X-RAY FLUORESCENCE ANALYSIS OF A SEDIMENT CORE FROM LEONARD LAKE, MENDOCINO COUNTY, CALIFORNIA

Characterizing climate change during the Holocene is vital to understanding present and future mechanisms that drive climate variability. Leonard Lake is a small (8.7 ha) lake located in Mendocino County, California that is ideally suited for paleoenvironmental study given its geographic location and relatively undisturbed watershed. In 2014 a 5.2 meter sediment core was raised from the lake with the aim of reconstructing past climate of the region. Analyses include magnetic susceptibility, gamma-ray wet bulk density, loss-on-ignition, macroscopic charcoal, and X-ray fluorescence (XRF). An age-depth model was created using CLAM (v 2.2) based on four AMS radiocarbon determinations, ²¹⁰Pb, and ¹³⁷Cs.

Here we present newly calibrated results from contiguous one-centimeter resolution portable X-ray fluorescence (p-XRF) measurements on the Leonard Lake core. Each centimeter was analyzed in 4 voltage settings at 30 seconds each. These settings optimize the analyzer’s sensitivity for particular groups of elements. In order to reliably compare elemental concentrations, XRF measurements have been calibrated using matrix-specific Certified Reference Materials (CRMs). Discrete loose-powder samples were analyzed to quantify effects of interstitial water and varying particle size in the sediment. Loose-powder samples were transformed into pressed-powder samples and reanalyzed to determine effects of void spaces between particulates in the samples. Results from these two analyses were then used to calibrate down core elemental concentrations (derived from “wet” core measurements) for increased accuracy and precision. Results show that several major lithophile elements (Ti, Al, Zr) correlate, and varying concentrations indicate changes in terrigenous input into the lake due to hydrologic variability in the region over the last ~3400 years. Poor correlation between lithophile elements and iron (Fe) suggests some portion of the iron is authigenic, likely associated with redox conditions in the lake. These results demonstrate the efficacy of using p-XRF on lake sediment cores to help reconstruct environmental change.