<P> The "greening" of the continents occurred during Devonian time . The covering of the planet's continents with massive photosynthesizing land plants in the first forests may have reduced carbon dioxide levels in the atmosphere . Since CO is a greenhouse gas, reduced levels might have helped produce a chillier climate . Evidence such as glacial deposits in northern Brazil (located near the Devonian South Pole) suggest widespread glaciation at the end - Devonian, as a broad continental mass covered the polar region . A cause of the extinctions may have been an episode of global cooling, following the mild climate of the Devonian period . The Hangenberg event has also been linked to glaciation in the tropics equivalent to that of the Pleistocene ice age . </P> <P> The weathering of silicate rocks also draws down carbon dioxide from the atmosphere . This acted in concert with the burial of organic matter to decrease atmospheric carbon dioxide concentrations from about 15 to three times present levels . Carbon in the form of plant matter would be produced on prodigious scales, and given the right conditions, could be stored and buried, eventually producing vast coal measures (e.g. in China) which locked the carbon out of the atmosphere and into the lithosphere . This reduction in atmospheric CO would have caused global cooling and resulted in at least one period of late Devonian glaciation (and subsequent sea level fall), probably fluctuating in intensity alongside the 40ka Milankovic cycle . The continued drawdown of organic carbon eventually pulled the Earth out of its Greenhouse Earth state into the Icehouse that continued throughout the Carboniferous and Permian . </P> <P> Magmatism was suggested as a cause of the Late Devonian extinction in 2002 . The end of the Devonian Period was a time of extremely widespread trap magmatism and rifting in the Russian and Siberian platforms, which were situated above the hot mantle plumes and suggested as a cause of the Frasnian / Famennian and end Devonian extinctions . The Viluy and Pripyat - Dniepr - Donets large igneous provinces were suggested to correlate with the Frasnian / Famennian extinction and the Kola and Timan - Pechora magmatism was suggested to correspond to the end Devonian - Carboniferous extinction . Most recently, scientists have confirmed a correlation between Viluy traps (in the Vilyuysk region) on the Siberian Craton and the Kellwasser extinction by Ar / Ar dating . The Viluy Large igneous province covers most of the present day north - eastern margin of the Siberian Platform . The triple - junction rift system was formed during the Devonian Period; the Viluy rift is the western remaining branch of the system and two other branches form the modern margin of the Siberian Platform . Volcanic rocks are covered with post Late Devonian--Early Carboniferous sediments . Volcanic rocks, dyke belts, and sills that cover more than 320 × 10 km, and a gigantic amount of magmatic material (more than 1000 × 10 km) formed in the Viluy branch . Ages show that the two volcanic phase hypotheses are well supported and the weighted mean ages of each volcanic phase are 376.7 ± 3.4 and 364.4 ± 3.4 Ma or 373.4 ± 2.1 and 363.2 ± 2.0 Ma, which the first volcanic phase is in agreement with the age of 372.2 ± 3.2 Ma proposed for the Kellwasser event . However, the second volcanic phase is slightly older than Hangenberg event which place at 358.9 ± 1.2 Ma . Viluy magmatism may inject enough CO and SO 2 into the atmosphere to generate a destabilised greenhouse and ecosystem, such as rapid global cooling, sea - level falls and marine anoxia occur during Kellwasser black shale deposition . </P> <P> Other mechanisms put forward to explain the extinctions include tectonic - driven climate change, sea - level change, and oceanic overturning . These have all been discounted because they are unable to explain the duration, selectivity, and periodicity of the extinctions . </P>

What was the cause of the devonian extinction