Dr. Alessio Rovere and PhD student Thomas Lorscheid standing by the boulder called “bull”. | Photo: Elisa Casella

19.10.17 | Over 100,000 years ago, boulders weighing many tons were washed onto a cliff in the Bahamas. It has been long debated whether the rocks were actually transported by ancient “super storms”. Dr. Alessio Rovere, along with scientific colleagues from MARUM – Center for Marine Environmental Sciences of the University of Bremen and from the Leibniz Centre for Tropical Marine Research (ZMT), have now discovered that storms with intensities that are not uncommon today combined with a few meters increase in sea level would be sufficient to achieve the feat. The journal Proceedings of the National Academy of Sciences published their results on October 30, 2017.

Coastal regions worldwide are threatened by climate change and sea-level rise. Warmer climate conditions can increase the strength of storms and cause floods to occur more often – a growing hazard for the populations, the infrastructures and industries near the coasts. It is therefore important to improve our predictions regarding the magnitude that extreme storms will increase in intensity and frequency in the future. For this, climate researchers rely on weather observations and model calculations, but also look beyond these into the past.

Super storms in the past

The last warm period, between around 128,000 and 116,000 years ago, is often used as a measure for comparison. This was the last period in Earth’s history during which the climate was warmer than in the pre-industrial era. Sea level was higher than it is today. It is thought that the North Atlantic was characterized at that time by “super storms”, which were stronger than any storms that have occurred since instruments have been used to record them. On the east coasts of Bermuda and the Bahamas, various deposits that include giant boulders testify to erstwhile storm waves that engulfed the land. Dr. Alessio Rovere, leader of the Junior Research Group “Sea Level and Coastal Changes” at MARUM – Center for Marine Environmental Sciences of the University of Bremen and the Leibniz Centre for Tropical Marine Research (ZMT), has studied this in some detail together with his colleagues. “We wanted to know how much energy is required for a storm wave to move such huge boulders from the cliff edge to their present positions, and whether modern storms possess the energy to accomplish this feat.”

Storm waves transport boulders

On the Bahamian island of Eleuthera seven massive boulders are positioned atop a 15-meter-high cliff. According to a number of studies they were transported there by super storms. They are so gigantic that it is almost inconceivable that they could have been deposited there by waves. Two of the larger boulders are known in the region by the names “cow” and “bull”. Rovere and his team have measured these two boulders, estimated their volumes and densities, and calculated a weight of 383 tons for the “cow” and 925 tons for the “bull”. The researchers fed the data collected on location into their computer model and adjusted for sea level at the time of deposition, which was up to 15 meters higher than today. Using their wave model they simulated waves that were generated by three modern-era storms that hit Eleuthera especially hard: the “Perfect Storm” of 1991, Hurricane Andrew a year later, and Hurricane Sandy in 2012. The team of researchers carried out a scenario in the model where these storm waves strike the cliffs of Eleuthera, in order to calculate the wave height and length, as well as current speed, required to move the rocks.

Lessons for the future from waves in the distant past

“Our results indicate that a super storm was not necessary to explain the present positions of the two boulders,” says Rovere. “Even if we assume a sea level only six meters higher than that of today, waves such as those produced by Hurricane Sandy would have been sufficient to transport the boulders “cow” and “bull” to their present positions.” So for future storms it can be concluded that, even if the intensity of storms does not become greater, the rise of sea level alone would be enough to increase the frequency of strong waves during a storm as well as the current speed, which increases the wave energy.

Original publication:

Giant boulders and Last Interglacial storm intensity in the North Atlantic
Alessio Rovere, Elisa Casella, Daniel L. Harris, Thomas Lorscheid, Napayalage A. K. Nandasena, Blake Dyer, Michael R. Sandstrom, Paolo Stocchi, William J. D’Andrea, Maureen E. Raymo, DOI: 10.1073/pnas.1712433114

Participating institutes:

Lamont-Doherty Earth Observatory, Columbia University, USA
Leibniz Centre for Tropical Marine Research (ZMT), Bremen
MARUM – Center for Marine Environmental Sciences, University of Bremen
NIOZ Royal Netherlands Institute for Sea Research, The Netherlands
University of Auckland, New Zealand
University of Queensland, Australia

This is a joint press release by MARUM – Center for Marine Environmental Sciences of the University of Bremen and the Leibniz Centre for Tropical Marine Research (ZMT)

© Jana Stone, MARUM