By measuring the ratio of cadmium to calcium in the shells recovered from an ocean core, we can tell where the bottom water came from when the foraminiferan was living. Ocean sediments also contain ground-up rock, which is transported and deposited by ocean currents, just as wind carries airborne dust to be deposited on ice sheets. The mineralogy of the ground-up rock can be used to identify where it came from. For example, a layer of hematite-rich sediments in ocean cores near Bermuda indicates that ocean currents were transporting material from the east coast of Canada to Bermuda when the sediments in the layer were deposited. To determine what the temperature of the ocean surface was in the past we can use organic compounds made by phytoplankton.
Phytoplankton live near the ocean surface where there is light for photosynthesis.
Some phytoplankton produce compounds know as alkenones which are straight chains of carbon atoms. Along these chains of carbon there can be two or three double bonds. The number of double bonds depends on the water temperature. The double bonds are thought to keep the cell membrane pliable in cold water. When the phytoplankton die, the alkenones fall to the bottom and become incorporated into the sediment.