Geotimes - May 2004 - Ice in the greenhouse?

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Paleoclimate
Ice in the greenhouse?

New sediment cores from the New Jersey coastal plain suggest that large and rapid sea-level changes occurred during the Late Cretaceous. This evidence is prompting researchers to propose that the greenhouse world of that epoch, long thought to be ice-free, may have been chillier than previously predicted.

Crocodile in Greenland: In 1998, researchers found fossils of cold-blooded, crocodile-like reptiles in the Arctic, supporting the idea that Late Cretaceous climate was one of the warmest greenhouse periods in Earth’s history. New evidence is suggesting large polar ice sheets may have existed at the same time, creating a paradox represented in this artist’s interpretation. Courtesy of Jim Wright.

Kenneth Miller of Rutgers University and colleagues report in the March GSA Bulletin that sea level changed several times between 90 and 65 million years ago, by as much as tens of meters in less than a million years. “We have provided the best-dated Late Cretaceous sea-level record,” Miller says, “and conclude that large, rapid sea-level changes occurred.”

The researchers write that only the waxing and waning of large continental ice sheets could explain such drastic change, and therefore the findings require either that ice sheets existed in the hothouse Cretaceous “or our understanding of causal mechanisms for global sea level is fundamentally flawed.” Thus the finding adds yet another element to the paradox of Late Cretaceous climate.

“Ninety-nine percent of all geologists have assumed that the Cretaceous was an ice-free greenhouse world,” Miller says. However, in 1996, some researchers suggested that the Late Cretaceous could actually have been glacial, but the evidence for it was spotty and equivocal.

Meanwhile, support for an ice-free Cretaceous remained strong: isotopic evidence of warm surface- and deep-sea temperatures at high latitudes, and fossil evidence of large crocodile-like, cold-blooded reptiles in the Arctic, dinosaurs in Antarctica and forests at latitudes far higher than present-day tree lines. These and other studies suggest that Earth’s average global surface temperature was at least a balmy 10 degrees Celsius warmer than today.

Miller’s team cored sediments from the New Jersey coastal plain during two terrestrial legs of the Ocean Drilling Program. By looking at compaction, subsidence and seismic activity, as well as sediment loading and distribution patterns, the team says it was able to rule out the effects of tectonics and sediment supply, which can affect interpretation of sea- level change. Additionally, because a smattering of other datasets for the time period show drops in sea level, the team says the New Jersey cores reflect global sea level at the time.

“This is a real good step towards trying to come up with a true global sea-level curve for the Cretaceous,” says Brian Huber, a paleobiologist at the Smithsonian National Museum of Natural History who studies Cretaceous climate.

The Rutgers team compared the sea-level changes found in the cores with oxygen isotope values recorded in the shells of microscopic marine organisms at the same time. The isotopic signature of oxygen in seawater is affected by ice volume, salinity and temperature and gives scientists an indicator of past climate, including how much of Earth’s water was tied up in ice.

However, because many factors affect oxygen isotope values, and because a complete oxygen isotope record is lacking for most of the Cretaceous, uncertainty still remains about the true implications of the oxygen isotope values. “It’s going to be a real challenge to sort out what is the temperature record versus possible effects of ice volume variation — of course, this has been the challenge for a long time,” Huber says.

Miller agrees, saying that it is still uncertain that the isotopic records require ice. However, given what is now known about sea level, both researchers say that large rapid changes would require ice. So the question remains of how ice sheets and cold-blooded creatures could coexist in polar regions. Huber also notes that the study is from only one region of the world: “We need to demonstrate the timing and synchroneity of these events at other sites elsewhere in the world.”

But, Huber says, the key to solving the paradox will lie in deciphering the climate of the Turonian age in the Late Cretaceous, a time of extreme global warmth around 90 million years ago, for which a well-sampled oxygen isotope record exists. If researchers can show a large global sea-level change during an extremely warm interval such as in the Turonian, then “we’ve got a real problem because it’s very hard to envision ice as a causal mechanism, yet there’s no other mechanism we know,” Huber says. “So what else is it?”

Sara Pratt
Geotimes contributing writer

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