The study, published today in the Proceedings of the National Academy of Science, suggests the die-offs happened in conjunction with significant natural and anthropogenic events, such as large-scale weather patterns and changes in land use since European settlement.
The findings could also mean the effects of more recent bleaching events in 2016 and 2017 have been underestimated because of the long-lasting impacts of earlier die-offs, said lead researcher Tara Clark, a marine palaeoecologist at the University of Queensland and Griffith University.
Dr Clark said growing evidence suggested the Great Barrier Reef is in decline, but information about disruption events and the how the reef responds to stress only goes back around 30 years.
To understand how the reef had changed over a longer period of time, Dr Clark and colleagues dated samples of dead Acropora branching corals from a number of sites around the Palm Islands in the central Great Barrier Reef.
The technique — which is used to estimate when the coral died — allowed researchers to look back over a century of coral activity.
While earlier studies suggested branching corals were common in many parts of the reef, Dr Clark said their team's surveys showed these reef builders now make up less than 5 per cent of the coral population at the sites studied.
"There were some skeletons we picked up that were huge — the branches were as thick as your arm, these were massive branching corals so that's a very different shift in terms of community structure at a lot of these places," she said.
Timeline from reef to graveyard
One site in particular — Pelorus Island — experienced a huge die-off of branching coral between the 1920s and 1950s.
Dr Clark said there weren't many environmental records from that time, but the die-off may have been caused by a combination of a shift towards a wetter weather pattern and an increase in sugar cane plantations and grazing in the area.
"What we suspect is there was potentially a lot of land clearing going on, you get this shift in the Pacific Decadal Oscillation, with huge rainfall and a huge amount of sediment being delivered out the Herbert River, just north of Pelorus Island," she said.
"Because Pelorus is the closest reef to the Herbert River, it's possible that these plumes came out and wiped them out then, and they've not been able to recover."
Instead, the reef is now dominated by algae, and small stony coral known as Pavona.
At the other four sites, the die-offs were more recent, taking place between 1970 and 2000.
In particular, there were peaks in coral death that corresponded to mass bleaching events that occurred with warmer sea surface temperatures around 1983, 1987, 1994 and 1998.
Dr Clark said what was most worrying was that at many of these 'death assemblages' of dead coral, there was very little sign of living Acropora, suggesting that these more sensitive coral species have not bounced back.
"I think it's cause for concern because if there was nothing [living] there, then the impact of these larger bleaching events that happened more recently [in 2016 and 2017] has been underrepresented and underestimated," she said.
She said long-term monitoring programs are critical to understanding of how the state of coral has changed over time.
"Being able to use these methods to look at disturbance and recovery and work out which reefs are vulnerable, which haven't come back from some of these disturbances, is really important," she said.