New magnetostratigraphy for the Olduvai Subchron in the Koobi Fora Formation, northwest Kenya, with implications for early Homo
A problematic magnetostratigraphy for the Koobi Fora Formation has contributed to debates on the evolutionary implications for early hominin fossils. To address this, 50 independent samples distributed over a nearly 63-m-thick interval were collected from the lower-middle KBS Member type section in fossil collection Area 102, northeast Turkana Basin. Characteristic directions obtained by thermal demagnetization define a coherent magnetostratigraphy that is supported by alternating-field studies on 28 sister specimens and the prior tephrochronological framework. Two long polarity intervals were recognized, each 30-40 m in thickness, and interpreted as the upper part of the normal polarity Olduvai Subchron and the overlying reverse polarity Matuyama Chron. The end Olduvai consists of a normal-reverse-normal polarity sequence occurring over a thickness of at least 1 m but perhaps up to 5 m, suggesting that this subchron has a short reverse interval in its uppermost part. Such a fine-scale structure also has been reported from several other sites, like the Pliocene-Pleistocene boundary and point stratotype section at Vrica, Italy, which serves as a basis for formally delimiting three temporally discrete polarity subintervals for the Olduvai Subchron. These paleomagnetic results that place the upper boundary of the Olduvai at ∼ 48 m above the base of the KBS Member, coupled with published radioisotopic dates, firmly secure the age of partial cranium KNM-ER 3733 in the interval 1.78-1.48 Ma, with an interpolated age of ∼ 1.7 Ma, giving this fossil the most unambiguous numerical-age constraints, as compared to the oldest Homo cranial remains from Europe and Asia. Nonetheless, assured placement of the top of the Olduvai Subchron in the KBS Member is not sufficient in the face of other uncertainties to influence conventional interpretations of the timing and direction for the global dispersal of early Homo erectus.