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Wet phases in tropical southern Africa during the last
glacial period
D Williamson, Joel Guiot, G Buchet, Y Garcin, A Vincens
To cite this version:
D Williamson, Joel Guiot, G Buchet, Y Garcin, A Vincens. Wet phases in tropical southern Africa during the last glacial period. Geophysical Research Letters, American Geophysical Union, 2006, 33 (7), �10.1029/2005GL025531�. �hal-01457691�
Wet phases in tropical southern Africa during the last glacial period
Yannick Garcin,1Annie Vincens,1 David Williamson,1Joe¨l Guiot,1and Guillaume Buchet1
Received 19 December 2005; revised 9 February 2006; accepted 1 March 2006; published 4 April 2006.
[1] A 45,000-year high-resolution sedimentary record
from Lake Masoko in Tanzania shows the climate in this part of East Africa to have been characterized by a short and less severe dry season during the Last Glacial Maximum. Moisture and lake-level proxies, pollen assemblages and magnetic susceptibility, indicate that rainfall in the Masoko area was strongly controlled by low-latitude insolation (i.e., precessional forcing). Such observations contrast with the climatic patterns previously reconstructed in East Africa for the last glacial. Indeed, widespread dry conditions are generally observed and are attributed to the predominant control of high latitude glacial forcing over insolation forcing on the tropical hydrology. However, during the Younger Dryas cold event, out of phase with regional insolation behaviour, wetter conditions prevailed in this area, suggesting that the rainfall belt over Africa was probably shifted to the South, bringing moisture to the southernmost tropics. Citation: Garcin, Y., A. Vincens, D. Williamson, J. Guiot, and G. Buchet (2006), Wet phases in tropical southern Africa during the last glacial period, Geophys. Res. Lett., 33, L07703, doi:10.1029/2005GL025531.
1. Introduction
[2] The tropics are the main source of heat and water
vapour for the earth’s atmospheric convection. For this reason, knowledge of their long-term behaviour is essential for the reconstruction of climate. Over the last three decades, past environmental and climate studies performed on sedimentary archives recovered from lakes, peat bogs and swamps across tropical Africa have shown that during the cold Last Glacial Maximum [LGM = 19 – 23 calendar kilo years before present (cal. ka BP)] and Younger Dryas (YD = 11.5 – 12.8 cal. ka BP), most of the East African lakes from the Intertropical Convergence Zone Area were low [Barker and Gasse, 2003; Gasse, 2000; Roberts et al., 1993; Thevenon et al., 2002] and dry [Johnson et al., 1996]. The few paleoclimatic records available show that during these two key episodes, tropical Africa in both hemispheres was drier with lower land [Bonnefille et al., 1992; Powers et al., 2005] and sea-surface [Sonzogni et al., 1998] temperatures than now. However, during the LGM, summer insolation under the control of orbital precession [Berger, 1978] was at a maximum in the southern tropics, and might normally have been expected to strengthen the summer African monsoon [Kutzbach et al., 1993], resulting
in higher rainfall inland. Such patterns are observed in the American tropics where contrasting LGM climates in the southern (wetter) and northern (drier) tropics attest to a strong solar radiation forcing [Baker, 2002]. The dry LGM observed at some sites in tropical Southern Africa has been generally associated with a dominance of glacial boundary conditions forcing (ice volume, CO2and sea level) over low
latitude insolation forcing [Barker and Gasse, 2003; Gasse, 2000].
