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Tephrochronology of Bed I, Olduvai Gorge: an Application of Laser- Fusion 4°Ar/39Ar Dating to Calibrating Biological and Climatic Change

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Tephrochronology of Bed I, Olduvai Gorge: an Application of Laser- Fusion 4°Ar/39Ar Dating to Calibrating Biological and Climatic Change Quaternary International, Vol. 13/14, pp. 37-46, 1992. 1041b-6182/92 $15.00 Printed in Great Britain. All rights reserved. © 1992 INQUA/PergamonPress Ltd TEPHROCHRONOLOGY OF BED I, OLDUVAI GORGE: AN APPLICATION OF LASER- FUSION 4°AR/39AR DATING TO CALIBRATING BIOLOGICAL AND CLIMATIC CHANGE R.C. Walter,* P.C. Manegat and R.L. Hays *Geochronology Center, Institute of Human Origins, Berkeley, CA 94709, U.S.A. t Department of Geology and INSTAAR, University of Colorado, Boulder, CO 80309, U.S.A. ~Department of Geology, University of lllinois, Urbana, IL 61801, U.S.A. New geochronological data indicate that hominid-bearing deposits of middle to upper Bed I at Olduvai Gorge comprise an extremely brief interval of time, from about 1.80 to 1.75 Ma. Dates for lower Bed I suggest that the age of the lower boundary of the Olduvai subchron is >1.98 Ma, over 100 ka older than the currently accepted value. The results of this study enable rates of biological, archaeological, geological, and climatic change during Bed I times to be determined with great precision. The new date for the base of the Olduvai subchron has important implications for calibrating the age of marine and continental deposits that rely on the Olduvai subchron for chronologic control. INTRODUCTION rocks of the Pan-African Mozambique Belt (Hay, 1976). The entire depositional sequence is roughly Olduvai Gorge is the premier fossil hominid site 100 m thick, and is composed of lacustrine, lake in East Africa. The extensive paleontological and margin, fluvial, and alluvial deposits with numerous archaeological collections from this site provide drama- interbedded primary and reworked tephras. The tic evidence for early hominid evolution (cf. Hay, 1976; deposits are divided into seven beds, but stratigraphic Leakey, 1967, 1971; Tobias, 1967). In addition, the correlations are locally complicated by faulting, abrupt deposits of Bed I contain a record of paleoenvironmen- facies changes, and disconformities (Hay, 1976). tal change that can be related to global paleoclimate Bed I contains five lithofacies (lava, lacustrine, lake through correlation of the Olduvai geomagnetic event margin, alluvial fan, and alluvial plain deposits), and (subchron) with marine deposits of comparable age six widespread tephra units that permit stratigraphic (Kappleman, 1986). correlations to be made throughout the Gorge (Tufts Despite extensive dating studies at Olduvai Gorge IA-IF) (Hay, 1976). The eastern alluvial fan facies is over the past 30 years (Curtis and Hay, 1972; Evernden composed of thick volcaniclastic sediments and debris and Curtis, 1965; Leakey et al., 1961, 1962) the detailed flows derived from the adjacent Ngorongoro volcanic chronology of the deposits is largely unknown. This is highland. This clastic wedge thins to the west where it because most of the tephra units are reworked interdigitates with fossiliferous lake margin sediments volcaniclastic deposits, and contain detrital minerals around the junction of the Main and Side Gorges (Loc. that cause spuriously old K-Ar results (Hay, 1976). 45; Fig. 1). Farther to the west, lake margin sediments The purpose of this manuscript is to present new grade into fine grained lacustrine deposits. In the isotopic age data and to elaborate on the dating results alluvial fan facies (eg. Loc. 29) (Hay, 1976), Bed I, recently reported for Bed I based on 4°mr/39Ar from Tuff IB to Tuff IF, is roughly 30 m thick. In the analyses using the laser-fusion technique (Walter et al., lacustrine deposits 10 km to the west (Loc. 80) the same 1991). This technique is well suited for detecting and stratigraphic interval is only 6 m thick (Fig. 1). avoiding contamination (Bogaard et al., 1987; Lo Bello In pioneering dating studies, Evernden and Curtis et al., 1987). We have now dated all the major tephro- (1965) determined K-Ar dates on tephra samples from stratigraphic units in Bed I, and constructed a detailed Bed I and Bed II. Subsequent work by Curtis and Hay age framework for the associated faunal and archaeolo- (1972) showed that most of the tephras produced gical discoveries. anomalously old dates due to detrital contamination. Only Tuff IB, an ignimbrite, produced consistently BACKGROUND reliable dates of about 1.84 + 0.03 Ma (Curtis and Hay, 1972) (corrected for revised 4°K decay constants of Olduvai Gorge is a 20 km long, narrow, deeply-cut Steiger and Jager, 1977). valley in the eastern Serengeti Plain. The Gorge Paleomagnetic studies by Gromm6 and Hay (1963, exposes Pleistocene to Holocene sediments that overlie 1967, 1971) revealed a period of normal polarity Plio-Pleistocene ignimbrites and lava flows. These recorded by the lava and tufts in lower Bed I to lower volcanic deposits unconformably overlie metamorphic Bed II. K-Ar dates (Curtis and Hay, 1972; Evernden 37 38 R.C. Walter et al. "AT E A S T L U I I I I I -T Fal :ond . ault Loc. 66b Loc. 80 Loc. 45 Loc. 29 ~~ Tuff IF ............ TufflF (1.74) ...... Tuff IF (1.75) ....... (1.74) I_._ Tuff ID Tuff ID (1.76) Tuff IC (1.78) (1.75) Tuff IA (1.98) k~Tuff IC (1.76) Tuff IB (1.79) /~ OH-5, 0H-62 I ) (1.76) ------4 ,mite OH-7 ~ Tuff IB (1.87) I lU 4-- CFCT (2.02) Tuff IA (1.98) Tuff IB ~-~-- Naabi (2.03) 15m Upper Beds i Tuff ~ Paleomagnetic site Beds Ill & IV i Lava (1.74) SCLF date (Ma) 1.87) i Bed II ~ Ignimbrite OH-5 Hominid Level f (~) ~-Ignimbrite (2.06*) Bed I ~ Pan-African FIG. 1. A palinspastic reconstruction of Olduvai Gorge stratigraphy, with summary SCLF results for key Bed I sections. The view is of the north wall of the main Gorge. Numbers above the section refer to geological localities of Hay (Hay, 1976; Leakey, 1971). Major tephrostratigraphic units are shown, along with a compilation of the known magnetostratigraphy (Gromm6 and Hay, 1963, 1967, 1971; Hay, 1976), and the stratigraphic position of several key hominid discoveries (Leakey, 1971). and Curtis, 1965) placed this interval of normal polarity measured on a MAP-215 noble gas mass spectrometer within the Matuyama reversed chron. Based on these fitted with a Johnston electron multiplier operating at a observations, the Olduvai subchron was established gain of about 30,000. and subsequently dated to between 1.87 and 1.67 Ma The analyses and data collection are fully automated, (Mankinen and Dalrymple, 1979). with one analysis lasting about 20-30 min. Typical system blank volumes of 4°Ar, 39Ar, 3SAr, 37Ar, and METHODS 36Ar, which are automatically and routinely measured every 3 samples, are 4, 2, 0.08, 0.3, and 0.3 × 10 -17 Samples for 4°Ar/39Ar analyses were irradiated for moles, respectively. Sanidine from the Fish Canyon 0.25-0.5 hr at 8 MW at the Omega West research Tuff is used as the neutron fluence monitor, which has a reactor of the Los Alamos National Laboratory, which reference age of 27.84 Ma (Deino et al., 1990). Errors has a fast neutron fluence of 5.7 × 1013n cm-lsec -1. for each analyses (lo) reflect errors in J as well as in Cadmium shielding was used to reduce the thermal the determination of Ar isotopic ratios, which in turn neutron production of 4°Ar. After irradiation, the propagate errors in discrimination (D) and errors in Ar samples were transferred to a copper holder and loaded beam intensities from sample and blank (Deino et al., onto the extraction line for overnight bakeout at ca. 1990). The weighted mean age and uncertainty, which 200°C. are the preferred age and error estimates (Tables 1 and Presently, up to 220 individual samples can be loaded 2), are computed using an inverse variance weighting at one time. Single grain samples are fused with an 8 W factor, which uses deviations about a weighted mean to argon-ion laser. The volume of each grain is typically determine the weighted uncertainty (Deino et al., 1990; < 1 × 10 -3 cm 3, corresponding to a sample mass of Samson and Alexander, 1987). Results of the SCLF < 2.5 mg per grain. Abundances of the Ar isotopes are analysis are graphically displayed using either an Ar Tephrochronology of Bed I, Olduvai Gorge 39 isotope correlation diagram (Bogaard et al., 1987) or a 10 tephrostratigraphic units and lavas spanning the probability-density function (cf. Deino and Potts, thickness and lateral extent of Bed I. In this study, 1991). correlative Bed I tephras from various depositional facies were sampled to test stratigraphic correlations NEW RESULTS and the concordance of the results. Table 1 and Fig. 1 summarize the results of the Naabi Ignimbrite single-crystal laser-fusion (SCLF) 4°Ar/39Ar dating The Naabi Ignimbrite, beneath Bed I in the western study, which was performed on over 30 samples from part of the Gorge (Hay, 1976), is one of the oldest TABLE 1. Summary of Mean SCLF Ages for Bed I Tephras and Lavas Laser-Fusion Ar40/39 Results Inverse Ar Isotope Correlation Results Unit Locality Sample # L # Ca/K Age (Ma) + n Age (Ma) _+ (Ar40/36)i +_ MSWD n Tuff IF 80 OG89-15 2474 0.22 1.740 0.016 6 1.77 0.04 283 15 0.4 6 Tuff IF 45c OG89-8 2463 0.33 1.741 0.018 9 1.74 0.02 298 19 3.3 9 Tuff tF 45c OG89-8 2465 0.25 1.760 0.055 4 1.79 0.15 293 8 2.0 4 Tuff IF 40 OL86-47C 1461 0.22 1.761 0.013 10 1.79 0.03 267 22 0.9 10 Tuff IF 5 OG89-35 2484 0.37 1.740 0.023 7 1.76 0.02 268 14 0.8 7 Avg/SD 1.748 0.011 1.770 0.021 Wtd Mn/SEM 1.749 0.005 36 1.759 0.009 Tuff IE 29E OG89-32 2480 0.35 1.734 0.030 5 1.69 0.04 324 16 0.2 5 Tuff IE 29E OG89-32 2481 0.37 1.839 0.023 5 1.78 0.03 346 19 0.7 5 Tuff IE 29E OG89-32 2482 0.35 1.795 0.017 5 1.79 0.03 302 9 1.5 5 Avg/SD 1.789 0.053 1.75 0.054 Wtd Mn/SEM 1.797 0.030 1.76 0.031 Tuff IE* 29E OG89-32 2480-82 0.35 1.811
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