A Staircase Through Time
In the intermontane basins of Northern Thailand, the area holds a subtle but detailed climate archive. Along rivers like the Ping, a major tributary of the Chao Phraya, you can find a series of step-like landforms called river terraces. These are not man-made structures but the remnants of ancient floodplains. As a river cuts deeper into its valley over geological time, it abandons its old, higher floodplain, leaving it behind as a terrace. In Lamphun Province, this process has created at least of at least three distinct terrace levels.
These terraces are composed of sediments (gravel, sand, and silt) transported and deposited by the river thousands of years ago. The oldest and highest terraces contain a record of river behavior and regional climate, stretching back approximately 50,000 years. This timeframe places the deposits squarely within the last glacial period, an era of global climate instability. Each layer within these terraces records the river's power and the intensity of the Asian monsoon that fed it.
Reading the Climate Record
Scientists use a technique called Optically Stimulated Luminescence (OSL) to date these ancient river deposits. OSL dating measures the last time that grains of sand, specifically quartz, were exposed to sunlight. When a sand grain is buried, it is shielded from light and begins to absorb natural background radiation from the surrounding sediment, trapping energy in its crystal lattice. In the lab, scientists can release this stored energy with a laser, producing a light signal whose intensity reveals how long the grain has been buried. This method effectively resets a clock to zero at the moment of deposition.
The data from Lamphun's terraces correlate with global climate cycles known as Marine Isotope Stages (MIS). The 50,000-year record here primarily covers MIS 3 (a variable period from roughly 59,000 to 29,000 years ago) and MIS 2 (the Last Glacial Maximum, about 29,000 to 14,000 years ago).
The composition of the sediment layers tells a clear story. Thick beds of coarse, rounded gravels indicate a powerful, high-energy river. This points to periods of intense rainfall and a strong summer monsoon, capable of moving large stones. In contrast, layers of finer sand and silt signify a much weaker river with lower discharge, corresponding to periods of a weaker, drier monsoon. Studies show that during the warmer phases of MIS 3, the monsoon was relatively strong, leading to significant sediment deposition. During the peak cold of the Last Glacial Maximum, the monsoon weakened considerably, altering the river's behavior and the type of sediment it left behind. This natural archive in Lamphun links high-latitude ice ages and the tropical monsoon system.