Flood basalts are large volumes of basalt erupted from deep mantle plumes, which cover large areas of land with volcanic basalt rock in a relatively short time, geologically speaking (hundreds of thousands to millions of years). Like other volcanic eruptions they also release climate altering carbon dioxide and sulphur dioxide gasses, and do so in very large volumes. This is thought to have a very profound effect on the climate, and flood basalts are often closely linked to mass extinction events, for example the Siberian Traps Flood Basalt is associated with the End Permian Extinction, the Mid-Atlantic Flood Basalt with the End Triassic Extinction, and the Deccan Traps Flood Basalt with the End Cretaceous Extinction. The youngest known flood basalt is also the youngest; the Columbia River Flood Basalt, in the Pacific Northwest of the United States, erupted between 17 and 5 million years ago, from a north-south trending fissure system than covered parts of eastern Washington, eastern Oregon, western Idaho, and northern Nevada. This is coincident with a wider outbreak of volcanism in the area, associated with the subduction on the Cascade Arc, which fuelled volcanism in central Oregon and northern Nevada, and the Yellowstone–Snake River Plain Hotspot, which fuelled volcanism in western Idaho and eastern Wyoming. For this reason, plus difficulties constraining the dates of the basalt eruptions, the Columbia River system is not universally accepted as a true-mantle-plume-driven flood basalt. Nor is the Columbia River Flood Basalt associated with an extinction event, though its onset does appear to be correlated with the Middle Miocene Climate Optimum, 16.5 million years ago, a period of rapid warming associated with a spike in atmospheric carbon dioxide, that saw the retreat of the Polar Ice Caps, and major climatic perturbations in many areas.
In a paper published in the journal Science Advances on 19 September 2018, Jennifer Kasbohm and Blair Schoene of the Department of Geosciences at Princeton University publish a new chronological sequence for the Columbia River Flood Basalts, based upon Uranium-Lead Zircon dating.
Zircons are volcanic minerals that form within molten rock as it cools. Like other such minerals, zircons will incorporate some elements present in the melt, but not others, notably they will incorporate uranium, but not lead. This is important because uranium is an unstable element, and over time undergoes fission, with the uranium atoms breaking down to produce, amongst other things, lead atoms. This is very useful because nuclear fission occurs at a steady rate, unaffected by conditions such as temperature or pressure, so that by comparing the proportion of uranium to lead within a zircon crystal.
One problem with this is the remarkable stability of zircon crystals. Zircons form in cooling magma and volcanic melts where the right elements are present, but unlike other such minerals can survive being reheated past the temperature at which they originally formed. This enables zircons to survive the melting of rock along subductive plate margins, and subsequent re-eruption through volcanoes along these margins. The durability of zircons has proved extremely useful to scientists studying the early evolution of the Earth, as the oldest minerals on the planet are zircons far older than their host rocks, but is a serious problem in an area like the Columbia River Flood Basalts, where material from a deep mantle plume has erupted through recent volcanic rocks associated with a subductive plate margin, where most of the material being subducted was older volcanic rock, something that has made it very hard to date the Columbia River sequence.
In order to resolve this problem Kasbohm and Schoene took samples from eight horizons within the Colombia River Sequence, then tested a large number of zircons (20-40) from each sample, selecting the youngest age present in each sample to represent that part of the sequence. Using this method, they were able establish that the upper 72% of the Steens Basalt, the oldest rocks of the sequence, erupted between 16 653 000 and 16 589 000 years ago, the overlying Imnaha Basalt erupted around 16 572 000 years ago, the Grande Ronde Basalt, which overlies the Imnaha Basalt, finished erupting around 16 066 000 years ago, and the lowest 77% of the Wanapum Basalt, close to the top of the sequence, had erupted by 15 895 000 years ago.
