The Sandstone That Preserved a Ghost Coast: Western Australia's Shark Bay Stromatolites

At Hamelin Pool in Western Australia, the water is twice as salty as the open ocean. Beneath the surface, mounds of sediment rise from the shallows like loaves of bread, their surfaces dark and leathery. These are living stromatolites—the same kind of structures that appear in the 3.5-billion-year-old rocks of the Pilbara.

The Living Rock

A stromatolite is not a single organism. It is a layered community of microbes—mostly cyanobacteria—that trap and bind sediment grains into domes, columns, and mats. The cyanobacteria photosynthesise during daylight, producing oxygen as a waste product. At night, other microbes take over, precipitating calcium carbonate that cements the sediment into rock.

The result is a structure that grows a fraction of a millimetre per year. A stromatolite one metre tall at Hamelin Pool may be several thousand years old. But the lineage is far older. The same process built the earliest known fossils on Earth, preserved in the Warrawoona chert of the Pilbara, 1,600 kilometres to the north-east.

What makes Shark Bay exceptional is that this ancient process still happens here. Almost everywhere else on Earth, grazing snails and burrowing worms destroy microbial mats before they can lithify. But Hamelin Pool is too salty for those animals. The hyper-saline water—up to twice the salinity of seawater—is a refuge from the biological competition that elsewhere erased the stromatolite way of life.

A Window Into the Archaean

The Hamelin Pool stromatolites are not identical to their Archaean ancestors. The microbial communities are different; the seawater chemistry has changed; the atmosphere now contains 21 percent oxygen. But the fundamental process—a microbial mat trapping sediment and precipitating carbonate—is the same.

Visiting Hamelin Pool is as close as a human can get to standing on a 3.5-billion-year-old shoreline. The water is shallow and warm. The air smells of salt and sulphur. The stromatolites sit just below the surface, their dark caps exposed at low tide. They look like nothing else on Earth.

The oldest known stromatolites, found in the Pilbara's Dresser Formation, are 3.48 billion years old. The ones at Hamelin Pool began growing after the last ice age, about 7,000 years ago. The gap between them is the story of life itself.

The Chemistry of Preservation

The same conditions that allowed the Hamelin Pool stromatolites to form also explain why the ancient ones survived. In both cases, the key is rapid cementation. The microbial mats secrete a sticky mucus called extracellular polymeric substance—EPS—that traps sediment grains. Then, as the microbes metabolise, they alter the local chemistry, causing dissolved calcium carbonate to precipitate directly onto the grains.

This cementation happens within days of the sediment being trapped. It binds the layers into a firm structure that resists erosion. When the mat is buried by new sediment, the cemented layers remain intact, preserving the texture of the microbial community.

The Pilbara stromatolites survived for billions of years because they were repeatedly buried by volcanic ash and silica-rich fluids that replaced the original carbonate with chert—microcrystalline quartz. The Hamelin Pool stromatolites are not being replaced by chert. But they are being cemented by aragonite, a form of calcium carbonate, which gives them a similar resilience.

A Shrinking Refuge

The Hamelin Pool stromatolites are not permanent. The bay is shallow, and the water chemistry that sustains them depends on a narrow range of conditions. A change in sea level, a shift in rainfall patterns, or the introduction of grazing organisms could collapse the ecosystem.

Already, the living mats are restricted to a small fraction of their former extent. Core samples show that stromatolites were once widespread across the bay's floor. Today, they survive only in the most saline reaches of Hamelin Pool, where the water is too harsh for snails.

This makes Shark Bay a fragile archive. It preserves not just a living fossil, but the only place on Earth where the Precambrian is not yet extinct. When the snails finally arrive—or the salinity drops—the ghost coast will vanish. The rocks will remain, but the living process will be gone. And the Pilbara stromatolites will once again be the only record of how life first learned to build stone.