пятница, 17 апреля 2020 г.

Climate change and the collapse of Angkor Wat

Built using a thousand elephants and 300,000 laborers, the opulent temple city of Angkor has been near-deserted for centuries, yet its grandeur and mystery now attract a million visitors each year. The question is, why was it deserted at all? An answer is finally emerging.

Climate change and the collapse of Angkor Wat
On a floating platform, Penny and team take a drill core sample from an overgrown reservoir.
The 12th century tower S1, of the Prasat Suor Prat group, looms in the background
[Credit: Louise M Cooper]

In the basement of the University's Madsen Building are archives of Cambodia's environmental history, including dozens of drill cores in a cool room, that were extracted from the city of Angkor. These drill cores have seen scientists reconsider the downfall of the world's largest pre-industrial city.

Taken from just two meters beneath the earth's surface, the cores tell the story of how land at Angkor has been used over thousands of years. "Their layers are like pages in a book," says Associate Professor Dan Penny from the School of Geosciences.

"Once we bring a core home from Angkor and split it open, going back through each of those pages is like going back through time," he says, his detailed descriptions conveying a deep knowledge of this ancient city, one he has explored for more than 18 years.

A wonder of the ancient world

Over five centuries, Angkor grew to cover more than a thousand square kilometers, comparable in size to modern day Los Angeles, though with a much lower population density.

The accepted view has been that Angkor collapsed suddenly in 1431, following an invasion by inhabitants of the powerful city of Ayutthaya, in modern day Thailand. Penny and his colleagues put this theory to the test when, in 2016, they took a dozen drill cores from the earth beneath Angkor's temple moats.

From these cores Penny extracted microscopic evidence of past environmental change. In particular, he examined pollen grains from plants and charcoal derived from residential fires, while also measuring rates of erosion and sedimentation.

"We were looking for what people were doing in the landscape. How they used fire, how plants were changing, when occupation was intense and when it decreased," he says. "We certainly didn't find evidence of the sacking in 1431, and a sudden abandonment of the city. It was instead a very prolonged diminution in the commercial and ritual core of the city."

Penny's findings suggest the central city elite left Angkor gradually, attracted, perhaps, to the better located and more profitable trading centers on the Mekong Delta.
That word again: climate

Additional evidence provided by ancient tree rings suggests climatic variation may have been the nail in the coffin. Tree ring cross-sections taken from long-lived conifers indicate a major drought in Angkor towards the middle of the 14th century, followed by intense monsoons and then another big drought.

Climate change and the collapse of Angkor Wat
Split sediment cores reveal layers of history. In the petri dish are ceramic fragments,
sometimes found in these environmental archives [Credit: Louise M Cooper]

"The problem was not drought or flood, but variability between both," Penny says. The collapse of the water network—due to a combination of intense summer monsoon rains and lack of maintenance—likely hastened the city's desertion.

Penny began looking at Angkor as a postdoctoral researcher in 1999, and soon began collaborating with archaeologist Roland Fletcher. Now a professor, Fletcher was at Angkor exploring questions about urbanism and the demise of cities. Penny was researching environmental change at pre-Angkor sites along the Mekong Delta. Their skills were perfectly matched to explore the rise and fall of Angkor.

"It's not a purely urban story, nor is it a purely environmental one," says Penny. "It's a mixture of the two."

Penny returned from Edinburgh University to join the University of Sydney in 2001. Along with Fletcher, he is now a director of the Angkor Research Program, which brings together academics from across the University to better understand this once great city. Rather than focus on the monuments, the program delves into "the stuff that's no longer there."

"We're interested in what goes on between the monuments. We're interested in water, in forests, in soils," all of which draws on Penny's skills in paleo-botany and sedimentology.

Knowing the past to save the future

Why is this important? There are a number of cases, particularly in the tropics, where large, low-density cities failed at least partly because of stress generated by climate variability, Angkor among them.

"That rings quite significant alarm bells because we are moving into a century full of climate variability and more frequent climate extremes," says Penny. "More than half of humanity lives in cities, so understanding the fundamentals of urban resilience in the context of climate change is very important."

Penny points out that when we talk about the collapse of societies, we see it as an end point.

"But it isn't an end point for its people," he says. "It's part of a transformation as populations adapt to changing environments or circumstances. In the case of the Khmer people, the decline of Angkor saw their society transform from one huge agrarian kingdom to become much smaller trading cities along the Mekong Delta."

