This expedition from Alaska to Tahiti is part of the GEOTRACES research program, but what does that mean?

GEOTRACES is a global collaboration of oceanographers seeking to better understand ocean chemistry. “We are like the CSI of oceanographers,” says Co-chief scientist Phoebe Lam of the University of California Santa Cruz.

“Events and processes in the ocean leave behind chemical clues. GEOTRACES is about finding them, using them to reconstruct what happened and, ultimately, better understand how the world works.”

Members of the GEOTRACES GP15 science party and crew coordinate deck operations aboard the Research Vessel Roger Revelle en route to Alaska. Photo: Alex Fox

The clues GEOTRACES focuses on are called trace elements and isotopes. These chemicals can be nutrients, tracers of current and past ocean processes or contaminants from human activity. Where they go and how long they take to get there impacts the global climate as well as the ocean ecosystems humans depend on.

GEOTRACES scientists hunt down these clues on oceanographic research expeditions all over the world. Scientists from 35 countries have contributed to the program. The expeditions crisscross the globe along carefully considered routes designed to capture the full range of ocean conditions—from deep to shallow and from huge plankton blooms to turquoise marine deserts.

Yellow lines represent completed expeditions, red lines are planned expeditions (including GP15) and black lines are expeditions completed as part of a collaboration with another research program. Image: GEOTRACES

Reconstructing ocean events and cycles from the chemicals in seawater requires knowing how much of some element can be found in a given part of the ocean, but also figuring out how it got there and where it’s going. Understanding what drives the existing patchwork of elements in the ocean can also allow scientists to predict how those patterns and cycles might change in the future.

Another distinguishing feature of GEOTRACES research expedition is that they study a wide range of trace elements and isotopes all at once. “With GEOTRACES you have all these different scientists measuring very specific things, and they’re important, but when you join all the data together the whole is much greater than the sum of its parts,” says Chief Scientist Greg Cutter of Old Dominion University. The full suite of measurements provides context that allows for a richer interpretation of any one stream of data.

Despite knowing how important these chemicals are to marine life, in many cases little is known about where they come from, where they’re going, how they get there and what chemical reactions occurred along the way.

GEOTRACES attacks this knowledge gap by studying the distribution and movement of these key trace elements and isotopes at the global scale.

What are trace elements?

Trace elements are rare by definition, but some trace elements are essential nutrients for living things while others are toxic. For example, both humans and phytoplankton, the microscopic marine plants at the base of the ocean food web, need iron. Without iron, humans become anemic and phytoplankton can’t use sunlight to grow. On the other hand, heavy metals like lead or mercury are toxic.

Chief Scientist Greg Cutter secures the GEOTRACES CTD Rosette, one of the most important instruments on board. Photo: Alex Fox

“Just like the plants in your garden, certain nutrients are needed for plant growth in the ocean,” says expedition Co-chief Scientist Karen Casciotti of Stanford University.

“We’re interested in what controls the delivery of those nutrients, because they can help us understand why there are huge booms of marine life in some places but not others,” says Casciotti.

In the ocean, iron can be the difference between a saltwater desert and a feeding bonanza complete with whales, fish and seabirds. On the global scale, the presence or absence of these rare nutrients limits how much life the oceans can support and helps determine where it is found.

This has very real consequences for humans as well as whales and fish. A productive year in the oceans could mean a boost to the global fishing economy, but it also helps determine how much carbon dioxide is absorbed by the oceans each year.

4 dolphins portside kc cropped
Pacific white-sided dolphins take flight alongside the Research Vessel Roger Revelle. Photo: Kevin Cahill

When phytoplankton turn sunlight into food they suck up carbon dioxide, one of the primary greenhouse gases contributing to climate change. But phytoplankton also need iron to make the chemical reaction, called photosynthesis, go. An ocean full of carbon dioxide is no use without iron. By limiting the amount of carbon dioxide phytoplankton can use for photosynthesis, trace elements like iron can influence global climate.

The importance of trace elements as nutrients is well established, but where they are and how they got there is in some cases a mystery. To trace the path of these essential trace elements chemical oceanographers can use isotopes.

What are isotopes and what do they have to do with oceanography?

Many elements on the periodic table have isotopes—atoms with the same name and chemical properties, but different masses. Their non-standard masses make them distinctive and allow them to be picked out by scientists looking to do some chemical detective work.

The scientists of GP15 each collect their share of seawater brought up from the depths by the CTD Rosette. Photo: Alex Fox

When isotopes are radioactive, they can be used like stopwatches. Their predictable rates of decay allow scientists to infer how long it took the radioactive isotope to get where it is or how long it has been there.

Other isotopes can be used like fingerprints that identify their source. For example, certain isotopes only come from underwater cracks in the earth’s crust called hydrothermal vents.

A third use of isotopes is as a way of tracking chemical or biological processes. For example, some organisms prefer one isotope over another. So, if an oceanographer sees a certain ratio of isotopes in the water, it tells them something about what various marine critters are up to.

GEOTRACES seeks to establish the distribution of key trace elements and isotopes in the ocean while also uncovering the processes that brought them there. This approach has the power predict the patterns of marine life these elements underpin, as well as the ocean’s responses to our rapidly changing world.

The U.S. GEOTRACES program is funded by the U.S. National Science Foundation.

GP15 blog posts written by Alex Fox unless otherwise stated.

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GEOTRACES GP15 is supported by the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this material do not necessarily reflect the views of the National Science Foundation.

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