Deciphering clues to prehistoric climate changes locked in cave deposits

It turns out that the steady dripping of water deep underground can reveal a surprising amount of information about the constantly changing cycles of heat and cold, precipitation and drought in the turbulent atmosphere above. The analysis of a stalagmite from a cave in north east India can detect the link between El Nino conditions in the Pacific Ocean and the Indian monsoon, a new study has found.

When the conversation turns to the weather and the climate, most people’s thoughts naturally drift upward toward the clouds, but Jessica Oster’s sink down into the subterranean world of stalactites and stalagmites.

That is because the assistant professor of earth and environmental sciences at Vanderbilt University is a member of a small group of earth scientists who are pioneering in the use of mineral cave deposits, collectively known as speleothems, as proxies for the prehistoric climate.

It turns out that the steady dripping of water deep underground can reveal a surprising amount of information about the constantly changing cycles of heat and cold, precipitation and drought in the turbulent atmosphere above.

As water seeps down through the ground it picks up minerals, most commonly calcium carbonate. When this mineral-rich water drips into caves, it leaves mineral deposits behind that form layers which grow during wet periods and form dusty skins when the water dries up.

Today, scientists can date these layers with extreme precision based on the radioactive decay of uranium into its daughter product thorium. Variations in the thickness of the layers is determined by a combination of the amount of water seeping into the cave and the concentration of carbon dioxide in the cave’s atmosphere so, when conditions are right, they can provide a measure of how the amount of precipitation above the cave varies over time. By analyzing the ratios of heavy to light isotopes of oxygen present in the layers, the researchers can track changes in the temperature at which the water originally condensed into droplets in the atmosphere changes and whether the rainfall’s point of origin was local or if traveled a long way before falling to the ground.

The value of this information is illustrated by the results of a study published May 19 in the journal Geophysical Research Letters by Oster’s group, working with colleagues from the Berkeley Geochronology Center, the Smithsonian Institution National Museum of Natural History and the University of Cambridge titled “Northeast Indian stalagmite records Pacific decadal climate change: Implications for moisture transport and drought in India.”

In the study, Oster and her team made a detailed record of the last 50 years of growth of a stalagmite that formed in Mawmluh Cave in the East Khasi Hills district in the northeastern Indian state of Meghalaya, an area credited as the rainiest place on Earth.

Studies of historical records in India suggest that reduced monsoon rainfall in central India has occurred when the sea surface temperatures in specific regions of the Pacific Ocean were warmer than normal. These naturally recurring sea surface temperature “anomalies” are known as the El Niño Modoki, which occurs in the central Pacific, and the Pacific Decadal Oscillation, which takes place in the northern Pacific. (By contrast, the historical record indicates that the traditional El Niño, which occurs in the eastern Pacific, has little effect on rainfall levels in the subcontinent.)

When the researchers analyzed the Mawmluh stalagmite record, the results were consistent with the historical record. Specifically, they found that during El Niño Modoki events, when drought was occurring in central India, the mineral chemistry suggested more localized storm events occurred above the cave, while during the non-El Niño periods, the water that seeped into the cave had traveled much farther before it fell, which is the typical monsoon pattern.

“Now that we have shown that the Mawmluh cave record agrees with the instrumental record for the last 50 years, we hope to use it to investigate relationships between the Indian monsoon and El Niño during prehistoric times such as the Holocene,” said Oster.

The Holocene Climate Optimum was a period of global climate warming that occurred between six to nine thousand years ago. At that time, the global average temperatures were somewhere between four to six degrees Celsius higher than they are today. That is the range of warming that climatologists are predicting due to the build-up of greenhouse gases in the atmosphere from human activity. So information about the behavior of the monsoon during the Holocene could provide clues to how it is likely to behave in the future. This knowledge could be very important for the 600 million people living on the Indian subcontinent who rely on the monsoon, which provides the area with 75 percent of its annual rainfall.

“The study actually grew out of an accidental discovery,” said Oster. Vanderbilt graduate student Chris Myers visited the cave, which co-author Sebastian Breitenbach from Cambridge has been studying for several years, to see if it contained enough broken speleothems so they could use them to date major prehistoric earthquakes in the area.

Myers found a number of columns that appear to have broken off in the magnitude 8.6 earthquake that hit Assam, Tibet in 1950. But he also discovered a number of new stalagmites that had begun growing on the broken bases. When he examined these in detail he found that they had very thick layers and high concentrates of uranium, which made them perfect for analysis.

