An ocean floor on a mountainside
Along the Wae Lare river on the island of Flores, a unique geological formation rises to 500 meters above sea level. This is the Flores pelite formation, a thick sequence of fine-grained sedimentary rock that was once mud on the deep ocean floor. Its existence on a mountainside results from immense tectonic forces. Flores is located at a complex junction between the Eurasian and Indo-Australian plates. The collision of the Australian continental crust with the island arc has been forcing the island upwards for millions of years.
The uplift in central Flores has been measured at approximately 800 meters over the last 2.5 million years, a rate of about 0.32 millimeters per year. This slow, persistent process lifted ancient marine deposits high into the air, exposing them to the elements. The formation is primarily composed of pelite, a sedimentary rock formed from consolidated clay and silt. Within these layers, scientists have found an almost unbroken record of the region's climate history, stretching back into the Pliocene epoch.
Reading the climate record
Each thin layer in the pelite formation acts as a page in a climatic diary. The sediments preserve chemical signatures, known as proxies, that reflect environmental conditions at the time of their deposition. Scientists analyze these proxies to reconstruct the intensity of the Australian-Indonesian monsoon system over geological time.
One important proxy is the element thorium, specifically the isotope 232Th, which is delivered to the ocean exclusively from the weathering of continental rocks. Higher concentrations of thorium in a layer indicate periods of intense rainfall and increased river runoff from the surrounding landmasses. By measuring the changing concentrations of these elements up through the sediment core, researchers can map out the fluctuations in monsoon strength. This data provides a high-resolution look at past climate, showing how the monsoon system responded to global climate shifts, such as the ice ages of the Pleistocene. The record provides a long-term record on the dynamics of the Intertropical Convergence Zone, the band of tropical rainfall that governs the climate for a massive portion of the world's population.