Ocean gobbles carbon at different rates

climate change

Dead plankton does not sink at the same rate everywhere in the Pacific Ocean, say researchers. The new findings will boost our understanding of the supply chain to the world's biggest carbon sink – the bottom of the ocean.
The team of more than 40 scientists have led an unprecedented expedition to capture plankton sinking through what is known as the twilight zone, en-route to bottom of the ocean. The twilight zone is the considerable layer of water between the sun-lit surface waters – where tiny floating plants photosynthesise – and the deep ocean. It lies between 100 metres and 1000 m beneath the surface.
These tiny plants absorb carbon dioxide as they photosynthesise, and when they die, they begin their descent from the top layer of the ocean. This "marine snow" can either sink permanently to the bottom and store the carbon it contains there, or it can be consumed and digested by animals and bacteria along the way. In this second case, the carbon contained in the plants is recycled and may eventually make its way back to the surface and into the atmosphere.
The twilight zone, which is host to the tiny animals that feed on the plants, therefore plays a critical role in the oceans' long-term carbon capture.
"Unless the carbon that gets into the ocean goes all the way down into the deep ocean and is stored there, it can still make its way back into the atmosphere," says Ken Buesseler, from the Woods Hole Oceanographic Institution near Boston, US. "Without this long-term storage, there is little influence on atmospheric levels of carbon dioxide, a greenhouse gas that impacts the earth's climate," he adds.
In the zone
Buesseler and colleagues used automated plankton traps to study how tiny floating marine plants die and sink or are eaten by animals. The traps, dubbed "Twilight Zone Explorers", were programmed to stabilise at a certain depth and floated there in the same currents that carry the plankton.
The researchers found that only 20% of the total carbon suspended in surface waters near Hawaii made it through the twilight zone. The other 80% was consumed along the way, never making it to the bottom. In contrast, half of the surface carbon made it to the bottom in the northwest Pacific.
Buesseler says he is not sure why there is such a difference between the two regions, which were selected because of the team's familiarity with them.
One possibility is that plankton in the Pacific are more dense. Waters in the northwest Pacific are full of silica, which some types of plankton use to make their shells. So it could be that the northwest Pacific plankton sink faster (and drag the carbon they contain with them) leaving bacteria less time to digest them.
Another explanation could be that the lower temperatures in the northwest Pacific slow the breakdown of the dead plankton, giving it a greater chance of settling permanently on the bottom, they say.
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