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Testing the carbon isotope ratios of ancient strata or fossils can be helpful in accessing the climate conditions and biological productivity at the time these were laid down. The use of carbon isotope in this way is based on the principle that photosynthetic organisms, such as algae, preferentially uptake the lighter and more common carbon-12 while leaving behind the heavier carbon-13. During a mass extinction, there is less preferential uptake of carbon-12, and this is reflected in the sediments.
Analysis of carbon isotope ratios is common in accessing the impact of mass extinctions, though the exact relationship of carbon isotope ratios to productivity isn't completely understood. Analysis of these isotopes seem to suggest that life underwent five large extinctions in the last half-billion years, though three of these were notably more significant than the other two. All these mass extinctions were corroborated by abrupt decreases in biodiversity in the fossil record. Variations in carbon isotopes over time are known as incursions and excursions, respectively.
Besides accessing mass extinctions, carbon isotope ratios are also used to estimate the origin of life. Recently, carbon isotope evidence pointed to an extremely early origin of photosynthetic cyanobacteria, the first known living organisms, to as long as 4.3 billion years ago, just 100 million years after the initial liquefaction of water and about 267 million years after the formation of the Earth itself. If true, this is fascinating, as earlier estimates of the origin of life placed it much later, around 3.6 billion years ago. If life formed so soon after the initial formation of the Earth, then why does it seem so rare in the cosmos in general? Perhaps most life in the universe just consist of microbes, but if so, it may seem unusual that none of these microbes have yet evolved into intelligent beings that have visited us.
Carbon isotope ratios can also be used to access the degree of circulation in the oceans of millions of years ago. When circulation is low, biomaterial rich in Carbon-12 sinks to the sea floor and stays there. This makes subsequent organisms at the top comparatively rich in Carbon-13. When circulation is good, Carbon-12 from the bottom is brought back up to the top, and organisms have a normal ratio of Carbon-12 to Carbon-13.
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