EVIDENCE OF VOLCANICALLY INDUCED TURNOVER IN MAMMALIAN COMMUNITIES OF THE JOHN DAY BASIN, OREGON

The John Day Formation in eastern Oregon preserves a unique and remarkably complete record of volcanic and volcaniclastic sedimentary rocks spanning about 20 million years. The 400 m of the Turtle Cove Member (30.8-25.9 Ma) preserve evidence of diverse mammalian communities perturbed by at least ten volcanic events, including five dated tuffs and the large Picture Gorge Ignimbrite (PGI). The Turtle Cove Member provides a unique opportunity to investigate the long-term impacts of repeated volcanic perturbations on local mammalian communities.

Through the GeoCorps America program, a partnership between the Geological Society of America and the National Park Service, we conducted research related to rates of extirpation and appearance of species through the Turtle Cove Member. If the rates of extirpation or appearance in the geologic subunits associated with a volcanic event are higher than local background rates, then that event caused a major, long-lasting impact on the regional ecosystem.

We examined and independently identified 1,879 fossil mammal specimens representing 96 species from the classic John Day Basin, not relying on previously published faunal lists. Specimens were studied from classic sites within the vicinity of the Sheep Rock Unit of John Day Fossil Beds National Monument, including material from all stratigraphic units in the Turtle Cove Member (A-K2). For each unit, we calculated rates of extirpation and appearance, using a method based on the Three Timer extinction and origination rates. Results indicated a high rate of extirpation (mu = 0.45) and appearance (lambda = 0.54) for unit F, the unit associated with the PGI. These rates were substantially higher than any of the other units in the Turtle Cove Member. Our results suggest that the PGI, a potential super-volcanic eruption, had a great impact on the local ecosystem, while other volcanic events had little to no lasting impact.

Understanding the long-term impacts of volcanism on these ancient mammalian communities is important for understanding the resilience of modern mammalian communities. Ancient mammalian communities appear to be resilient to most volcanic eruptions, thus we would expect modern mammalian communities to respond similarly. However, we do not expect mammalian communities to be resilient to large magnitude eruptions.