5,700 years of sea level change in Micronesia suggests humans arrived much sooner than we thought

Sea levels in Micronesia have risen much faster than previously thought over the past 5,000 years, according to our new study released today Proceedings of the National Academy of Sciences.

This sea level rise is reflected in the accumulation of mangrove sediments on the islands of Pohnpei and Kosrae. The finding could change our minds about when humans migrated to remote Oceania and where they might have traveled from.

Impressive travelers

While significant advances in linguistic, bioanthropological, and archaeological research in the region have been made in recent decades, the exact pattern and timing of human habitation in remote Oceania is still a matter of debate.

Humans began migrating to remote Oceania — the area of ​​the “open” Pacific Ocean east of New Guinea and the Philippines — around 3,300 to 3,500 years ago. This migration required tremendous sea voyages over long distances never seen before in human history.

The Micronesia region stretches for many thousands of kilometers and contains thousands of low-lying atolls. Many of these atolls formed around 2,500 years ago when sea levels in the region stabilized near where they are today.

Before that, the sea level might have been up to two meters higher than today. Humans could only successfully colonize these atolls after sea levels had dropped and stabilised.

But there are also older and higher volcanic islands in Micronesia. In remote Oceania, these higher islands were more desirable for habitation than low-lying atolls because they have more reliable freshwater sources, better-developed soils for agriculture, and are less prone to storm surges.

We looked at published settlement ages in the western part of outlying Oceania and found that high islands tend to have earlier settlement ages compared to atolls, which is what we would expect. But we don’t see this pattern in Micronesia: the high islands of Pohnpei and Kosrae show settlement ages about 1,000 years later than other similar islands. Why?

Mangrove Notes

Deep in the mangrove forests of Pohnpei and Kosrae, previous researchers found mangrove sediments up to five meters deep. The only explanation for such deep mangrove sediments is continued sea level rise.

Mangroves live on the coast between low and high tide. Therefore, as sea levels rise, organic carbon and sediment accumulate under the mangrove forests, creating deep soils.

We visited the mangroves on Pohnpei and Kosrae and collected sediment cores to find out how old the sediments were underneath. Our new data, as well as previous work, show that the oldest mangrove sediment is about 5,700 years old.

From this we calculated that the sea level has risen by about four meters in the past 5,700 years. The most likely cause of this increase is that the islands are sinking: the land is sinking relative to the sea surface.

In our new study, we propose that this sea-level rise has obscured the archaeological record at Pohnpei and Kosrae. Consequently, as with other high islands, evidence of earlier occupation may lie underwater today.

It is possible that humans settled this region of Micronesia much earlier than previously thought, which also raises the question of whether humans traveled from the west or from the south to reach these islands.

A testament to rising seas

The UNESCO World Heritage Site of Nan Madol on Pohnpei may also be a testament to rising sea levels. Nan Madol is an impressive collection of abandoned megalithic structures made of dark basalt columns and crushed coral.

This place has been dubbed the “Venice of the Pacific” because of the characteristic network of waterways surrounding the buildings, resembling canals and islets.

Our records of sea level rise from the mangrove sediment show that sea levels were almost a meter lower than they are today when Nan Madol was built (around 1180-1200 AD).

We think it unlikely that Nan Madol was built with views of canals and islands. Rather, the channels and islets are the result of nearly 1,000 years of sea level rise.

Much like island nations today, large stone walls may have been built to protect the site from waves that slowly pushed higher and higher.

This article was republished by The Conversation under a Creative Commons license. Read the original article.