Even by the standards of enigmatic ancient ruins, in madol it’s strange. Built primarily of “logs” made of volcanic rock, the site consists of dozens of small man-made islands separated by tidal channels. It is built on the shores of the Micronesian island of Pohnpei, which has an enigmatic history, apparently remaining unpopulated as the northern and southern islands were settled during Polynesian expansion.
Now, a team of researchers offers a unique explanation for many of these oddities: Pohnpei Island is slowly sinking, evidence of earlier settlement beneath the waves. And, if your estimate of its sinking is accurate, Nan Madol would have been above the waves at the time of its construction.
Ups and downs
Human expansion into the scattered islands of the Pacific began more than 3,000 years ago and took place primarily along two parallel routes north and south of the equator. The southern route was populated by the ancestors of Polynesians, while the northern route was derived from people who probably originated from the Philippines. There were islands between the two along the equator, but they were not settled until about a thousand years later, when the descendants of the first wave spread out from the islands they had initially populated.
One explanation for this involves changes in sea level. Many of the islands between the northern and southern expanses are low-lying atolls, which could have been completely underwater at the time. That’s because sea levels in the equatorial Pacific were higher as the Earth’s crust adjusted to redistributing water away from the huge ice sheets of the last glacial period. From this vantage point, ocean levels in this region gradually dropped, revealing more of these atolls and making them easier to colonize.
That doesn’t explain everything, however. Pohnpei and Kosrae islands lie in the region, and both are centered on volcanic peaks that would have extended well above sea level all along. However, there is no indication that they were settled before other islands in this region. Approximately 3,000 years after their settlement, a new culture arrived and built important urban centers: Nan Madol in Pohnpei and Leluh in Kosrae.
Both sites have a similar construction. The large basalt columns, such as those found at Giant’s Causeway or Devils Tower, are arranged a bit like the logs in a log cabin. Coral blocks are also used. While Leluh was built on a small island off the coast of Kosrae, Nan Madol is on the water, with buildings separated by canals, earning it the nickname Venice of the Pacific. While the materials and architecture are similar to each other and shared with other sites in the Pacific, the channels are unique to Nan Madol.
The new work began with a look at the sediments formed in the mangroves. Mangroves only grow within a limited range relative to the area’s high tide mark and will trap sediment in that region. If tidal levels are stable, that limits sediment formation to a range of less than a meter around the high tide mark. However, working with sediments on these islands, the research team found mangrove sediments up to six meters thick in areas around the islands. This implies that the islands have been steadily sinking, allowing sediment to build up on top of previous layers.
Tracking these sediments at multiple sites, combined with carbon dating of materials in the sediment, allowed researchers to reconstruct sea level over the past 5,000 or so years. These show that the islands probably sank during most of that period. So while the ocean level in the area may have been falling for part of the period, local sea levels were actually rising as the islands slowly slipped out to sea. A GPS station at Leluh indicates that the subsidence has continued to the present.
(The dip may be related to the fact that these islands are the product of hotspot volcanism, driven by a plume of hot material in the mantle. But the Pacific plate has now shifted, so the hotspot is no longer was under these two islands. .)
Overall, the researchers conclude that the two islands could have been colonized during the initial expansion into the Pacific. But if people stayed on shore as they did on many other islands, the remnants of early settlements would be under water today. People would no doubt move inland for things like farming, but these activities may not have left anything to survive as an artifact.
As for Nan Madol, reconstruction of sea levels suggests that they were around one meter lower on the site when construction began. This would have left the “channels” completely dry except at extremely high tides. Except during storm surges, the worst flooding would have been a few inches. Even several hundred years later, when the rulers who built Nan Madol were displaced and the site abandoned, the canals would have been dry at low tide. So they probably shouldn’t look like channels at all.