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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