From an aesthetic perspective, black shales are not the prettiest rock. They don’t form soaring spires and walls like the granite in Yosemite, or the colorful, variegated landscape like the Jurassic sandstones of the desert southwest. Usually, they just weather into bland, monotonous, dark hillslopes. In fact, the bane of my recent research as a geologist has been finding the good stream cuts and valleys where the shales are actually exposed. From an economic and evolutionary perspective, though, black shales are fascinating. The reason black shales are, well, black, is that they were deposited under low-oxygen conditions, allowing high quantities of planktonic algae, which are normally degraded by microbes in the water column and sediment, to be preserved. This organic carbon gives the sediment (and ultimately the rock) its distinctive color. Upon burial and heating, the carbon turns into oil and gas; black shales are the source of our current carbon-based economy. How, why and where black shales form is thus a main thrust of petroleum geology.
Hopefully as a society we are in the process of transitioning towards renewable energy sources, and future interest in black shales will be limited to those interested in their evolutionary importance. My research focuses on the early emergence of animals, an event known as the Cambrian radiation, about 540 million years ago. During a brief (by geological standards) ~20 million year window, essentially every major animal group appeared in the fossil record. Interestingly, rocks before the Cambrian show abundant evidence that they contained relatively little oxygen. The deep ocean before the Cambrian is characterized by abundant and persistent black shales—indicating low oxygen conditions. In my field area in the Yukon Territory of arctic Canada, for instance, entire hillsides consist of black shale.
Monotonous black shales in the Precambrian Fifteenmile Group, Ogilvie Mountains, Yukon Territory, Canada, with two geologists for scale.
Crossing into the Cambrian, though, widespread black shale horizons become much less common, and for the most part the ocean appears fairly well oxygenated. Because oxygen so strongly affects marine animal life, many geologists have suggested that the Cambrian radiation is related to the oxygenation event that occurred around the Precambrian-Cambrian boundary. Study of modern low-oxygen analogues, such as offshore San Diego, may help to inform our understanding of what happened in the ancient past. Growing recognition that oxygen-deficient zones in the modern ocean are expanding has led to increased interest among oceanographers in studying these settings (including many aims of the San Diego Coastal Expedition). My research has been focused on using new information from these settings to understand how oxygen levels affect organismal biology, and then to apply these lessons to the geologic record. Reciprocally, times of oxygen stress and warming identified in the fossil record may be able to help us understand how organisms in the modern ocean will respond to current global warming and expansion of oxygen-deficient zones on timescales not measureable by ecological experiments.
Working with the OMZ team on both legs of this cruise, seeing the animals that live in the OMZ, and learning how an oceanographic cruise works has been one of the coolest experiences I’ve had in science. It’s been especially great to be able to learn from scientists like Chief Scientists Frieder and Grupe and Dr. Levin, and I feel really lucky to have been invited on the cruise.
Nightime deployment of the multi-corer to sample Oxygen Minimum Zone sediment.
It’s been quite a different experience from my normal geological fieldwork, though. As geologists we are used to working with the rhythms of daylight. In the field we’re usually up around first light, hiking to the outcrop, staying on the rock as long as light allows, and then back to camp, often by headlamp, to sit around the fire and talk about the day, the rocks we’ve seen and rocks we dream of seeing. Long days, for sure, but over when the sun goes down. On the Melville, the science never stops. When the OMZ team is up, it’s time to go. My first multi-core was a night deployment. We caught a nap after dinner and then woke up at 2:00 AM to work through the dawn. In theory I knew the night-time work would be happening, but it doesn’t quite prepare you…on the plus side, the breakfast prepared by the awesome chefs on the Melville certainly beats the freeze-dried meals we eat for weeks on end in arctic Canada! Now that I’m getting used to the rhythms of science on the Melville, the experience just keeps getting better, and I feel like I’m making some progress towards a realistic view of ancient low-oxygen seafloors.
- Dr. Erik Sperling, Postdoctoral Researcher at Harvard University