Australian Institute of Marine Science (AIMS) researchers have opened a window into the past by exposing ancient mangrove forests entombed beneath the Great Barrier Reef.

AIMS biologist Dr Dan Alongi said the expedition was surveying the impact of nutrients on coastal inshore areas when scientists unearthed mangrove forests in old river channels they believe may snake for 30 kilometres to the edge of the continental shelf.

Scientists have long theorised that sea level rose very gradually over several thousand years, but these remnant mangrove forests tell another story.

While it was previously known that relic river beds exist beneath the Great Barrier Reef, formed 9000 years ago when the sea level was lower than the continental shelf, their significance was never studied.

"When we took the first samples it was difficult to believe… we stood amazed wondering what exactly we were dealing with. We thought it was cyclone debris, but it was far too deep to be a modern event," said Dr Alongi.

AIMS researchers cored 1-2 metres of sediment and found remnant mangrove 70 centimetres below the surface of the present seafloor.

These core samples of mud are an evolutionary time frame. The evidence will help to establish the state of the reef and nutrient sediment information as it existed prior to human activity.

Dr Alongi said the mangroves were incredibly well preserved; a fact most likely attributed to the antibiotic properties in the concentrated tannins. "The cores still have the characteristic smell of tannins, that’s why we thought they were young.

"Within the cores were intact root systems and parts of trees including twigs and branches that radiocarbon dating put between 8550 and 8740 years of age.

"There’s such an abrupt change in core composition from mud-like substance to intact mangrove branches…from the modern to the ancient, that it suggests a large climate change happened," said Dr Alongi.