MUDDY WATERS: PARTICLE SIZE DISTRIBUTIONS IN A HOLOCENE LACUSTRINE DELTA

We extracted three 1-2 m long sediment cores from the distal portion of a Holocene lacustrine delta in upstate New York (Otsego Lake, 42° 43' 10.81" N, 74° 55' 29.88" W), at distances of 40, 60, and 100 m from the gravel topset-foreset-bottomset transition, which was close to the subaerial-to-subaqueous boundary on the delta. Our intent was to document stratigraphy and identify features in the cores related to flood discharges. We hypothesized that grain size should vary directly with discharge, and thus, grain size variations could serve as a proxy flood record. Given the large number of reservoirs and lakes in central New York, the effort could open up a significant library of flood records for the Holocene.

We find two moderately extensive layers—an organic-rich muddy unit overlying a gastropod-carbonate rich mud layer. We analyzed sediment smears on glass slides under the microscope at 10 power magnification for particle size distributions. ImageJ, a freeware developed by National Institutes of Health, contains macros for particle detection and measurement and provided a systematic method for characterizing particles. We sampled sediment cores every 2 cm for grain size distributions. Because of significant and varying amounts of opaque and isotropic material in the slides, we conducted a test of particle detection under cross polarized light. Grain sizes vary from 1-100 micrometers on the slide smears. Distributions for plane and cross polarized light are substantially uncorrelated with each other. Plane polarized light microscopy reveal power law distributions of probability density functions for cumulative particle sizes (average coefficient and exponent in the power law are 1.6 and -1.4 respectively). Cross polarized light reveal particles dominated by quartz and calcite mineralogy, and yield a wider variation in distributions. For power law fits to the cumulative distributions in cross polarized light, the average coefficient and exponent in the power law are 6.0 and -1.6 respectively. Size-depth diagrams show a decrease in particle size with distance from the delta front, consistent with a stream competence hypothesis. Particle size variation within a single core suggests several larger events have occurred over the time period required to accumulate 1.8 m of sediment.