ADVANCES AND FUTURE DIRECTIONS IN TEPHROCHRONOLOGY, WESTERN U.S

Tephra layers are the closest proxy for time horizons in the geologic record. They are usually conspicuous, and can be dated and correlated directly by isotopic and chemical methods. Thus, tephra layers provide a ready means by which associated deposits can be correlated and dated.

Tephra layers have long been used as stratigraphic markers, and stratigraphic continuity remains the only infallible criterion for their correlation. Advances in tephrochronologic methodology over the past 60 years have often followed on the heels of new analytical techniques, making correlations across great distances possible. Development of isotopic dating methods in the late 1940's and 1950's provided a stimulus to tephrochronology, complementing relative-age correlation with numerical ages. Field characteristics employed in earlier studies were augmented in the 1950's and 1960's by systematic microscopic petrography and mineralogical analysis. Development of the electron microprobe in the mid- to late 1960's revolutionized tephrochronology, allowing analysis of individual glass shards and mineral grains for major and minor elements, thus providing chemical “fingerprints” for tephra layers. Development of ever more sensitive analytical techniques since the1970's has made it possible to analyze individual glass shards for minor and trace elements, providing even greater resolution. Parallel development in laser-fusion 40Ar/39Ar isotopic dating has yielded high-precision dates for numerous key tephra in the western U.S.

Future scientific opportunities and developments in tephrochronology are likely to come from new advances in isotopic and other dating techniques, iterative testing of multiple dating and correlation methods in geologic studies, improved techniques of separating, identifying, and analyzing very small tephra particles, and cross-dating of chronostratigraphic or chronobiologic dating methods: for example, cross-dating of tephrochronologic, magnetostratigraphic, dendrochronologic, varved and oxygen-isotope records. These advances will be most successful and significant if made in the context of broader studies of climate and other types of environmental change--perhaps as part of a new, global “Manhattan Project”.