The reservoir is the result of a dramatic removal of carbon 34 million years ago that transitioned Earth away from a greenhouse planet into the ice-capped one it is now.

Up to 600 million parts per million of carbon was removed from Earth's atmosphere when it was a much warmer and humid planet.

Lead researcher in the study from the Australian Centre for Excellence in Antarctic Science and Institute for Marine and Antarctic Studies at UTAS Dr Katharina Hochmuth said it was a significant finding.

"For this to happen, the carbon that was keeping the atmosphere warmer didn't just disappear off the planet. It had to end up somewhere else on it," Dr Hochmuth said.

"Initially, I was looking at how to construct what the seafloor looked like at specific times in the past and we basically just stumbled upon this massive heap of sediment at this location."

"It's the shells of marine organisms which are built up from carbon stored at the sea floor."

For about four to seven million years, the gradual cooling of the climate combined with the change in ocean currents drove an intense hub of carbon-based biological activity in the oceans.

The effect of the ancient change in climate was significant enough to transform marine wildlife in the Southern Ocean and generate the growth of ice sheets in Antarctica.

It also extended grasslands across various continents and triggered the emergence of modern-day mammals.

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Dr Hochmuth said the discovery proved the Southern Ocean was very effective at regulating climate.

"It shows there is a potential the ocean can reduce the temperature of our planet significantly," Dr Hochmuth said.

"It just needs a couple of million years, and sadly we don't have that kind of time."

Today, the oceans remain one of the major sinks of atmospheric carbon, drawing down around 90 per cent of the excess carbon in the atmosphere.

Dr Hochmuth said the next step was to get their hands on some samples, which posed a challenge as it required drilling at deep levels.

"This reservoir holds a detailed archive of a very tumultuous period in the planet's history," she said.