Paleoclimatic variations in West Africa from a record of late Pleistocene and Holocene lake level stands of Lake Bosumtwi, Ghana

Shanahan, T. M.; Overpeck, Jonathan T.; Wheeler, Charles W.; Beck, J. Warren; Pigati, Jeffrey S.; Talbot, Michael R.; Scholz, Christopher A.; Peck, John A.; King, John W.

doi:10.1016/j.palaeo.2006.06.007

Cited by 140 publications

(98 citation statements)

References 41 publications

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“…(1) infer increased runoff from 14.5 ka, followed by aridity around 12.9-11.5 ka, a pattern that is consistent with the Atlantic dust flux record (2) and lake fluctuations and sedimentation rate in Lake Bosumtwi, Ghana (13,31). In this context, it is difficult to interpret the 12.9-11.5-ka cluster of ages for Komadugu floodplain sediments (23) as representing lakefull conditions, although clearly some moisture was present in this portion of the Lake Mega-Chad catchment.…”

Section: West African Monsoon Dynamicssupporting

confidence: 68%

West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad

Armitage

Bristow

Drake

2015

Proc. Natl. Acad. Sci. U.S.A.

135

116

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From the deglacial period to the mid-Holocene, North Africa was characterized by much wetter conditions than today. The broad timing of this period, termed the African Humid Period, is well known. However, the rapidity of the onset and termination of the African Humid Period are contested, with strong evidence for both abrupt and gradual change. We use optically stimulated luminescence dating of dunes, shorelines, and fluviolacustrine deposits to reconstruct the fluctuations of Lake Mega-Chad, which was the largest pluvial lake in Africa. Humid conditions first occur at ∼15 ka, and by 11.5 ka, Lake Mega-Chad had reached a highstand, which persisted until 5.0 ka. Lake levels fell rapidly at ∼5 ka, indicating abrupt aridification across the entire Lake Mega-Chad Basin. This record provides strong terrestrial evidence that the African Humid Period ended abruptly, supporting the hypothesis that the African monsoon responds to insolation forcing in a markedly nonlinear manner. In addition, Lake Mega-Chad exerts strong control on global biogeochemical cycles because the northern (Bodélé) basin is currently the world’s greatest single dust source and possibly an important source of limiting nutrients for both the Amazon Basin and equatorial Atlantic. However, we demonstrate that the final desiccation of the Bodélé Basin occurred around 1 ka. Consequently, the present-day mode and scale of dust production from the Bodélé Basin cannot have occurred before 1 ka, suggesting that its role in fertilizing marine and terrestrial ecosystems is either overstated or geologically recent.

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Section: West African Monsoon Dynamicssupporting

confidence: 68%

West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad

Armitage

Bristow

Drake

2015

Proc. Natl. Acad. Sci. U.S.A.

135

116

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“…New field data supported by numerous AMS age determinations confirm that the change coincided with a major lacustrine transgression following a lowstand contemporaneous with the YD [Shanahan et al, 2006]. High lake levels after the YD [Talbot and Delibrias, 1980;Shanahan et al, 2006], and the development of a diverse forest vegetation [Maley, 1991] at the time of sapropel accumulation, indicate that the climate was humid, but the lack of well-developed lamination suggests little seasonality in the runoff that supplied clastic sediment to the lake. Furthermore, the apparent absence of significant vertical mixing indicates that surface winds were too slack and seasonal temperature contrasts too small to cause significant overturn of the water column at that time [Talbot et al, 1984;Talbot and Johannessen, 1992].…”

Section: Lake Bosumtwimentioning

confidence: 83%

An abrupt change in the African monsoon at the end of the Younger Dryas

Talbot

Filippi

Jensen

et al. 2007

Geochem Geophys Geosyst

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[1] High-resolution studies of variations in the elemental and stable carbon-and nitrogen-isotope composition of organic matter in cores from Lakes Malawi, Tanganyika, and Bosumtwi (tropical Africa) indicate an abrupt change in the wind-driven circulation of these lakes that, within the limits of available chronologies, was contemporaneous with the end of the Younger Dryas in the northern hemisphere. The change was also coincident with shifts in surface winds recorded in cores from off the west and northeast coasts of Africa. A range of other proxies indicate that these changes in wind regime were accompanied by a marked increase in precipitation in the northern tropics. Africa south of 5°-10°S, on the other hand, initially suffered drought conditions. Together, the evidence suggests an abrupt northward translation of the African monsoon system at circa 11.5 ± 0.25 ka B.P. The data assembled here contribute to a growing body of work showing that the Younger Dryas was a major climatic excursion in tropical Africa. Furthermore, they add substance to recent suggestions that climatic events in the southern hemisphere may have played a significant role in the abrupt demise of the Younger Dryas.

