Benthic foraminifera fossils. - Image Credit: Wikipedia

The deep ocean floor provides another clue of what was happening in the atmosphere and in the oceans at the time the sediments were deposited. Sediment cores extracted from the ocean floor provide a continuous record of sedimentation dating back many hundreds of thousands of years and even millions of years in certain places. A sediment core from the equatorial eastern Pacific Ocean reveals the climate history as far back as 5 million years.

This analysis is possible because microscopic marine organisms, such as foraminifera, are found in ocean floor sediments. They obtain their oxygen content from seawater to make carbonate shells. You already have learned that the ratio of ¹⁸O to ¹⁶O is higher in cooler climates. Therefore, the ocean floor sediment cores would contain microscopic shells with more of the heavier oxygen (¹⁸O) in a cooler climate.