POLLEN RAIN OF THE PAST 200 YEARS IN PUGET SOUND, WASHINGTON

Three mud cores from Hood Canal and three from Puget Sound (2-3 m in length) were dated using lead-210 isotope dating. The object of the pollen work was to determine when in this sequence was the settlement period-- the main disturbance period marked by a drop in conifer pollen values and increases of alder pollen and weeds. Cores were from water depths of between 60 and 100 meters, and were 2-3 meters in length. Preparation of samples (1 cc of mud) included treatment with HCl, HF, KOH and acetolysis. The greatest problem we found was the tendency of amorphous carbon found in every sample to clump with the pollen and sediment particles. We used sodium metaphosphate for de-clumping. We added a spike of Lycopodium spores for determining influx and concentration. Our results reflect the general pollen trends identified by Margaret Davis who published on short cores from Lake Washington in 1973, and demonstrated the drop in conifer pollen and increase in alder and weed pollen types. Our results: 1) Floristically, cores from Hood Canal are similar to those from Puget Sound with only minor differences. Almost all samples contain abundant pollen grains; marine microfossils including dinoflagellate algae and forams occur in low amounts but regularly in all cores. 2) General characteristics: wind-pollinated pollen of woody plants dominates; herb pollen and spores are in low frequency. Key taxa are Thuja type (TCT), Pseudotsuga, Pinus, Abies, Picea, and Alnus), 3) We identify three zones, although the boundaries between the zones are more or less different among the cores, partly relating to rates of deposition. These are: (a) a lower zone of dominant conifer pollen in which alder pollen values are below 20% (ca 1800-1930), (b) a middle zone of transition in which conifer pollen values are declining (~60-40%) and alder pollen is rising (~20-40%; ca 1930-1950), and (c) an uppermost zone (1950 to present) showing the “disturbance signal” in which alder pollen (~40-60%) dominates over conifer pollen. The disturbance signal is less striking in Puget Sound and Hood Canal than it is in Lake Washington (M. Davis's data) and Hall Lake (M. Tsukada's data) near Seattle where the coring sites were close to the timberlands. In conclusion the initial effect of lumbering activities on the pollen rain is most clear in the marine record between around 1930-1950, and the increase of alder and weeds continues to the present.