LUMINESCENCE DATING AND PALYNOLOGY OF THE SODA LAKE CLAY DUNE COMPLEX, CARRIZO PLAIN, SAN LUIS OBISPO COUNTY, CALIFORNIA

LUMINESCENCE DATING AND PALYNOLOGY OF THE SODA LAKE CLAY DUNE COMPLEX, CARRIZO PLAIN, SAN LUIS OBISPO COUNTY, CALIFORNIA RHODES, Dallas D., Department of Geology and Geography, Georgia Southern University, Statesboro, GA 30460, DRhodes@GeorgiaSouthern.edu; RICH, Frederick J., Department of Geology and Geography, Georgia Southern University, Statesboro, GA 30460 The extensive clay dune system developed around Soda Lake and smaller pans on the floor of the Carrizo Plain contain a climate change record spanning the last ~10 ka. Lake levels and dune activity are inversely correlated. Increased precipitation raises groundwater and lake levels, inhibiting deposition on the dunes. Samples of the aeolian sediment have been dated through optically stimulated luminescence. Pollen contained in the same samples were extracted and identified, with plant taxa being similar to those found on the plain today. Although diversity is low, dominant forms include insect-pollinated composites, chenopods, and pines. A sample (coded BDRP) collected near the middle of the dune ridge fringing the largest (North Basin) pan was dated at 9.62 +/- 0.70 ka (the oldest date obtained thus far). The BDRP pollen sample contained pine and oak and bore many large clusters of composite pollen, suggesting that the pollen did not travel far and probably accumulated where the plants grew, i.e. the dune was partially vegetated. Another sample from the base of the dune surrounding the second largest (South Basin) pan (SB1-11.25) yielded a date of 9.17 +/- 0.68 ka. These dates establish that the two largest units in the Soda Lake clay dune system existed and were active soon after the end of the Younger Dryas. Active deposition on the clay dune system continued throughout much of the Mid-Holocene. An additional date at 6.17 +/- 0.45 ka from the South Basin dune (SB1-9.55) yields an average vertical accretion rate of about 5 cm / 100 yr of compacted sediment on the dune. A thermal luminescence date on the North Basin dune (3.3 +/- 0.20) is evidence that the dunes remained active until a high stand of Soda Lake near the end of the Iron Age Neoglacial (based on radiocarbon dates of seeds obtained from lake cores at 2.85 +/-0.7 to 2.24 +/- 0.9 ka). Morphologic evidence(compound and complex dune forms) show that parts of the dune were reactivated when the water level fell. More than 90% of the modern dune complex is stabilized or undergoing active erosion.