In search of the past: Ancient coral bommie rediscovered

In search of the past: Ancient coral bommie rediscovered

The mid-late 14th century was a time of great change – Medieval European society suffered political strife and one of the most devastating pandemics in recorded history, the Black Death, scourged the population.

On the other side of the world, beneath the sunlit tropical waters of the Coral Sea, new life was emerging. On the southern Great Barrier Reef (GBR), a Porites sp larva drifted from the surface waters and settled on what is now known as “Sanctuary Reef”. As the centuries passed, the tiny colonial polyps laid down a skeleton of calcium carbonate, at a rate of approximately 9 mm/year, emerging in the twentieth century as a massive Porites bommie, nealy 6 metres in height.

The Sanctuary Reef bommie is nearly 6 metres in height, and began its life over 600 years ago. Image: AIMS

From the outside, the modern-day bommie has the usual appearance of a mature stony coral community – a mixture of coral, sponges and a host of other reef organisms. Each year the coral polyps added to the structure further expanding the boulder-like façade. Yet, hidden deep within each layer of growth is a story – with details about past environmental conditions including water temperature– similar to tree rings. This treasure trove of information was locked away until November, 1985.

As part of a broader Great Barrier Reef coral core sampling program, Dr Peter Isdale, former Research Scientist at the Australian Institute of Marine Science (AIMS), led a team of scientists to Sanctuary Reef and extracted four cores from the massive Porites bommie. Once the cores were brought back to the Institute, x-rays of the coral core slices revealed the annual density bands characteristic of this massive coral species.  Knowing the date of collection of the core and by counting the annual bands back through the core, they found that the coral started laying down skeleton in the mid-late 14th century.  Immediately, this became one of the longest continuous annual growth records obtained from a living coral.

Dr Peter Isdale's fieldnotes book from 1985 contains a mud map of the bommie on Sanctuary Reef, as well as the position of coral cores extracted via its upper surface.

Fast forward to 2016, and scientists from AIMS’ Long-term Monitoring Program, out on a regular research trip rediscovered the prized relic. Armed with a very crude 1985 map of Sanctuary Reef and a drawing of the Porites colony, as well as a guess on the likely location from Google Earth, the team successfully located the large bommie. Scientists were excited to find that 30 years post-sampling, the Sanctuary colony was still largely covered with living coral tissue.

The present day Sanctuary Reef bommie is teeming with life. The location of the original cores can be seen in a crevice between live coral tissue on the bommie's upper surface. Image: Amanda Delaforce

Since the early 1980s, scientists at the Australian Institute of Marine Science have collected cores from long-lived massive Porites colonies at locations throughout the GBR, as well as from Western Australian reefs.  Using specialised equipment, scientists carefully extract a circular ‘slice’ of history, typically 5-10 cm in diameter.  Over the years, AIMS has amassed a collection of nearly 300 long cores from 56 reefs along Australia’s east and west coasts.  These are supplemented by more than 250 short cores and 500 small (less than 1 metre) colonies.  This collection has resolvedly become highly regarded nationally, and garnered international significance as the world’s largest coral core archive.  

AIMS Senior Principal Research Scientist, Dr Janice Lough, examines coral core slices to learn about past climates and coral growth rates. Image: AIMS

These natural coral reef history books contain a wealth of information about past annual coral growth rates and environmental conditions.  This historical perspective significantly extends our knowledge of reefs - exisiting long before observational records of marine climate were documented.  Such corals tell us that reefs are currently experiencing rapid changes in their marine environment and reveal growth responses that appear unprecedented over the past several centuries.

The oldest relic in AIMS Coral Core Archive is the Sanctuary Reef Porites bommie - a natural wonder that continues to stand firmly amidst a rapidly changing world.