The ocean is a major carbon sink that has absorbed about a quarter of all carbon dioxide emissions since the Industrial Revolution. It is generally understood that atmospheric carbon dioxide dissolves in surface waters, where photosynthesis converts it to organic carbon. As it sinks or progresses through the food chain, much of this carbon ends up sequestered in the deep ocean.
Most of the organic carbon in surface waters descends first to the twilight zone, which extends from about 100 to 1,000 meters deep and harbors myriad life forms, such as bioluminescent fish and sea jellies. Traditionally, scientists believed that twilight zone denizens got all their carbon from large, fast-sinking organic carbon particles, as well as from frequent feeding trips to shallower waters. However, these mechanisms appear to be insufficient to meet the ecosystem’s needs.
New research by Bol et al. challenges traditional ideas of how organic carbon descends from surface waters while adding to a growing body of evidence that small, slow-sinking particles—as opposed to larger, fast-sinking ones—play a bigger role than previously thought.
To examine the potential role of small-particle organic carbon, the team analyzed data collected by autonomous underwater Seagliders in the twilight zone above the Porcupine Abyssal Plain, southwest of Ireland. Five Seagliders crisscrossed the region for 1 year, taking daily measurements with sensors that detect light scattered by carbon particles, thereby revealing their concentration.
The analysis showed that a significant amount of small-particle organic carbon does indeed descend to the twilight zone. With at least 2–4 grams of particulate organic carbon flowing through a 1-square-meter area of water per year, the researchers calculated that these small particles account for at least 5%–25% of the annual carbon flow from surface waters to the twilight zone in the region.
The researchers found that the greatest flux of small-particle organic carbon to the twilight zone occurred in winter and spring. Their data add new support to earlier evidence that a mechanism known as the “mixed-layer pump” drives transport of these small particles.
The mixed-layer pump arises from changes in the depth of the ocean’s uppermost layer, where winds and waves mix surface waters to form a mass of uniform temperature and salinity. Over the Porcupine Abyssal Plain, strong storms cause this mixed layer to extend deeper into the ocean in winter and spring, whereas in the summer it remains shallow. This variation in depth appears to drive net transport of small-particle organic carbon into the twilight zone.
This study marks the first use of daily particulate organic carbon measurements taken over a full year. The findings suggest that future research could make further use of optical sensors on autonomous gliders to help clarify organic carbon transport and the role of the mixed-layer pump around the world. For example, such efforts could address the fate of small-particle organic carbon that reaches the twilight zone and reveal how much it actually contributes to carbon sequestration, especially in the context of global climate change. (Global Biogeochemical Cycles, https://doi.org/10.1029/2018GB005963, 2018)
—Sarah Stanley, Freelance Writer