Peter deMenocal and Frank Brown (Univ.of Utah)
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Reference: deMenocal, P. B., and F. H. Brown, Pliocene tephra correlations between East African hominid localities, the Gulf of Aden, and the Arabian Sea, in Hominid Evolution and Climatic Change in Europe, vol. 1, edited by J. Agusti, L. Rook and P. Andrews, 1999.
Introduction
The Pliocene-Pleistocene chronology of hominid and other vertebrate evolution in East Africa is largely constrained by isotopic dating and regional intercorrelation of volcanic ash layers. Some eruptions were of sufficient magnitude or duration that their widespread tephra dispersal define a series of dated marker horizons throughout the fossil-bearing sedimentary deposits of Tanzania, Uganda, Kenya, and Ethiopia [Brown, 1982; Feibel et al., 1989; Haileab and Brown, 1992; Haileab and Brown, 1994; Pickford et al., 1991; WoldeGabriel et al., 1994]. Although many of the larger eruptive events have been dated directly the ages of many tephra are only constrained by indirect stratigraphic interpolation between dated levels. The geochemical compositions of volcanic glasses from each eruption are unique, providing a definitive means to establish broad tephrostratigraphic correlations linking the regional climatic, tectonic, and biologic histories of this distinctive archive of Earth history.
This same tephrostratigraphic approach has been used to extend
the East African tephra correlations into the continuous and well-dated
marine sediment record of regional and global paleoclimate variability
[Brown et al., 1992; Sarna-Wojcicki et al., 1985].
These authors identified several megascopic volcanic ash layers
within Deep-Sea Drilling Project (DSDP) sites from the Gulf of
Aden, nearly one thousand kilometers northeast of hominid
localities in Ethiopia and Kenya. Major element chemistries of
volcanic glass shards extracted from these marine sediments were
used to establish precise tephrostratigraphic correlations into
the fossil-bearing East African sedimentary sequences. Moreover,
controversy concerning the ages of specific eruptive events which
then defined key temporal junctures in early hominid evolution
could be tested using the independent marine sediment chronostratigraphic
framework [Brown et al., 1992; Sarna-Wojcicki et al.,
1985].
Tephra Transport to the NW Indian Ocean
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Boreal summer surface wind field over the Northwestern Indian Ocean. Volcanic ash from explosive rhyolitic source volcanoes in Ethiopia, Kenya, and Tanzania is transported to the NE by the Findlater jet (arrow) which is associated with the summer Indian monsoon. These winds transport volcanic ash particles to marine sediments up to some 1500 km distant in the Gulf of Aden and the Arabian Sea. A sequential chemical extraction technique (below) concentrates and isolates the rare (<<1%) and small (<100 µm) volcanic glass shards found in distal deep-sea sediments which can then be analyzed for major and trace element abundances by electron microprobe. The resuling geochemical "fingerprint" allows quantitative correlation to ash layers which are used to date hominid and other fossils preserved in terrestrial sequences throughout NE Africa. |
Tephra Extraction from Deep-Sea Sediment
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Volcanic ash shards were concentrated and extracted from Arabian Sea (Site 721) deep-sea sediments using a sodium polytungstate (Na2WO4) heavy-liquid separation step (density=2.2 g/cc) to separate ash shards and other detrital mineral grains (r >2.5 g/cc) from lighter but very abundant diatoms and radiolaria (r=1.92.2 g/cc). Samples were added to a 2.2 g/cc Na2WO4 solution, sonicated, then centrifuged for 3 minutes. The basal 1 cm of the centrifuge tube (now containing the shards and other detrital mineral grains) was frozen in liquid nitrogen and remaining supernatant (containing diatoms and radiolaria) was decanted and the tube walls were rinsed with deionized water. After thawing the shard-bearing Na2WO4 slurry was then captured and rinsed using a 0.2 µm filter. The final concentrate typically comprised <1% of the original dry bulk sediment mass. After drying these samples were then mounted for microprobe analysis. |
Geochemical Tephra Correlation
Ash shards extracted from deep-sea sediments can be geochemically correlated with ash layers in terrestrial fossil-bearing sequences. Major element chemistries (Fe, Al, Si, Ti, and Ca) from microprobe analyses of the marine tephra are compared to tephra chemistries obtained from several different tephra compositions from terrestrial sequences in Ethiopia, Kenya, and Tanzania.
A quantitative match between the deep-sea composition and a candidate terrestrial tephra is made using the "Similarity Coefficient" statistic which roughly similar to a multiple linear regression correlation coefficient. An SC value of 1.0 indicates a perfect, identical match; values >0.95 are chemically indistinguishable based on the analytical errors of the microprobe measurements. The terrestrial-marine beta-Tulu Bor tephra match shown above had an SC value of 0.98, indicating an excellent geochemical match.
Terrestrial-Marine Correlations
Comparison of radiometric and stratigraphic (interpolated) ages for East African tuffs between 4.0-3.4 Ma and their orbitally-tuned ages derived from the marine sediment chronostratigraphy at Sites 721 and 722 in the Arabian Sea. Tephra correlations between East Africa, the Gulf of Aden, and the Arabian Sea sediments were accomplished using major element oxide compositions (see deMenocal and Brown, 1999; Figures 610; Tables 25). Note that the radiometric and orbitally-tuned ages agree to within their joint error, or generally within 0.01 Ma. New, orbitally-tuned ages can be applied to the Lokochot and Lomogol Tuffs for which direct radiometric age determinations have not yet been possible. Placement of the fossil hominid cranium A.L. 417 (Australopithecus afarensis ) within the context of the eolian dust record at Site 721/722 was constrained by its stratigraphic position relative to the Sidi Hakoma Tuff (SHT, =b-Tulu Bor).
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