Maurice Taieb

Early Pleistocene Presence of Acheulian Hominins in South India

Dates from a site in southeast India imply an early migration of Homo through Eurasia about 1.1 to 1.5 million years ago. South Asia is rich in Lower Paleolithic Acheulian sites. These have been attributed to the Middle Pleistocene on the basis of a small number of dates, with a few older but disputed age estimates. Here, we report new ages from the excavated site of Attirampakkam, where paleomagnetic measurements and direct 26Al/10Be burial dating of stone artifacts now position the earliest Acheulian levels as no younger than 1.07 million years ago (Ma), with a pooled average age of 1.51 ± 0.07 Ma. These results reveal that, during the Early Pleistocene, India was already occupied by hominins fully conversant with an Acheulian technology including handaxes and cleavers among other artifacts. This implies that a spread of bifacial technologies across Asia occurred earlier than previously accepted.

The sensitivity of a Tanzanian crater lake to catastrophic tephra input and four millennia of climate change

Diatom genera in many large East African lakes change little throughout the Holocene period suggesting relatively stable ecological conditions and some resilience to environmental change. Ecosystem stability is less common in smaller, more sensitive lakes, such as those within volcanic craters, where external impacts can cause abrupt and rapid fluctuations. A 4100-year diatom and cyanobacteria pigment record from Lake Massoko, a volcanic crater lake in southern Tanzania, is used to illustrate important switches in resource ratios following tephra deposition 1190 years ago. It is hypothesized that the tephra reduced the rate of P diffusion from the sediments and increased the Si:P ratio in the lake. A period of acute change in planktonic diatom communities resulted from the tephra impact and lasted c. 110 years. The magnitude of the change shown by the diatoms and their slow recovery from the tephra may be due in part to a coincident fall in lake level caused by a reduction in regional rainfall. The statistical significance of the tephra impact relative to that of catchment and climate change has been tested using variance partitioning and rate-of-change analysis. Multiproxy indicators show an important period of positive water balance 1700 ago and a relatively dry episode persisting between 1000 and 400 years ago. The lake ecosystem is shown to be highly sensitive to both climate change and tephra deposition.

Pollen-based vegetation changes in southern Tanzania during the last 4200 years: climate change and/or human impact

The age-constrained pollen data of a sedimentary sequence from the crater Lake Masoko, southern Tanzania (9‡20PS, 33‡45PE, 770 m), display a continuous record of vegetation for the past 4200 years. This record provides evidence that wetter Zambezian woodlands always occupied this area during the late Holocene, reaching a maximum extent between 2800 and 1650 cal yr BP related to increase in summer monsoon intensity. However, three main episodes of decline have been detected, between 3450 and 2800 cal yr BP, between 1650 and 1450 cal yr BP and from 1200 to 500 cal yr BP, for which a climatic interpretation, decrease in the summer monsoon strength, was preferentially advanced. The first is synchronous with lowstand of many tropical African lakes and, so, mainly induced by increased aridity. In contrast, the abrupt change in the pollen record at 1650^1550 cal yr BP is marked by a large extension of grasslands at the expense of arboreal cover, further by an increase in Ricinus communis and an intensification of burning. It could thus indicate local clearance of vegetation by man. However, at the same time, the decline of montane forest suggests the impact of a more regional change. During the last episode, between 1200 and 500 cal yr BP, dry climatic conditions are inferred from a combination of pollen, diatom and magnetic proxies, although the occurrence of Late Iron Age settlements in the region means that local human interference cannot be excluded. This study illustrates the difficulties in deciphering ecological and anthropological changes from pollen data in African tropical regions. A 2003 Elsevier B.V. All rights reserved.

