Quaternary alkalinity fluctuations recorded in rock varnish microlaminations on western U.S.A. volcanics

Rock varnish contains an alternating micron-scale layering of manganese-rich and manganese-poor microchemical laminations. Holocene varnishes have lower Mn:Fe ratios when exposed to alkaline environments and higher Mn:Fe ratios when exposed to less-alkaline environments. This is consistent with abiotic and biotic mechanisms of varnish formation, as alkalinity inhibits the concentration of manganese in both models. Mn:Fe variations with depth in varnishes in the western U.S. are interpreted as fluctuations in alkalinity or local microenvironmental influences. If enough varnishes are sampled from different rocks in a region, local effects can be ruled out. KAr dating of volcanics in the Coso, Cima, Lunar Crater and Crater Flat volcanic fields in the western Basin and Range provide numerical age control throughout the Quaternary, and allow the number of alkalinity fluctuations to be assessed for given periods of time. Alkalinity fluctuations in varnishes are likely tied to fluctuations in the abundance of alkaline aerosols, which are in turn related to fluctuations in the cover of water over playas and the cover of vegetation over soils. Microchemical laminations in varnish, therefore, have the potential to serve as an indicator of environmental fluctuations for the entire Quaternary.