Light shed on reef recovery
A new Great Barrier Reef study shows how the reef copes with rapid sea-level rise.
The study of coral cores from the Great Barrier Reef by scientists at the University of Sydney has shown that rapid sea-level rise and increased sediment came close to killing the reef 7,000 years ago.
Using unprecedented analysis of 12 new drilled reef cores with data going back more than 8,000 years, the study shows that there have been three distinct phases of reef growth since the end of the Pleistocene era about 11,000 years ago.
“We wanted to understand past reef resilience to multiple environmental stresses during the formation of the modern reef,” said lead author Kelsey Sanborn, a PhD student in the School of Geosciences at the University of Sydney.
The study, published in Sedimentary Geology, revealed a period between 8,000 and 7,000 years ago when the reef growth slowed as it was exposed to multiple stresses, including likely increases in sediment and nutrient flux on the reef.
As the ice caps and glaciers melted at the beginning of the Holocene era, sea level rapidly rose by up to seven metres every thousand years, flooding the continental shelf until about 7,000 years ago. At the same time, it is estimated sea-surface temperatures likely increased several degrees between 8,000 and 6,000 years ago.
“We are fortunate that the sea level stabilised about 6,000 to 7,000 years ago. If it had continued to rise as fast as it had been, the reef might not have survived given its slow growth rate,” Ms Sanborn said.
Associate Professor Jody Webster, a co-author of the paper, said that there are multiple environmental drivers affecting both the historic and modern reef system.
“We need to understand the past in order to predict the future. This paper and Kelsey’s broader research examine how sea level, surface temperature, sediment in the water, nutrient influx and energy inputs into the reef system affect its vulnerability to environmental change,” he said.
“The reef system survives because of a delicate balance these environmental factors. Anthropogenic climate change is threatening to interfere with this balance.”
A surprising result of the research is evidence that initial reef growth occurred on the low-energy, leeward side of the reef, ahead of growth on the high-energy, windward side.
“This is contrary to established models of reef growth,” Ms Sanborn said.
“In those models, the part of the reef exposed to higher energy inputs from waves and wind were thought to have been cleared of land-based ecosystems, clearing the way for reef development.”
The paper proposes a new model that needs further testing in other regions of the Great Barrier Reef and reef systems around the world. What it establishes is that the more protected parts of the reef might have been more suitable for early coral development.
“This provides new constraints on how we understand changes in the environment controlling reef development through sea-level rise and inundation,” Ms Sanborn said.