Inside of an Old Icehouse Planet, Carbon, Climate Change, and Ocean Anoxia

New research reveals a time of fast global warming and climate change in a glacier Earth similar to today 304 million years ago.

Within 300,000 years, atmospheric CO2 levels quadrupled, seas became anoxic, and biodiversity on land and at sea declined.

Carbon and climate change

(Photo : Mario Tama/Getty Images)

"It 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, as per ScienceDaily.

Although several other 'hyperthermal' or rapid warming events are known in Earth's history, this is the first identified in an icehouse Earth, when the planet had ice caps and glaciers, comparable to the present day.

It shows that an icehouse climate may be more sensitive to changes in atmospheric carbon dioxide than warmer conditions when CO2levels are already higher.

The work is published this week (May 2) in Proceedings of the National Academy of Sciences.

Montanez's group researched the time between 300 million and 260 million years ago when Earth's climate transitioned from a glacial icehouse to a hot, ice-free greenhouse.

They demonstrated in 2007 that the climate swung back and forth multiple times throughout this time span.

Recently, Montanez's' team and others were able to zero in on a change that occurred 304 million years ago, known as the Kasimovian-Gzhelian boundary, or KGB.

To determine atmospheric CO2 at the time, they employed numerous proxies, including isotope ratios and essential minerals from rocks and plant fossils, as well as modeling.

Dead zones of the deep ocean

Marine anoxia, or a decline in dissolved oxygen in the water, is one of the repercussions of global warming.

Melting ice caps bring freshwater onto the ocean surface, obstructing deep water circulation and cutting off oxygen supplies. Marine life dies in the absence of oxygen.

The absence of oxygen is reflected in uranium isotopes absorbed into rocks developing at the ocean's bottom.

The researchers were able to derive a proxy for the quantity of oxygen - or lack thereof - in the water when those rocks were put down by detecting uranium isotopes in carbonate rocks in modern-day China.

They believe that around 23% of the world's seabed has become anoxic dead zones.

Marine anoxia mechanisms

They ran climate model simulations with the fully coupled Community Earth System Model (CESM), version 1.2, and a KGB event paleogeography to investigate the potential processes that may have been responsible for considerable ocean anoxia during the KGB warming, as per PNAS.

Our models indicate that ocean deoxygenation in the Northern Hemisphere was caused, at least in part, by increased thermocline stratification and decreased deep meridional overturning circulation.

Increased surface stratification lowers mixed-layer depths, lowering ocean-interior oxygen concentrations.

Surface stratification caused by warming in the northern high latitudes also induces closure of deep convection, resulting in a 61 percent drop in maximum overturning in the Northern Panthalassic Ocean.

These findings imply that a reorganization of circulatory modes may have had a role in the induction of anoxia, while more research is needed to evaluate the consequences on the world's oceans' redox landscape.

Furthermore, the increased amount of anoxia reported at the KGB per degree of heat, when compared to other C-perturbation occurrences documented in greenhouse settings, can be related to a variety of factors.

First, the logarithmic relationship between temperature change and atmospheric pCO2 indicates that the same rate of C injection may generate a bigger temperature rise under icehouse settings than in greenhouse conditions when the baseline, prewarming pCO2 is low.