2. Lake Masoko Record
[3] The long terrestrial record from Lake Masoko
(920.00S, 3345.30E, 840 m a.s.l.) provides new insight into environmental and climate changes for the late Qua-ternary in East Africa. The Masoko maar-crater was formed 50 ka ago in the Rungwe volcanic highlands, 30 km north of Lake Malawi [Gibert et al., 2002]. Regional climate is strongly seasonal and controlled by the strength of the African Monsoon and the migration of the Intertrop-ical Convergence Zone (ITCZ, Figure 1) [Nicholson, 1996]. The rainfall (2,400 mm yr1) occurs during the November to May that coincides with the insolation maximum in the southern tropics. The remaining months are dry, dominated by strong trade winds with up to 100 successive days without rainfall. The Masoko catchment is covered by dense Brachystegia/Uapaca semi-deciduous woodlands, typical of a Zambezian Miombo vegetation [Vincens et al., 2003] characteristic of a severe dry season (>5 months). The lake is sensitive to rainfall changes and its level today fluctuates seasonally between 1 to 2 m.
[4] Here, we present pollen analyses (resolution: 1
sam-ple per 10 cm, i.e., 350 years) coupled with low-field magnetic susceptibility record (resolution: 1 sample per 0.8 cm, i.e.,30 years) performed on a composite sediment core taken from the deepest central part of Lake Masoko (38 m depth). Methods for pollen analyses follow Vincens et al. [2003] and those used for magnetic susceptibility are described by Garcin et al. [2006] and Williamson et al. [1999], and are accompanied by other intensive sedimentological and magnetic measurements supporting our interpretations.
[5] The lake sediments show a constant
accumulation-rate of lacustrine biogenic elements associated with clastic and organic inputs from the catchment. Numerous tephras and turbidites interrupt the lacustrine sedimentation and correspond to event deposits. In order to isolate the undis-turbed sedimentation from volcanic deposits, seismites and other accumulations related to lake-slope instabilities, we have removed all these event-layers from the record. Chro-nology is based on twenty-five accelerator mass spectrom-etry (AMS) radiocarbon ages [Garcin et al., 2006; Gibert et al., 2002].
1Centre Europe´en de Recherche et d’Enseignement de Ge´osciences de
l’Environnement, Centre National de la Recherche Scientifique, UMR 6635, Universite´ Paul Ce´zanne, Aix-en-Provence, France.
Copyright 2006 by the American Geophysical Union. 0094-8276/06/2005GL025531
[6] Pollen analysis (Figure 2a) shows the response of the
vegetation to climate changes and offers further details of changes in the hydrological regime, particularly the duration of the dry season. The sediment low-field magnetic suscep-tibility (clf, Figure 2c), is mainly dependent on the
concen-tration of detrital (Ti-)iron oxides held in the terrigenous clastic inputs. Multi-parameter magnetic measurements, scanning electron microscope observations and geochemical analyses performed on both catchment material and lake sediment indicate that the magnetic signal originates from the coarse 20 –500 mm multi domain titanomagnetite (Fe3-xTixO4) [Garcin et al., 2006; Williamson et al.,
1999]. Field-survey observations on the catchment show that the transfer of this heavy mineral by soil erosion from the crater flanks to the deep lake is primarily constrained by changes in lake-level. Indeed, the strong hydraulic conductivity of the catchment materials (>1 m h1) prevents runoff erosion processes on the crater-slopes. There-fore, due to the small catchment area (20 ha against 36 ha for the lake surface area), the inputs of terrigenous materials into the lake are rather controlled by lakeshore dynamics than by runoff processes on the whole catchment. As observed today, high amplitude seasonal lake oscillations coupled with wind-driven turbulence at the lakeshore during the dry season allow the transport of the titanomagnetite to the deep lake. Lake lowstands act in the same way because they reduce the distance between the lakeshore terrigenous source and the deep lake, enhancing the supply of dense catchment material. Therefore, the sediment magnetic susceptibility can be inter-preted in terms of hydrological regime: high susceptibilities indicate stronger seasonal fluctuations and/or lake lowstands, while low susceptibilities indicate lower seasonal fluctuations and/or lake highstands.
