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Marine snow falls heaviest at the Equator

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Organic matter drifts down to the equatorial ocean floor in distinct patterns.

Scientists have mapped heavy marine snow fall in the equatorial ocean.

According to the Renaissance mathematician Evangelista Torricelli, who discovered atmospheric pressure, “We live submerged at the bottom of an ocean of air.” If the atmosphere is an ocean, then the ocean is also an atmosphere, with its own turbulence and microclimates. And the parallels between these two great fluid environments of our planet go further. When Japanese scientists took a dive into the ocean in a submersible in 1952 and their lamp revealed a flurry of shining white flakes falling towards the depths, they were going to name it only one thing.

This week, scientists report the most in-depth (and at-depth) analysis of this ‘marine snow’ — in the region that experiences the heaviest falls. For it is more than a mesmeric curiosity. The origins and fate of these oceanic snowflakes — in reality various forms of organic matter ranging from dead plankton to plant and animal detritus — help to determine what happens to carbon in the deep ocean. Carbon that makes it all the way to the depths without being released on its journey is effectively sequestered from the atmosphere for hundreds of years.

Writing in Nature Geoscience (R.Kikoetal.NatureGeosci.http://doi.org/cdz6;2017), the scientists describe how they scanned the avalanche of marine snow that makes slow and steady progress towards the depths of the equatorial Atlantic and Pacific oceans. They discovered particularly heavy clouds of the material at depths of between 300 and 600 metres. This is where zooplankton (drifters) and nekton (swimmers) head from the surface during the daytime. The snowy scene, the scientists conclude, is largely made up of the faeces released by these organisms.

The study overturned one common assumption that is included in many models of ocean carbon transport. The researchers found that most of the organic matter that reaches the bottom arrives as a veil of relatively slow-moving small particles, rather than the assumed faster-falling and larger aggregates, which seem to disintegrate steadily as they sink.

The scientists also noted another fascinating effect. Strong and deep equatorial currents stop the snow drifting north or south towards the poles. Instead, it falls as a narrow curtain of flakes drifting down the darkness of the marine sky.

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Marine snow falls heaviest at the Equator. Nature 550, 158 (2017). https://doi.org/10.1038/550158b

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