THE APPLICATION OF TEPHROCHRONOLOGY TO THE STUDY OF CLIMATE CHANGE IN MONTANE ENVIRONMENTS IN THE WESTERN UNITED STATES
Holocene paleoenvironmental records in California, the Great Basin, Nevada, and the Pacific Northwest are often anchored by the presence of the widespread Mazama ash (~6,750 yr B.P., 14C), and in some cases by its precursor, the Tsoyowata ash (~7,000 yr B.P., 14C), which has a more limited areal distribution. Both of these tephra originated from Mount Mazama (now Crater Lake). Tephra erupted from Glacier Peak and Mount St. Helens also provides late Pleistocene age control over a reduced geographic area. Chronologies in these areas are further refined through the use of more locally derived tephra such as the Mono-Inyo Craters and Little Glass Mountain ash beds.
Putative tephrochronologic correlations of eruptive events to the Greenland ice core records provide not only precise age control. Studies of microfossils deposited with the volcanic ash can provide information on seasonality. In addition, the spatial distribution of some extensive tephra deposits allows for the stratigraphic correlation of lakes across many different environments and altitudes, as well as with the marine record.
Attempts to identify individual, but chemically highly similar volcanic ash beds from sites with multiple, closely spaced eruptive events, such as Mono Craters are in progress (e.g. LA-ICP-MS). Discrete identifications and more precise age determinations of these tephra supported by micropaleontologic proxy evidence will lead to improved reconstructed climate histories.