[7] From 45 to 23 cal. ka BP, the low frequencies of
Zambezian tree taxa suggest that the Masoko area was dominated by open savannah vegetation characteristic of a semiarid climate with a very long dry season. The rather high susceptibility values during this dry period could mark a period of enhanced seasonality and/or a lake lowstand, but also of strengthened soil erosion on the catchment associ-ated with the reduced tree cover. The general upward decrease of the high amplitude susceptibility variability at this time is probably a result of the geomorphic evolution of the crater, which smoothed the slopes reducing their sensi-tivity to erosion pulses. Woodland development started
about 23 cal. ka BP, at the time of the LGM, with the increase of tree taxa such as Macaranga, Ulmaceae (Celtis and Trema) and then Moraceae (Myrianthus, Trilepisium, Milicia and Ficus). Such vegetation requires a humid climate for its development, with a shorter (<3 months) and less severe dry season [Berg and Hijman, 1989; Polhill, 1966; Radcliffe-Smith, 1987; White, 1983]. The lowest susceptibilities are observed during the LGM, suggesting a stable lake highstand. The slow decrease of the woodland Figure 1. (a) Present ITCZ seasonal migration over Africa
(gray lines). This belt of moisture and low pressure moves across the equator, roughly following the Sun’s zenith point. (b) Location of the study area (star) in East Africa. Black surfaces are the main regional lakes including the Lake Malawi.
Figure 2. Comparison of Masoko paleoenvironmental data with reconstructed climate-proxy data. (a) Percentage pollen diagram of selected taxa for the Lake Masoko. Sums based on 500 pollen grains. (b) January solar insolation for 9S [Berger, 1978] (gray line). (c) Masoko magnetic susceptibility, 50-year averages (turbidites- and tephras-free record, reversed scale). Triangles are Masoko calibrated radiocarbon ages [Gibert et al., 2002]. (d) Lake Malawi TEX86 surface temperature [Powers et al., 2005].
(e) Oxygen isotopes of the GISP2 ice core from Greenland [Stuiver and Grootes, 2000]. The LGM and the YD intervals (black labels) are depicted with vertical gray bands.
L07703 GARCIN ET AL.: WET PHASES IN TROPICAL SOUTHERN AFRICA L07703
trees after 19 cal. ka BP follows the enhancement of the susceptibility signal. This indicates respectively more open vegetation and increase of seasonal lake fluctuations reflect-ing a change toward drier conditions. The progressive reforestation after 15 cal. ka BP coincides with the onset of the ‘African Humid Period’ [deMenocal et al., 2000] and terminates in peak of Macaranga frequencies around 12.7 cal. ka BP, followed by peaks of Moraceae and Ulmaceae between 12.5 and 12 cal. ka BP. The occurrence of Urticaceae taxa during this time, which indicates closed habitat [Friis, 1989], supports the episode of reforestation. Magnetic susceptibility abruptly falls at 12.9 cal. ka BP and remains low until 11.7 cal. ka BP. The culmination of this episode of wooded vegetation and susceptibility changes is synchronous with the YD event and suggests wetter con-ditions with a short dry season at least as well marked as the LGM. The end of this episode around 11.7 cal. ka BP coincides with the beginning of development of Uapaca woodlands (Uapaca kirkiana and Uapaca nitida) and the sharp increase in susceptibility, synchronously with the Holocene period onset. Uapaca woodlands are characteris-tic of contrasted climate with a severe dry season (>5 months) [Carter and Radcliffe-Smith, 1988; White, 1983]. The persistence of Uapaca woodlands during the Holocene suggests the occurrence of a well-established seasonal climate, associated with pronounced migrations of the ITCZ. The generally high Holocene susceptibilities indicate high amplitude seasonal lake oscillations with wind-stress during the dry season.
[8] At Masoko, climatic conditions during the
mid-Holocene were closer to those of the period 25 – 40 cal. ka BP than those of the LGM. However, the scarcity of Uapaca between 25 and 40 cal. ka BP indicates drier and probably cooler conditions than during the mid-Holocene, which favoured the development of open wooded grass-lands rather than woodgrass-lands.
