Carbon, climate change and ocean anoxia in an ancient world of icehouses

A new study describes a period of rapid global climate change in an ice-covered world much like the present, but 304 million years ago. In about 300,000 years, atmospheric carbon dioxide levels have doubled, the oceans have become anoxic, and biodiversity has plummeted on land and at sea.

“This was one of the fastest warming events in Earth’s history,” said Isabel Montañez, Distinguished Professor in the Department of Earth and Planetary Sciences at the University of California, Davis.

Although several other “hyperthermal” or rapid warming events are known in Earth’s history, this is the first identified in an Earth icehouse, when the planet had ice caps and glaciers, comparable to the present day. It shows that a cooler climate may be more sensitive to changes in atmospheric carbon dioxide than warmer conditions, when CO2the levels are already higher. The book is published this week (May 2) in Proceedings of the National Academy of Sciences.

Montañez’s lab studied the period from 300 million to 260 million years ago, when Earth’s climate changed from a glacial cooler to a hot, ice-free greenhouse. In 2007, they showed that the climate oscillated several times during this period.

More recently, Montañez’s team and others were able to zero in on a transition 304 million years ago, the Kasimovian-Gzhelian or KGB boundary. They used several proxies, including carbon isotopes and trace elements from rocks and plant fossils, and modeling to estimate atmospheric CO2 at the time.

The researchers estimate that around 9,000 gigatons of carbon were released into the atmosphere just before the KG boundary.

“We don’t have a rate, but it was one of the fastest in Earth history,” Montañez said. This doubled the atmospheric CO2from about 350 parts per million, comparable to modern pre-industrial levels, to about 700 ppm.

Deep ocean dead zones

One of the consequences of global warming is marine anoxia, or a drop in dissolved oxygen in the ocean. Melting ice caps release fresh water to the surface of the ocean, creating a barrier to deep water circulation and cutting off the supply of oxygen. Without oxygen, marine life dies.

The lack of oxygen leaves its mark in the uranium isotopes embedded in the rocks that form at the bottom of the ocean. By measuring uranium isotopes in carbonate rocks in present-day China, researchers have been able to get an approximation of how much oxygen — or lack thereof — was in the ocean when those rocks were deposited.

About 23% of the world’s seabed has become anoxic dead zones, they estimate. This matches other studies showing large biodiversity losses on land and at sea at the same time.

The effect of carbon release on ocean anoxia was significantly greater than that observed in other studies of rapid warming under “greenhouse” conditions. This may be because the baseline level of atmospheric CO2 was already much higher.

“If you raised CO2 of the same amount in a greenhouse world, there isn’t much of an effect, but coolers seem to be much more sensitive to change and marine anoxia,” Montañez said.

The massive release of carbon may have been triggered by volcanic eruptions that tore through carboniferous coal beds, Montañez said. The eruptions would also have started fires and warming may have melted the permafrost, leading to the release of more organic carbon.

Montañez is corresponding co-author of the paper with Jitao Chen, a former postdoctoral researcher at UC Davis and now at the Nanjing Institute of Geology and Paleontology, China and Xiang-dong Wang, Nanjing University, China. Additional co-authors are: Shuang Zhang, Texas A&M University; Terry Isson, Sofia Rauzi and Kierstin Daviau, University of Waikato, New Zealand; Le Yao, Yu-ping Qi and Yue Wang, Nanjing Institute of Geology and Paleontology; Sophia Macarewich and Christopher Poulsen, University of Michigan, Ann Arbor; Noah Planavsky, Yale University; Feifei Zhang, Jun-xuan Fan and Shu-zhong Shen, Nanjing University; and Ariel Anbar, Arizona State University.

The work was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences, and the National Science Foundation of the United States.

Teresa H. Sadler