A library of ancient climate
High in the Sierra de Guadarrama mountains, the Peñalara cirque holds a collection of over 20 small glacial lakes. The largest and most studied of these is the Laguna Grande de Peñalara, situated at an altitude of 2,017 meters. This entire landscape was carved by glacial action during the Quaternary period, which began around 1.8 million years ago. The geology is predominantly granite and gneiss, the result of tectonic collisions and magmatic intrusions that occurred between 360 and 250 million years ago.
The lake beds contain a detailed environmental record. Scientists use paleolimnology—the study of lake history through sediment—to understand past climates. By drilling into the lake bottom, they extract sediment cores that reveal thousands of years of accumulated material. Contained within these layers are microscopic algae called diatoms. These organisms have silica shells that preserve well in sediment, and different species thrive in specific conditions of temperature, pH, and ice cover. Analysis of the diatom species found in the Peñalara cores shows a major shift around 8,000 years ago, when the lakes became permanently free of year-round ice, signaling a new, warmer phase of the Holocene epoch.