All posts by LL

Ancient crystal growths in caves reveal seas rose 16 meters in a warmer world

The future of sea level rise may be written into the walls of coastal Spanish caves.

Mineral “bathtub rings” deposited inside the limestone Artà Caves on the Balearic island of Mallorca show how high seas rose during the Pliocene Epoch — a time when Earth was about as warm as it’s expected to get by 2100. Those mineral deposits suggest the planet’s seas were around 16 meters higher on average than they are today, researchers report August 30 in Nature.

That measurement provides the most precise peek yet into what may come as climate change causes ice sheets to melt and ocean waters to rise — a process that could happen over hundreds to thousands of years. Previous estimates of Pliocene sea levels gave similar results, but relied on more indirect dating methods or failed to incorporate information about the subsequent rise and fall of the Earth’s crust. The Artà analysis, however, takes that rise and fall into account. 

The new study’s combination of precision dating and corrected sea levels also may help answer a crucial question: How much of Earth’s biggest ice sheet melted during the Pliocene? These sea level measurements suggest that, while smaller ice sheets in Greenland and western Antarctica melted severely, only the parts of the massive eastern Antarctic ice sheet that jutted into the sea melted during that era, says Alan Haywood, a paleoclimatologist at Leeds University in England who was not involved in the study. It’s still unclear, however, how today’s unique warming patterns will affect ice sheets.

“Anything that gives us added information on how sensitive the ice sheets are … is going to be very important,” Haywood says.

aragonite crystals
These aragonite crystals formed 4.39 million years ago inside an overgrowth on a cave feature in the Theater Room in Spain’s Artà Caves. The locations and ages of such growths can help researchers track past sea levels.BOGDAN ONAC

Conditions during the Pliocene, 5.33 million to 2.58 million years ago, may offer the best example of what a human-addled climate will eventually look like (SN: 11/28/17). In the past, paleoclimatologists mainly used two approaches in reconstructing Pliocene sea level changes. One links ratios of two types of oxygen, or isotopes, in fossilized sea creatures to a global record of oxygen ratios and ice sheet cycles. The other uses the ages of ancient coral reefs to estimate ancient sea levels.

For the new study, researchers searched caves for evidence of past sea level change (SN: 4/15/13). “Caves are a very protected environment,” says study coauthor Oana-Alexandra Dumitru, a geochemist at the University of South Florida in Tampa who began collaborating with Mallorcan researchers as a graduate student. “We don’t worry about erosion and other weathering after deposition as much as you would about terrestrial or other records.” 

In the 1970s, Mallorcan researchers found aragonite and calcite deposits on stalactites and stalagmites in the Artà Caves. The deposits, called phreatic overgrowths, accumulate over years when brackish seawater laps against the rock. Similar features have been found in coastal caves on the Italian island of Sardinia and in Mexico and Japan. Early dating attempts in the 1990s and 2000s focused on samples only a few hundreds of thousands of years old. But Dumitru and colleagues used a dating method capable of reaching back further in time, comparing ratios of uranium to lead in the minerals, to determine when the much older Pliocene deposits formed.

stalactites
Delicate stalactites on the ceiling of a coastal cave on the coast of Mallorca, Spain, have some mineral overgrowth from a time when the sea surface was just grazing them. LUCAS LAURSEN

As Earth’s temperatures rose, higher levels of seawater washed into the caves. That rising water left behind mineral deposits at heights from 14.7 to 23.5 meters above today’s sea level, Dumitru’s team found.

One of the lower deposits corresponds with the mid-Piacenzian Warm Period, which lasted from about 3.3 million to 3 million years ago. Global temperatures during that time period were 2 to 3 degrees Celsius warmer than in modern, preindustrial times — and resemble leading forecasts for the year 2100. Global mean sea levels then were 16.2 meters higher than today, Dumitru and colleagues calculate. 

The higher of the six mineral deposits studied corresponds to the Pliocene’s warmest period about 4.39 million years ago, when temperatures were about 4 degrees C higher than in preindustrial times. That Artà sample suggests that seas were about 23.5 meters higher than today. “We still may not know exactly how much sea level rose,” Haywood says. But with results like these, “we’re getting more confidence that we’re in the right ballpark.”

