Exciting Times in the Carboniferous 

In the Carboniferous the UK was near the Equator. The rocks laid down indicate that sea levels changed from low to high repeatedly. And the best explanation for the cause of this is glacial maxima and interglacials, like that of the Pleistocene - but over a much longer period. 50 million years to be precise (in a geological sense - what's a couple of million years between geologists!). 340 to 290 Ma is the current estimate. In the maxima ice accumulated at the South Pole (the centre of Gondwana) and sea level fell; in the interglacials the ice melted and sea level rose.

Sedimentary evidence for global climates 320 Ma ago. As well as the large tracts of glaciogenic sediments, smaller occurrences and examples of polished rock surfaces over which ice had passed show the probable full extent (blue line) of ice sheets across the southern, Gondwana sector of Pangaea (Credit: after Fig 7.3, S104, Earth and Space, ©Open University 2007)

And then things changed. Ice-house world changed into greenhouse world. Temperatures rose and it became drier. THIS ARTICLE describes an ACADEMIC PAPER which gives an explanation for the change. Using samples from the Late Carboniferous of China, isotopes of carbon, oxygen and uranium were used to estimate the CO₂ content of the atmosphere and oxygen in the oceans. 

Between 304 and 303.5 Ma oxygen content dropped by about 30%. At about the same time the CO₂ concentration in the atmosphere doubled (350 to 700 ppm).

What caused the rise in CO₂? There is little evidence for volcanism at this time. The author of the article suggests massive burning of the coal forests of the tropics, encouraged by the high oxygen content of the atmosphere.

Rise in CO₂ results in lower oxygen in the oceans and atmosphere. The oxygen goes into changing dissolved iron (Fe²) into insoluble iron (Fe³) and the Permian has its red beds!