Early Warming System

Fossil reefs record punctuated bursts of sea-level rise during warming period.

An octocoral — a type of soft coral — called a golden sea spray
An octocoral — a type of soft coral — called a golden sea spray

Winter 2018
By Jade Boyd

Scientists from Rice and Texas A&M University–Corpus Christi’s Harte Research Institute for Gulf of Mexico Studies have found clues in long-dead reefs about the way Earth’s oceans may respond to climate change. Fossil evidence from drowned reefs off the Texas coast showed that sea level rose in sharp, punctuated bursts at the close of the last ice age.

“What these fossil reefs show is that the last time Earth warmed like it is today, sea level did not rise steadily,” said André Droxler, a study co-author and Rice marine geologist. “Instead, sea level rose quite fast, paused, and then shot up again in another burst and so on. This has profound implications for the future study of sea-level rise.”

A high-resolution 3-D map of Southern Bank off the South Texas coast clearly reveals terraces, which are a characteristic coral reef response to rising sea level
A high-resolution 3-D map of Southern Bank off the South Texas coast clearly reveals terraces, which are a characteristic coral reef response to rising sea level.

Given that more than half a billion people live within a few meters of modern sea level, punctuated sea-level rise poses a particular risk to those communities that are not prepared for future inundation. “We have observed sea level rise steadily in contemporary time,” said Jeff Nittrouer, a study co-author and Rice coastal geologist. “However, our findings show that sea-level rise could be considerably faster than anything yet observed, and because of this situation, coastal communities need to be prepared for potential inundation.” Each of the punctuated bursts found in the fossil record lasted less than 100 years, and some may have lasted just a few decades.

The study’s evidence came from a 2012 cruise by the Schmidt Ocean Institute’s research vessel Falkor. During the cruise, Droxler, Rice graduate student Pankaj Khanna, and Harte Research Institute colleagues John Tunnell Jr. and Thomas Shirley mapped 10 fossil reef sites with a state-of-the-art sonar called a multibeam echo sounder that created high-resolution 3-D images of the seafloor.

All the dead reefs had terraces. Khanna said the stairlike terraces are typical of coral reefs and are signatures of rising seas. For example, as a reef is growing at the ocean’s surface, it can build up only so fast. If sea level rises too quickly, it will drown the reef in place. If the rate is slightly slower, the reef can adopt a strategy called backstepping. When a reef backsteps, the ocean-facing side of the reef breaks up incoming waves just enough to allow the reef to build up a vertical step.

Khanna said it’s likely that additional fossil evidence of punctuated sea-level rise will be found in the rock record at sites around the globe. “Based on what we’ve found,” he said, “it is possible that sea-level rise over decadal time scales will be a key storyline in future climate predictions.”