Tephrochronology of the Lake Malawi Drill cores: a feasibility study.
2012-07
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Tephrochronology of the Lake Malawi Drill cores: a feasibility study.
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2012-07
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The Lake Malawi Drilling Project recovered sediment cores from two sites on Lake Malawi in 2005: one in the northern basin (site MAL05-2 at 9°58'56.60"S, 34°11'9.17"E) and one in the central basin (site MAL05-1 at 11°17'39.60"S, 34°26'9.00"E). These cores have been studied extensively for paleoenvironment and paleoclimate conditions. Uncertainty still exists in stratigraphic correlations between cores, and the age model is becoming more refined.
Cores from hole 2A and 2B contain several tephra layers, and 17 visible tephra layers were examined to correlate between the two sites. Tephra layers were sampled and analyzed for chemical composition using both electron microprobe analysis with wavelength-dispersive spectrometry (EMPA-WDS) and on a scanning electron microscope through energy-dispersive spectrometry (SEM-EDS).
SEM-EDS was found to produce results that were not sufficiently accurate to make correlations and did not accurately reflect the chemical composition of samples. Because of the higher counting time and current used in SEM-EDS, alkali elements experienced migration and thus had lower perceived concentrations. Other elements such as MnO and MgO had abundances at the detection limit of SEM-EDS (0.2 wt.%) and thus varied greatly from EMPA-WDS results.
Four visible tephra layers could be correlated between holes 2A and 2B, based on EMPA-WDS compositions using single linkage Euclidean distance statistical analysis of geochemical data. The correlations established that identical tephra layers in holes 2A and 2B differed in burial depth by as much as 0.9 mblf (meters below lake floor).
A 23 m segment of drill site 1C was also examined for visible tephra layers and cryptotephra layers. Tephra from 23.72 to 46.29 mblf were examined in search of ash from the Youngest Toba Tuff (YTT) eruption (~74 ka). This depth corresponds to ~55 ka to ~85 ka based on the current age model for the drill site. The discovery of YTT ash would provide an isochronous marker and would improve the age model for this drill site. Three visible tephra layers were found and examined for chemical compositions, however only two possibly correlate with ash from site 2 and none correlated to the YTT. Three cryptotephra were found using a density separation method. Sediment was extracted and the fraction larger than 25 μm was density separated using sodium pentatungstate. Tephra was then identified based on optical properties and counted using a petrographic microscope, and converted to a count of shards/g. Each cryptotephra layer was analyzed using EMPA-WDS.
Cryptotephra layers were also examined by X-ray fluorescence (XRF) to evaluate the utility of this rapid, non-destructive, and inexpensive approach. A phonolitic to trachytic cryptotephra layer (~1,000 shards/g), a rhyolitic cryptotephra layer (~3,000 shards/g), and a thin visible layer (phonolitic to trachytic, ~0.1 cm thick, >15,000 shards/g) were scanned with XRF. Three different XRF scanning resolutions were attempted (0.5 cm, 0.2 cm, and 0.05 cm). The cryptotephra layers were not detected at any scanning resolution; however the thin visible layer was detected at all scanning resolutions.
The cryptotephra layers were all analyzed by EMPA-WDS to determine chemical composition. Of the three cryptotephra layers found in hole 1C, two were phonolitic to trachytic (typical of the Rungwe Volcanic Province eruptions) and one was rhyolitic. The rhyolitic layer, found at 28.10 mblf, correlates to the YTT eruption. One other cryptotephra layer correlates to layers from site 2, and the other layer does not correlate using geochemical data and cluster analysis (single linkage Euclidean distance). The rhyolitic cryptotephra layer correlates to the YTT (~74 ka) based on the geochemical results from EMPA-WDS. The cryptotephra layer matches the chemical composition of YTT ash published in Smith et al., 2011. This is a significant discovery because the depth at which the cryptotephra layer was found, 28.10 mblf, corresponds to an age of ~60 ka using the model by Scholz et al. (2011) and therefore indicates the need for a revised age model for this portion of the drill core. This discovery also extends the distal extent of the YTT ash from its source in the Indonesian archipelago to ~7,300 km. The most distal known extent prior to this discovery was ~4,300 km from Toba in the Arabian Sea (Schulz et al., 2002). The presence of YTT ash in Lake Malawi more than doubles the known area of YTT ash, and creates strong evidence for finding the YTT ash in other areas of East Africa.
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University of Minnesota M.S. thesis. July 2012. Major: Geological sciences. Advisor: Dr. Thomas Johnson. 1 computer file (PDF); ix, 107 pages, appendices A-K.
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Chorn, Benjamin. (2012). Tephrochronology of the Lake Malawi Drill cores: a feasibility study.. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/139966.
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