AN INVESTIGATION OF THE YOUNGER DRYAS IMPACT HYPOTHESIS THROUGH ANALYSES OF LATE QUATERNARY LAKE SEDIMENT, LAGUNA SECA CHAPALA, BAJA CALIFORNIA, MEXICO

This study examines the sedimentology, stratigraphy, and mineralogy of deposits exposed along the margin of a dry lake basin, Laguna Seca Chapala, in central Baja California, Mexico that previous studies show span the Younger Dryas stadial. The Younger Dryas was an abrupt change in climate that occurred approximately 12.9 ka that is hypothesized to have been triggered by a cometary airburst or impact. A stratigraphy of a 3.4 m section was drawn of an outcrop of lacustrine sediment along the southwest margin of Laguna Seca Chapala. Sediment includes silty clay interbedded with five layers of coarse sand to silty sand that likely represent stages of lower lake levels. Samples were collected every 2 cm across two distinct grain-size boundaries that were bracketed by optically stimulated luminescence and radiocarbon dating reported by Davis (2003) to be between 15 ka and 8 ka. Samples were wet sieved for grain size, botanical, and microfossil analyses. The magnetic fraction was processed with a strong grade 42, neodymium magnet. Samples were examined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to search for potential impact markers including magnetic and carbon spherules. Results show that the floated fraction decreased in mass and volume with age, the oldest samples having the smallest amount. SEM analysis of spherules collected in the floated fraction show possible carbonate spherules coated with very fine-grained silicate minerals. The magnetic fraction (primarily detrital magnetite) increased with depth. The magnetic fraction of each sample layer was sieved to isolate the varying sizes and analyzed under an optical microscope in order to find and select possible magnetic spherules with the selected candidates scheduled for further analysis utilizing SEM microscopy. Future testing will include collecting additional samples from Laguna Seca Chapala in August 2015 as well as further examination by SEM for a more precise look at any spherule candidates.