In Australia, ancient rocks and an ancient mystery
Geobiologists know that you don’t need a time machine to glimpse billions of years into the past, and they also know you don’t need a spaceship to reach an alien landscape. A plane ride to Australia will do just fine.
That’s why MIT researchers in the lab of Roger Summons, professor of geobiology in the Department of Earth, Atmospheric and Planetary Sciences, have been gathering and studying samples from Hamelin Pool — a salty, warm enclosed pool within Shark Bay in northwestern Australia.
The shallow water margins of Hamelin Pool are dotted with stubby brown columns that, despite appearances, are some of the most stunning specimens of geobiological interest on Earth. These structures are stromatolites, the constructions of a thin layer of photosynthesizing bacteria that grow upwards like corals to build rippled layers of cemented sand.
“Hamelin Pool is one of the few contemporary places where we can see stromatolites forming; it’s become a touristic 'Mecca' for those geologists interested in Earth's history,” Summons says.
The cyanobacteria building the stromatolites in Hamelin Pool evolved from bacteria that lived on the Earth 3.5 billion years ago, making them descendants of the oldest photosynthetic life on the planet. Cyanobacteria are the most primitive of organisms capable of oxygenic photosynthesis and are thought to have caused the initial rise of atmospheric oxygen — in other words, they helped create the very air we breathe today.
“I’ve always been interested in the different kinds of microbial communities found there and how we might view them and study them as analogues for ancient counterparts,” says Summons, who hails from Australia himself and has been studying Hamelin Pool microbes for nearly 30 years.
But the stromatolites here are distinctive for another reason, he continues: the ooids they contain. Under a microscope, ooids look like egg-shaped pebbles. When sliced, these tiny grains display an internal geometry of concentric layers around a nucleus; these layers have been shown to comprise calcium carbonate and organic matter. Ancient ooids are preserved in rocks called oolitic grainstones or oolites.
“They form in warm tropical environments when the seawater chemistry is just right,” Summons says. “Hamelin Pool is one of the few places you can find ooids forming at the present time.”
So why are oolites so special? Possibly because they are found in rocks of all ages going back as far as 2700 million years ago and could help us tell more about what occurred on the coastlines of ancient continents.
Around 540 million years ago, complex life took off running. In some tens of millions of years — a short time when you’re talking about the history of the Earth — a vast array of species rapidly evolved and flourished. Scientists call this burst of biodiversity, which produced many of the major groups of animals that exist today, the Cambrian Explosion. But they’re still not sure what caused it. A likely candidate is a sudden increase in atmospheric oxygen, creating a friendlier environment for oxygen-breathing creatures like ourselves.
As researchers search for causes, a large event known to geochemists as the Shuram Excursion catches the eye. Roughly 580 million years ago — just before the Cambrian Explosion — there seems to have been significant environmental change on a global scale. In rocks around the world, researchers have found an eyebrow-raising drop in the abundance of the carbon-13 isotope in limestones — a key indicator of the environmental conditions where the rocks formed.
“There was a major disruption of the carbon cycle,” Summons says. “It seems to have been a major weathering event; the only way you can get such a thing is when you have weathered an unusual amount of organic carbon.”
In 2006, MIT Professor Daniel Rothman, WHOI scientist John Hayes, and Summons published a paper positing this idea: oceans full of organic carbon, which lacks carbon-13, are suddenly filled with oxygen that oxidized the carbon, producing carbon dioxide that forms carbonate rocks low in carbon-13. Thus, the existing data on the Shuram Excursion could point towards a global rise in oxygen, they postulated.
In some places, including Death Valley and parts of Oman, the Shuram Excursion is recorded in oolites. Geologists love oolites because it’s easy to tell whether or not they are “pristine,” meaning that they haven’t been subjected to heat, pressure or recrystallization in hot water that could interfere with the signals (like carbon isotopes) that researchers track. That’s why it’s important to learn about how ooids form and how to “read” them so that we can use them more effectively to decipher what happened. And places like Hamelin Pool — as well as other field sites including Highborne Cay and Cat Island, in the Bahamas — are critical to Summons’ efforts to do just that.
“By having sites where we see ooids forming in different parts of the world, we can look for commonalities and look for differences that might help us better understand what’s going on,” he says. “Sedimentary rocks have the ability to tell us about our own origins, and to tell us about the history of the Earth in ways that might allow us to look into the future.”
So breathe deep, and thank whatever it was that gave us this Earth and this air. Meanwhile, the Summons Lab and geobiologists all over the world will continue to search far, wide, and deep to find and connect the pieces of the very large, very old puzzle that is our planet’s story.