THE APPLICATION OF TEPHROCHRONOLOGY TO THE STUDY OF CLIMATE CHANGE IN MONTANE ENVIRONMENTS IN THE WESTERN UNITED STATES

Age control in paleolimnology is critical to tracking rates of environmental change. For latest Pleistocene and Holocene studies, ¹⁴C is the most common dating method. Limitations, however, commonly include 1) lack of suitable material for dating in some deposits; 2) the presence of ¹⁴C-dead “old carbon” in groundwater and(or) local sediment sources; and 3) deposits older than ~50,000 years, which is beyond the range of ¹⁴C dating. Tephrochronology is a powerful tool for augmenting age control provided by ¹⁴C dating, as well as extending chronologies to Neogene (and older) lacustrine sediments. In the original sense, tephrochronology uses tephra, either in discrete layers or isolated shards (cryptotephra), to connect stratigraphic sequences by providing precise tie-points, and provide ages for those sequences.

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.