Tephrostratigraphy and geological context in paleoanthropology

Abstract

The fossiliferous and artifact‐rich sites of East Africa, which are central to our understanding of early hominid evolution, also preserve a detailed record of explosive volcanism. The products of these eruptions, ash, lapilli, and pumice, are collectively known as tephra. They drifted down from the skies or washed down rivers in later rainy seasons, and now provide a key to both dating and correlation, and with them the establishment of a geologic framework for evolution. While some tephra can be directly dated, particularly through mineral phases they contain, the glass component of most ashes has a geochemical fingerprint that is unique to a particular eruption. That fingerprint defines an isochronous marker, a layer in time. By identifying characteristic geochemical signatures from far‐flung localities, geologists can correlate sequences, establish relationships in time, and compile long‐term records from local sections. Much of our understanding of the pattern and timing of Plio‐Pleistocene evolution in East Africa is based on this tephrostratigraphic framework. © 1999 Wiley‐Liss, Inc.

Plio-Pleistocene microtephra in DSDP site 231, Gulf of Aden

“…The microscopic tephrostratigraphy of DSDP Site 231 confirms the evidence for three pulses of three pulses of explosive volcanic activity from 4.0 to 3.2 Ma, c. 2.5 Ma and 1.7 to 1.3 Ma described in the Turkana Basin (Haileab and Brown, 1994;Feibel, 1999). The first peak in volcanic activity is seen with widespread, large volume tephra deposits between 4.0 to 3.2 Ma in both the Turkana Basin and DSDP Site 231.…”

“…5). In the Turkana Basin the highest Plio-Pleistocene tephra frequency is recorded between 1.75 and 1.25 Ma (equivalent to one eruption every 3 ka), with many additional tephra deposited throughout the interval 2.0-1.25 Ma (Feibel, 1999;McDougall and Brown, 2006). A similar pattern is documented near Konso, Ethiopia, where rifting and volcanic activity intensified in the early Pleistocene (between 1.9 and 1.4 Ma) in the southern Main Ethiopian Rift (Katoh et al, 2000;Nagaoka et al, 2005;WoldeGabriel et al, 2005).…”

“…Averaged geochemical data are presented for a given tuff where the shard population is unimodal, polymodal distributions were first divided into modes principally on the basis of iron and aluminum content before averaging (Table 2). Several samples yielded polymodal shard populations which most likely indicate incorporation of tephra erupted from several sources, and mixed within the time interval of sedimentary sampling resolution (Feibel, 1999;Shane, 2000). Alternatively polymodal shard geochemistry may result from compositional gradients within the magma of an individual eruption (e.g., Hildreth, 1981).…”

“…The Turkana Basin and Omo Valley preserve the most complete record of eruptions in the East African Rift System with over 150 tuffs (Feibel, 1999). However, variable fluvial transport and accommodation space influences the preservation of tephra such that thickness of a tephra deposit may be unrelated to erupted volume (Feibel, 1999;Pyle, 1999). The record of microtephra abundances in DSDP Site 231 offers another perspective on tephra fluxes from the East African Rift System (Fig.…”

“…These tephra provide initial age control for the sedimentary archive of terrestrial dust and plant leaf waxes in the core, documenting variable environmental aridity and vegetation type in East Africa during the last 4 Ma (deMenocal and Bloemendal, 1995;Feakins et al, 2005). Hundreds of additional silicic eruptions are recorded in East Africa throughout the last 4 Ma (e.g., de Heinzelin, 1983;Brown and Feibel, 1986;Harris et al, 1988;Brown et al, 1992;Feibel, 1999;Katoh et al, 2000). Since many of these Plio-Pleistocene tuffs originate from volcanoes in the Main Ethiopian Rift (Hart et al, 1992), we suspect that tephra from many additional eruptions may have reached DSDP Site 231.…”