LASER DIFFRACTION GRAIN SIZE ANALYSIS OF LITTLE LAKE, OREGON

Paleolimnology is the study of lake sediments and the significant paleoenvironmental records preserved in them. Examining these sediments and organic remains in them gives us a better understanding of how vegetation, climate and forest-fire systems respond to changes similar in scale to those predicted for the near future. Although these studies are common, the linkage between these systems and landscape evolution is less understood. This project used a 16 meter core from a previously studied lake to test the relationship between changes in median particle-size and other paleoenvironmental proxy. Little Lake is located in western Oregon, in the Coastal Range forests between the Pacific Ocean coast and the Cascade Mountains at 44°10'03” N, 123°35'01”W. Age control for the core was derived from previously published age-depth models, three AMS radiocarbon ages and the Mazama tephra. Particle-size analysis was performed every ten centimeters using a Malvern Mastersizer 2000E laser diffraction particle-size analyzer after pretreatment for organic matter. In general, median grain-size coarsens upward but there are periods when there is little change or the variance changes. The following means (m) and standard deviations (sd) are reported in microns and the description of variance (low, moderate and high) is relative to this core only. Between ~30.0 and 23.5 ka BP sediment is fine with low variability (m= 5.6; sd=0.37). Between ~23.5 and 16.0 ka BP sediment is also fine with slightly more variability (m=5.8; sd=0.48). Between ~16.0 and 11.0 ka BP median grain-size increases from 6.0-9.0 microns with moderate variability (m=7.0; sd=0.95). Between ~11.0 and 9.0 ka BP median grain-size increases from 8.6-14.0 microns with high variability (m=10.5; sd=2.3). Between ~9.0 and 5.0 ka BP median grain size is slightly coarser with high variability (m=12.8; sd=2.0). Between ~5.0 and 2.0 ka BP median grain-size changes little but the variance decreases (m=12.1; sd=0.90) Between ~2000-1000 ka BP median grain-size increases from 15-21 microns with the most variability (m=16.9; sd=2.6). Median grain-size decreases after ~1000 ka BP from 12-8.5 microns with more moderate variability (m=9.5; sd=1.4). Some of these periods correlate well with previously published periods of climatic, vegetation and forest fire frequency change.