As Penny's work continues on these so-called middle period cities on the Mekong Delta, his team is also exploring the collapse of civilizations in the Maya territories of Belize, Mexico and Guatemala. Happening at the end of the first millennium, the collapse of the Maya was also triggered by drought. In his Central American work, the focus is on cities that survived.

"What is it about these cities that makes them able to survive the profound changes in climate, whereas cities only tens of kilometers away were destroyed or abandoned?" he asks.

Small-picture thinking

Penny does field work in the tropics of Asia and America every year to collect drill cores. Back in Sydney, he spends much of his time in the laboratory or at the microscope set up on his desk, surrounded by slide boxes, methodically examining every sample. While this kind of scientific work takes patience, what Penny does is laced with moments of pure satisfaction, like finding a pollen grain from a crop plant that someone tended 900 years ago.

"Those connections to places and peoples that are long gone are really exciting."

* This article was originally published here

Mahogany tree family dates back to last hurrah of the dinosaurs

Mahogany is a commercially important wood, valued for its hardness and beauty. The United States is the world's top importer of the tropical timber from leading producers like Peru and Brazil. Unfortunately, mahogany is harvested illegally a lot of the time.

Mahogany tree family dates back to last hurrah of the dinosaurs
The holotype of Manchestercarpa vancouverensis is a section of fruit showing fleshy mesocarp,
leathery meso- carp, thick-walled endocarp, and subapically attached seed
[Credit: University of Kansas]

For science, mahogany is important, too -- the fossil presence of the mahogany family is a telltale of where tropical forests once stood. Until recently, paleobotanists had only found evidence the mahogany family extended back to the Paleocene (about 60 million years ago).

Now, a new paper written by University of Kansas researcher Brian Atkinson in the American Journal of Botany shows the mahogany family goes back millions of years more, to the last hurrah of the dinosaurs, the Cretaceous.

"For understanding when many of the different branches of the tree of life evolved, we're primarily dependent on the fossil record," said Atkinson, an assistant professor of ecology & evolutionary biology and curator in the Biodiversity Institute's Division of Paleobotany. "In this case, Meliaceae, the mahogany family, is an ecologically and economically important group of trees. A lot of researchers have used this group as a study system to better understand the evolution of tropical rainforests. This work is the first definitive evidence that the tropically important trees were around during the Cretaceous period, when we first start to see the modernization of ecosystems and modern groups of plants."

Atkinson's new work pushes back the fossil record for Meliaceae by 15 to 20 million years, the Campanian stage of the Late Cretaceous, from between 72-79 million years ago. The well-preserved mahogany specimen Atkinson analyzed was discovered just off Vancouver Island in Canada.

"The rock that contained the specimen was collected by a local fossil collector, Graham Beard, who is the director of the Qualicum Beach Museum of Natural History," Atkinson said. "He collected it years ago, but I was actually interested in the rock that has this fossil in it for something else. And as I kept preparing this rock, more for the other fossils were in there, this thing showed up by surprise. So, it was kind of found by accident."

To pinpoint the fossil's identity, Atkinson carefully studied the structure of the fossilized fruit and also analyzed phylogenetic information to figure out its relationship to other species in the mahogany family.

"I combined the molecular data from living representatives of the mahogany family with the morphology of the fossil, as well as the morphology of living species," he said. "And then I subjugated that combined dataset to phylogenetic analyses, which allows us to reconstruct evolutionary relationships. Based on this analysis, we found the fossil is closely related to this genus called Melia, which is living today."

The KU researcher gave the oldest-known mahogany fossil the scientific name Manchestercarpa vancouverensis -- the species name signifies where the specimen was discovered, and the genus is named after an esteemed colleague in the field.

"I named it after a prolific paleobotanist who's really improved our understanding of the evolution of flowering plants through the fossil record," Atkinson said. "So, I named it in honor of Steve Manchester, who's at the University of Florida Museum of Natural History."

While it's noteworthy that Atkinson has pushed back the origin story of mahogany, he stressed it also helps improve our understanding of the rate of early flowering plant evolution and, in turn, our grasp of larger modern ecosystems.

"They're our most diverse group of plants on Earth, and so there's a whole lot to explore," he said. "And there are some cool things you can do methodologically that you might not be able to do with other groups of plants. I can really ask some exciting paleontological and general evolutionary questions with this group."

* This article was originally published here


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