Rock Varnish on HuaHilai and Mauna Kea Volcanoes, Hawai'i

Tropical rock varnishes found on Hualalai and Mauna Kea Volcanoes, Hawai'i, vary systematically with time and environment. Radiocarbon dating of encapsulated organic matter, (K+ + Ca2+)/Ti4+ ratios, and Zn, Cu, and Ni trace element concentrations in rock varnish are consistent with lava flowages established by K-Ar and 14C dating, where samples are collected from arid microsites well away from the soil surface. However, inaccurate ages are obtained from rock varnish in subsurface locations and from sites with abundant lichens, cyanobacteria, and fungi that chemically erode varnish. In contrast with continental deserts, Hawaiian varnishes commonly interfinger with and are less common than rock coatings of amorphous silica. Laboratory experiments on Hawaiian rock varnishes indicate that K and Ca are preferentially leached relative to Ti over time and at higher temperatures. The location of in situ leaching has been identified in Hawaiian varnishes as porous textures without abundant detrital grains. THE STEADY GROWTH AND IMPROVEMENT of stratigraphic dating methods have been fundamental components ofQuaternary research during the past two decades. In contrast, surface exposure dating (SED) methods have been a weak sister; they attempt to measure the time since a land surface formed. Even though erosion of landforms is an ongoing process, SED methods utilize fossil remnants of "original surfaces" to date the landform (e.g., patches of glacial polish). Radiometric dating of lava flows and other stratigraphic units can offer valuable insights into rates of earth-surface processes (e.g., Dohrenwend et al. 1987). However, datable materials that provide surface exposure dates have a limited 1 This study has been supported by the National Science Foundation under Presidential Young Investigator Award EAR 87-57014 and a grant from the National Geographic Society. Manuscript accepted 10 April 1991. 2 Department ofGeography, Arizona State University, Tempe, Arizona 85287-0104. 3 Regional Facility for Radioisotope Analysis, University of Arizona, Tucson, Arizona 85721. 4U.S. Geological Survey, Federal Center, MS-903, Denver, Colorado 80225. 5 U.S. Geological Survey, 971 National Center, Reston, Virginia 22092. 6 Crocker Nuclear Laboratory, University of California, Davis, California 95616. distribution-that is, until the recent revolution in SED methods. SED methods can be classified as biological, cosmogenic, and weathering. Biological SED methods rely on the growth of trees or lichens to constrain the age of the underlying deposits. Cosmogenic SED methods measure the time-dependent buildup of stable and unstable isotopes that occur in situ in minerals because of nuclear reactions caused by cosmic rays. Weathering SED methods, in contrast, measure the time-dependent decomposition and diagenesis of minerals in terrestrial weathering environments. Originally proposed by Lal and Peters (1967), the measurement ofin situ cosmogenic isotopes was not possible before the advent of accelerator mass spectrometry (AMS) (Brown 1984, Elmore and Phillips 1987). Helium-3 is the stable cosmogenic isotope that has received the most attention (Cerling 1990, Kurz et al. 1990). Chlorine-36 (Phillips et al. 1990), lOBe and 26Al (Brown 1987, Nishiizumi et al. 1989), 14C (Jull et al. 1989), and 1291 (Fabryka-Martin et al. 1985) have seen the widest application of a large suite of radioactive cosmogenic isotopes. Other in situ cosmogenic isotopes that are being actively studied include 41Ca and 21Ne (Elmore and Phillips 1987, LaI1988).