Flipping Through the Pages of Time, Slowly

“I look at the geological record as a history of the world imperfectly kept…only here and there a short chapter has been preserved; and of each page, only here and there a few lines.”

- Charles Darwin “On the Origin of Species” (1859)

Imagine a vast book, with pages the size of Ohio, and even larger. Look at the edge of that volume, laying flat, with sheaves of edges layered one above the other. You can almost sense the piled narrative hidden within, pick up on the heft and depth and duration of the story to be told.

Lay that book out upon the landscape, with the illustrations and lines of text receding into the distance. You peel back the cover and begin to read—from the most recent description of events on the first page before you, down to the beginning of the plot far below.

In terms of geologic books, then, one “murder” mystery of pre-history is the riddle of mass extinction. The fossil record shows repeated episodes through deep time where large numbers of living creatures simply vanish. Their rocky remains are buried in earlier, or deeper strata, but are nowhere to be found in later stratigraphy, the geologic layers closer to the surface.

Was it Colonel Mustard in the library with the candlestick? That’s the question, after a fashion: Who, or what, did the deed?. Much consideration and interest in recent years has been given to a large weapon and the single assassin. Who killed the dinosaurs? Well, there’s strong evidence that an asteroid hit Earth about 65 million years ago, leaving an impact crater near the Yucatan peninsula called Chicxulub Crater. Many geologists and paleontologists who study “macrostratigraphy,” the way the layers of sediment tell their immense story, think the Chicxulub event led directly to the extinction of the dinosaurs and many other species, right down to snails and plankton. Through the course of time, there have been dozens of other continent-wide extinction events, with at least five in the last 500 million years (just the last chapter or two as a geologist reads).

Shanan Peters ’98, an assistant professor of geology at the University of Wisconsin-Madison, wanted to look at the wider story of mass extinctions, and started doing a body count—remember, you can only read this book from the end back to the beginning, from the surface to the earlier depths. His research has unearthed, if you will, some significant findings, and earned him equally significant recognition as one of geology’s rising stars. In fact, just last year Peters received the Hodson Award of the Palaeontological Association of Great Britian, given annually to a paleontologist under the age of 35 who has made a significant early contribution to the science.

Peters knew, starting with his work as a Denison undergrad with professors like Ken Bork, that there were places around the globe where earlier layers gave the careful reader a peek ahead, or actually back closer to the start of the story. Through his graduate work at the University of Chicago and a post-doc at the University of Michigan, he got a look at parts of the narrative revealed in Montana’s Rocky Mountains, around the globe to India’s eroded strata, and off into desert regions of Egypt.

Along with the glimpses into the deeper past, he was working to find new approaches to “quantifying the rock record”— to read between the lines, in a way, using chemistry in the rocks and details of the smallest fossils embedded there to figure out just what happened when 75 to 95 percent of the plants and animals on the planet seemed to suddenly vanish.

Peters’ work, funded in part by the National Science Foundation, led to a report published in June by the noted science journal Nature, which says not that the butler did it, but that what appears to be sudden and dramatic extinction events may be the result of something as slow and steady and inevitable as the rise and fall of sea levels.

Peering directly at the make-up of the sediments themselves, Peters is able to show whether fossils are embedded either in material made substantially from the breakdown of shells and skeletal material during their cycle of life and death, leaving behind much silica -- or are primarily the aftermath of simple sediments washed down from the land.

A broad inland continent wide sea, like the one once spreading across the central United States, will drain and change as tectonic plates push up and when climactic changes lock up more polar ice, further lowering sea levels. As the seas dry up, the animals and plants, from the largest shark to the smallest shrimp or algae must move, or die, and sometimes both. They disappear quickly from the fossil record, but as a result of processes that take long stretches of time to play out.

Under the title "Environmental determinants of extinction selectivity in the fossil record," the study looks at the chemistry and make-up of the pages of time themselves, and fossils embedded there serving as virtual page numbers. With the help of a global compendia of marine animal fossils complied by Jack Sepkoski of the University of Chicago, those page numbers help compare the chemical traces of ocean levels to "shifts in shallow marine environments that play out over the space of an entire continent," Peters observes. Page by page, through 4,000 samples from over 500 American sites stretching from the Appalachians to the Rockies, Peters' analysis looks back through Darwin's "imperfectly kept" record of geologic time in sedimentary layers, showing how the rise and fall of the earth's oceans can readily account for the mass extinctions as they occur again and again over millenia.

Peters is quick to point out that there are still asteroids and volcanic eruptions in the story, and biological factors of evolution like competition and disease play their part. “This work links them and smaller events in terms of a forcing mechanism, and it also tells us something about who survives and who doesn't across these boundaries,” Peters explains. “These results argue for a substantial fraction of change in extinction rates being controlled by just one environmental parameter.”

With concerns over global climate change and ocean levels high in today’s world, Peters research into the deeper reaches of earth’s history also carry a current relevance. Extinctions today and sea level rise may leave a mark on the earth that will be studied in ages to come – we hope.