CITATIONS

O.A. Dumitru et al. Constraints on global mean sea level during Pliocene warmth. Nature. Published online August 30, 2019. doi:10.1038/s41586-019-1543-2.

First published by Science News: [html] [pdf].
An edited version appeared in the September 28, 2019, print edition.

See also my 2013 story on the this research for Science News: “Cave Detective Hunts For Clues To Past Sea Level.”

Taming the genie in the forest of the devil’s trees

It wasn’t long after the cairns appeared in the forest that women from surrounding villages began using them in a purification rite that ended in leaving underwear on the stone mounds. The cairns were new to the forest, but the women’s purification rite was not. In the ritual, older Berber women guided younger women into the forest, and the younger women washed themselves under the open sky and prepared their spirits for finding a lover. Forest rangers had built the cairns to mark the borders of Morocco’s national forests. They were designed to protect argan trees – which some Berber call the “tree of the devil” ­– from use and harvesting. But the local women turned the cairns into something else.

When Morocco’s government established Souss-Massa National Park in 1991, the Berber people were already familiar with temporary prohibitions on forest use, says anthropologist Romain Simenel of the Institute of Research for Development in Marseille, France. But they were accustomed to setting the prohibitions themselves, through a system called agdal, which involves religious stories laden with mischievous genies who curse parts of the forest, and community rituals that reopen the way to harvesting or grazing among the argan trees.

Instead, national authorities were now insisting on prohibiting access to a core zone of the argan forest, allowing limited access to a second zone, and leaving a third zone to more community-led use. They sought to protect the forest from both desertification and local land management decisions. But the genies in the argan forest are not easy to tame.

Continue reading Taming the genie in the forest of the devil’s trees

In search of the missing

A BBC Radio 4 series, Spain’s Lost Generations, looks
at the ongoing legacy of Spain’s civil war and dictatorship. Matt Elton spoke to its presenter, 
Lucas Laursen 

Which groups of people does this series deal with, and what happened to them?

Our series deals with people killed or otherwise disappeared during the Spanish Civil War and the subsequent dictatorship. More than 100,000 men and women are still missing. One episode focuses on people executed by Franco’s regime, and on the families who are only in the past decade or so recovering those remains from mass graves. The other is about people affected by the state-initiated theft of babies, which started during the war and continued through the early years of democracy. In some cases these people are now beginning to recover their identities and their families.

How did you find out about their stories?

Thanks to a growing civil society movement that began in the early 2000s, more Spaniards are learning where their lost loved ones are buried – or, in the case of some stolen babies, where they are now living. More than 8,000 bodies have been recovered from mass graves since the first exhumation in 2000, and we followed several families we met via those organisations through different stages of their journeys of recovery.

We recorded at a ceremony in Guadalajara, during which the organisation returned the remains of 22 people to living relatives. Unfortunately, I know of far fewer reunions of families with stolen babies, but we tried to follow some of those threads, too, and I suspect we’ll hear about more those in the future.

Are there any cases you found particularly striking or moving?

The most moving moments for me came when I met people who had lost family members. Some had been children during the war, while others had lost babies at the same Madrid hospital where my daughter was born. Meeting these people in person helped me experience how the human toll of the Spanish Civil War and the subsequent dictatorship and the healing it needs are still unfinished.

What do these stories tell us about the wider legacy of the civil war?

I hope listeners will hear just how unfinished that business is. We spoke with government bureaucrats and policymakers, activists and legal experts, each with their own prescription for healing. The generation of people doing the recovering isn’t composed of those who survived the war or lived under the dictatorship, but the generation afterwards. That taught me something about how slow people are to overcome fear, and how slow democracies are to take root. Perhaps, after hearing these voices, listeners will have their own ideas about what it takes to mend a rift like Spain’s – and how important it is to avoid them in the first place.

First published by BBC World Histories: [pdf].

Listen to the radio documentaries for BBC Radio 4 here: Franco’s Disappeared and The Missing Children.