Geobiology Fieldwork 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.

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.