Australia: a love letter across time
Brooke Johnson, DPhil candidate and College Lecturer in Earth Sciences
Australia is old beyond imagining and its memories run rich and deep within its rocks, sleeping snug below glowing red deserts. Australia remembers a time before people, and animals, and plants. Australia remembers the first wriggling bacteria in the slick mud of a warm watery world, but the memories in the rocks go back even further than that. There are single microscopic crystals, a fraction of the size of a grain of sand, hidden in rocks that are already so ancient the word loses meaning, that remember deeper still. They tell of the time before those first bacteria, a time before even the primal vestiges of the first continents existed. Single tiny crystal motes that remember the very first flows of liquid water as it trickled across a new planet, whose surface was still scarred and torn by the brutality of the early solar system and the violent birth of our moon. Australia is OLD and Australia remembers. These stories want to be shared; they’ve been waiting billions of years to be heard and whisper these secrets with a voice like the sigh of waves from long vanished oceans. You just have to listen to the rocks to hear it.
To a geologist like me, interested in the deep murky roots of the story of Earth and the life that inhabits it, Australian rocks are an incomparable treasure. To a young rock obsessed version of me, looking at pictures of these rocks in my geology books, was like peering into some lost alien world. It never occurred to me that a wee working-class lad from Cleveland (UK, not Ohio) would get to work with such cool rocks, trying to decipher mysterious messages from the distant past.
Following a somewhat circuitous route, I became a geologist, and started my PhD at the University of Oxford, studying a box of Australian rocks. The samples had been pulled by drill core from two kilometres beneath the blistered and heat-scarred outback, and sent to my advisor, Dr Nick Tosca, by Dr Amber Jarrett of Geoscience Australia. The rocks were a type of sediment known as ‘black shale’ — the rocks that produce oil — and a pretty standard type of marine sediment. What made these oil shales special was not just how oil rich they were, some of them dribbled oil when cut, but their incredible age, around 1.3 billion years old.
Oil is produced when organic matter in the form of algae, bacteria and bits of plants are buried on the seafloor under oxygen free conditions, and then cooked gently at 15⁰⁰C for a few million years. However, 1.3 billion years ago, the continents were barren, and the sea was dominated by bacteria. For there to be this much oil in the rocks, it meant there had to have been a phenomenal number of microbes inhabiting the ancient seas of northern Australia.
1.3 billion years ago, northern Australia had been a sub-tropical inland sea. With no large organisms to eat them, the bacteria inhabiting the water were free to die and accumulate in vast abundance on the dark sea floor. Repeat this process for millions of years and you end up with a thick pile of oil rich rocks. There had been so much organic matter packed into these sediments, you could still smell it.
Scientists can search oil rich rocks for biomarkers — tiny fragments of organic molecules that can only be produced by living organisms. Using this method, Dr Amber Jarrett discovered that among the bacterial biomarkers were steranes, biomarkers that are only produced by eukaryotes. This group contains all life on Earth that is not a bacteria (or their sisters, the archaea): everything from single celled algae and amoebas through to all plants, fungus and animals, including you. The presence of steranes meant that the oldest single celled ancestors of all eukaryotes were living in the seas of northern Australia 1.3 billion years ago. But if eukaryotes have been around for so long, why did large animals and plants only appear in the last 550 million years?
The apparent late arrival of animals and plants in Earth history has been a dilemma since the time of Darwin. Currently there are two broad ideas that suggest why this apparent delay happened. The first is that an environmental factor — the current favourites being a lack of oxygen and nutrients — meant early eukaryotes could not evolve and remained second place to bacteria for around a billion years. The second idea is that it simply takes a long time to evolve complexity. To test the first idea, myself and Dr Rosalie Tostevin collected additional rocks from the Northern Territory Geological Survey (NTGS) drill core storage facility in Darwin, and brought them back to Oxford for analysis.
A drill core is essentially a tube of rock, sometimes several kilometres long, that is extracted from the Earth in sections by diamond drill. The NTGS storage facility is like an aircraft hangar crammed floor to ceiling with pallets stacked with cores in metal cases. Each pallet holds 200 metres of core, and there are close to a thousand pallets at the Darwin facility alone, that’s over 100,000 kilometres of rock from all over the Northern Territory and covering several billion years of Earth history. Thankfully we only needed to study and sample about 5 kilometres’ worth of the material. It still took nearly a month to get all the samples and data we needed.
Sampling consisted of collecting chunks of rock every 10 meters or so, cutting them in half and then taking the bit we wanted. Dr Rosalie Tostevin did the sampling and cutting, and I did the core logging. A core log is a fancy graph: height is up the Y axis and represents time, oldest rocks at the bottom, increasing grain size goes left across the X axis. As grain size and texture in a sediment are key environmental indicators, a lot of environmental change can be recorded quickly and in an intuitive way. The sampling and logging area was next to the storage hanger and open to the hot dry air, though thankfully a roof keeps out the intense sun.
It was actually quite pleasant to examine the cases of ancient rock while listening to the exotic birds chirp and call in the distance. Less pleasant was having to pick out the possum droppings and various large creepy crawlies that were hidden in the core cases. Some of the material we sampled had never been fully studied and it was exciting and humbling to know that we were among the first humans to ever see these little slices of deep time. To me they seemed to feel somehow heavier than they should, dense with the memories of a time so distant it might as well be the other side of the universe.
If field sampling is the glamorous and adventurous side of geology, sample prep and analysis is the dull slog no one talks about. We had 500+ samples that needed to be prepped, powdered, thin sectioned, and then analysed in multiple ways. Although there were some parts that were enjoyable, especially grinding rock samples to the width of a human hair and then peering at the resulting thin sections under a microscope. Aside from the valuable data, rocks under the microscope look amazing.
That analysis has taken the best part of the last three years, but it has been worth it. We now have one of the richest data sets in the world about an interval of time that is both immensely important in the story of life, but also very poorly studied.
The project has exceeded our expectations: the rocks of Australia were eager to spill their secrets. We have begun to answer our initial research questions and have started to rebuild the world of the first eukaryotes. But this world is nothing like we imagined and surprises us with every new piece of data. However, if you want to read about what we learned from the lost world of the early eukaryotes, you will have to wait until our first paper drops in January. Australia’s rocky core is patient and doesn’t mind though, what’s a few more months compared to the billions of years it has already been waiting to share its secrets.
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