Fossil reefs show sea level rose in bursts

by Mary Caperton Morton
Friday, January 12, 2018

Off the coast of Texas, a collection of fossil coral reefs sits under 60 meters of water — relics from 20,000 years ago, when the sea surface was much lower than today. In a new study, researchers created high-resolution maps of the reefs that suggest they drowned as sea levels rose in rapid bursts — each lasting decades to centuries — instead of at a steady rate, as has long been assumed.

Using shipboard multibeam sonar, a team led by Pankaj Khanna of Rice University in Houston mapped 10 fossil reef sites between 50 and 80 kilometers offshore of Corpus Christie, Texas, producing high-resolution three-dimensional images of the seafloor. “The coral reefs' evolution and demise have been preserved,” Khanna said in a statement. “Their history is written in their morphology — the shapes and forms in which they grew.”

All 10 reefs mapped by the study were terraced, each with as many as six stairstep-like features that are known to form as sea levels rise. “In our case, each of these steps reveals how the reef adapted to a sudden, punctuated burst of sea-level rise,” Khanna said. “The terraces behind each step are the parts of the reef that grew and filled in during the pauses between bursts.”

The study, published in Nature Communications, offers some of the first evidence of sea-level rise occurring over decadal timescales rather than over centuries or longer. “It is possible that sea-level rise over decadal timescales will be a key storyline in future climate predictions,” Khanna said. Many projections of future sea-level rise, such as those from the International Panel on Climate Change, divide the expected rise over the next century by 100 to derive a projected steady rate of rise per year.

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