Reading between the rocks: Studying the Late Ordovician mass extinction on Anticosti Island (Québec, Canada)

The following post is based on a research project funded by a 2019 GSA Graduate Student Research Grant. Joshua’s research proposal was titled “Reinterpreting the Late Ordovician Mass Extinction from a Sequence Stratigraphic Perspective.” GSA will begin to accept applications for 2021 Graduate Student Research Grants on 1 December 2020.

By Joshua Zimmt, Graduate Student, Finnegan/Marshall Lab, University of California Museum of Paleontology

A storm battered island amid the waters of the Gulf of Saint Lawrence in eastern Quebec would seem to be strange place to find a paleontologist, who one might think would be more at home along a mountain road cut or coastal plain cliff. But beneath the vast expanse of boreal forest and bogs that cover this remote island lies a hidden treasure trove, for those keen enough to search for it. This isolated island, Anticosti Island (from the name given to the island by the Innu tribe, Notiskuan, or “where bears are hunted”), is composed of a succession of fossiliferous rocks deposited in a tropical ocean from 450 to 435 million years ago. You can find fossils nearly anywhere you look on Anticosti if you know what to look for– fossils litter the coastlines, falling out of towering coastal cliffs, they line the walls of river-cut canyons, and even can even be found in the gravel used to cover the island’s few roads!

Anticosti Island possess a truly exceptional fossil record, but it would also be difficult to find a more isolated spot in eastern North America. With the nearest major airport 300 miles away in Québec City and the strong currents around the island, travel options to Anticosti are limited. The number of shipwrecks due to the strong currents around Anticosti has earned the island the nickname of the “Cemetery of the Gulf”. Once on the island, traversing the unpaved logging roads that wind through the boreal forests is only possible with a four-wheel drive equipped truck (a luxury, given that expeditions in the 1980’s required boats to travel around the island). Yet for the past 150 years, paleontologists from around the world have made the (sometimes perilous) journey to Anticosti Island, attracted by the allure of one of the most spectacular Early Paleozoic fossil records in the world. The diversity of fossils on Anticosti is spectacular, with a diverse Paleozoic marine fauna composed of baseball-sized brachiopods, meter-long trilobites, and beautiful coral reefs, all of which have been preserved in exquisite detail. But the island is notable for another important reason- it is home to one of the best-preserved records of the one of the largest mass extinctions in Earth’s history.

Approximately 445 million years ago, the Earth’s greenhouse climate was punctuated by an abrupt cooling event and a major drop in global sea level. Over the next ~1.4 million years, referred to as the Hirnantian stage, nearly 80% of global species diversity would be eliminated in an event known as the Late Ordovician mass extinction Where the fossil record of the extinction is well-preserved, the Late Ordovician mass extinction is expressed as two clusters of last occurrences, the first at the base of the Hirnantian, coinciding with the onset of global cooling and sea level fall, and the second in the upper Hirnantian, coinciding with global warming and sea level rise. These clusters are commonly interpreted at face value as two pulses of rapid extinction. However, the drivers of the Late Ordovician mass extinction remain enigmatic, in part due to the large fluctuations in sea level that coincided with the extinction event.

With a drop in sea level equivalent (or greater) in magnitude to the 400-foot drop in sea level during the last ice age, much of the Hirnantian is represented by a hiatus, or temporal gap, in the stratigraphic record around the world. This is particularly problematic for studying the fossil record, as major changes in sea level can overprint the biological signal of the fossil record. The distribution of fossil occurrences in the fossil record is strongly influenced by the structure of the stratigraphic record. As sea level changes, species will track their preferred environments across the shelf. Sea level-driven lateral shifts in environments and associated facies will vertically juxtapose facies with different fossil faunas. Such juxtapositions can produce clusters of last occurrences in the fossil record that may appear to represent extinction pulses, but in fact represent an environmental shift. The structure of the stratigraphic record can thus distort the true tempo and timing of extinction events in the fossil record, complicating efforts understand their drivers, such as during the Late Ordovician mass extinction.

