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Squeaky or sticky sand shows beaches most at risk

“We have these high shell-content sands in [Sydney’s] northern beaches – that really needs to be given some thought,” Angus Gordon, a prominent coastal engineer, said. “The dissolution will lead to the breakdown of productive offshore reefs.”

Massive surf causing beach erosion at Narrabeen beach in 2015

Massive surf causing beach erosion at Narrabeen beach in 2015Credit:Nick Moir

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Most of eastern Australia’s beaches are dominated by quartz-like sands, although the strip from Sydney’s North Head to Palm Beach has a higher carbonate content. Narrabeen, for instance, can reach as high as 70 per cent carbonate sand, Mr Gordon said.

Oceans have become about 25 per cent more acidic since carbon dioxide emissions began rising with the burning of fossil fuels in the industrial era. Seas are now more acidic than for at least two million years as oceans absorb about one-third of human-related CO2, scientists say.

Ana Vila-Concejo, an associate professor at Sydney University, said acidification was one of several challenges brought by climate change that would affect beaches. Average ocean pH was about 8.1.

“Carbonate dissolution will occur at pH 7.5-7.6, which in a generic context will be reached within 100 years more or less,” she said. “However, there are studies that forecast net dissolution of sediments as early as 2050 for certain environments.”

Beach erosion at Stockton near Newcastle, another region where carbonate-rich sands may see faster depletion.

Beach erosion at Stockton near Newcastle, another region where carbonate-rich sands may see faster depletion.Credit:Mitchell Harley

Professor Vila-Concejo was part of a team that published new research in the Scientific Reports journal on Thursday about improving the estimate of how carbonate sands move.

The work used new mathematical models to show how small changes in the rate carbonate sediment settles could lead to big changes in how the sand is transported by oceans. The study focused on Queensland’s Heron Island and also Oahu, Hawaii.

The researchers found the availability of carbonate sands on the seafloor was probably overestimated by more than one-fifth while the transport of such sediment was underestimated by one-tenth. Gaining a better fix on the shape of the carbonates – typically coarser and less dense than quartz sand – was key.

“It’s a really inaccurate science,” Professor Vila-Concejo said, adding poor accounting contributed to both bad coastal management but also a misunderstanding of how much sand was available for global construction and manufacturing.

Tristan Salles, another of the paper’s authors, said keeping tabs on carbonates would probably become more important.

“If islands and atolls are at risk from erosion caused by sea-level rise, it will be vital to understand how the sand protecting them will respond to the ocean currents, waves and high-energy sea swells battering them,” said Dr Salles, also from Sydney University’s School of Geosciences.

Andrew Short, an emeritus professor at Sydney University specialising in coastal morphology, said Australia’s southern and west coasts also had relatively high carbonate levels compared with the global average.

How climate change affected carbonates was “a concern for half our beaches”, he said.

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