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Carbon isotope ratios in ancient strata can reveal climatic conditions and biological productivity during mass extinctions. They also provide evidence for an early origin of photosynthetic cyanobacteria and can estimate ocean circulation in the past.
Testing the carbon isotope ratios of ancient or fossil strata can be useful in gaining access to climatic conditions and biological productivity at the time they were established. The use of the carbon isotope in this way is based on the principle that photosynthetic organisms, such as algae, preferentially take up 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, although the exact relationship between carbon isotope ratios and productivity is not fully understood. Analysis of these isotopes seems to suggest that life has experienced five major extinctions in the last half-billion years, although three of these have been considerably more significant than the other two. All of these mass extinctions have been confirmed by sudden decreases in biodiversity in the fossil record. The changes of carbon isotopes over time are known as incursions and excursions, respectively.
In addition to accessing mass extinctions, carbon isotope ratios are also used to estimate the origin of life. Recently, carbon isotope evidence has pointed to an extremely early origin of photosynthetic cyanobacteria, the earliest known living organisms, up to 4.3 billion years ago, just 100 million years after the initial liquefaction of water and about 267 million years ago. years after the formation of Earth itself. If true, this is fascinating, as early estimates of the origin of life placed it much later, about 3.6 billion years ago. If life formed so soon after Earth’s initial formation, then why does it seem so rare in the cosmos at large? Perhaps most of the life in the universe consists only of microbes, but if that’s the case, it may seem unusual that none of these microbes have yet evolved into intelligent beings to visit us.
Carbon isotope ratios can also be used to access the degree of circulation in the oceans millions of years ago. When circulation is low, the carbon-12-rich biomaterial sinks to the seabed and stays there. This makes organisms later on at the top relatively rich in carbon-13. When circulation is good, carbon-12 from below is carried up and organisms have a normal carbon-12 to carbon-13 ratio.
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