A 22 kyr BP sedimentological record of Lake Rukwa (8°S, SW Tanzania): environmental, chronostratigraphic and climatic implications

Abstract The sedimentological study of a 12.8 m long core (R96-I) from Lake Rukwa (Tanzania, 8°S) provides a new record of past lake-level fluctuations that took place in response to changes of the regional climate since the last glacial period. From 21 to 15 cal kyr BP, nearshore and swamp/marsh environments are evidenced from clastic deposition, macrophytes debris and Ca–Mg carbonate enrichments. A transgressive sequence, starting around 15 cal kyr BP, lead to the humid lacustrine optimum between 15 and 7 cal kyr BP. From 12 to 10 cal kyr BP, anoxic lake bottom environments were favored by the concomitant effects of high primary productivity, increased subsidence and inputs of volcanic ash in the lake. The Middle Holocene (7–3 cal kyr BP) is characterized by high concentrations of silt, carbonates, and low organic content, which indicate the occurrence of relatively oxic, shallow and saline depositional environments, especially around 7 and 3.4 cal kyr BP. Although grainsize and TOC profiles suggest that shallow environments likely persisted in the Upper Holocene, low Mg concentration values for the last 3 kyr may indicate a trend toward relatively more dilute environments. The two main Lake Rukwa low-stand periods, at 21–15 and 7–3 cal kyr BP, correspond remarkably well with the insolation maximum at the Equator, which occurred at 17 kyr BP for the spring equinox, and at 6 kyr BP for the autumn equinox, respectively. We suggest that, during these periods of minimum inter-hemispheric insolation gradients, monsoon circulation weakened in south equatorial regions, and the ITCZ was located north of 8°S in east Africa.

Equatorial extension of the younger Dryas event: rock magnetic evidence from Lake Magadi (Kenya)

Abstract Magnetic parameters measurements (magnetic susceptibility χ and Isothermal Remanent Magnetizations (IRMs) and sedimentological analyses have been performed on Late Pleistocene/Holocene laminated deposits from Lakes Natron and Magadi (Kenya). Accelerator Mass Spectrometry (AMS) 14 C dating on the Total Organic Matter (TOM) locates a continuous laminated sequence in the 12-10 ka B.P. time interval. The results show the occurrence of a two-stepped lacustrine optimum. A first humid period, resulting in a high-stand above a 635 m altitude geographical barrier, reached its maximum between 12 and 11 ka B.P. Between 11 and 10.7 ka B.P., the lake level dropped below the 635 m altitude barrier, which resulted in the separation of Lake Natron and Lake Magadi. A second maximal high stand period occurred from ca. 10 ka B.P., at the beginning of Holocene times. The 11-10 ka arid event recorded in Lake Magadi confirms previous studies which suggested an equatorial extension of the Younger Dryas event. Magnetic parameters and microfacies analyses suggest that this event was produced by a progressive and general weakening of monsoonal rainfall in East Africa.

Taphonomy and diagenesis in diatom assemblages; a Late Pleistocene palaeoecological study from Lake Magadi, Kenya

Many fossil diatom assemblages do not possess a direct modern analogue as a result of taphonomic processes and diagenesis within the assemblage. Some of these problems are illustrated with reference to core material collected from hypersaline Lake Magadi, Kenya, which during the Late Pleistocene experienced major fluctuations in water chemistry and depth. Competing multiple hypotheses are proposed for no analogue assemblages, with selection between these hypotheses being based on the results of interdisciplinary research.

Solar and ENSO signatures in laminated deposits from lake Magadi (Kenya) during the Pleistocene/Holocene transition

Abstract Sedimentological and geochemical analysis of two cores (8.7 and 9.6 m) taken from lake Magadi (Kenya, 1°50′S, 36°15′E) revealed a laminated sequence, particularly between a depth of 10 and 280 cm. This unit documents a wet and high lake level and was dated by accelerator mass spectroscopy 14C dates on total organic matter, at between 12,000 and 10,000 years B.P. Investigation of these laminated deposits shows a pair of alternating microfacies. The first lamina is dark and rich in lacustine organic matter; the second is light and rich in detritus, carbonates and magadiite. Assuming continuous regular sedimentation, counting of laminae controlled by 14C dates gives an average cyclicity of 4.30 yrs for the deposition. Spectral analysis, applied to the thickness variation of laminae, demonstrated other periods of around 6.45 years, 8.23 years, 8.79 years, 9.44 years, 13.82 years, 18.94 years, 24.18 years and 30 years. This multiannual climatic variation could be related to the sunspot cycle and/or its double (11 and/or 22 years), and the ENSO cycles. These periodicities also reflect the interdependence of sedimentation and cyclic astronomical phenomena that, in part, regulate the climate.

