Thursday, August 9, 2018

Massive drought or myth? Scientists spar over an ancient climate event behind our new geological age | Science | AAAS

Drought records in stalagmites from Mawmluh Cave in India's Meghalaya state are ambiguous.


Massive drought or myth? Scientists spar over an ancient climate event behind our new geological age

Last month, the International Commission on Stratigraphy (ICS), the bureaucracy that governs geological time, declared we are living in a new geological age. No, it's not the Anthropocene, the much-debated proposal for a geological division defined by human impact on Earth. The new age anointed by ICS is called the Meghalayan, based on signs in the rock record of a global drought that began about 4200 years ago. It is one of three newly named subdivisions of the Holocene, the geological epoch that began 11,700 years ago with the retreat of ice age glaciers. And the name will now filter its way into textbooks.

Many scientists say, however, that the "4.2-kiloyear event" was neither a global drought nor fixed to that moment in time. "The whole idea of defining the subdivision of the Holocene with a break at 4.2 seems a bit baseless," says Raymond Bradley, a climatologist at the University of Massachusetts in Amherst. Jessica Tierney, a paleoclimatologist at the University of Arizona in Tucson, says ICS, following the lead of some paleoclimate scientists, mistakenly lumped together evidence of other droughts and wet periods, sometimes centuries away from the 4200-year-old event, to mark the beginning of the Meghalayan. This is a "paleoclimate white whale," she says.

The first clues to the Meghalayan came from archaeology. In the early 1990s, Harvey Weiss, an archaeologist at Yale University, was excavating a compelling story of drought-induced collapse in Mesopotamia. At Tell Leilan, an ancient city of the Akkadian Empire in northeastern Syria, he found evidence that drought pushed people out of the city 4200 years ago. The signal repeated across much of Mesopotamia.

It was a good starting point for Mike Walker, a geologist at the University of Wales in Lampeter who a decade ago began the effort to divide the Holocene. Scientists commonly talk about an early, middle, and late Holocene—tracking the glaciers' retreat and partial return—but with wildly different time spans in mind. ICS asked Walker to standardize those divisions for the sake of clear scientific communication. But although abrupt changes in the rock record mark earlier geological divisions, such changes are scarce in the relatively calm Holocene.

Finding a date to divide the early and middle Holocene, now dubbed the Greenlandian and Northgrippian, was easy, Walker says. About 8200 years ago, an outburst of freshwater from naturally dammed glacial lakes poured into the North Atlantic Ocean. The floods are believed to have disrupted a conveyor belt of ocean currents, leading to signals of global cooling that can be reliably found in ice cores, lakebeds, and cave rocks.

But the second division proved harder. "We were struggling," Walker says. Fortunately, some paleoclimate scientists were picking up where Weiss's archaeological work left off. Signs of a 4200-year-old drought were emerging in the Mediterranean, the Americas, and Asia, where researchers linked it to a weakened monsoon. In 2012, paleoclimatologists reported an analysis of a stalagmite from Mawmluh Cave, a limestone complex in Meghalaya state, a wet part of northeastern India. Stalagmites, calcium carbonate formations on the floors of caves, grow drip by drip as mineral-rich rainwater seeps in. In the Mawmluh stalagmite, a shift in oxygen isotopes seemed to show a stark drying around 4200 years ago, a clear signal of centuries-long drought. Walker thought the stalagmite signal could serve as the perfect geological exemplar, or "golden spike," marking the beginning of the Meghalayan. In June, a few dozen geologists from ICS and its parent body ratified Walker's proposal for the new ages with little dissent.

Paleoclimatologist Ashish Sinha is surprised that ICS used the stalagmite for its golden spike—and few know it better, as it was his lab at California State University in Dominguez Hills that found and analyzed it. His team could date only a few of the stalagmite's layers, and water had partially dissolved the rock close to the drying event, potentially blurring the record. An unpublished analysis of other Meghalayan stalagmites by paleoclimate scientists from Xi'an Jiaotong University in China adds to the doubts: It found a steady weakening of the monsoon over more than 600 years, rather than a sudden drought 4200 years ago. The closest thing to a sharp drought can be seen 4000 years ago, in a few decades-long events. These excursions could be said to match the golden spike "to an extent," says Gayatri Kathayat, who led the research, "but not entirely."

Elsewhere in the world, the 4200-year-old event is even less apparent, according to a team at Northern Arizona University (NAU) in Flagstaff. Over the past few years, the NAU team has amassed 550 published paleoclimate records of temperature and moisture change during the Holocene, based mainly on stalagmites, lake sediments, and ice cores. Graduate student Hannah Kolus scrutinized the records in vain for significant changes in global temperature or moisture about 4200 years ago. "You don't see that signal at all," Kolus says.

The archaeological evidence is also far from definitive, adds Mark Altaweel, an archaeologist at University College London. He says political collapse, not drought, may have doomed some settlements in Mesopotamia. And in ancient Egypt, Greece, and elsewhere the evidence of a global drought is even murkier, adds Guy Middleton, an archaeologist at Charles University in Prague. "Nothing happened as suddenly or as synchronously as made out." The drought makes no sense as a marker, he says. "It is new mythmaking."

Walker wishes Kathayat's new stalagmite records had been published in time for their proposal. But he thinks that, though scattered in time and space, the signs of drought are good enough to define a new division that geologists can use to clarify their discussions of the Holocene. "The fact that this is extremely damned close is encouraging for us," Walker adds. For Bradley, it shows the stark division between ICS, which studies Earth's deep history, and scientists who study the recent past. "[We're] on totally different pages, really totally different books," Bradley says.

Critics of the Meghalayan will have plenty of time to bolster their arguments because for now, debate is over. To prevent continual spats, ICS freezes discussion for a decade after it ratifies a boundary. "It gives time for new ideas to bed down," Walker says.

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Paul Voosen

Paul Voosen

Paul Voosen is a staff writer who covers Earth and planetary science.

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