A staircase carved by ice ages
The flat plains surrounding Vientiane are not as simple as they appear. They form a subtle geological staircase of river terraces, former floodplains of the Mekong River abandoned as the river cut deeper over millennia. These terraces are a direct record of the planet's glacial cycles. During past ice ages, vast quantities of water became locked in continental ice sheets, causing global sea levels to drop. The Last Glacial Maximum, which peaked between 26,500 and 20,000 years ago, lowered sea levels by as much as 125 meters.
This drop dramatically steepened the Mekong's gradient on its path from the Tibetan plateau to the South China Sea. With a steeper path, the river's flow accelerated, giving it immense erosive power. It carved downward into its bed, leaving its former, higher floodplain high and dry. When the ice ages waned and sea levels rose, the river's energy decreased, and it deposited sediment, building up its new floodplain. This process, repeated over multiple glacial-interglacial cycles, created the series of terraces visible today. The highest and oldest terraces near Vientiane are ancient Ice Age floodplains, while the lowest level is the active floodplain of the modern Mekong.
A clock in the sediment
Geochronologists determine the age of these terraces using a method called Optically Stimulated Luminescence (OSL). This technique works on mineral grains like quartz and feldspar, which are abundant in the Mekong's sandy deposits. While buried, these grains are exposed to natural background radiation from elements in the surrounding soil, which causes electrons to get trapped in the minerals' crystal structures. The number of trapped electrons increases over time.
Scientists can measure the accumulated radiation dose by taking sediment samples in complete darkness and exposing them to light in a laboratory. This stimulation releases the trapped electrons, which emit a faint light signal, luminescence. The intensity of this light is proportional to the radiation dose the mineral has absorbed since its last exposure to sunlight. By measuring the dose and the background radiation rate of the burial environment, researchers can calculate the last time the grain of sand saw the sun, effectively dating when the river deposited it. OSL dating has been successfully applied to Mekong River sediments, allowing scientists to establish a chronology for the formation of these ancient landscapes. This method allows for dating beyond the limits of radiocarbon, reaching back hundreds of thousands of years.