Desert varnish—the thin, dark red or black coating found on exposed rock surfaces in arid environments—was long thought to be a purely chemical deposit. However, recent micro-analytical studies have revealed that the varnish is primarily formed by manganese-oxidizing bacteria, such as *Metallogenium* and *Pedomicrobium*. These microscopic organisms concentrate dissolved manganese from the atmosphere and bind it to the rock surface using a sticky extracellular polymeric matrix. This biological concentration process is extremely slow, often requiring thousands of years to produce a visible layer. Crucially, the bacteria concentrate manganese at levels up to one hundred times its concentration in the surrounding environment, which explains why the varnish is so rich in manganese even when the local soils and dust contain only trace amounts of the element. Because this bacterial activity is highly sensitive to moisture, the rate of varnish deposition increases significantly during periods of elevated humidity, leaving behind distinct layers that paleoclimatologists use to reconstruct ancient rainfall patterns.
According to the passage, what is the direct cause of desert varnish containing high concentrations of manganese even in areas where the element is scarce in local soils?
- AA purely chemical precipitation of dissolved manganese from the atmosphere during periods of elevated humidity
- The biological concentration of manganese by bacteria at levels far exceeding its concentration in the surrounding environmentAnswer
- CThe presence of highly concentrated manganese deposits on the rock surface, which stimulates the growth of the bacteria
- DThe rapid binding action of a sticky extracellular polymeric matrix on dry rock surfaces