Rock varnish microlamination dating
What the exact role of the bacteria plays in the growth of rock varnish is still not clear.
Microscopic view of a cross section of rock varnish layers.
In varnish collected from the Cima volcanic field, California, distributions of the minor elements used in cation-ratio (CR) dating reflect varnish stratigraphy as defined by the major elements Mn, Fe, and Si.
There is also a growing body of evidence that indicates varnish microstratigraphy carries climate record: Mn-poor yellow layers formed during dry periods, Mn-rich black layers deposited during wet periods.
Cation-ratio dating of rock varnish is an empirical surface-exposure dating method based on decreases in the cation ratio (Ca K): Ti over time.
Rock varnish microlamination (VML) dating uses the slow buildup of 'varnish' or dark coating on subaerially exposed rock surfaces.
It is the world's slowest-accumulating sedimentary deposit at around ~1μm per 1000 years.
It is made up of clays and iron and manganese oxides.