The Slowest Painters on Earth
The striking, dark sheen on the sandstone formations of Bardenas Reales is not a simple stain left by water or weather. It is desert varnish, a thin, layered veneer created by a patient community of microorganisms over thousands of years. This coating is a living biofilm, composed primarily of clay particles cemented to the rock by oxides of manganese and iron. The architects of this natural art are colonies of bacteria and fungi living on and within the rock surface.
The process relies on wind-blown dust, which delivers clay and trace elements to the rock. Specialized manganese-oxidizing microbes, including cyanobacteria from the Xenococcaceae family, are important. These microorganisms absorb manganese from the dust and, through their metabolic processes, oxidize it. This biological process concentrates manganese at levels 50 to 60 times greater than in the surrounding earth's crust. The resulting manganese oxides, along with iron oxides, act as a glue, binding the fine clay particles to the sandstone and creating the dark, shiny patina.
A Climate Record Written in Varnish
The growth rate of desert varnish is exceptionally slow, accumulating at a rate of less than 1 to 40 micrometers every thousand years. A typical coating is rarely more than 200 micrometers thick, about the width of two human hairs, regardless of its age. This means even the thinnest layer is an immense span of time.
Scientists can analyze the microscopic layers, or microlaminations, within the varnish to reconstruct past environmental conditions. The ratio of manganese to iron oxides changes depending on the climate. Black, manganese-rich layers correspond to wetter periods, which are more favorable for the manganese-concentrating bacteria to thrive. Orange or red-brown layers are richer in iron and indicate drier conditions. By studying this chemical microstratigraphy, researchers can read a long-term climate record that can extend back up to 250,000 years. The varnish is a shield, case hardening that makes the underlying rock more resistant to erosion.