Because of the large amount of water running into the cave, the stalagmite they choose to analyze had grown about 2.5 centimeters in 50 years. (If that seems slow, compare it with growth rates of a few millimeters in a thousand years found in caves in arid regions like the Sierra Nevada.) As a result, the annual layers averaged about 0.4 millimeters thick — wide enough for the researchers to get seven to eight samples per layer, which is slightly better than one measurement every two months. The amount of information about the climate that scientists can extract from the stalagmites and stalactites in a cave is amazing. But the value of this approach increases substantially as the number of caves that can act as climate proxies increases.

It is not a simple task. Because each cave is unique, the scientists have to study it for several years before they understand it well enough to use it as a proxy. For example, they must establish how long it takes water to move from the surface down to the cave, a factor that can vary from days to months.

Efforts to use the mineral deposits in caves as climate proxies began in the 1990’s. Currently, there are only a few dozen scientists who are pursuing this line of research and they have analyzed the mineral deposits from 100 to 200 caves in this fashion.

Story Source:

The above story is based on materials provided by Vanderbilt University. The original article was written by David Salisbury. Note: Materials may be edited for content and length.

Journal Reference:

1. Christopher G. Myers, Jessica L. Oster, Warren D. Sharp, Ralf Bennartz, Neil P. Kelley, Aaron K. Covey, Sebastian F.M. Breitenbach. Northeast Indian stalagmite records Pacific decadal climate change: Implications for moisture transport and drought in India. Geophysical Research Letters, 2015; DOI: 10.1002/2015GL063826

 

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Dry Heat

Last week, Lake Mead, which sits on the border of Nevada and Arizona, set a new record low—the first time since the construction of the Hoover Dam in the 1930s that the lake’s surface has dipped below 1,080 feet above sea level.

The West’s drought is so bad that official plans for water rationing have now begun—with Arizona’s farmers first on the chopping block. Yes, despite the drought’s epicenter in California, it’s Arizona that will bear the brunt of the West’s epic dry spell.

The huge Lake Mead—which used to be the nation’s largest reservoir—serves as the main water storage facility on the Colorado River. Amid one of the worst droughts in millennia, record lows at Lake Mead are becoming an annual event—last year’s low was 7 feet higher than this year’s expected June nadir, 1,073 feet.

If, come Jan. 1, Lake Mead’s level is below 1,075 feet, the U.S. Bureau of Reclamation, which manages the river, will declare an official shortage for the first time ever—setting into motion a series of already agreed-upon mandatory cuts in water outlays, primarily to Arizona. (Nevada and Mexico will also receive smaller cuts.) The latest forecasts give a 33 percent chance of this happening. There’s a greater than 75 percent chance of the same scenario on Jan. 1, 2017. Barring a sudden unexpected end to the drought, official shortage conditions are likely for the indefinite future.

Why Arizona? In exchange for agreeing to be the first in line for rationing when a shortage occurs, Arizona was permitted in the 1960s to build the Central Arizona Project, which diverts Colorado River water 336 miles over 3,000 feet of mountain ranges all the way to Tucson. It’s the longest and costliest aqueduct in American history, and Arizona couldn’t exist in its modern state without it. Now that a shortage is imminent, another fundamental change in the status quo is on the way. As in California, the current drought may take a considerable and lasting toll on Arizona, especially for the state’s farmers.

“We need to stop growing alfalfa in the deserts in the summertime.”

Robert Glennon, water policy expert at the University of Arizona

“A call on the river will be significant,” Joe Sigg, director of government relations for Arizona Farm Bureau, told the Arizona Daily Star. “It will be a complete change in a farmer’s business model.” A “call” refers to the mandatory cutbacks in water deliveries for certain low-priority users of the Colorado. Arizona law prioritizes cities, industry, and tribal interests above agriculture, so farmers will see the biggest cuts. And those who are lucky enough to keep their water will pay more for it.

According to Robert Glennon, a water policy expert at the University of Arizona, the current situation was inevitable. “It’s really no surprise that this day was coming, for the simple reason that the Colorado River is overallocated,” Glennon told me over the phone last week. Glennon explained that the original Colorado River compact of 1922, which governs how seven states and Mexico use the river, was negotiated during “the wettest 10-year period in the last 1,000 years.” That law portioned out about 25 percent more water than regularly flows, so even in “normal” years, big reservoirs like Lake Mead are in a long-term decline. “We’ve been saved from the disaster because Arizona and these other states were not using all their water,” Glennon said.

