Research Highlight: Growing Glaciers in a 'Supergreenhouse'

Researchers at Scripps Institution of Oceanography at UC San Diego are digging deep beneath the ocean floor to study a global warming event that took place on Earth nearly 91 million years ago. This extreme warming event in Earth’s history raised tropical ocean temperatures about 10°C (18° F) higher than exist today, thus creating an intense “supergreenhouse” climate. Despite these extremely warm conditions, new research suggests that large glacial ice sheets existed and were able to grow.

Scripps researchers were surprised by the physical evidence they uncovered that indicates an ice sheet about 50 to 60 percent of the size of the modern Antarctic ice cap existed for about 200,000 years during the Cretaceous Period, 91 million years ago.

“The common assumption that substantial ice could not have existed during past super-warm climates is apparently wrong,” said Richard Norris, a Scripps professor of paleobiology, who counted himself among the doubters before this study.

The presence or absence of sea ice during past global warming events has major environmental implications today, specifically in terms of sea level rise and global circulation patterns. As humans continue to add large amounts of carbon dioxide and other greenhouse gases that accelerate the heating of the atmosphere and oceans, research on Earth’s past climate conditions is critical to predict what will happen as the planet’s climate continues to warm.

To estimate the growth and eventual melting of large ice sheets during the event known as the Cretaceous Thermal Maximum, Scripps researchers, Andre Bornemann, Oliver Friedrich and Richard Norris, extracted and analyzed geochemical data from exceptionally well-preserved fossils of ancient marine organisms.  The microfossils, about the size of a grain of salt, are known as foraminifera and were extracted from a mud core drilled 520 meters (1,706 feet) below the seafloor in the western equatorial Atlantic.

By comparing stable isotopes of oxygen (d¹⁸O) in bottom-dwelling and near-surface foraminifera, researchers can analyze the past chemical composition of the ocean to infer about the growth and eventual melting of ice sheets. Forams soak up the available oxygen in the seawater during their life and, as a result, a record is preserved within their shells. As changes in ocean temperature and evaporation occurred, a shift in isotopic composition consistent with the formation of sea ice was detected.

The Cretaceous Thermal Maximum was a major turning point in the history of Earth's climate with average ocean temperatures reaching a hot tub-like 35-37°C (95-98.6°F). At the same time, tropical breadfruit trees flourished in Greenland and alligators thrived in the Canadian Arctic.

Researchers are still unsure where such a large mass of ice could have existed in the Cretaceous or how ice growth could have started.  The authors suggest that climate cycles may have favored ice growth during a few times in the Cretaceous when natural climate variations produced unusually cool summers. Likewise, high mountains under the modern Antarctic ice cap could have been potential sites for growth of large ice masses during the Cretaceous.

The Scripps team’s conclusions, recently published in the journal Science, are consistent with independent studies from Russian and other American research teams that show sea level fell by about 25 to 40 meters (82 to 131 feet) at the same time that the ice sheets were growing during the Cretaceous Period.  Sea level is known to fall as water is removed from the oceans to build continental ice sheets; conversely, sea level rises as ice melts and returns to the sea.

Norris said that this seemingly paradoxical finding reveals the need for continued strengthening of climate models to help scientists understand Earth’s hugely complicated climate system.

-- Annie Reisewitz