ordovician
The Ordovician is the second period during the Palaeozoic and it began 488.3 Mya and ended 443.7 Mya. Typically the climate in Britain would have been moderate to warm, but it is characterised by the large glaciation during the end of the period, the abundance and diversity of marine life and also by the mass extinction event. It has three epochs called the lower, middle and upper, and the geology of the period can be found mainly in Wales, the Lake District, the Isle of Man and along the Highland Boundary fault in Scotland. Typically the sediments deposited were mainly marine in nature and consisted of conglomerates, grits, shales, greywackes, mudstones and radiolarian cherts rich in fossils. It is not uncommon to see the shales from the era metamorphosed to slates, especially in the Lake District and Wales.
At the start of the Ordovician, England was located 60 °S and was part of the supercontinent, Gondwana. This broke up during the end of the Ordovician, forming Avalonia, a smaller continent that England was joined to which moved north to 35 °S. England was covered in the shallow seas of the Iapetus Ocean, with deeper water over the northern part of the country, mainly the Lake District, where evidence of England's underwater past is present in the form of thick muddy sediments. The Ordovician was also a time of extensive volcanism, evidence of this can been seen in the Lake District, where ancient volcanic island arcs are found.
At the start of the Ordovician, England was located 60 °S and was part of the supercontinent, Gondwana. This broke up during the end of the Ordovician, forming Avalonia, a smaller continent that England was joined to which moved north to 35 °S. England was covered in the shallow seas of the Iapetus Ocean, with deeper water over the northern part of the country, mainly the Lake District, where evidence of England's underwater past is present in the form of thick muddy sediments. The Ordovician was also a time of extensive volcanism, evidence of this can been seen in the Lake District, where ancient volcanic island arcs are found.
CO2 levels in the Ordovician were 14 - 16 times higher than today's, we know this from carbon isotope measurements of Ordovician soils, this was due to the large amount of volcanic activity, not a lot of weathering due to flooding and little vegetation to photosynthesise. This then increased the greenhouse effect and temperatures increased around the globe. These warm temperatures were perfect for storms, and evidence of these are seen in the form of tempestites, storm deposits, which are commonly found in Ordovician rocks.
However, there is also evidence of a cooler climate, especially in higher latitudes. How glaciation would occur with such a high percentage of CO2 in the atmosphere can be explained by the decrease in volcanism during the end of the Ordovician, which would also decrease the amount of CO2 in the atmosphere, increasing orogeny would mean an increase in weathering and so also
remove CO2 from the atmosphere. When snow started to fall this would increase the albedo effect and so there would be still lower temperatures. This cooling would then have made the oceans cooler and more habitable for photosynthesising organisms, increasing O2 and decreasing CO2 in the atmosphere and so increasing the overall effect. This period of glaciation is now thought to have lasted for the last 500,000 years of the Ordovician and also caused a decrease in global sea levels which may be one of the reasons for the mass extinction, with 60% of marine invertebrates and 25% of all families going extinct. Evidence for the glaciation is found in the Geology of the Sahara and Mauriania. This ended the Ordovician period and the Silurian began.
However, there is also evidence of a cooler climate, especially in higher latitudes. How glaciation would occur with such a high percentage of CO2 in the atmosphere can be explained by the decrease in volcanism during the end of the Ordovician, which would also decrease the amount of CO2 in the atmosphere, increasing orogeny would mean an increase in weathering and so also
remove CO2 from the atmosphere. When snow started to fall this would increase the albedo effect and so there would be still lower temperatures. This cooling would then have made the oceans cooler and more habitable for photosynthesising organisms, increasing O2 and decreasing CO2 in the atmosphere and so increasing the overall effect. This period of glaciation is now thought to have lasted for the last 500,000 years of the Ordovician and also caused a decrease in global sea levels which may be one of the reasons for the mass extinction, with 60% of marine invertebrates and 25% of all families going extinct. Evidence for the glaciation is found in the Geology of the Sahara and Mauriania. This ended the Ordovician period and the Silurian began.