The atmosphere during the Devonian period
During the Devonian (approximately 416- 359 million years ago), trees and seed plants evolved and expanded to allow for the formation of complex forest ecosystems. This large expansion of life on land had drastic effects on the atmosphere. First, this expansion of life resulted in the removal of carbon dioxide from the atmosphere to sustain these plants. Second, this led to the increased transportation of organic carbon from land to marine areas. This resulted in increased organic carbon burial in the oceans. The transfer of carbon from the atmosphere to rocks and sediments resulted in global cooling and eventually caused the development of continental ice sheets at high latitudes during the Carboniferous Period which persisted for millions of years into the Permian Period. During this Paleozoic icehouse, atmospheric carbon dioxide levels were similar to recent pre-industrial levels.
Marine carbonates are rocks and sediments that record the seawater carbon isotopic composition (isotopic signature used to measure the ratio of the stable isotopes Carbon-12 and Carbon-13). They are often interpreted as a reflection of the biologic pump- the process that moves organic carbon from the surface ocean and deposits it as sediment in the deep ocean. Changes in the seawater carbon isotopic composition reflect changes in Earth's atmosphere as the carbon dioxide concentration of the surface ocean and atmosphere are in equilibrium over long periods of time.
Another aspect of the expansion of plants on land during the Devonian is the likely increase of continental weathering rates as tree roots fragmented rock and acids secreted by terrestrial plants weathered rocks chemically. The chemical reactions that occur as silicate rocks are weathered is an additional way that atmospheric carbon dioxide is removed and stored in rocks. One way to measure continental weathering rates is by measuring strontium isotopes in seawater as marine carbonates record those changes as well. Changes in the amount of Strontium-87 and Strontium-86 in seawater tend to indicate changes in continental weathering rates. Currently, it is known that the seawater Sr isotopic composition increased during the Middle Devonian and while it indicates increased continental weathering, it is very difficult to link these. Most previous studies did not link both variations in the Sr isotopic composition and the carbon isotopic composition together at a high enough resolution to draw definitive conclusions regarding the increase of weathering rates and atmospheric carbon dioxide drawdown.
This aspect of the project will combine both the carbon isotopic composition and Sr isotopic composition from marine carbonates to obtain a high resolution record of continental weathering and then couple it to the record of organic carbon burial and global cooling. In this way, the variations in the Sr isotopic composition can be linked to the organic carbon burial and global cooling found during the Devonian period. The samples for this aspect of the project will be obtained in Iowa, in the Cedar Valley. Middle Devonian limestone is found throughout northeastern Iowa in quarries and outcrops. This analysis will provide insight on this specific topic, but will also add to what is currently known about Iowa's geologic past. The analysis of these samples will be made possible with the help and supervision of Dr. Jennifer Stern at the NASA Goddard Spaceflight Center.
Marine carbonates are rocks and sediments that record the seawater carbon isotopic composition (isotopic signature used to measure the ratio of the stable isotopes Carbon-12 and Carbon-13). They are often interpreted as a reflection of the biologic pump- the process that moves organic carbon from the surface ocean and deposits it as sediment in the deep ocean. Changes in the seawater carbon isotopic composition reflect changes in Earth's atmosphere as the carbon dioxide concentration of the surface ocean and atmosphere are in equilibrium over long periods of time.
Another aspect of the expansion of plants on land during the Devonian is the likely increase of continental weathering rates as tree roots fragmented rock and acids secreted by terrestrial plants weathered rocks chemically. The chemical reactions that occur as silicate rocks are weathered is an additional way that atmospheric carbon dioxide is removed and stored in rocks. One way to measure continental weathering rates is by measuring strontium isotopes in seawater as marine carbonates record those changes as well. Changes in the amount of Strontium-87 and Strontium-86 in seawater tend to indicate changes in continental weathering rates. Currently, it is known that the seawater Sr isotopic composition increased during the Middle Devonian and while it indicates increased continental weathering, it is very difficult to link these. Most previous studies did not link both variations in the Sr isotopic composition and the carbon isotopic composition together at a high enough resolution to draw definitive conclusions regarding the increase of weathering rates and atmospheric carbon dioxide drawdown.
This aspect of the project will combine both the carbon isotopic composition and Sr isotopic composition from marine carbonates to obtain a high resolution record of continental weathering and then couple it to the record of organic carbon burial and global cooling. In this way, the variations in the Sr isotopic composition can be linked to the organic carbon burial and global cooling found during the Devonian period. The samples for this aspect of the project will be obtained in Iowa, in the Cedar Valley. Middle Devonian limestone is found throughout northeastern Iowa in quarries and outcrops. This analysis will provide insight on this specific topic, but will also add to what is currently known about Iowa's geologic past. The analysis of these samples will be made possible with the help and supervision of Dr. Jennifer Stern at the NASA Goddard Spaceflight Center.