3. Discussion and Conclusion
[9] A comparison of the Masoko susceptibility time
series with the precession dominated January insolation (austral summer) at 9S shows that the main periods of a long dry season and a short dry season described above are roughly in phase with the insolation minima and maxima (Figures 2b and 2c). At orbital scale, such an impact of the insolation on the tropical climates as already been demon-strated in several parts of Africa [Partridge et al., 1997; Thevenon et al., 2002; Trauth et al., 2003]. A well-marked wet event at Masoko is out of phase with the insolation but coincides with a sharp minimum in the Greenlandd18O data (Figure 2e), indicating the Younger Dryas cold event, known as a massive discharge of ice sheet meltwater on North America [Broecker et al., 1988].
[10] In East Africa, the ITCZ seasonal migration controls
the modern rainfall, which in turn controls lake-levels and vegetation evolution. Our data suggest that during the last 45 ka, the ITCZ dynamics were significantly modified; firstly by low-latitude insolation changes and secondly by Northern Hemisphere glacial conditions. The regional pale-oclimatic data located north of Masoko, even in southern latitudes, indicate lowstands and drier climates during the LGM and YD [Bonnefille et al., 1992; Gasse, 2000;
Johnson et al., 1996; Roberts et al., 1993; Thevenon et al., 2002]. In the Masoko area, however, moister conditions and weaker seasonal contrasts are inferred during these two episodes. We therefore suggest that a southward shift of the meteorological equator occurred in East Africa before and during the YD (15 to 11.7 cal. ka BP), resulting in a longer period of rain south of 8S, with possibly 2 wet seasons – as observed today near the geographical equator. Although a full picture of the mechanism still remains to be established, we propose that the abrupt cooling of the Northern Hemi-sphere during the YD may have reversed the interhemi-spheric thermal gradient, shifting the meteorological equator to the South. During the LGM, the Masoko climate was probably controlled by solar radiation forcing, because in phase with the summertime insolation maximum but also partly by glacial forcing, as observed during the YD, both acting to increase moisture in this area. The moister LGM and YD climates at Masoko support a latitudinal southward shift of the equatorial climatic belt in East Africa. Similar ITCZ shifts have already been highlighted by climatic recon-structions from South America [Lea et al., 2003].
[11] Theses results are not coherent with the LGM
regional lake lowstands as reconstructed from the diatom-inferred water conductivity in the relatively deep freshwater Lake Masoko [Barker et al., 2003], or the frequency of light-demanding diatom assemblages in the large Lake Malawi [Gasse et al., 2002; Johnson et al., 2002]. In both lakes, apparent increases in water conductivity and depth of the photic zone (that varies seasonally by a factor 4 in Lake Malawi [Hecky and Kling, 1987]), as well as changes in planktonic productivity are also constrained by the seasonal wind regime, namely the stability of the lake water column during the dry season, regardless of the lake-level. Water column mixing processes in the Masoko and Malawi oligotrophic freshwater lakes may have modified the ben-thic and planktonic diatom assemblages in other ways than formerly assumed. For example, a relationship between increases in nutrient inputs, benthic productivity and lake stratification during wet intervals may have caused the observed changes.
[12] Alternatively, an impact of the regional cooling
during the LGM [Powers et al., 2005] (Figure 2d) on the Rungwe area should not be excluded. This cooling could have modified the hydrology of these highlands, by increas-ing local rainfall – in association with evaporation changes over Lake Malawi.
[13] Acknowledgments. We thank M. Taieb and the ECC-RUKWA project team, who collected the cores in 1994 and 1996, P. E. Mathe´, L. Bergonzini, S. Kajula, M. Decobert for technical help, and S. Brewer, P. Barker, M. R. Talbot, M. A. J. Williams and two anonymous reviewers for constructive comments. We acknowledge the support of COSTECH and IRA from Tanzania, the French CLEHA-ECLIPSE-INSU program, the RESOLVE project of the French Ministry of Research, the PNEDC/INSU project ECHO and the EU project MOTIF (EVK2-CT-2002-00153).
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