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Fossil Coral Dating Reveals Critical Clues About Sea-level Change

Fossil corals on Great Inagua Island, Bahamas were planed off by wave erosion when the sea level fell. Samples from these and many other corals were accurately dated by WHOI geochronologist Bill Thompson for this study. The hammer and pen are for scale and indicate the presence of two different coral species. (Photo by H. A. Curran, Smith College.) Click to enlarge.

with contributions from / Published September 16, 2011

New evidence of sea-level oscillations during a warm period that started about 125,000 years ago raises the possibility of a similar scenario if the planet continues its more recent warming trend, says a research team led by the Woods Hole Oceanographic Institution (WHOI).

In a paper published online in the September 11 edition of the prestigious international journal Nature Geoscience, the researchers—including H. Allen Curran, William R. Kenan, Jr., Professor Emeritus of Geology; and Brian White, professor emeritus of geology, of Smith College—reported data from an improved method of dating fossil coral reef skeletons in the Bahamas. By calculating more accurate ages for the coral samples than previously possible, they found that about 120,000 years ago sea levels were considerably less stable than earlier believed—oscillating up and down by 4 to 6 meters (13–20 feet) over a few thousand years.

The team consisted of WHOI geochronologist William G. Thompson, lead author of the study; Curran and White of Smith; and Mark A. Wilson of The College of Wooster. The latter three are experts on fossil coral reef sites and stratigraphy in the Bahamas. “What we discovered on the western coast of Great Inagua Island was a fossil coral reef that was eroded to a flat surface when sea level dropped,” says Curran. “Then when sea level began to rise, a new reef grew on the planed surface of the older one.” The age difference between the two reefs is about 4,000 years, a short period for such a substantial oscillation of sea level.

Curran noted that background work leading to the present study began in the 1980s with detailed mapping of the Cockburn Town fossil coral reef on San Salvador Island. Smith geology students worked on the initial mapping and have contributed to previous studies of both fossil and modern reefs in the Bahamas. Most recently, Sarah Hale ’07 and Rachel Herrmann ’07 assisted with fieldwork on San Salvador for the present study.

Rachel Herrmann (top) and Sarah Hale, both geology majors in the Smith College class of 2007, mark sample sites on the fossil coral reefs on San Salvador Island. Hale and Herrmann did fieldwork here in January 2006 as part of the Smith Interterm Bahamas geology course held at the Gerace Research Centre. (Photos by H. A. Curran, Smith College.) Click photos to enlarge.

“If today’s ice sheets continue to melt, we may be headed for a period of ice sheet and sea-level change that is more dynamic than current observations of ice sheets suggest,” says Thompson. The polar ice caps currently are shrinking and sea level is rising at a rate of about 30 centimeters (1 foot) per century. “How much sea level will rise over the next century or two is a crucial question for the significant part of the world’s population that lives in coastal zones.”

A better understanding of sea-level change in the past can help to inform predictions for the future. Historical records, such as those from tide gauges, extend back only a century or so. “The geological record offers a longer perspective on rates of change,” says Thompson, “and sea-level changes during previous warm intervals are especially relevant to today’s situation.” Sea levels during the last interglacial are known to have been about 6 meters (20 feet) higher, on average, than they are today. “The real surprise is that sea levels were oscillating during this period.”

To get more accurate age estimates from the geological record, Thompson developed an advanced way of interpreting the uranium and thorium isotope ratios that have been traditionally used as a coral dating method. “The geologic evidence for sea-level change at our sites was convincing,” says Curran, “but we couldn’t absolutely prove sea-level oscillation without more precise dating.”

Although no two intervals of geologic time are identical, sea-level history of the last interglacial has been thought to be generally similar to that of our present interglacial. The significance of this study, according to Curran, is that “we have shown that sea level can fluctuate significantly over relatively short interglacial time spans. If changing climate leads to smaller ice sheets in the future, this may result in sea-level instability similar to that documented in our study.”

The work was supported by the WHOI Ocean and Climate Change Institute, the Comer Science and Education Foundation, and the National Science Foundation. Previous work for this study was supported by the Keck Geology Consortium, of which Smith College was a founding member, and by grants to Smith managed by Curran and White.

To read the published study in Nature Geoscience visit their Web site.