Investigating The Use Of Scanning X-Ray Fluorescence To Locate Cryptotephra In Minerogenic Lacustrine Sediment: Experimental Results

Methods to isolate and analyze low concentrations of tephra-cryptotephra-are destructive, time consuming, and can be prohibitive when sample size is limited, when looking for tephra over long stratigraphic intervals, or when sediments are minerogenic. Therefore, a more rapid, non-destructive approach to detecting cryptotephra would allow for wider application of tephrochronology and for more complete evaluation of tephra content within sedimentary profiles. In this experiment, we test the ability of scanning X-ray fluorescence to detect tephra glass shards with different composition, concentration, and grain-size in minerogenic lacustrine sediment. Synthetic sediment cores spiked with tephra were created in centrifuge tubes, which provided a simple means to introduce tephra in known positions and to replicate the process of analyzing real sediment cores. Background sediment was added incrementally and spun in a centrifuge to create a series of 20 laminations in 4 synthetic cores. Rhyolitic and basaltic tephra were added between laminations with different concentrations and using two grain-size ranges (< 53 ae m and 105-177 mu m). The synthetic cores were split and analyzed on an XRF core scanner, which produced a signal of element composition every 100 mu m. Ti, Mn, and Si produced the strongest response to the rhyolitic tephra, and Ti, Mn, Fe, and Cu were most diagnostic of the basaltic tephra. Element ratios were also used to accentuate the difference in composition between tephra and the background sediment. We were able to identify a distinct elemental response across a few cryptotephra horizons, but in general the signal of tephra attenuated quickly with decreasing concentration. Comparison of the signal between different tephra grain-size fractions showed that grain-size was inversely related to the strength of the elemental response. We also compared these experimental results to XRF scans of a lake sediment core where basaltic and rhyolitic cryptotephra layers had previously been identified using conventional methods. The rhyolitic tephra did not produce a distinct elemental response, but the basaltic tephra was identified in the XRF data. These experiments provide new perspectives on the application and limitations of scanning XRF for cryptotephra studies.