When something turns pink in nature, it is usually not a good sign. But strange pink sand washing up on South Australia’s beaches has exposed an ancient Antarctic mountain range thought to be buried under ice.
When pink streaks first appeared in the sand of Petrel Cove, a remote beach that flows into the Southern Ocean, scientists in Australia quickly figured out what the colored sand was made of, a mineral called garnet. However, they were surprised by its age and where it came from. derived from.
“This journey started with the question of why there was so much garnet on the beach at Petrel Cove,” says geologist Jacob Mulder from the University of Adelaide.
“It’s fascinating to think that we have been able to trace tiny grains of sand on a beach in Australia to a previously undiscovered mountain range beneath the Antarctic ice.”
The Earth’s crust is constantly eroding and reforming, with loosened sediments being carried away by the wind and water being deposited elsewhere to form new land. If geologists are lucky, they can make connections between deposits of similar ages with similar properties over vast distances and long periods of time.
Garnet is a fairly common mineral, deep red in color. It crystallizes at high temperatures, usually where large mountain belts are scouring upwards from colliding tectonic plates. This makes it perhaps the most important mineral for inferring how and when mountains formed, as the presence of the crystals indicates the pressure and temperature history of the metamorphic rocks in which they form.
The team’s lutetium-hafnium dating showed that some of the garnet found at Petrel Cove and in nearby rock formations matched the timing of local mountain building events in South Australia.
But their results indicate that it largely formed about 590 million years ago, some 76 to 100 million years before the Adelaide Fold Belt formed, and billions of years after the Gawler Craton crustal block formed.
“The garnet is too young to come from the Gawler Craton and too old to come from the eroding Adelaide Fold Belt,” explains Sharmaine Verhaert, a geology graduate student at the University of Adelaide who led the research.
Instead, the garnet likely formed at a time when the South Australian crust was “relatively cool and non-mountainous,” Verhaert says.
Garnet is usually destroyed by long-term exposure to waves and currents, so the researchers also thought it likely surfaced locally, even if it originally formed millions of miles away, millions of years ago.
Their research uncovered a grand solution, one that connects the pink sand at Petrel Cove to layers of nearby glacial sedimentary rock and to distant garnet deposits previously found in a spur of the Transantarctic Mountains in East Antarctica.
The rock outcrops protrude from a thick ice sheet that otherwise completely obscures the area below, making it impossible to sample geology beyond the exposed peaks of a mountain range thought to lie beneath. The hidden mountain belt is believed to be 590 million years old, just like the garnet analyzed in this study, but researchers have not been able to get a good look at it.
Connecting the dots to ice flow indicators in the sedimentary rocks of the South Australian glacier, Verhaert and colleagues think that garnet-rich glacial sands from the Antarctic mountains – which have yet to see the light of day – have been ground down by an ice sheet moving north. -west during the late Paleozoic Ice Age, when Australia and Antarctica were connected in the supercontinent Gondwana.
‘The garnet deposits were then stored locally in glacial deposits along the southern Australian margin,’ explains geologist Stijn Glorie of the University of Adelaide, ‘until erosion occurred. [once again] freed them and the waves and tides concentrated them on the South Australian beaches.
An epic journey through land and time.
The research was published in Communication Earth & Environment.