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Paleoceanography 3: Climate Proxies

This is Jim Fiorendino, your host for On the Ocean. How can we study changes in Earth’s climate that occurred millions of years ago? Fortunately, there are clues left behind by ancient life.

The shell, or test, of Syracosphaera azureaplaneta. This phytoplankter is a coccolithophore, which means it forms intricate tests made of calcite plates. These plates remain in sediments after phytoplankton die. Image from: https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/2018/newoceanplan.jpg

Phytoplankton are microscopic marine organisms capable of photosynthesis, meaning they create food for themselves using CO2 in their environment and light energy. Some compounds phytoplankton produce, like lipids, are resilient and difficult to break down. Lipids are a biomarker, meaning they are produced by living things and their presence or characteristics are indicative of processes within an organism or its environment. When phytoplankton die and degrade, lipid biomarkers remain for scientists to find and study. Lipid biomarkers are also known as “molecular fossils” which complement “hard” fossils like shells, bones, and teeth.

The structure of various unsaturated alkenones. Unsaturated alkenones have double bonds. A diunsaturated alkenone has two double bounds, a triunsaturated alkenone has three double bonds, and a tetraunsaturated alkenone has four double bonds. The number of double bonds in alkenones changes with temperature. Figure from Conte et al. (1998).

The structure and composition of lipid biomarkers are clues to what Earth’s climate was like when those molecules were produced. Alkenones are a type of lipid produced by a few species of phytoplankton. Their defining feature is a very long chain of carbon atoms. The number of double bonds in alkenones is one clue scientists use to study Earth’s climate history. Double bonds in alkenones decrease at higher temperatures to maintain the fluidity of cell membranes; the degree of unsaturation (or number of double bonds) in alkenones is therefore an indicator of ocean temperature. Additionally, the ratio of carbon isotopes in alkenones is indicative of Earth’s atmospheric CO2 levels. Isotopes are atoms of the same element with different atomic mass. Phytoplankton can be picky eaters, preferring to utilize lighter forms of carbon if possible. A high ratio of light carbon to heavy carbon in alkenones means the ocean and atmosphere had more CO2. Using these tools, scientists produce records of Earth’s climate history, including both ocean temperatures and atmospheric CO2 concentrations.

This has been On the Ocean, a program made possible by the Department of Oceanography and a production of KAMU-FM on the campus of Texas A&M University in College Station. For more information and links, please go to ocean.tamu.edu and click On the Ocean.

Featured image from: Mikkel Juul Jensen/SPL/Cosmos (left) and Aphelleon/Shutterstock (right)

Script Author: James M. Fiorendino

Contributing Professor: Dr. Yige Zhang