A library of ancient life
In the subtropical Hoshitate-ura mangrove forest on Iriomote Island, layers of mud hold a genetic library of past ecosystems. Scientists use a technique called sedimentary ancient DNA (sedaDNA) analysis to read this library. By drilling into the sediment and extracting cores, they access a vertical timeline where the deepest layers are the oldest. Preserved within this mud are fragments of DNA from plants and animals that lived thousands of years ago.
This method provides a detailed record of local vegetation, often with greater taxonomic resolution than traditional analysis of pollen or microfossils. In the Iriomote study, researchers could identify specific plant species that populated the area through different eras. The DNA fragments, often less than 100 base pairs long, are extracted, amplified, and sequenced, allowing scientists to reconstruct the plant community composition of the past. This genetic record offers direct evidence of how the local environment responded to large-scale climatic shifts over millennia.
A current of change
The data from the Iriomote sediment core shows the history of northward migration. Around 6,000 years ago, during a warm period in the Holocene epoch, mangrove species began to appear and then flourish farther north than their previous ranges. This ecological shift was not random; it corresponds with evidence of a strengthening Kuroshio Current.
The Kuroshio Current, sometimes called the Black Current because of the deep blue color of its water, is a powerful western boundary current in the Pacific Ocean. It transports enormous amounts of heat from the tropics toward the pole, significantly influencing the climate of Japan. The ancient DNA record indicates that as this current intensified 6,000 years ago, the warmer waters it carried enabled temperature-sensitive mangrove forests to expand their territory northward along the Ryukyu island chain. This migration, captured in the genetic fragments buried in the mud, acts as a biological proxy for the behavior of a major ocean current deep in the past.