Imagine you were trying to understand how much warmer your hometown has gotten over the last hundred years due to anthropogenic climate change, but all you had to answer this question was one random temperature observation per decade. The signal you are looking for over the whole time period may only be one degree Celcius, but it would be masked by year-to-year shifts that could be over fifty times that as you compare winter nights to summer afternoons.
We have a similar challenge in geology. This field is famous for identifying large environmental and climatic changes deep in Earth’s past. From the cataclysmic impact that wiped out the dinosaurs (and tons of other species) in a relative instant, to the slow march of oxygen buildup in our atmosphere and oceans, our science often comes down to reading changes as we step through successive layers of rock. But just as the specific conditions on a blustery evening in January have a heavy influence on the temperature recorded, the particular environment that lays down a rock layer imparts its own influence in recording biological diversity, water depth, and ocean chemistry. So how can we know if we are witnessing true global changes in Earth’s deep past or a relatively static period recorded under different local conditions?
This project is a detailed treatment of this problem of reading change from records that are highly variable. To achieve a satisfying answer in the climate example, the trick is to add in context that comes from seasons and weather. We do the same thing for rocks in this study. We outline image analysis and statistical techniques so that each observation relating to fossil diversity or ocean chemistry is accompanied by relevant information about conditions that deposited each rock you’re looking at. Just as you might adjust each point your random temperature record based upon the weather at the time it was recorded, we set proper expectations for observations of Earth’s history based upon the conditions that created each rock layer, to leave more accurate records of stasis and change.
In taking this approach when studying Earth’s history, some periods that we previously thought were times of major upheaval will appear more static. Viewing Earth history with this added context, however, does not rob us of opportunities to learn about important events in our planet’s ancient past. When we are able to peel back false indicators of global change, we have a much clearer window in to the effects imparted on Earth’s environments, climate, and life by more everyday dynamics—like the ancient reefs we study in the paper. The tools we develop here are not just for this study. They are a framework for reading any rock record more honestly. Every layer is both a signal and a story about the conditions that preserved it, and learning to tell those apart is how we sharpen our view of deep time.