21 October 1977, Volume 198, Number 4314 Pollen Influx and Volcanic Ash The pollen content of Mazama and Glacier Peak ashes reveals details of their depositional chronologies. Peter J. Mehringer, Jr., Eric Blinman, Kenneth L. Petersen Glacier Peak, Washington, and Mount Mazama (Crater Lake), Oregon, are two of the many Cascade Range volcanic vents that produced widespread ash de- posits in northwestern North America. These volcanoes have been objects of study for many decades, and the ashes are routinely used as chronological contained in lake sediments of Lost Trail Pass Bog, Bitterroot Mountains, Mon- tana (Fig. 1), were studied in more detail. Specifically, we hoped to test the poten- tial of pollen influx as a tool with which to estimate the durations of ashfalls and the intervals between them. We chose Lost Trail Pass Bog for this Summary. Pollen influx can be used to estimate the duration of short-term depo- sitional 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 illustrat- ed that (i) two falls of Glacier Peak ash, which occurred about 11,250 14C years ago, were separated by 10 to 25 years; and (ii) volcanic ash from a major eruption of Mount Mazama (about 6700 14C years ago) first fell in the autumn and 4.6 centi- meters 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 eco- systems 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 Cas- cade Range volcanoes. markers over distances of more than 1000 (Glacier Peak ash) and 2000 kilome- ters (Mazama ash) (1). Our initial exami- nation of the fossil pollen content of these ashes at several localities revealed considerable variation in both pollen numbers and types, and we believed that such variability might provide clues to the duration, season, and ecological ef- fects of the ashfalls. Therefore, ashes 21 OCTOBER 1977 SCI E NCE firmed by refractive index, phenocryst suite, and microprobe analysis of major elements (3). At Lost Trail Pass, Glacier Peak ash (5.73 to 5.77 meters in depth) is repre- sented by two distinct ashes separated by 7.5 millimeters of lake sediment. Both ashes are laminated, but a turbation zone occurs within the upper ash (Fig. 2). Lake sediments below and above the two ashes gave radiocarbon ages of 11,200 ± 100 (sample WSU-1548) and 11,300 ± 230 (WSU-1554) years. These are younger than the ages of 12,000 to 13,000 14C years most often cited for Gla- cier Peak ash (1). It is now clear, how- ever, that several eruptions of Glacier Peak produced volcanic ashes of dif- ferent geographic ranges and ages (4) and that the Lost Trail Pass Glacier Peak ashes are referable to the uppermost of three main Glacier Peak ash deposits (5). The occurrence of lake sediments be- tween ashes provided the opportunity to measure the interval between ash falls by pollen influx. The Mazama ash (5.05 to 5.13 m in depth) consists of at least 23 graded lami- nations from 0.5 to 10 mm thick. Lake sediments on either side of the ash were radiocarbon-dated at 6700 + 100 (WSU- 1552) and 6720 ± 120 (WSU-1553) years ago. The 6700-year date compares favor- ably with other dates for Mazama ash. The thickness of the ash allowed us to investigate ashfall duration through total pollen content and the pollen sequence within the ash. Pollen Influx Pollen influx is an estimate of the num- ber of pollen grains incorporated into sediments with a particular surface area within a particular time. It may vary with vegetation type and density and is there- fore an important tool for reconstructing vegetational history from fossil pollen assemblages. Pollen influx per square centimeter per year can be estimated if the deposition rate and the number of pollen grains per volume of lake sedi- ment are known. For example, if 1 cm3 of lake sediment containing 100,000 pol- P. J. Mehringer is a professor and E. Blinman and K. L. Petersen are teaching assistants in the lzcpart- ment of Anthropology, Washington State Uiiversi- ty, Pullman 99164. 4W 't.'s7if study because the Glacier Peak and Ma- zama ash layers (Fig. 2) in our 10-cen- timeter-diameter lake sediment cores represented primary deposition, as in- dicated by the lack of detrital materials from surrounding soils. Also, their boundaries were distinct, they had clear laminations indicating deposition into standing water, and they were well dated (2). Identification of both ashes was con- on March 27, 2016 Downloaded from on March 27, 2016 Downloaded from on March 27, 2016 Downloaded from on March 27, 2016 Downloaded from on March 27, 2016 Downloaded from