Researchers Outline and Quantify Critical Mechanisms Involved in the Ocean Carbon Cycle

Oceans in the Earth have an outstanding natural capability to draw carbon from the atmosphere and store it deep within the ocean waters, establishing a significant control on the global climate.

A surfer in the waves of the Pacific Ocean of the coast of California. A new Rochester study has found that factors such as wind, currents, and even small fish play a larger role in transferring and storing carbon from the surface of the ocean to the deep oceans than was previously thought. (Image credit: Getty Images photo)

For example, a major part of the carbon dioxide released when humans burn fossil fuels is absorbed and stored in the ocean through a set of processes that constitute the ocean carbon cycle. However, the fast rate at which carbon dioxide emissions are increasing implies that the future of the cycle is questionable, particularly when most of the important processes are not yet understood well.

In a recent publication in the journal Nature, Tom Weber, an assistant professor of Earth and environmental sciences at Rochester, and his team summarized and measured important mechanisms involved in the ocean carbon cycle, particularly the “biological pump.” Their fresh understandings can be used to direct climate computer models to better estimate the impacts of climate change on a warming world.

Sinking Deeper into the Ocean

The biological pump expresses the sum of all the biological processes that transport carbon dioxide from the atmosphere to the deep ocean. Small marine plants, called phytoplankton, use carbon dioxide from the surface ocean to synthesize biomass. The biomass joins together into particles, which subsequently gets submerged to the deep ocean. The particles break down in the deep ocean, emitting carbon dioxide.

The net effect is the ‘pumping’ of CO2 from the atmosphere to the deep ocean.

Tom Weber, Assistant Professor, Earth and Environmental Sciences, University of Rochester.

As the particle goes down deeper into the ocean, it will take more time to return the carbon to the surface and back into the atmosphere. For instance, carbon discharged at depths of a few hundred meters is returned back to the atmosphere within a time period of about 10 years, but if particles get submerged deep into the ocean—at a depth of over 1,000 m—their carbon can be stored for up to 100 decades before being circulated back to the surface.

Particle Injection Pumps

Scientists earlier assumed that the transfer of particles from the surface to the deep ocean took place merely by sinking under the force of gravity, which was considered as the “biological gravitational pump” by Weber and his colleagues. However, over the past few years, researchers have identified other processes that are essential in transporting carbon from surface waters to the deep ocean.

As summarized in the study, these involve the physical stirring of the ocean by the wind, by extensive ocean currents, and by biological transport through animals like small fish that consume the biomass particles at the surface and excrete them at depth. The scientists call these processes together as “particle injection pumps” (PIPs) as they can “inject” particles to much deeper depths—in relation to mere gravitational settling—before decomposition takes place and the carbon is discharged.

It’s a much more efficient way of pulling carbon from the surface into the deep waters,” states Weber.

Weber and his team merged observational evidence and new model estimates to measure, for the first time, the amount of carbon transported by the PIPs. They discovered that PIPs are a much more powerful factor than assumed earlier: as a whole, they account for the same amount of carbon storage in the ocean as in the biological gravitational pump.

The Ocean Carbon Cycle and Climate Change

Since the ocean carbon cycle is influenced by environmental variations in temperature, light, and nutrient availability, the scientists can use their new outcomes to enhance climate models and better predict how the ocean carbon cycle will react to future global climate change, Weber says.

If we want to have some predictive power with respect to the biological pump, we need to understand all the mechanisms and equip our global climate models with a complete representation.

Tom Weber, Assistant Professor, Earth and Environmental Sciences, University of Rochester.

The ocean carbon cycle is predominantly impacted by climate change due to warming ocean waters. The deep ocean is composed of cold, dense, and nutrient-rich water while the ocean surface is lighter and warmer. In order to sustain biological productivity, wind mixes the ocean waters, blending them to make the nutrient-rich water come up to the surface.

However, when temperatures of the ocean increase because of climate change, the density difference between the water in the ocean surface and the water in the deep ocean increases, rendering it more difficult for the ocean to blend, Weber reports. “Satellite records show the overall productivity of the surface ocean is declining because the stirring of nutrients is becoming less efficient.

His new work adds another “wrinkle to the problem,” he states. Prior assumptions of the biological pump denoted that a limited ocean mixing rate would delay productivity but “not really affect other processes in the biological pump: once you produce the particles, gravity alone would make them sink and decompose.” The new perception, however, shows that a delay in mixing will also reduce the PIPs, which are essential to the ocean carbon cycle as “very efficient export mechanisms that get the particles nice and deep where the carbon can be stored longer,” Weber says.

In case particles are not submerged deep into the ocean, this can, in turn, lead to climate change. “If carbon dioxide is released at shallower depths, it escapes quicker into the atmosphere, meaning more carbon dioxide in the atmosphere where it contributes to global warming.”

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