Microfossils are the remains of microscopic ocean organisms. These organisms built their shells from chemicals present in seawater while they were alive. Oxygen atoms got locked into the shells as they were built. Scientists analyze the ratio of ¹⁸O to ¹⁶O atoms in the shells of microfossils to deduce the salinity level that existed at the time the organisms were living. Scientists can track changes in salinity by observing changes in oxygen isotope ratios in shells that formed at different times.

To find shells that formed at different times, geologists drill into the ocean floor and bring layers of sediments to the surface encased in tubes. These are called cores. Sedimentologists and paleontologists observe samples from the cores under high-powered microscopes to detect microfossils. The deeper the microfossils are found, the older they are.

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