(Image: Adriana Dutkiewicz et al./University of Sydney)
The of the ocean floor sediments (shown above) shows we have little clue about how the world鈥檚 works.
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the carbon emitted into the atmosphere by human activity ends up in the ocean. It dissolves into the surface water, where it is taken up by phytoplankton during photosynthesis.
鈥淥nce they die their remains sink to the bottom and they lock away the carbon,鈥� says at the University of Sydney in Australia. Or at least, that鈥檚 what people thought.
Dutkiewicz has now shown that this process seems to depend on water temperature 卢and salinity 鈥� something her team discovered when they digitally mapped the seafloor.
Previous maps of seafloor sediments were drawn by hand, and the last one was completed in the 1970s. So Dutkiewicz and colleagues looked at sediment descriptions and samples taken by scientific cruises from as long ago as the 1950s. From more than 200,000 data points, they focused on the 15,000 highest quality ones, and used a computer model to turn them into a detailed map of the world鈥檚 ocean sediments.
Blotchy sediment
The map threw up surprises. It found there is 30 per cent more clay on the seafloor than previously thought, and 25 per cent less of a sediment called diatom ooze 鈥� the dead remains of a type of phytoplankton called diatoms.
And what researchers thought were continuous belts of different sediment types turned out to be separate pockets, blotched around the ocean floor.
Dutkiewicz then compared the map with data related to the temperature and salinity of the ocean surface, as well as regions with regular large blooms of diatoms 鈥� the most common type of phytoplankton.
鈥淭he map demonstrates that the diatoms on the seafloor don鈥檛 correspond to seasonal blooms on the surface,鈥� says Dutkiewicz. Instead, the amount that makes it to the bottom depends on the surface water having a low salinity and temperatures of between 0.9 and 5.7掳C, but high concentrations of nutrients.
Without these surface conditions, fewer diatoms make it to the bottom. 鈥淲e know they come from the surface, but they suffer some sort of fate before they get to the seafloor. They are not preserved,鈥� she says. 鈥淭hey get recycled in the upper surface somewhere.鈥�
And that means that rather than being locked away on the ocean floor for millennia, the carbon re-enters the atmosphere sooner. But exactly how long before it makes it back into the atmosphere is unknown.
Puzzle piece
鈥淚t鈥檚 one piece of the puzzle,鈥� says at the University of Tasmania. 鈥淚t鈥檚 possible that there鈥檚 a large proportion of material that could sink below 300 or 400 metres and that would take the carbon out of circulation for decades or centuries.鈥�
Next, Dutkiewicz plans to look at the relationship between temperature and diatom accumulation over tens of millions of years by drilling cores of ocean sediment.
But the fact that what makes it to the bottom seems to depend on temperature is a worry in the face of soaring temperatures, says Dutkiewicz.
In the longer term, things are even grimmer since diatoms only live in cold water. 鈥淲ith warming, the diatoms will be replaced with small phytoplankton that produce less oxygen and have less mass so deposit less carbon on the seafloor,鈥� she says.
Journal reference: Geology,
