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The deep sea is often thought of as a tranquil, benign environment - but a study led by a scientist at The Australian National University shows that the ocean depths are an extremely dynamic environment that may even experience 'seasons'.

Dr Michael Ellwood headed up a research team that discovered seasonal fluctuations in the carbon levels of certain rock sponges that grow at depths greater than 500 metres in the South Pacific.

The team's findings are published in the journal Limnology and Oceanography.

"Corallistes undulates is a large, cup-shaped sponge that lives attached to rocks in the deep ocean," Dr Ellwood said.

"Sponges are animals that grow by filtering particles out of the water around them to aid in the building of their glass skeletons. The fact that they must draw their sustenance from the water about them means that they act as environmental recorders, keeping 'histories' of the ocean about them."

The scientists used dating techniques to determine that their samples of the sponge were between 135 and 160 years old - an age range than matched closely to the number of silica density bands in the sponge, which were identified using x-ray technology.

Like the rings of a tree, these bands represent roughly a year of growth for the animal.

"This silica banding is quite remarkable, because the deep ocean is often thought of as being benign. What this shows is that the levels of chemical elements near the sea floor are constantly in flux, suggesting a much more dynamic environment," Dr Ellwood said.

"But our research also shows that there are annual patterns in the fluctuation of trace elements, which suggests that something akin to 'seasons' might be occurring under the ocean.

"We're not sure what is causing this yet. It could be the result of lifecycles at the surface of the ocean, where phytoplankton live, breed and die, before sinking to the depths and enriching the water with nutrients.

"It could also be the result of annual patterns in deep ocean currents. We'll need to do more research before we know for sure."

Dr Ellwood said the results are exciting, as they point to the possibility that sponges could be used for more research into the history of the oceans and the climate above.