Dino-Era Earth Had Polar Ice, Low Sea Level, Study Says

Mark Schrope
for National Geographic News

November 29, 2005

A new analysis of deep sediment cores taken from the New Jersey coast
has revealed unexpected ice in the otherwise warm dinosaur-ruled world
50 to 100 million years ago.

Data from the cores also show dramatically lower sea levels for the period than previously thought. Finally, the study suggests that the rate of sea-level rise has doubled during the last 150 years.

The new report, published in the November 25 issue of the journal Science, spans the past 540 million years and is based largely on studies of 11 core samples each 1,600 feet (500 meters) long.

The greatest difficulty in establishing long-term sea level changes has been the complexity of removing outside influences from the calculations, according to lead author Ken Miller.

Miller, a geologist at Rutgers University in Piscataway, New Jersey, says events such as the rise and fall of continental landmasses can affect the data.

"There's no place that's completely stable," Miller said. But New Jersey's instabilities during the past billion years have been fairly predictable, so they can be filtered out reliably using ever-improving methods.

This makes cores from the area one of the best available records of long-term sea level change, he said.

Questioning Authority

The researchers dated sections of the cores using common radiological methods and other techniques. They then analyzed the samples in a variety of ways to determine sea level at a specific period.

For example, the type of material, such as sand or finer marsh sediment, found at a particular depth was used to determine the sample location's proximity to the ocean.

If the location was submerged at a particular time, sea-life fossils could indicate how deep the water was, because certain animals are found only at specific depths.

One of the study's most striking finds was that ice existed during a period of time geologists had been convinced was ice-free.

About 100 to 50 million years ago, a time referred to as the Greenhouse world, the atmospheric concentration of carbon dioxide was high enough to make the air and oceans significantly warmer than today. Scientists long believed polar ice caps were nonexistent during this period.

But Miller's study revealed that during the Greenhouse world there were, by geological standards, rapid rises and falls in sea level. The changes were as much as 80 feet (25 meters) in less than a million years.

Miller believes these fluctuations can only be explained by significant quantities of ice in Antarctica—about a third of what the planet holds today. Freezing and melting ice would rapidly change sea level at various times.

"[That theory] is somewhat heretical from a geologist's point of view," Miller said. With melting and freezing cycles, though, the planet would still have been ice-free for about 80 percent of the time.

"I don't disagree that there may have been ice around during that [presumed] ice-free world," says Michael Kearney, a geographer at the University of Maryland in College Park who studies sea level change.

"How much ice is the question, and where," he said.

But Kearney says direct evidence for the ice in Antarctica might never be found, because all signs of the ice would likely have been destroyed.

Double Trouble

Another major finding from the New Jersey study is evidence that sea level during the Cretaceous period, from 100 to 65 million years ago, was much lower than previously thought.

Miller and his team suggest sea level reached a peak of 330 feet (100 meters) during the Cretaceous. Previously agreed upon estimates had set the figure in the 820-foot (250-meter) range.

"That was one piece of information I had to convince myself was even true," Miller said. "I believed 250 [meters] like it was the King James Bible."

Scientists believe sea level during this period was controlled largely by tectonic activity under the oceans. Shifting tectonic plates would create mountains that displace the oceans and heat that expands the water, both of which would force water farther inland on the continents.

If sea level was as low as the new study suggests, then tectonic changes such as seafloor spreading must have been proceeding at a much slower rate than thought, Miller said.

Miller's research also sheds new light on present-day sea-level rise. Data on the ebb and flow of tides and other records have reliably set sea-level rise during the past 150 years at about 0.08 inches (2 millimeters) per year.

Comparing this rate to preindustrial sea-level rise would give scientists some indication as to whether human contributions to climate change are affecting sea level.

From their core samples, Miller and his team were able to tease out a 0.04-inch-per-year (1-millimeter-per-year) average rate of sea-level rise for the last 5,000 years up to about 200 years ago.

Other teams, such as those analyzing coral records in the Caribbean, have found similar rates for this time period. The finding suggests sea-level rise has doubled since the onset of industrialization.

Miller says it would be difficult to explain this increase by any means other than human activities leading to greenhouse warming.

Kearney says a doubling of sea-level rise is a plausible conclusion of the New Jersey study.

"I think it's something to worry about," he said. "It doesn't take a big rise [to affect coastal land]. If you double it, you're going to have problems."

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