Changes to Earth’s 'twilight zone' could cripple the ocean’s future

Welcome to the twilight zone. It’s a real place — but under the sea.

The mesopelagic zone, also known as the “twilight zone,” is a mysterious region of the ocean between 200 and 1,000 meters in depth. It’s sandwiched between the sunlit surface and pitch-dark deep ocean.

A new study shines a light on its formation over geologic time scales, revealing the process is intimately linked with global temperature.

Currently, this is a boon to the creatures that live there. But, as anthropogenic climate change heats up the ocean, this finding suggests the twilight zone and its integral role in the carbon cycle may be negatively affected.

This is bad news not just for the animals that live there, but for people too — a twilight zone in balance is essential for keeping billions of tons of carbon out of our planet’s atmosphere.

What’s new — This finding was published Thursday in the journal Science. Ultimately, the study suggests that as global temperatures cooled over the past 15 million years, the biological carbon pump, which transfers carbon in the form of organic matter from the surface to the deep ocean, became more efficient.

More organic matter sinking to greater depths means more food to support life below the surface. “We show that this, in turn, represented an evolutionary boost for life in the twilight zone, which became more abundant and diverse in step with global cooling,” co-lead author Flavia Boscolo-Galazzo, a postdoctoral fellow at the University of Bergen in Norway, tells Inverse.

Some background — The twilight zone is a critical component of a complex process that removes carbon from the atmosphere called the biological carbon pump. Phytoplankton at the surface use carbon dioxide in photosynthesis. When phytoplankton die or are eaten by zooplankton, all of the dead bits, known as particulate organic carbon, sink to the twilight zone.

There, most of it becomes food for fish and bacteria. The rest drifts down to the deep ocean floor where it is locked away in sediment.

Animals leave the twilight zone daily to feed on prey in the sunlit surface zone, which also moves carbon from the shallower part of the ocean to the deep. This movement is likely one of the largest daily migrations on the planet.

All told, the biological carbon pump stores anywhere between 2 and 6 billion metric tons of carbon each year, which prevents the Earth from being 6 to 11 degrees Fahrenheit warmer.

The animals that live and feed in the twilight zone, including jellies, plankton, fish, and deep-diving seals, make up the intricate marine food web. The twilight zone is key to the balance of food webs in the deep open, Lihini Aluwihare, a professor of chemical oceanography at the University of California San Diego, tells Inverse. She was not involved with this study.

“Small changes in flux to the twilight zone could have quite important ramifications for the robust community of organisms that inhabit this region,” Aluwihare explains.

How they did it — In this study, the researchers used a combination of geochemistry and data modeling.

The geochemical information was measured from the shells of tiny, single-celled organisms called foraminifera that collect on the ocean floor over millions of years. Deep-sea cores of sediment contain foraminifera, so their shells are chemically analyzed to reconstruct temperature and dissolved carbon in seawater.

This information was then used in a model simulation to illustrate the relationship between temperature and the formation of the twilight zone over 15 million years.

What they found — While it was previously theorized that temperature controls how much organic matter makes it to the deep ocean from the surface, this study was able to test that hypothesis directly over multi-million-year timescales.

When the climate was warmer 15 million years ago, “organic matter was degraded very efficiently at the surface with very little leftover sinking deeper,” Boscolo-Galazzo says. On the other hand, when it is cooler, they found “organic matter is preserved longer and can sink to greater depths.”

The amount of organic matter moving through the layers of the ocean increased globally as the temperature cooled over the last 15 million years, leading to a transformation and expansion of the twilight zone.

Why it matters — The carbon cycle of the ocean is critical to humanity because it controls the amount of carbon dioxide in the atmosphere. It is also critical to ocean life and fisheries.

“Deep-living fish account for a billion tonnes of biomass and are a major food source for whales and dolphins and also large diving fish like tuna and swordfish,” Boscolo-Galazzo says.

If the existence of the twilight zone as we know it is closely tied with temperature, it could be severely impacted by warming temperatures.

“When the climate is warmer, a weak biological pump would allow less CO2 to be sequestered into the deep ocean away from the atmosphere, hence reinforcing global warming,” Boscolo-Galazzo says.

What’s next — For as important as the twilight zone is, it is still very mysterious. “Like many regions of the subsurface ocean we cannot observe it effectively,” says Aluwihare, although she notes our ability to explore the deep ocean is improving. “For many geochemists like me,” she says, “we want to know how important this region is for respiring carbon and nutrients and how carbon and nutrients get there.”

Boscolo-Galazzo says their study reveals that the twilight zone may be more vulnerable to climate change than previously thought: “Our study challenges this view, and calls for a better understanding of deep ocean life sensitivity and resilience to global climate change.”

Abstract: Theory suggests that the ocean’s biological carbon pump, the process by which organic matter is produced at the surface and transferred to the deep ocean, is sensitive to temperature because temperature controls photosynthesis and respiration rates. We applied a combined data-modeling approach to investigate carbon and nutrient recycling rates across the world ocean over the past 15 million years of global cooling. We found that the efficiency of the biological carbon pump increased with ocean cooling as the result of a temperature-dependent reduction in the rate of remineralization (degradation) of sinking organic matter. Increased food delivery at depth prompted the development of new deep-water niches, triggering deep plankton evolution and the expansion of the mesopelagic “twilight zone” ecosystem.