A handful of times in Earth history are characterized by greatly elevated rates of extinction – the so-called ‘mass extinctions’. While the era-bounding event at the end of the Cretaceous is now well known to result from the impact of a large extraterrestrial object, the environmental scenarios at other extinctions are less clear. “Two decades ago, people focused on one environmental change at a time. One group, studying temperature, might conclude that a change in temperature is what killed the animals. Another looked at a declining oxygen levels in the ocean, and then say, they suffocated; a lack of oxygen killed them,” says Thonis Family Professor III Zunli Lu. Two recent projects from the Lu lab take a more integrated approach, recognizing that a concatenation of factors is more likely to explain extinctions than a single isolated cause. By studying several aspects of the Earth system together, a fuller picture of environmental conditions and how they might have affected life emerges.
In the first study, published in Nature Geoscience last November, Prof. Lu and colleagues investigate conditions during the first great mass extinction, at the end of the Ordovician, where growing evidence points to cooling associated with glaciation driving the loss of taxa. Lu’s analyses of I/Ca ratios of carbonates (the iProxy) combined with computer simulations show that changes in ocean circulation driven by global cooling resulted in an increase in oxygen saturation in the shallow ocean, reaffirming cold temperatures as the primary driver of extinction in shallow marine organisms. But more surprising was the discovery of a synchronous decrease in oxygen content in the deeper ocean. A growing body of work, including that led by the Lu team, is now challenging conventional views of the glacial ocean: “Global warming causes the oceans to lose oxygen and thus impact marine habitability, potentially destabilizing the entire ecosystem,” Lu says. However, “in recent years, mounting evidence points to several episodes in Earth’s history when oxygen levels also dropped in cooling climates.”
On the other end of the spectrum, Prof. Lu and colleagues from Stanford University are taking the lead on a new $2 million grant from the National Science Foundation’s Frontier Research in Earth Sciences (FRES) program to untangle the root cause of major extinctions related to hot intervals, or hyperthermals, in Earth history. Mass extinctions in the late Devonian, the end-Permian, and the end-Triassic are all linked to times of elevated pCO2 and temperature, the last two (at least) due to massive, large-igneous-province volcanism. How warm temperatures specifically kill animals though is less clear, and the team will be investigating the connections between warming, deoxygenation, and metabolic demand to explain the patterns of extinction across these intervals. Prof. Lu will once again bring his iProxy approach to marine hypoxia to help quantify how its extent changes across the extinctions. Together with Earth system model results, the data will help to constrain marine habitat loss and explain the resulting extinction. This project is just getting underway, and will have clear implications for the projected impacts of rising pCO2 on marine organisms today.