Microprobe Elemental Mapping of Varnish Chemistry

Figure Caption    Electron microprobe element maps (128×128 µm) of rock varnish from Death Valley, California. The inset image at lower right is an optical microstratigraphy seen in varnish ultra-thin section under a polarized light microscope. As seen from the probe maps, silica (Si), aluminum (Al), magnesium (Mg), and potassium (K) achieve their highest concentrations in the yellow layer at the top of the varnish microstratigraphy (interpreted to represent the Holocene). In contrast, maganese (Mn), barium (Ba), calcium (Ca), and phosphorus (P) achieve their highest concentrations in the dark layers (interpreted to represent the last glacial time). Iron (Fe) shows the smallest glacial to interglacial change in concentration. With some exceptions, sulphur (S) is not generally associated with barium in varnish microlaminae during both glacial and interglacial time. The line profile of barium oxide content was taken along a vertical traverse and the probe mapped portion of the varnish microstratigraphy is marked by a square. This varnish contains a layering sequence of LU-1/LU-2/LU-3/LU-4 (WP6), suggesting a minimum varnish-based age estimate of 60 ka. Please see Liu (2003) for more information about varnish layering pattern interpretation and age dating.