The last capital of the Khmer Empire, Angkor was the world’s most extensive city in the 1200s, home to hundreds of thousands of people in its urban core and comparable numbers of rice farmers in the surrounding area. But Angkor mysteriously declined in the 1400s, and some archaeologists have suggested that the site was abandoned suddenly, possibly due to a military defeat. But new sediment analyses indicate that the city’s ruling elites gradually abandoned Angkor starting in the early 1300s, researchers report online February 25 in the Proceedings of the National Academy of Sciences.

That’s when geologic signs of human activity began to decline at Angkor Thom, a walled city within Greater Angkor, the team found. Evidence of burning, forest disturbances and soil erosion from farming — gleaned from remnants of plants, pollen and minerals that had accumulated from the surrounding area in sediment taken from Angkor Thom’s moat — decreased throughout the 1300s, say geoscientist Dan Penny of the University of Sydney and his colleagues.

Pollen remains indicate that floating swamp vegetation covered the moat by around 1400, apparently because no administrative officials remained to coordinate the structure’s maintenance, the team says.

Previous research by Penny and his colleagues suggested that the breakdown of the city’s expansive water system, partly due to rapid shifts between periods of drought and heavy rainfall, hastened the city’s demise in the 1400s (SN: 11/10/18, p. 11). Now the scientists suspect that the steady departure of bigwigs caused the water system to fall into disrepair and ultimately fail.

Reasons for Angkor’s ruling class to leave may have included establishing or joining cities

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closer to the coast and profitable sea trade routes. As a result, by the 1400s, any invaders would have encountered little resistance at Angkor, the researchers suspect.