TESTING THE YOUNGER DRYAS IMPACT HYPOTHESIS USING ELEMENTAL AND MAGNETIC SPHERULE ANALYSIS OF 12.9KA LAKE SEDIMENT FROM LAGUNA CHAPALA, BC, MEXICO

Lacustrine sediments deposited in Laguna Seca Chapala, located in central Baja California, Mexico, span the Younger Dryas stadial. The Younger Dryas was an abrupt change in climate that occurred approximately 12.9 ka and is hypothesized to have been triggered by a cometary airburst or impact. Evidence of the impact has been found by others at nearly 40 localities in North America. In North America, the YDB is coincident with the extinction of more than 40 species of megafauna and the demise of the Clovis paleoIndian culture.

This research examines the elemental composition and magnetic fraction of the lacustrine sediments for indicators of an impact fallout horizon in the sedimentary sequence. We collected 49 samples (spanning 2 cm each) across two distinct grain-size boundaries that were bracketed by optically stimulated luminescence (OSL) and radiocarbon dating reported by Davis (2003) to be between 15 ka and 8 ka. X-ray diffraction analysis shows the sediment contains dominantly quartz, sodic plagioclase, a small amount of microcline, and minor calcite. The samples were homogenized and leached using double-distilled nitric acid and microwave digestion.. After digestion, the leachates were diluted with 18MΩ water and analyzed by ICPMS for Ir, Fe, Ni, Co, Mn and Ti.

Results show that the samples have high concentrations of Fe, likely due to detrital magnetite. All samples contained <0.1ppb of Ir with no apparent spikes in concentration. Mn, Co, and Ni do show spikes in concentration at two sample intervals (162-164 and 174-176cm below the base of the overlying modern dune sands). The cause of those spikes is not known, but they do not appear to be related to a possible fallout layer. Both of the spikes fall above a layer dated at 12.79±0.78ka by OSL dating. Results indicate that Laguna Chapala sediments do not preserve evidence of meteoric impact.