Zircons are volcanic minerals that form within molten rock as it cools. Like other such minerals, zircons will incorporate some elements present in the melt, but not others, notably they will incorporate uranium, but not lead. This is important because uranium is an unstable element, and over time undergoes fission, with the uranium atoms breaking down to produce, amongst other things, lead atoms. This is very useful because nuclear fission occurs at a steady rate, unaffected by conditions such as temperature or pressure, so that by comparing the proportion of uranium to lead within a zircon crystal.
One problem with this is the remarkable stability of zircon crystals. Zircons form in cooling magma and volcanic melts where the right elements are present, but unlike other such minerals can survive being reheated past the temperature at which they originally formed. This enables zircons to survive the melting of rock along subductive plate margins, and subsequent re-eruption through volcanoes along these margins. The durability of zircons has proved extremely useful to scientists studying the early evolution of the Earth, as the oldest minerals on the planet are zircons far older than their host rocks, but is a serious problem in an area like the Columbia River Flood Basalts, where material from a deep mantle plume has erupted through recent volcanic rocks associated with a subductive plate margin, where most of the material being subducted was older volcanic rock, something that has made it very hard to date the Columbia River sequence.
In order to resolve this problem Kasbohm and Schoene took samples from eight horizons within the Colombia River Sequence, then tested a large number of zircons (20-40) from each sample, selecting the youngest age present in each sample to represent that part of the sequence. Using this method, they were able establish that the upper 72% of the Steens Basalt, the oldest rocks of the sequence, erupted between 16 653 000 and 16 589 000 years ago, the overlying Imnaha Basalt erupted around 16 572 000 years ago, the Grande Ronde Basalt, which overlies the Imnaha Basalt, finished erupting around 16 066 000 years ago, and the lowest 77% of the Wanapum Basalt, close to the top of the sequence, had erupted by 15 895 000 years ago.
Map of Columbia River Flood Basalts and regional volcanism. The map shows the areal extent of each formation of the Columbia River Flood Basalts, and the legend provides the volume contribution of each formation. Stars represent geochronology sample collection sites; dashed lines enclose areal extent of source dike swarms. The Prineville Basalt and Picture Gorge Basalt are coeval with the Grande Ronde Basalt, represent 1.4% of the total Columbia River Basalt volume, and are grouped with the Grande Ronde Basalt for all volume estimates presented here. Kasbohm & Schoene (2018).
This implies that 95% of the total volume of the Columbia River Flood Basalts erupted within 758 000 years, between 16 653 000 and 15 895 000 years ago. If correct, this means that the Columbia River Basalts were erupted at a remarkably high rate even for flood basalts, with peak rates of between 20 and 40 cubic kilometres of material per year being produced. This compares to estimates of peak eruption levels for the Deccan Traps Flood Basalts of between one and two cubic kilometres per year, for the Central Atlantic Flood Basalts of three to five cubic kilometres per year, and for the Siberian Traps Flood Basalts of between one and four cubic kilometres per year.
Such a large volume of volcanic material being erupted over such a short time would have had a very strong effect on the atmosphere, potentially doubling the amount of carbon dioxide in a relatively short period of time. This correlates well with the onset of the Middle Miocene Climate Optimum at 16.5 million years ago. The Middle Miocene Climate Optimum was followed my the Middle Miocene Disruption, which began around 14 million years ago, when falling carbon dioxide levels led to plunging temperatures, and an extinction event that removed many warm adapted groups from higher latitudes. This suggests that the rapid eruption of volcanic material created a spike in atmospheric carbon dioxide that lasted around a million years after the eruptions ceased.
See also...
Such a large volume of volcanic material being erupted over such a short time would have had a very strong effect on the atmosphere, potentially doubling the amount of carbon dioxide in a relatively short period of time. This correlates well with the onset of the Middle Miocene Climate Optimum at 16.5 million years ago. The Middle Miocene Climate Optimum was followed my the Middle Miocene Disruption, which began around 14 million years ago, when falling carbon dioxide levels led to plunging temperatures, and an extinction event that removed many warm adapted groups from higher latitudes. This suggests that the rapid eruption of volcanic material created a spike in atmospheric carbon dioxide that lasted around a million years after the eruptions ceased.
See also...
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