February 17, 2005
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.
"This sharp boundary between these ancient mangroves and the
overlying modern mud can tell us something fundamental about how
quickly the water rose over time."
Dr Alongi estimates that the shift in sea level occurred over a
geologically short time-span, from a few centuries or even decades.
Research with fellow AIMS scientists will help to paint a more
accurate picture of this timeframe, and investigate the concentration
of background radionuclides in the top layers of sediment. These
measurements will help pinpoint the period over which the sea level
rose.
Knowing how rapidly the seascape changed in the past helps us
predict future changes with global warming," said Dr Alongi.
