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Scientists Piece Together Ancient Sea Level Record Via Mallorcan Cave Formations

USF Geosciences Professor Bogdan Onac says findings provide new insight into how high and how fast sea levels can rise

Phreatic Overgrowths on Speleothems exactly at sea level, pictured here above the diver and submerged stalagmites in the Galeria de les Delicies in Coves del Drac, Mallorca, Spain, provides high resolution reconstruction of previous sea level highstands. Credit: Antoni Cirer
Underwater caves on the Mediterranean island of Mallorca are yielding climate secrets from an important era when sea levels were higher than they are today, potentially foreshadowing what is to come amid climate change, reports a team of scientists that includes USF Geosciences Professor Bogdan Onac.

In a study that graces cover of the November issue of Nature Geoscience, an international team of researchers from USF, the University of New Mexico, the University of Balearic Islands (Spain), Boston College, University of Iowa, and University Rome Tre (Italy) uses unique cave formations on the Spanish island called phreatic overgrowths on speleothems – known as POS. These can only form in coastal caves at the water surface, making them an accurate marker for sea level as time goes by.

To collect the scientific evidence, Onac and fellow researchers entered or/and dove into the caves to collect samples and measure their elevation. The mineralogical and crystallographical investigations on the mineral samples were performed at Onac’s lab on the Tampa campus and Fornós’s lab at the University of Balearic Islands, whereas uranium dating to determine the ages of the formations was performed at the University of New Mexico labs of Professors Yemane Asmerom and Victor Polyak. The team then used nine different glacial models to evaluate the new data and make their findings.

Image from the Galeria de les Delicies in Coves del Drac. Credit: Miquel A. Perello

In past climates, glacial periods were interrupted by warm climate intervals that significantly melted the large ice sheets roughly every 100,000 years. Sea level increases during these warm periods are known as interglacials.

The research found that the phreatic overgrowths on speleothems from that period are a more accurate record of sea-level change than corals, which has been studied worldwide as a potential indicator of such past fluctuations. And with these newer and more accurate ways to measure past change, the scientists are reevaluating predictions on how sea levels might rise in the current era of warming.

‘‘This is alarming, because it suggests that if global temperatures continue to rise, sea levels could eventually reach higher than scientists previously estimated,“ Onac said.

The research was prominently featured on the cover of Nature Geoscience.

In the paper, the scientists report how the Earth’s climate changed from a cold glacial to a warm interglacial period dating back more than 120,000 years ago. That last interglacial period was the last time sea level was as high as present-day sea-level – providing scientists with a window to the past to understand present-day and especially future conditions.

For the last 12,000 years, Earth has been in the Holocene interglacial period. The researchers say their work now establishes a better understanding of what occurred in the last interglacial period: warming temperatures caused the melting of the polar ice sheets early in the interglacial period, followed by gradual ice sheet growth and sea level drop. Previous studies have put sea level rise during this past warm period between three and 15 meters.

“We wanted to better understand the magnitude, timing, and stability of sea level, since such information is critical for future projections of sea level change under a 1.5 to 2ºC global warming scenario,” Onac said.

This study provides exceptionally accurate timing of the sea level history during the above mentioned period and shows that it rose to six meters above present sea level about 127,000 years ago, it would have gradually fell to two meters by 122,000 years ago, and would have stayed at that elevation for the remainder of the sea level highstand to 116,000 years ago,” Polyak said.

“The results suggest that if the pre-industrial temperature will be surpassed by 1.5 to 2ºC, sea level will respond and rise two to six meters (7 to 20 feet) above present sea level.”

The research reported in this study is the result of a collaborative National Science Foundation project awarded to USF and UNM. The researchers (including two graduate USF students) are continuing their work in the Mediterranean under a new National Science Foundation Grant.

Read the paper at https://www.nature.com/articles/s41561-018-0222-5.

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