Post-Ice Age Sea Level Rise Happened In Bursts

The coral on these reefs has long since drowned as the waters became too deep, but their terraced shapes reveal how water levels rose after the Ice Age. P. Khanna/Rice University

When the world's oceans last rose, 15,000 years ago as the last Ice Age ended, sea levels didn't increase smoothly. Instead the rise occurred in bursts, fossil corals from the Gulf Of Mexico reveal. The causes of that period of warming were different from those today, but the work may still help scientists predict the form of future changes to the oceans.

Efforts to reconstruct ancient sea levels have been hindered by a lack of fine-grained data, leaving palaeogeologists to present predictions in linear graphs. This has influenced future estimates. Professor Andre Droxler of Rice University said in a statement that Intergovernmental Panel on Climate Change reports, for example, “had to simply take the projected rise for a century, divide by 100 and say, 'We expect sea level to rise this much per year.'”

Droxler is part of a team that mapped 10 fossil reefs, discovered off Texas in the 1930s, using multi-beam echo sounders. These reefs once hosted tropical corals, the kind that need plenty of sunlight to grow, rather than deep-sea black corals. They now lie in roughly 60 meters (195 feet) of water, 50-80 kilometers (30-50 miles) off Corpus Christi, too deep for such corals to thrive, and much deeper than the reefs threatened by Hurricane Harvey.

Droxler calculated that the water depth would have allowed the coral to begin forming between 19,000 and 11,000 years ago, as the world exited the last Ice Age and the planet's glaciers began to melt. The reefs are terraced, showing how the corals reached up towards the light as water rose above them, until eventually, the surface rose too far or too fast for them to keep up.

The shapes of the reefs have been preserved in magnificent detail, and Droxler and colleagues report in Nature Communications what they reveal about the way the waters rose.

According to study author Pankaj Khanna, when sea levels rise slowly, reefs keep up using what is called backstepping, producing a shape that looks like a set of stairs. "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 reefs are widely spaced, with terraces at consistent depths, indicating their stop-start growth was not caused by local factors. Some reefs had six terraces, showing the sea levels of the era rose in at least that many bursts over a period of a century or less.

The positive side of this story is that ecosystems have coped with more rapid increases in sea levels than we thought, but the implications for coastal communities are frightening, as inundations could happen terrifyingly fast.

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