When it comes to taking carbon out of the atmosphere, the ocean biosphere plays a big role. It starts near the sunlit surface, where plankton are able to photosynthesize, converting carbon dissolved in the water into tissue or combining it with calcium to form shells. When these plankton die, they sink, carrying their carbon with them. They may be eaten or converted back to dissolved carbon as they sink, but a small fraction eventually makes it to the ocean floor and becomes subsumed into the seabed—taking this carbon out of the atmosphere for good.
This process is the so-called biological pump, and it’s crucial to our understanding of the Earth’s warming climate. A new study reveals that the strength of the biological pump can vary dramatically between seasons, which could call into question some previous estimates of annual carbon export from the ocean surface.
Fassbender et al. analyzed 7 years of near-continuous observations from a buoy at Ocean Station Papa, a spot in the northern Pacific Ocean roughly 1200 kilometers west of Canada’s Vancouver Island. They supplemented these observations with data from research vessels, robotic gliders, and National Oceanic and Atmospheric Administration satellites.
Their measurements show that the biological pump is active in the spring and summer, as growing organisms feast on carbon, counteracting physical and chemical processes that would otherwise cause carbon to be released from the ocean to the atmosphere. In autumn and winter, biological activity slows dramatically, and it is the physical and chemical processes that cause the ocean to absorb carbon from the atmosphere. This seasonal variability means that for some locations, estimates of the biological pump’s strength that are based on a single season’s measurements may not accurately represent its year-round, overall strength.
They also found that calcium carbonate particles—the stuff that makes up tough shells—were roughly twice as abundant as the global average. This suggests that organisms with calcium carbonate shells play an unusually large role in the carbon cycle here.
The authors say this study shows how complex the ocean’s carbon cycle can be over a range of time scales and that extended records of year-round observations are required in order to truly understand it for any given spot in the ocean. (Global Biogeochemical Cycles, doi:10.1002/2015GB005205, 2016)
—Mark Zastrow, Freelance Writer
Citation: Zastrow, M. (2016), Moored ocean buoy tracks marine carbon cycle variations, Eos, 97, doi:10.1029/2016EO047783. Published on 14 March 2016.