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

Dorn, R.I., ]990. Quaternary alkalini ty f luctuations recorded in rock varnish microlaminat ions on western U.S.A. volcanics. Palaeogeogr., Palaeoclimatol. , Palaeoecol., 76:291 310. Rock varnish contains an a l te rnat ing micron-scale layering of manganese-r ich 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 consis tent with abiotic and biotic mechanisms of varnish formation, as alkalini ty inhibits the concentra t ion of manganese in both models. Mn:Fe variat ions with depth in varnishes in the western U.S. are interpreted as f luctuat ions in alkalinity or local microenvironmental influences. If enough varnishes are sampled from different rocks in a region, local effects can be ruled out. K Ar dating ofvolcanics in the Coso, Cima, Lunar Crater and Crater Flat volcanic fields in the western Basin and Range provide numerical age control th roughout the Quaternary, and allow the number of alkalini ty f luctuations to be assessed for given periods of time. Alkalini ty f luctuations in varnishes are likely tied to f luctuations in the abundance of alkaline aerosols, which are in turn related to f luctuations in the cover of water over playas and the cover of vegetat ion over soils. Microchemical laminat ions in varnish, therefore, have the potential to serve as an indicator of environmental f luctuat ions for the entire Quaternary. I n t r o d u c t i o n Alternating manganese-rich and manganesepoor microlaminae have been observed in marine (Margolis and Glasby, 1973), lacustrine (Moore, 1981), subsurface (Alhonen et al., 1975) and in "rock varnish" terrestrial (Perry and Adams, 1978; Potter, 1979; Dorn and Oberlander, 1982) ferromanganese deposits. Rock varnish occurs in virtually all terrestrial weathering environments but is noticeable in deserts, hence the common name of ~desert varnish" (Dorn and Oberlander, 1982). Because of its biogeochemical stability in most arid and semiarid microenvironments, it has the potential to record long-term paleoenvironmenta] fluctuations in regions where such records are often missing. This paper reports on new studies of these micron-scale microchemical laminations in rock varnish, and indicate that a long-term record of alkalinity fluctuations is recorded. The rock varnishes studied here consist of about 15-20% manganese and iron oxides, with clay minerals being the dominant constituent composing up to 2/3rds of the varnish (Potter and Rossman, 1977, 1979). The remainder of the varnish is composed of over 40 minor and trace elements (Dorn and Oberlander, 1982). Microchemical laminations in ferromanganese deposits have different rates of accretion, 0031-0182/90/$03.50 (" 1990 Elsevier Science Publishers B.V.