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“…If this low depth did not prevented anoxic bottom waters during Igornay deposition (Marteau, 1983), in Lally and Muse oil-shales, the absence of benthic fauna and the occurrence of phytoplankton and fishes fossils suggest that the anoxic part of the water column was very close to the bottom of the lake (Gall, 1979;Marteau, 1983). A decrease of the lake level could thus have caused the disappearance of anoxic conditions in the bottom waters such as in the Lake Bosumtwi (Ghana) during the Younger Dryas (Shanahan et al, 2006). Therefore, the increasing trend of biodegraded lacustrine OM concentration can be explained by the shallowing of the lake, which would have, in fine, prevented the presence of perennial anoxic bottom waters.…”

Section: Organic Matter Sources and Maturity Of Oil-shalesmentioning

confidence: 99%

Palaeoenvironmental control on primary fluids characteristics of lacustrine source rocks in the Autun Permian Basin (France)

Garel¹,

Béhar²,

Schnyder³

et al. 2017

Bull. Soc. géol. Fr.

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-The Autunian (Late Gzhelian-Sakmarian, ∼295 Ma) of the Autun Basin (Saône-et-Loire, France) is well known for its lacustrine oil-shales and boghead that were exploited during the 19th and 20th centuries. A study of the kerogen geochemistry, palynofacies, hydrocarbon (HC) distribution of pyrolysates and kinetics on these organic-rich sediments gave indications on the control of lacustrine primary fluid characteristics by palaeoenvironmental settings. Palynofacies results have shown a Botryococcus algae dominance in the boghead, whereas the oil-shales were dominated either by a mixing of terrestrial particles and lacustrine phytoplanktonic/bacterial organic matter (OM), or by a lacustrine OM that suffered biodegradation within the water column. During the Autunian, an increasing trend of degraded lacustrine OM concentration is observed in the successive oil-shales. Geochemical and kinetics analyses showed that, except for the boghead, hydrogen index values were < 700 mg/g TOC and activation energies diagram showed multi-energetic profiles, unexpected for such a lacustrine OM. Along with pyrolysates primary fluids ones, these results unraveled two groups of oil-shale samples. On the one hand, samples dominated by well-preserved lacustrine and terrestrial OM produced primary fluids dominated by n-alkanes with (n-C 6 -n-C 14 ) /n-C 14þ n-alkanes ratio depending on the proportion of terrestrial OM. They were only encountered in the three oldest oil-shales. On the other hand, the HC profile of fluids originating from samples dominated by degraded lacustrine OM, which was obtained for the first time in this study, show low n-alkanes concentrations and a relatively prominent hump. These samples were only present in the three youngest oilshale levels. Finally, the boghead showed a link between strong concentration of the Pila genus of Botryococcus algae and a primary fluid dominated by n-C 6 -n-C 14 n-alkanes. By comparing these data with published palaeoenvironmental studies, it appears that the kind of dominant OM, and thus the HC distribution of primary fluids, is controlled by the lake level: a lower depth preventing the settlement of anoxic conditions in the bottom waters, and the subsequent good preservation of the OM. During the Autunian, the lake level decreased, consequence of a reduced tectonic activity and of the beginning of a dryer period. Finally, these changes will have caused a strong decrease of the nutrient influx to the lake, favoring the bloom of Botryococcus algae that accumulated to form the boghead. cinétiques ont montré que, excepté pour le boghead, les valeurs d'index d'hydrogène étaient < 700 mg/g COT et les énergies d'activation montraient des profils pluri-énergétiques, inhabituels pour une MO lacustre. Ajoutés à ceux des fluides primaires de pyrolysats, ces résultats ont mis en évidence deux groupes d'échantillons de schistes-bitumineux. Le premier est marqué par une dominance de la MO lacustre et terrestre bien préservée et produit des fluides primaires dominés par les n-alcanes ave...

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Paleoclimatic variations in West Africa from a record of late Pleistocene and Holocene lake level stands of Lake Bosumtwi, Ghana

Cited by 140 publications

References 41 publications

West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad

West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad

An abrupt change in the African monsoon at the end of the Younger Dryas

Palaeoenvironmental control on primary fluids characteristics of lacustrine source rocks in the Autun Permian Basin (France)

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