David Williamson

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.

13C/Palynological evidence of differential residence times of organic carbon prior to its sedimentation in East African Rift Lakes and peat bogs

Abstract Most terrestrial plants producing large amounts of organic matter in the East African Rift follow the Calvin (C3) photosynthetic pathway. Their end products have δ13C values of ca. −27 ± 2‰ (vs. PDB). On the contrary, most Cyperaceae (notably Cyperus papyrus and C. latifolius) are characterized by higher 13C contents ° 13 C = −10.5 ± 1‰ ) in relation to their Hatch and Slack (C4) photosynthetic cycle. In consequence, δ13C values in total organic matter (TOM) from peat bog or lake cores essentially responded to the proportion of detritus from C4-Cyperaceae. Immediate evidence of the development or disappearance of Cyperaceae around lake margins or in peat bogs can be found in pollen assemblages. Lag times between pollen signals and correlative °13C shifts in TOM from cores are therefore indicative of the residence time of organic matter prior to its sedimentation. Delayed sedimentation of TOM will result in 14C anomalies which depend on several parameters, most of them being site specific as shown by examples from a peat bog in Burundi and from southern Lake Tanganyika. An independent assessment of the chronology by high resolution paleomagnetic correlations indicates a ca. 1.5 ka apparent 14C age of TOM in Lake Tanganyika at the Pleistocene-Holocene transition.

Geomagnetic moment variations in the last 70,000 years, impact on production of cosmogenic isotopes

Abstract On various time scales, the intensity of the Earths magnetic field presents large amplitude oscillations due to secular variation of the non-dipole field (short term regional influences) or to variations of the moment of the main dipole (long term global influence). Here we introduce variations of the relative intensity of the local geomagnetic field reconstructed from a sedimentary sequence deposited during the Late Pleistocene in a maar crater lake located in Western Europe: Lac du Bouchet, Massif Central, France. The comparison of these results with absolute palaeointensities and virtual dipole moment (VDM) values obtained from volcanic rocks, as well as with relative palaeointensity variations measured on marine sedimentary sequences, lead to conclude that the geomagnetic dipole was drastically reduced between 43 and 30 ka. In the second part of this article, we discuss the relation of this geomagnetic crisis with the increase of atmospheric concentration of cosmogenic isotopes, recently documented for the relevant time interval. The modulation of the magnetospheric shielding by geomagnetic dipole fluctuations can be retained as the probable origin of such global atmospheric changes.

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.

Lithostratigraphy, volcanism, paleomagnetism and palynology of Quaternary lacustrine deposits from Barombi Mbo (West Cameroon): preliminary results

We present preliminary results from the study of 23.50-m core from Lake Barombi Mbo, representing the last 25,000 years. The lake is in an explosion crater formed during Quaternary time. The very laminated sediment is composed mostly of clay containing 5–10% organic carbon. Each couplet is commonly composed of a basal lamina rich in quartz, plant debris, muscovite and sponge spicules, and of a more clayey upper lamina often with siderite. A perturbed section near the base of the core, before ca. 21,000 yr B.P., could be the result of a violent release of gas, such CO2, comparable to the recent Nyos gas eruption. The paleomagnetic studies exhibit high-frequency oscillations interpreted as paleosecular variations of the local geomagnetic field. This first record obtained on the African continent can be closely compared to the type record obtained in Western Europe. The pollen results demonstrate the presence of a forest refuge in West Cameroon during the last great arid period, ca. 18,000 yr B.P. When equatorial forest was broken up, elements of montane vegetation spread to the lowlands. These phenomena resulted from a drying and cooling climate.

AMS-14C chronology of 40.0 cal ka BP continuous deposits from a crater lake (Lake Massoko, Tanzania): Modern water balance and environmental implications

Abstract Lake Massoko, Tanzania (8°20′S, 33°45′E; 870 m a.s.l.) is a freshwater maar lake belonging to the Rungwe volcanic area, 15 km northwest of the Livingstone Mountains and Lake Malawi. The rainfall regime is driven by the yearly oscillation of the Intertropical Convergence Zone, which determines a wet summer and a dry and windy winter. The presence of Lake Malawi to the south, and high ranges to the north result in a higher annual precipitation than that observed at a regional scale. Air masses overloaded with humidity bypassing Lake Malawi are submitted to ascending currents, producing rainfall up to 2400 mm yr−1 in the Lake Massoko area. Important exchanges between the lake and groundwater reservoirs have been demonstrated from hydrological and isotopic balances of Lake Massoko on a regional scale. These exchanges are superimposed on groundwater circulation. The hydrological setting of Lake Massoko is also influenced by the tectonics of the region dominated by the northwest/southeast tilt of tectonic blocks. The sedimentary sequences of Lake Massoko cored in 1995 and 1996 under the EEC-Project RUKWA [ RUKWA Project – Final Report, 1997 ] likely register the palaeoenvironmental changes on the catchment. The sediment record is nearly continuous due to the near-constant high water level of the lake. In contrast with many other lacustrine sequences, in which reduced deposition rates, discrepancies and/or gaps characterise arid periods such as the Last Glacial Maximum and the Younger Dryas event, all the AMS-14C dates on authigenic organic samples fit with the stratigraphy, and the palaoenvironmental record of Lake Massoko exhibits a constant mean sedimentation rate of 0.628 mm yr−1 for the last ca. 40 cal ka BP.

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.

Evidence for an early land use in the Rhône delta (Mediterranean France) as recorded by late Holocene fluvial paleoenvironments (1640-100 BC)

Abstract The overall objective of this paper is to describe the late Holocene (1640–100 BC) sedimentary and biological evolution of the Rhône–delta–plain, to interpret the sedimentary facies and palynofacies as the result of the effects of fluvial dynamic fluctuations and relative sea level change and to evaluate the paleohydrological constraints in the development of the land use and settlements of the Camargue. Focus is made on the upper part of VIII core drilled on NE of the Vaccares lagoon. By combining sedimentology, palynology, magnetic susceptibility and archeological data, this study allowed to identify the superposition of three types of paleo-environments (marsh, fluvial floodplain, levee/crevasse splay). This sequence indicates a gradual extension of fluvial environments between the end of the second millennium BC and the 1st century BC. The variability of fluvial dynamic is evident during this period with important flood events which contrast with periods of low flow. Pollen record can be a good marker of the fluvial dynamic variability. The expression of the riparian tree pollen grains in the coarser floodplain deposits could correspond to increased fluvial influence and probably to erosion of riverbank during flood events. The local plants are associated to the low energy sedimentary environments. Focuses are made on the relations between the evolution of the environment and land use. The development of the cereal culture in the floodplain of the Rhone delta has been demonstrated between 1640–1410 and 100 BC. The last alluviation of the Rhone perturbs the research of the archaeological sites in the central part of the delta but the existence of the rural villages from the first part of the first millennium BC is highly possible.

QUATERNARY PALAEOCLIMATES IN THE EASTERN CANARY ISLANDS

Abstract In the eastern Canary islands, aeolian formations interbedded with palaeosols have been studies. The sequences cover the last 40 ka, as evidenced by one 14 C accelerator analysis and several classical radiocarbon dates. As in the Sahara, at least two wet episodes characterize isotopic stage 3. A short pedogenesis corresponds to the first deglaciation step at 15 ka. After a new aeolian episode, the Holocene wet phase begins at around 10 ka. The aeolian sedimentation is of Saharian dust, in particular during the stage 3 dry episode(s). U Th and OSL dating are current.