Twilight zone transport

Science 316, 567–570 (2007)

Credit: MARY SILVER, UNIVERSITY OF CALIFORNIA, SANTA CRUZ

Carbon that reaches the deep ocean is stored and does not re-enter the atmosphere for centuries, mitigating its short-term contribution to global warming. Microorganisms inhabiting the mesopelagic or 'twilight zone' between about 100 and 1,000 metres consume carbon as it sinks, however, making it available as a greenhouse gas.

Now, an international team led by Ken Buesseler of Woods Hole Oceanographic Institute has discovered that the efficiency of carbon transport to the deep sea depends critically on the type of microorganisms in the murky mesopelagic. Using a newly designed sediment trap that hovers at specific depths, they measured the transfer through the twilight zone of sinking carbon-containing particles in Hawaiian subtropical and Northwest Pacific subarctic waters.

In the Northwestern Pacific, where diatoms thrive on the nutrient-rich waters, carbon reached the deep ocean with an efficiency of 46 to 55%. In nutrient-poor Hawaiian waters, dominated by smaller, shell-less phytoplankton, the transport efficiency was only 20%. Extrapolated globally, the difference in carbon sequestration between the nutrient-rich and nutrient-poor waters is equivalent to nearly half of all human-generated carbon emissions. As the oceans warm, nutrient supply is expected to decrease, which will favour smaller phytoplankton and less carbon storage in the deep sea.

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Smalley, E. Twilight zone transport. Nature Clim Change 1, 2 (2007). https://doi.org/10.1038/climate.2007.6

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