They are now. Since around 2000, Arizona has been withdrawing its full allotment from the Colorado River, and it’s impossible to overstate how important the Colorado has become to the state. About 40 percent of Arizona’s water comes from the Colorado, and state officials partially attribute a nearly 20-fold increase in the state’s economy over the last 50 years to increased access to the river.

On April 22, Arizona held a public meeting to prepare for an eventual shortage declaration, which could come as soon as this August. The latest rules that govern a shortage, established in 2007 by an agreement among the states, say that Arizona will have to contend with a 20 percent cut in water in 2016 should Lake Mead fall below 1,075 feet, which will decrease the amount available to central Arizona’s farmers by about half. At 1,050 feet, central Arizona’s farmers will take a three-quarters cut in water. At 1,025 feet, agriculture would have to make due largely without water from the Colorado River. That would probably require at least a temporary end to large-scale farming in central Arizona. Below 1,025 feet, the only thing Colorado River states have agreed to so far is a further round of negotiations. In that emergency scenario, no one really knows what might happen. More

 

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This machine makes salty water drinkable

The American engineers who traveled to rural India two years ago believed they were going to help poor villagers get rid of microbes in their drinking water. But soon after their arrival, they began hearing about a different problem: salt.

“People kept talking about the salt in the water,” recalled Natasha Wright, a doctoral candidate who was part of the team from Massachusetts Institute of Technology that made the journey in 2013. “The groundwater beneath the villages was brackish.”

Those complaints inspired new technology that could some day supply water to thirsty villages and drought-stricken farms in other parts of the world. The MIT team developed a solar-powered water desalination system that uses the sun’s energy to turn brackish liquid into contaminant-free water safe for drinking and for crops.

While there are dozens of different desalination systems in use around the world, MIT’s is uniquely designed to be small, relatively cheap and 100-percent solar-powered, making it suitable for remote areas where the electricity supply is unreliable or non-existent, Wright said.

The panel of judges last month deemed the machine’s potential so impressive that they gave the inventors the $140,000 “Desal Prize,” an award sponsored by Securing Water for Food, a joint project of the U.S. Agency for International Development and the governments of Sweden and the Netherlands. Some 68 engineering teams from 29 countries competed in the contest, hosted by the Interior Department’s Bureau of Reclamation in Alamogordo, N.M.

“Providing a sustainable water supply is important for the West, the country and the world,” Esteva Lopez, the department’s reclamation commissioner, said after the top prize was awarded to MIT and its research partner, Jain Irrigation Systems.

Wright said she and fellow engineers from MIT’s Global Engineering and Research Laboratory became aware the extent of saltwater intrusion in northern and central Indian aquifers during visits to investigate solutions for widespread water contamination in India. In addition to problems with bacterial contamination, the groundwater in much of rural India is brackish, having a salt content lower than seawater but still high enough to cause problems. In some of the villages visited by the MIT researchers, locals were trying unsuccessfully to remove the salt using filters and chemicals.

“People complained about the salty taste,” Wright said, “and the salt ruined their cooking pots.”

Traditional desalination systems are expensive and require substantial amounts of electricity to operate, making them impractical for India’s remote farming communities. Instead, the MIT researchers designed a system that removes salt through a process called electrodialysis, using a series of electrodes and membranes to remove the salt. They added solar panels and batteries to run the pumps and charge the electrodes. Then, in a final step, they installed ultraviolet light arrays to kill any microbes remaining in the water.

The finished prototype is small enough to fit in a tractor-trailer and includes photovoltaic cells to supply the electricity. The system, when fully operational, can supply the basic water needs of a village of between 2,000 and 5,000 people, MIT officials said. Although the prototype was more expensive, Wright said the team is hopes to lower the costs of a village-sized unit to about $11,000.

Such a lower-power system is useful mainly for treating brackish water and not seawater, which contains far more salt. But the prototype now being tested could handle water that contains salt concentrations of up to 4,000 parts per million, meaning it would work in about 90 percent of India’s wells, Wright said. Seawater’s salt concentration averages about 35,000 parts per million.

“There are places where this kind of system won’t work, but the advantage is, it uses half the energy of other systems,” said Wright. And, thanks to solar cells, “you can be fully off the grid.” More

 

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