Measuring Ocean Iron to Improve Climate Models

Mathematical models can help us anticipate and plan, illuminating the future in areas as diverse as public health and meteorology – but their predictive ability is only as sound as the assumptions and data that underpin them. Senior Research Scientist Ben Twining is seeking to improve global climate models by studying a key nutrient that drives ocean productivity: iron.

“Iron controls biological production in a third of the global ocean, so we cannot understand how climate change scenarios are likely to unfold without taking a close look at the iron cycle,” Twining said. “Knowing the full picture of how life at the base of the food chain interacts with iron could change our conceptual understanding of the ocean and its future.”

This research takes Twining around the globe, and his current work to understand how ocean algae use iron is based in the Sargasso Sea. For more than 30 years, generations of scientists have studied this region through the Bermuda Atlantic Time Series, which runs monthly research cruises to record the progression of its seasonal cycles and observe the impacts of global climate change. The thorough scientific understanding of this area and the way its chemistry mirrors that of the global ocean make it the perfect place to study how iron moves throughout an ecosystem.

As the jet stream flows west, it carries iron-rich dust from the Sahara Desert and deposits it over the Sargasso Sea every summer. Much of the iron quickly sinks into the deep sea, but the remaining particles fertilize the phytoplankton that thrive in the upper, sunlit layer of the ocean. Through a series of research cruises in 2019, Twining and his colleagues closely followed the fate of this iron, tracing its journey through the seawater and – most interesting to Twining – the algae that consume it.

“Trying to figure out the iron cycle requires us to look at the very bottom of the food chain, but much of my motivation actually comes from the desire to help us understand fisheries and more visible ways the ocean impacts people,” Twining said. “This project hinges on the incredible importance of iron and other ocean nutrients, and it shows that understanding one spot in the ocean can help us understand the whole planet.”

Mapping the path iron takes through the water and algae of the Sargasso Sea is just the beginning. Professor Alessandro Tagliabue, an expert in modeling the iron cycle based at the University of Liverpool, is incorporating the data Twining and the others measure into a mathematical model of the Sargasso Sea. Ultimately, he will use it to fuel global climate models, which currently vary widely in how much iron they assume enters the ocean.

These assumptions can have big mathematical consequences. In a separate project, Tagliabue examined how predicted ocean changes will affect fisheries production in the equatorial Pacific Ocean. He and Twining discovered that the way the model calculated iron storage by phytoplankton completely changed whether fisheries productivity was expected to increase or decrease in the future.

“The path iron takes through the environment is complex, and models need to be sophisticated about this complexity in order to be accurate,” Twining said. “Having observational scientists and modelers working closely together on this project allows us to integrate our measurements into the bigger picture and makes the science stronger.”

This international effort is jointly funded by the National Science Foundation and the U.K.-based Natural Environment Research Council. The research team conducted a series of four oceanographic cruises in the Sargasso Sea last year, measuring several different types of iron in the water and collecting phytoplankton samples. The field team members from Bigelow Laboratory included two students – Research Experience for Undergraduates intern Gabby Kim, and master’s student Lauren Chaco. In addition, Dan Ohnemus helped write the proposal as a postdoctoral researcher with Twining, and he continues to help lead the project now as a faculty member at the University of Georgia.

Next, Twining and Ohnemus will travel to Chicago, where they will scan individual phytoplankton with x-rays at Argonne National Laboratory in order to reveal their iron content. The COVID-19 pandemic has disrupted many activities this spring, including scientific research. Their scheduled visit was postponed when the facility closed, and the final oceanographic cruise in the series, intended to take place in March, was canceled as well. Despite these interruptions, Twining and the rest of the research team have continued to move this research forward by focusing on the project’s data analysis and modeling components.

“As we learn more about the complexity of the world, we increasingly need mathematical models to tie everything together, because the whole picture is more complex than one person’s understanding can envelop,” Twining said. “This collaboration allows us to put our work into a larger context and improve our understanding of the world, and I think that's how science can best serve society.”