Geology grad school is inevitably fraught with muddy field trips, dirty and dusty field work, broken down cars and popped tires, adventures and misadventures. During my Master’s degree (2016-2018) I studied the formation of the Sierra Nevada mountain range in California. Here’s a peek into what it’s like to work in a hot valley sandwiched by the highest and lowest points in the US lower 48 (Mt. Whitney and Death Valley) in southeastern California, hunting for amoebas hiding in rocks that went extinct a quarter billion years ago.
Part 2
Sweat dripped down my brow and the July desert sun battered my back as I squinted at the unconformity. I was standing at the top of a slope of eroding shale, looking at a cliff of conglomerate that forms a massive mesa, appropriately called “Conglomerate Mesa." The bright purple and green shale, 260-million-year-old sediment probably deposited at the bottom of the sea floor, ran right into the thick pebbly cliff, rocks that were probably formed in a river about 250 million years ago. This type of a “contact” – one type of rock next to another – is pretty common in geology. But this one is unique. It represents a mass extinction, where 96% of all species were lost, and 99% of all ocean life.
It’s known as the “great dying”, and it’s the worst extinction event the world has ever seen, the border between the Permian and Triassic time periods. Life on earth nearly ended. Our planet almost reverted back to how it was before life – smoldering volcanoes and barren rocks (at least that’s just how we imagine it to be). But life didn't quite end, it was just set back about 300 million years.
So what caused this? And why didn’t it just completely wipe out all life? Most scientists agree that it was triggered by volcanic eruptions in Siberia that emitted carbon dioxide into the atmosphere. Bacteria responded to the carbon dioxide by emitting methane, which is a greenhouse gas, and caused temperatures to rise and the ocean to acidify.
Then, 200 million years later, it happened again. We’ve all heard of the K-T (Cretaceous-Tertiary) extinction – because that’s when all the dinosaurs died, along with 76% of all species. That extinction event was likely also affected by climate change and volcanic activity, and then we’re pretty certain that a meteor struck and was the straw that broke the camel’s back….. or the triceratops’ back.
There’s a theme to these extinctions: climate change due to the accumulation of greenhouse gasses such as carbon dioxide and methane. Hmm…. Sounds familiar. In fact, biologists think we might be on the brink of the next big extinction event. Global warming is accelerating at a rate that could doom humans in the near future (by geologic standards), as well as other critters.
Extinction events are spotted in the rock record by paleontologists when many fossilized species suddenly go missing. The extinction event represented in the rocks I’m studying – the P-T extinction – is noticeable in my field area by the disappearance of fusulinids. Fusulinids are little football-shaped marine critters that are related to present-day amoebas that floated around in the ocean in the Permian.
To me, fusulinids are like my cairns, my road signs: they tell me when I’m on track. Because they only appear in Permian rocks, I know that if I see fusulinids, I’m sampling the right stuff. So I suppose their extinction can be useful. Sometimes they look like they’ve been tossed around and deposited in a layer, which means they could have been “re-worked,” or eroded out of a lower layer and then re-deposited, which is tricky. But they don’t get transported too far, so you can assume you’re looking at rock that was deposited close to the age of the fusulinid’s life span.
The other awesome thing about fusulinids is, they can tell you a pretty precise age. It’s called biostratigraphy, and paleontologists have been matching tiny marine fossils to age slots for hundreds of years. They’ve tracked the physiological evolution of the tiniest critters to such a T, that if an expert looked at one of my fusulinids under the microscope, he would be able to tell me exactly which part of the Permian that animal lived in, and give me age constraints like 255-260 million years ago. That may sound like a big bracket, but trust me, for stuff that happened so long ago, it’s pretty damn precise.
But if I’m supposed to be tracking the deposition of the rocks, what do I do about the Permo-Triassic unconformity and extinction event?! There could have been over 10 million years missing from the geologic record here. There’s literally nothing I can do but stare at the rocks and think about greenhouse gasses flooding the ancient atmosphere and smothering almost all living things into oblivion. How many degrees warmer was it? More than two degrees C, the danger zone for modern global warming? Surely it was. But will we have to wait to find out?
Paleontologists recognize an extinction event when certain fossilized species suddenly go missing in the stratigraphic record. How will humans show up in the ancient record? Will our time period be long enough to be preserved at all?
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