Carbon speeds through the ocean’s twilight zone

In the ocean, carbon fixed by photosynthetic organisms in the upper sunlit water sinks through the “twilight zone,” or transition region, to the dark ocean below. The process reduces atmospheric carbon dioxide to levels that allow for life as we know it. But how much carbon is sequestered in the deep sea depends on the rate at which organic particles sink through the twilight zone, a parameter that varies with season and region depending on the local population of photosynthetic organisms. Although a 2008 paper reported that the sinking speed increases with depth, few studies have detected particle speeds in situ.

Now a team led by María Villa-Alfageme at the University of Seville in Spain has measured particle sinking velocities down to 600 m. The measurements were taken during three seasons in the North Atlantic Ocean at sites from Iceland to Bermuda. The abundances of polonium-210 and its parent nuclide lead-210 in the water column serve as a proxy for particle fluxes, and by extension velocities, because organic particles adsorb ²¹⁰Po and then get assimilated by microorganisms.

The researchers found that slowly sinking particles—those that descended less than 100 m/day—made up the majority of material exported from the surface region. But when phytoplankton bloomed during the spring in temperate areas, most of the sampled particles were sinking faster. Overall, throughout the year and across the North Atlantic, sinking velocities systematically increase by 30–90% as particles travel deeper toward the dark ocean. Such novel observations will enable scientists to better account for a major variable in carbon-cycle models. (M. Villa-Alfageme et al., Geophys. Res. Lett. 43, 8609, 2016 .)