One way to disentangle the biological and stratigraphic signature of the fossil record is to take a basin-wide perspective of both the fossil and stratigraphic records. However, given the major fall in sea level, most records of the Late Ordovician mass extinction are thin and isolated, and thus are not conducive to a basin wide analysis of the fossil record. In contrast, on Anticosti Island, the Late Ordovician mass extinction interval is preserved in 90 meters of section of highly fossiliferous rock at its thickest extent, with excellent exposure across the length of the 200-km long island. The outcrop belt on Anticosti runs roughly oblique to the paleoshoreline, preserving an east-west depth transect across the island. When you arrive at Port Menier on the western coast of Anticosti, you are surrounded by distal-ramp carbonates, deposited in calm waters far from the shoreline. As you travel east, you gradually move up the slope of the ancient basin, and by the time you reach the eastern coast, you find yourself among pure siliciclastic rocks deposited by an ancient river delta draining off the craton. This ability to study the Late Ordovician mass extinction across the entire basin, coupled with the exceptional fossil found across Anticosti, makes the record on Anticosti unlike anywhere else in the world and perfect for a basin wide assessment of the fossil and rock record.

To produce a comprehensive basin-wide analysis of the fossil record, I can use a sequence stratigraphic framework to align patterns of faunal turnover from individual stratigraphic sections across the island. Implementing a sequence stratigraphic framework is particularly important when studying the fossil record across an entire basin, where rapid lateral changes in facies can frustrate traditional lithostratigraphic correlation schemes. Once the data are within a basin wide sequence stratigraphic framework, I can then begin to investigate the pattern of faunal turnover on a basin wide scale to identify the most likely timing, tempo, and pattern of the Late Ordovician mass extinction on Anticosti Island. Incorporating geochemical proxy data into the analysis will provide an additional dimension to the project, allowing patterns of faunal turnover to be contextualized in terms of both sea level fluctuations and local climate change. Understanding the link between climate and mass extinction in the fossil record is crucial to predicting how global climate change may impact species diversity in the ocean. Through studying the fossil record, we can gain critical insight into how marine organisms and ecosystems respond to climate change, and furthermore explore how communities bounce back from the brink of extinction. Past mass extinctions serve as the only data we have on the relationship between biodiversity crises and global change: Carefully studying these events through the fossil record will provide us with valuable data that we can use to better understand our changing world.

While Anticosti Island provides one of the best opportunities to study the Late Ordovician mass extinction, arranging a field season on the island is a challenge. A single field season on Anticosti costs several thousand dollars, posing a major barrier to conducting research on the island for faculty and students alike. The research grants that I received from the Geological Society of America (the 2019 Marland Pratt Billings and Katharine Fowler-Billings Research Award and a 2019 ExxonMobil/GSA Student Geoscience Grant) have transformed my research, providing me with the funding I need to complete two full field seasons on Anticosti Island. In doing so, the grant has directly impacted the trajectory of my dissertation research, allowing me to pursue a comprehensive reassessment of Late Ordovician mass extinction, working with a fossil record unlike anywhere else in the world. Working in such a remote location like Anticosti has allowed me to grow and develop as both an individual and academic. Before my first field season on Anticosti, I had never been camping or conducted field work. Relying on my own knowledge and abilities in the field was both an important learning process and opportunity to push my own boundaries. Reflecting on the field season afterwards allowed me to assess my preparedness, field methods, and hypotheses, to better plan for the next trip to Anticosti. Next summer, I hope to return to Anticosti, fully equipped with a better understanding of myself and the island, to dive back into the Late Ordovician mass extinction!

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To learn more about this project and others like it, join the author, and thousands of other geoscientists, at GSA 2020 Connects Online. The author is co-chairing session T63. The Ordovician Earth: New Insights to Environmental and Biotic Responses in the Fossil and Rock Record. The abstract system is open and you can submit your research to this session or others like it.