Researchers from the Australian Institute of Marine Science (AIMS) were surveying the impact of nutrients on coastal inshore areas when they unearthed mangrove forests in old river channels they believe may snake for 30 kilometres to the edge of the continental shelf.

An ancient mangrove sample cored off Russell Island, North Queensland.

Scientists say the buried mangrove forests store vital information about the coastline as it existed prior to human activity.

"To find these ancient mangroves was quite unexpected and incredibly exciting," said Dr Dan Alongi, a biologist at AIMS, and one of the lead researchers who made the discovery.

"At first 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."

While it was previously known that relic river beds lie beneath the Great Barrier Reef, formed 9 000 years ago when the sea level was lower than the continental shelf, their significance was never studied until Dan Alongi and his team took their first core samples in July 2002.

Radiocarbon dating has put these deposits between 8 550 and 8 740 years old. (The cores were radio dated by Professor John Chappell of the Australian National University.)

Aboard the AIMS research ship, the RV Lady Basten, Dan set out on another fact-finding mission to source more core samples to study the genetic makeup of the mangrove species and compare it to its modern counterpart, as well as measure the age of upper sediments to determine the timeframe of the shift in sea level.

Scientists plotted the position of their original findings off the Russell River, so pinpointing the ancient river channels and capturing another snapshot of the past was trouble-free.

Ancient mangroves revealed

To probe the depths, Dan used a Kasten corer, a large square tube two metres long attached to a one and a half tonne weight of lead which is lowered down onto the seafloor. Sheer weight and gravity burrows the tube into the seascape. A trap door shuts and locks in thousands of years of information before being hauled to the surface.

The cores were as impressive as the original samples discovered two years earlier.

"The cores still have the characteristic smell of mangroves (attributed to the tannins), that's why we thought it was young. The layers abruptly change from reef, which looks like light grey mud, to intact mangroves which delve 70 centimetres deep."

A record of the sea change

"There's such an abrupt change in core composition…from the modern to the ancient, that it indicates a large climate change happened," said Dan.

Scientists have long theorised that sea level rose very gradually over several thousand years. These remnant mangrove forests suggest another story.

"The fact that we found ancient mangroves in such pristine condition, with such a sharp boundary between them and the overlying modern mud, tells us the water rose quickly over a geologically short time span."

Dan estimates this time span to be from a few centuries to, even, decades. Research with fellow AIMS scientists will help to paint a more accurate picture of this timeframe, and investigate the concentration of radionuclides in the top sediment. These measurements will help pinpoint the period over which seas rose.

Slow breakdown

"The mangroves appear to be very slowly decomposing, with the possibility that nutrients are percolating up through the mud influencing modern lagoon activity and promoting phytoplankton production. It's amazing to think that something buried 9 000 years ago is still breaking down and having an impact on the living world today," Dan said.

"These mangroves can tell us how rapidly the seascape changed in the past and just how rapidly it may in the future as global warming sets in."

"What this can tell us is whether or not we will have to deal in the future with a rapid die-off of mangrove forests, or a slow pace of change; dealing with a rapid change will obviously be more difficult than dealing with a slow change"