Ancient sand dunes now 100m above sea level. Optically stimulated luminescence dating shows they formed 120,000 years ago during the last interglacial, then were lifted by tectonic uplift.
A landscape lifted
Along the Tyrrhenian coast of Italy, about 100 kilometers south of Rome, lies a peculiar geological feature: a series of ancient, hardened sand dunes. These are not the soft, shifting dunes of the modern beach. Instead, they are consolidated "fossil dunes" that now stand up to 100 meters above the current sea level. They are prominent within the Circeo National Park, a protected area established in 1934 that covers a diverse area of forest, wetlands, and the imposing limestone promontory of Mount Circeo.
The story of these dunes begins around 120,000 years ago, during a period known to geologists as the Last Interglacial, or Marine Isotope Stage 5e (MIS 5e). During this time, global temperatures were warmer than today, and the global mean sea level was between 4 and 6 meters higher. The Tyrrhenian Sea coast was further inland, and these dunes formed along that ancient shoreline.
The Circeo fields are notable is their current elevation. Their position high above the waves is the result of tectonic uplift. The Italian peninsula is a geologically active region, squeezed between the African and Eurasian plates. The land here has been rising steadily, a process that has lifted these coastal dunes far from the sea that created them. The uplift rate in this part of Italy can vary, with some measurements in the wider region suggesting rates between 0.2 and 0.6 millimeters per year. Over millennia, this slow but persistent movement hoisted the entire landscape, preserving the 120,000-year-old dunes as records a past coastline.
Reading the grains of sand
Determining the age of a sand dune is a complex task. Scientists found the answer trapped inside individual grains of quartz. The method they used is called Optically Stimulated Luminescence (OSL) dating. OSL is a technique that can measure the last time a grain of sand was exposed to sunlight.
Here is how it works: while sand grains are buried, they are exposed to low levels of natural radiation from the surrounding sediment. This energy gets trapped in the crystal lattice of minerals like quartz. When scientists take a core sample from the dune, carefully protecting it from light, they can take it to a laboratory. There, they expose the sand grains to a specific wavelength of light, which releases the trapped energy as a luminescence signal. The intensity of this signal is proportional to the amount of radiation absorbed, which in turn corresponds to the time the grain has been buried.
By applying OSL dating to the Circeo dunes, researchers were able to establish their formation age at approximately 120,000 years, confirming they originated during the warm MIS 5e interglacial period. This technique provides a precise burial-time clock, turning the layers of sand into records coastal dynamics and sea-level changes from a distant geological epoch. The dunes are now covered in Mediterranean vegetation, unlike the barren, shifting sands that once defined them.
💡Fun Facts
The Last Interglacial period (MIS 5e) was so warm that sea levels may have peaked at 6 to 9 meters higher than they are today.
The nearby Mount Circeo is geologically distinct from the dunes; it is a limestone massif formed during the Mesozoic Era.
In 1939, a well-preserved Neanderthal skull was discovered in the Guattari Cave on Mount Circeo, indicating a deep history of hominin presence in the area.
The modern coastal dunes within Circeo National Park, which run for 22 kilometers, are one of the longest and best-preserved dune systems in Italy.