased as the subduction came to an end [51]. The Indian Ocean and the Atlantic Oceans have been a major depocentres for carbon, though subduction of crust rich in carbon has been limited mostly to small regions near Indonesia and Central America [47].

As the concentration of CO2 in the atmosphere declined following the docking of India with Asia [44] the climate began cooling, finally leading to glaciation of Antarctica by about 34 Ma, and it has remained more or less glaciated to the present. Negative feedbacks, such as the effects of declining temperature of the atmosphere and plant growth rates on weathering. [30]. There is a tendency for a balance to be reached between the outgassing from the crust and CO2 drawdown, resulting from these negative feedbacks, which the authors¹ say have been equal for the past 700,000 years. Temporal variations of plate tectonics [53] and outgassing rates are believed to possibly be related to large fluctuations of the Antarctic ice sheet that are known to have occurred over the last 34 My. The amount of CO2 in the atmosphere that remained relatively constant over the past 20 My implies the outgassing and rates of weathering over that period were in near balance.

With the exception of recent ice core data, knowledge of CO2 levels in the Cenozoic is limited to proxy measures that are imprecise. The authors¹ suggest that in spite of some discrepancies among the different proxy measures and even when the same proxy method is being used by different investigators, CO2 is indicated to have been in the early Cenozoic in the order of 1000 ppm, and over the last 20 My less than 500 ppm [2, 44].

See Source 1 for references used by authors.

Sources & Further reading

  1. Hansen, James et al., 2008, Target Atmospheric CO2: Where Should Humanity Aim? The Open Atmospheric Science Journal, 2008, 2, 217-231.


Author: M. H. Monroe
Last updated 16/05/2012

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