Thursday, 22 October 2015

The Uplifted Coral Reefs in North Queensland that were used to Discover Ancient Sea Levels

I have been told that when the graves in Port Douglas cemetery were dug there was a layer of hard coral under the sand.  The Port Douglas peninsula which is anchored at the end by Island Point was apparently once a coral reef.  Fortunately in some nearby places fossil coral reefs are exposed and the ancient corals can be seen.  The best place is Yule Point, which is 5 km south of Port Douglas.  
Most of the fossil coral reefs lie in shallow pools on the sand flats - click to enlarge
The fossil coral reef is probably 1-5 thousand years old as the sea was about a metre higher in coastal areas at that time.  It is not that sea level has fallen, rather the coastline has risen up a little.  The reason is iso-static rebound which is common phenomena in lands that were covered with ice sheets during the last ice age.  The weight of the ice sheets pressed the land down and when the ice melted, the ground slowly lifted up out of the sea and this process continues today in places such as Northern Europe.  Locally iso-static rebound works in the opposite direction with the submerged continental shelf being push down by the weight of the Coral Sea resulting in a slightly bending of the Australian Continent.  The downward thrust of the continental margins creates a small uplift at the edges of the exposed landmass.  This uplift raised many nearshore coral reefs up out of their tidal zone and created the fossil reefs that can be found today.
The irregular raised patches are fossil coral colonies
The fossil coral reefs are not that glamorous to look at but they are scientifically important.  The provide one of the best  lines of evidence for how high sea level once rose.  In particular scientists look for micro-atolls as these provide a very good record of sea level height.  Below is a micro-atoll growing in the rocks on the headland at nearby Oak Beach.  Normally they are completely round but the ones in this photo are compressed against stones.  See how precisely the cut-off level for coral growth is defined, this is what makes micro-atolls such good indicators of ancient sea levels.

A micro-atoll has a dead centre and a ring of living coral around the edge.
Oyster reefs and band of barnacles are also used but are less precise.  Barnacles usually grow in the over a wide vertical range on exposed rocks due to the splash zone so scientists seek out deep fissures in rocky coasts where waters are less likely to be splashed.   On Magnetic Island, a band of scattered barnacles occurs between the granite boulders that about 1 m above where barnacles are found today and this band has been carbon dated to approximately 4000 years before present.  Deep rocky fissures are rare on the coastline north of Cairns but do exist.

Barnacle covered cave wall near Port Douglas
On the wall of the fissure above which is about a metre wide and two metres tall are vast colonies of barnacles.  The living barnacles are purplish and dead barnacles are green with algae.  There is a distinct cut-off between the solid mass of barnacles in the lower part of the photo and the ribbed growth of barnacles at the top.  However if you look closely, there are a few scattered living barnacles on the green ribs which detracts from the ability to use barnacles for measuring sea level.

The following photos show some of the ancient coral colonies.  Erosion of the ancient coral surface is evident in most places and much of the original surface features have been replaced with textures created by erosion.

Rings show that this was a micro-atoll
This coral colony at nearby Little Reef Beach might be the same species.
Remains of a rosette coral 
A living colony off the beach at Little Reef Beach
One of the massive coral colonies even had the remains of  two burrowing clams in it
(centre of photo)
The round objects are the ends of stag horn coral and very hard and totally immobile
Being quite close to high tide level, there is little marine life on the uplifted corals, mainly hermit crabs and the occasional green sea cucumber.  Despite the apparently low levels of visible life, many species of migratory wading bird were busily feeding in this area.  On the sand flats away from the beach, each pool had its own ecology and had a different array of seaweeds and fauna.  The most interesting discovery were tiny starfish that occasionally encrusted blocks of ancient coral rubble.

Further Reading.

Sea levels of the Great Barrier Reef: assessing past changes using oyster bed deposits

This paper is short and easy and has a good graph of how corals, oysters and barnacles were used to understand ancient sea levels.

Post-glacial sea-level changes around the Australian margin: a review

This paper provides the full story but is rather heavy.