What method would be the best choice for a climatologist to find out what the ocean temperature were like during the Pleistocene ice ages 30 million years ago?

The correct answer to this open question is the following.

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The method that would be the best choice for a climatologist to find out what the ocean temperature was like during the Pleistocene ice ages 30 million years ago was to analyze alkenones molecules of phytoplankton in rocks of that period.

Scientists confirm that alkenones molecules can endure the test of time. They can resist for millions of years. That is why climatologists can use these molecules to understand weather patterns of the past, including the early periods of planet earth.