Chronological potential of palaeomagnetic oscillations recorded in late quaternary sediments from Lake Tanganyika

Abstract The measurements of Depositional and Post-Depositional Remanent Magnetizations (DRM and PDRM) in two piston cores collected at water depths of 140 m (core MPU3) and 422 m (core MPU12) in the southern Lake Tanganvika (Mpulungu Sub-Basin, Zambia) provide a palaeomagnetic record for south-east Central Africa, which spans over the last 25-5 ka. Ages (on a conventional 14C time scale) for the most characteristic inclination peaks were estimated by correlating the palaeomagnetic record with the Lake Barombi-Mbo (Cameroon) and the Western Europe Secular Variation type curves. AMS 14C ages on Total Organic Matter (TOM) measured in MPU12 core appear to be 2 to 4 ka older in the lower part of the record, from ca. 25 to 15 ka BP, i.e. during the last arid episode of the Late Pleistocene. These apparent 14C ages exceed PDRM acquisition lagtimes (evaluated to be about 800 years at most), and also account for residence time of carbon in TOM prior to its sedimentation. Therefore magnetostratigraphic correlations are believed to provide a better overall chronology for the Late Pleistocene and Early Holocene changes in Lake Tanganyika level. Major drops shown by breaks of deposition in core MPU3 (at 1000 cm and 825 cm sub-bottom) are respectively dated at ca. >21.5 ka BP and between 17 and 15 ka BP.

The 36C1 ages of the brines in the Magadi-Natron basin, East Africa

The depression in the East African Rift which includes both Lake Magadi and Lake Natron forms a closed basin within which almost all the dissolved chloride originates in precipitation, since there is no important source of very ancient sedimentary chloride. This provides an ideal setting for the evaluation of the 36Cl methodology as a geochemical and hydrological tracer. The main source of recent water, as represented by the most dilute samples measured, is characterized by a 36C1/C1 ratio of 2.5 × 10−14, in agreement with the calculated value expected in precipitation. Surface evaporation increases the chlorinity of the local freshwater inflow by about a factor of 110 without changing the isotopic ratio, indicating that little chloride enters the system in the form of sediment leachate. A second type of brine found in the basin occurs in a hot deep groundwater reservoir and is characterized by lower 36C1/C1 ratios (<1.2 × 10-14). By comparing this value with the2.5× 10−14 in recent recharge, one obtains an approximate salt accumulation age of 760 Ka which is consistent with the time of the first appearance of the lake. These older brines also have lower 18O and 2H values which indicate that they were recharged during a climatically different era. The 36C1/C1 ratios in the inflowing waters and in the accumulated brine, together with the known age of the Lake Magadi basin, may be used to estimate the importance of the hypogene and epigene, as opposed to the meteoric, mode of 36C1 production. Such a calculation shows that the hypogene and epigene processes together contribute less than 6% of the total 36C1 present in the lake.

Pleistocene lacustrine stromatolites, composed of calcium carbonate, fluorite, and dolomite, from Lake Natron, Tanzania : depositional and diagenetic processes and their paleoenvironmental significance

Abstract On the western edge of Lake Natron, Tanzania, the lower Pleistocene lacustrine series contains some stromatolitic layers which are mainly composed of fluorite rather than calcium carbonate, as is the case with most stromatolites. Moreover in the uppermost part of some layers, fluorite is replaced by dolomite, but preserving the petrographic texture. The uppermost part of the section (around 50 m above the present lake level) is capped by calcareous stromatolitic benches which were known to delimit the ancient shorelines. These stromatolites are unaltered. The composition of the stromatolitic bodies seems to be controlled by the lake chemistry, which is in turn related to the lake volume. Chemical variations of the lake water might reflect the lake level changes resulting from climatic oscillations and be recorded in the mineral composition of the stromatolites, and therefore provides a paleoenvironmental record of lake water composition as the lake was changing from a large freshwater lake to a playa-lake similar to the present Lake Natron. Our results illustrate depositional and diagenetic processes that may occur in alkaline environments such as possible pseudomorphism of the calcium carbonates, origin of the fluorite-ore and dolomitization.