Big extinctions don’t just wipe out a lot of species. They also send ecological cycles reeling for millions of years, a new study suggests.
Following massive die-offs, the natural processes that keep carbon flowing through marine ecosystems — from tiny photosynthesizers to big fish and between the bottom and the top of the ocean — get broken. For millions of years after at least two global disasters, marine communities were too unstable to keep the molecules churning, researchers report in the February issue of Geology. These “chaotic carbon episodes” could have big ramifications for extinctions in the modern era, say scientists from Brown University in Providence, R.I., and the University of Washington in Seattle.
In healthy marine ecosystems, diverse swimming predators, lazy filter feeders and myriad other organisms keep the carbon flowing nonstop, says study coauthor Jessica Whiteside, a paleobiologist at Brown. But after a global extinction, when only a few plants, animals or single-celled critters occupy each rung of the food chain, minicatastrophes like diseases or climatic shifts take big tolls, she says. “If you already have a weakened state of ecosystems, these things that would normally be minor variations now become wildly oscillating,” she says.
The team looked particularly at the diversity of ancient octopus-like animals called ammonites, which were ultimately wiped out by the same extinction that killed the dinosaurs. Ammonites often swam to catch prey as well as floated idly, snagging debris from ocean currents. But for millions of years after the end-Permian mass extinction 250 million years ago, as well as after the end-Triassic mass extinction 50 million years later, few swimming ammonites survived.
The devastation of ammonites and other life forms left its mark on the geologic record, Whiteside says. Scientists recognize five mass extinctions in Earth’s history. The worst — the end-Permian extinction — killed about 90 percent of ocean-dwelling species. Scientists discovered that after most of these calamities carbon molecules in the environment, on average, got lighter. The culprits behind this drop are major volcanic events like those associated with the end-Permian mass extinction. Volcanoes spew a relatively high proportion of lighter isotopes of carbon — atoms with fewer neutrons in their nuclei — into the atmosphere, says Jean Guex, a paleobiologist with Lausanne University in Switzerland.
“They are not volcanoes like people imagine,” says Guex, who was not involved in either study. “They are fissures which are extruding millions of cubic kilometers of basalts, inducing major atmospheric pollutions with sulfur and heavy metals.”
Tiny photosynthesizers gobble down the lighter carbon in the air, introducing it to the environment and, eventually, the geologic record. In a normally functioning ecosystem, that would cause the proportion of light carbon isotopes in the record to lighten and then eventually return to normal. But when Whiteside’s team eyed carbon isotopes in sedimentary layers from the periods following the end-Permian extinction and the end-Triassic extinction, carbon weights jumped back and forth from heavier to lighter isotopes. She argues that ecosystem instabilities could easily have triggered these wild fluxes. As living organisms died, they floated to the ocean bottom, carrying the lighter carbon molecules in their bodies with them. That left the carbon in the surface waters, on average, heavier, she says. When communities rebounded, carbon weights in the upper oceans dropped back down.
But Guex says that the real chaotic factor in this picture was the environment, not the carbon flows. Undersea volcanoes erupted on and off for millions of years following each extinction event, he argues. “The carbon isotopes mainly reflect these crises.”
Whiteside agrees that such churning could have played a role but cautions against ignoring the ecological factors. Minor volcanic eruptions would have affected weak communities much more than strong ones, she says.
The team’s conclusions are particularly relevant for the modern era — what many biologists have dubbed the sixth mass extinction, Whiteside notes. Computer simulations of climate change look 100 years into the future, but the impacts of species extinctions today may last long after human pollution has come and gone, she says. “It’s a cautionary tale for the species loss that’s going on right now.”
