Revised age of the Rockland tephra, northern California: Implications for climate and stratigraphic reconstructions in the western United States: Comment

Lanphere et al. (1999) presented new data for the age of the Rockland pumice tuff breccia of Wilson (1961) using the incremental-heating ⁴⁰Ar/³⁹Ar technique. Their age, ∼610 ka, is ∼200 ka older than zircon fission-track ages obtained on this tuff by Meyer et al. (1980; 1991). Application of new ⁴⁰Ar/³⁹Ar technologies to tephrochronometry is an important advance that allows more precise dating of widespread tephra layers. Although the new age may be correct, I urge caution in accepting it in place of grain-discrete fission-track dates and other data that suggest a younger age for this unit.