USING CRYPTOTEPHRA TO IMPROVE AGE MODELS OF SEDIMENTARY RECORDS: GEOCHEMICALLY FINGERPRINTING LAKE MALAWI TEPHRA

The Lake Malawi Drilling Project recovered seven cores from two locations on Lake Malawi in 2005, which have been extensively studied for paleoclimate information. Due to the long record of these cores (>140,000 years), age models are less constrained beyond the limit of radiocarbon dating. Tephrochronology, specifically cryptotephrochronology, was utilized to improve the age model of these cores. The multiple visible tephra layers as well as cryptotephra layers discovered provide useful isochronous stratigraphic markers and thus improved the age model for the Lake Malawi cores.

Cryptotephra from the Lake Malawi cores were recovered using an HCl wash, sieves, and a series of density separation. Electron microprobe analysis with wavelength-dispersive spectrometry (EMPA-WDS) was used to geochemically fingerprint tephra. Of the three cryptotephra layers found from hole 1C, one layer was geochemically fingerprinted as tephra from the Youngest Toba Tuff (YTT) eruption (75 ka, Toba caldera, Indonesia), a source ~7,300 km from hole 1C. The discovery of the YTT ash in Africa extends the known distal extent of ash from the Toba supereruption from ~4,300 km to ~7,300 km, and demonstrates the feasibility of finding cryptotephra beyond the known distal extent of eruptions to improve age models in sedimentary records.