Foraminifera are tiny marine protists that build sand-grain-sized shells, or "tests," from the mineral calcite, which is made of calcium, carbon, and oxygen. As they grow, they also incorporate small amounts of other elements, like magnesium and boron, into their shells. The amount of these trace elements varies based on environmental conditions, such as ocean temperature and pH. Because foraminifera have a long fossil record, scientists use the composition of fossil shells to reconstruct past ocean conditions and learn how Earth's environmental history has changed over time.

In this study, we investigated whether the amount of barium in foraminifera shells changes with surface ocean productivity, which refers to how much plant life (mainly tiny algae called phytoplankton) grows in the sunlit upper layer of the ocean. We then tested if the barium content in fossil specimens can be used to reconstruct past ocean productivity. Studying past ocean productivity is important because it helps scientists understand how marine ecosystems responded to past climates, shifts in ocean circulation, and atmospheric carbon dioxide levels over thousands to millions of years. Since ocean productivity plays a key role in the global carbon cycle—helping regulate Earth's climate by removing carbon from the atmosphere—reconstructing its past variations provides crucial insights into how the ocean might respond to future environmental variability.

We first established that the barium content of modern foraminifera shells positively varies with surface ocean productivity. This led us to hypothesize that fossil shells from the past should also reflect changes in surface ocean productivity. To test this hypothesis, we measured the barium content in fossil foraminifera from the tropical Pacific Ocean, dating from the present day back to around 21,000 years ago, during the Last Glacial Maximum. We specifically targeted a period of increased ocean productivity that occurred between 12,000 and 15,000 years ago. Our results showed that barium levels in fossil shells were elevated during this time, just as they are in modern foraminifera that record productivity. This confirms that certain species can reliably track past ocean productivity.

We also found that careful preparation of fossil shells is critical for accurate barium measurements. If parts of the shell with high barium are lost during cleaning, results can be skewed toward artificially lower barium values. Additionally, shells that have partially dissolved on the seafloor tend to lose their barium-rich portions, meaning only well-preserved specimens are useful for reconstructing past productivity.

An unexpected outcome of this study was the discovery that 3D reconstructions of shells, which we performed to assess the preservation of our fossil specimens, can be used to estimate past ocean carbonate ion concentrations. Past carbonate ion concentrations are linked to how much carbon is stored in the deep ocean. This finding provides researchers with another valuable tool for studying changes in the Earth's carbon cycle over time.

The broader impacts of this project include research support for a PhD student, who is currently employed as an environmental scientist in the private sector, and 5 undergraduate students. The undergraduates gained experience conducting laboratory research, performing geochemical analyses on both trace element and isotope ratio mass spectrometers, a laser ablation instrument, and with data analysis, figure generation, and data interpretation. These students have gone on to jobs at federal agencies, the private sector, and the academic setting. This project also included research support for international collaborations and the instruction of field methods to researchers from France, Taiwan, the United Kingdom, and India. 

Last Modified: 02/26/2025
Modified by: Jennifer Fehrenbacher