Sea Level More Variable Than Previously Thought

"People tend to associate substantial sea level changes like this with long-term changes in the Earth's climate, like the onset of an ice age," says Thompson, lead author on the paper and currently a post-doctoral researcher at Woods Hole Oceanographic Institute. "But we've shown this isn't necessarily true." Thompson carried out the research as part of his doctoral research at Lamont.

It is already widely accepted that sea level varies by as much as 120 meters between highs and lows of the 100,000 year-long glacial cycle, and sea-level cycles of several tens of meters occur about every 21,000 years, driven mainly by changes in the Earth's orbit. However, this study also uncovered evidence of large changes in sea level during the relatively warm, stable climate of a so-called inter-glacial period, such as the one we are living in now.

The evidence comes from the fossil record of corals in Barbados and Papua New Guinea. Scientists use the remains of coral reefs to determine sea level in the distant past because many species grow only at specific depths in the ocean. As coral grows, the living part, known as the polyp, extracts minerals from seawater to construct the hard shell that make up the bulk of a reef. Among the substance that coral takes in is a small amount of uranium, including the isotopes 234U and 238U.

These isotopes of uranium occur in seawater in a constant ratio. They are also radioactive and decay in a regular series of steps to produce, among other things, an isotope of the element thorium (230Th), which is not found in the coral when it first forms. By measuring the ratio between 230Th and 238U in a fossil coral and comparing it to the amount of 234U and 238U that remains, scientists are able to determine precisely when the coral formed. Moreover, by dating corals known to live within a few feet of the ocean surface they are able to give a remarkably accurate picture of sea level hundreds of thousands of years ago.

"Corals are as close to a direct measurement of sea level in the Earth's past as you can get," said Goldstein, an associate professor of geochemistry at Lamont.

But uranium-thorium dating has been hampered in the past by the assumption that the chemical composition of coral changes over time in a so-called "closed system" in which there are no other changes to isotopic ratios other than that caused by radioactive decay. However, that has not always proved to be the case, and researchers have been forced to disregard their data whenever calculations show that corals would had to have formed with uranium isotope levels not found in seawater. Often, this has resulted in 90 percent of coral age measurements being disregarded.

"For a long time people have known there are isotopic anomalies in corals that make no sense in a closed system," said Thompson, lead author on the paper and currently a post-doctoral researcher at Woods Hole Oceanographic Institute. "Trying to reconstruct the past using the old method is like looking at a distant object through a pair of dirty glasses." Thompson carried out the research as part of his doctoral research at Lamont.

The new method of dating fossil coral developed by Thompson and Goldstein, however, works under the principle that radioactive decay in coral occurs in an "open system" that is subject to additional changes in isotopic composition. The open-system model permits a much more detailed picture of past sea level fluctuations because it enables scientists to obtain information from coral samples that were previously rejected. By greatly increasing the number of samples that can be used to evaluate sea level, the open-system model is able to detect changes occurring over a much shorter time scale—two or three thousand years instead of 10,000 years.

In 2002, Thompson and Goldstein went to the Caribbean island of Barbados to collect fossils of the coral Acropora palmata, or elkhorn coral, from reefs 70,000 to 400,000 years old. Elkhorn coral is known to live within one or two meters of the ocean surface and when they applied their open system dating method to the samples, they discovered that sea level had undergone several relatively large fluctuations of as much as 35 meters during the two previous interglacial periods, known as Marine Isotope Stage (MIS) 5 and 7, as well as the glacial period MIS6. They also found that somewhat smaller fluctuations of between six and 30 meters occurring in just a few thousand years are surprisingly common in Earth's past.

"This demonstrates that past changes in sea level are much more frequent than had previously been thought to occur," said Gideon Henderson, a geochemist at Oxford University who authored a Perspectives article about the study that appears in the same issue of Science. "Rather than happening on a ten thousand year timescale, we now know that sea level oscillations can occur more frequently — on the thousand year timescale or shorter. This is telling us that the continental ice sheets, which drive sea level change, must respond to internal oscillations within the Earth's climate system, as well as to changes imposed by slow changes in the Earth's orbit."

Moreover, the results place renewed emphasis on concern that human-induced changes to the Earth's climate could result in changes to sea level. "A thirty meter sea level change corresponds to one quarter of the ice that melted when the last ice age ended," said Goldstein. "Now that we know rapid sea level changes are a common feature of the geologic record, we have to be even more concerned about the extent that human activities could also contribute to this affect."

The project was funded by grants from the Lamont Climate Center, the Comer Science and Educational Foundation, the Goodfriend Prize, and start-up funds from the Lamont-Doherty Earth Observatory and Columbia University. The National Science Foundation also supports the Lamont-Doherty Earth Observatory Deep-Sea Sample Repository, which curated some of the corals used in this study.

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