Congruent paleomagnetic and archeomagnetic records from the Western United States: a.d. 750 to 1450.

Two independently dated, high-resolution paleomagnetic records, one lacustrine and one archeological, record the passage across western North America of the same nondipole feature of the geomagnetic field during the time interval from A.D. 750 to 1450. Although these sequences indicate that correlation between paleomagnetic and archeomagnetic records is feasible under certain conditions, differences between the records underscore the difficulty of dating accurately an archeological site by correlation of a single archeomagnetic direction with a secular variation curve.

Pollen influx and volcanic ash.

Pollen influx can be used to estimate the duration of short-term depositional events. When applied to volcanic ashes, it may also provide information on the season and ecological effects of ashfall. In our initial application of the method to volcanic ashes from Lost Trail Pass, Bitterroot Mountains, Montana, we have illustrated that (i) two falls of Glacier Peak ash, which occurred about 11,250 (14)C years ago, were separated by 10 to 25 years; and (ii) volcanic ash from a major eruption of Mount Mazama (about 6700 (14)C years ago) first fell in the autumn and 4.6 centimeters of ash was deposited before the following spring. We also believe there is a reasonable probability that (i) about 1 centimeter of ash fell during the following year and about 1.7 centimeters fell the year after; (ii) in all, the sporadic primary Mazama ashfall lasted for nearly 3 years; (iii) Mazama ash resulted in low lake productivity, as measured by the occurrence of Botryococcus and Pediastrum; (iv) Mazama ash, perhaps through a mulching effect, may have produced increased vigor and pollen production in some sagebrush steppe genera; and (v) as measured by the records of fossil pollen and acid-resistant algae, effects on the aquatic and terrestrial ecosystems were short-lived. With refinement of the methods and broader geographic application, pollen influx studies may prove valuable for separating the regional and chronological details of tephra attributed to Mazama, Glacier Peak, and other Cascade Range volcanoes.