Oxygen Isotopes

How are oxygen isotopes used in the correlation of Quaternary sediments?

Chemostratigraphy is using stable isotopes in determining stratigraphic ages. Isotopes are variations in the number of neutrons of a particular element. Some isotopes, like oxygen ¹⁶O and ¹⁸O, have very well-defined ratios in nature.

In the ocean, some shell-forming creatures uptake oxygen, locking in the ¹⁸O/¹⁶O ratio. Other organisms fractionate oxygen isotopes as part of their sea-critter behaviours, so do not preserve the ratio at time of formation. These shells can be preserved independent of lithology, so make a handy way of tracking isotope ratios in marine sedimentary formations over geologic time.

The lighter isotope, ¹⁶O, is more abundant, but evaporates more readily than the heavier isotope ¹⁸O. In periods of warm climates with little glaciation, the evaporated ¹⁶O precipitates as rain, running off to the ocean, maintaining overall ¹⁸O/¹⁶O ratios. In periods of colder climates with serious glaciation, the the evaporated ¹⁶O precipitates as snow, getting locked up in ice caps, so the overall amount of ¹⁶O in oceans decreases. Climate change is geologically instantaneous and global, making this a good method for determining ages.

However, a raw number in isolation is useless, as any particular ¹⁸O/¹⁶O ratio has occurred many times in history. Instead, similar to magnetostratigraphy, the pattern of highs and lows of conformable succession of beds creates a pattern of values that is unique over geologic time. Using secondary methods, like biostratigraphy (even of the creatures whose shells are used for determining the oxygen isotope ratios), allows for matching a pattern segment into the larger sequence.

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