40Ar/39Ar Dating of Laetoli, Tanzania

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Abstract

40Ar/39Ar dating of Pliocene tuffs from Laetoli, northern Tanzania, has refined the geochronological framework of the Laetolil Beds and overlying strata. Dated units include the Lower and Upper Laetolil Beds (4.36−3.85 Ma and 3.85−3.63 Ma, respectively), the Lower and Upper Ndolanya Beds (3.58 and 2.66 Ma, respectively), the Naibadad Beds (2.155−2.057 Ma), and the Olpiro Beds (<2.057 Ma). Sedimentation rates of ∼50 cm/ka are obtained for the lower half of the Upper Laetolil Beds through Tuff 2, compared to a rate of ∼15 cm/ka for the upper half (Tuff 4 to Tuff 8). Accumulation of the Upper Laetolil Beds required less than 220 ka, corresponding to a minimum overall sedimentation rate of 22 cm/ka. The new dates provide a more precise assessment of the age of the fossil hominins at Laetoli. Australopithecus afarensis specimens from the Upper Laetolil Beds are constrained to 3.85−3.63 Ma, and a new interpolated age for the Footprint Tuff (Tuff 7) is 3.66 Ma. The Paranthropus aethiopicus maxilla (Silal Artum) from the Upper Ndolanya Beds at 2.66 Ma is amongst the oldest known specimen attributable to this genus.