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Abstract

The effects of precipitation changes on tropical East African ecosystems and human populations is poorly understood due to the complex interplay between global and regional processes and missing data from key regions and time periods. We generate a water-budget model for Lake Victoria, the largest tropical lake in the world, the source of the White Nile, and a region that supports some of the densest human populations in Africa, that assesses the impact of changing climate on lake levels and the rate of lake level change. Model results demonstrate that significant changes in the size and volume of Lake Victoria are possible in response to changes in temperature, precipitation, and orbital forcing. This modeling indicates that Lake Victoria can transition back and forth between modern lake levels and complete desiccation in centuries to a few millennia, which is rapid enough to allow for two previously observed desiccation events between 14-18 ka, during which time the lake drained and refilled twice. Combined observations from modeling and estimates of paleoprecipitation indicate that Lake Victoria was likely desiccated between 94-36 ka. This dry interval partially overlaps the megadrought (140-70 ka) identified in Lakes Malawi and Tanganyika further south, and the cooler, drier conditions identified in the Gulf of Aden between 75-50 ka. This prolonged desiccation was probably driven by eccentricity-enhanced precession and high-latitude forcing that affected the Congo Air Boundary convergence. Using future climate projections, our model also predicts that at current rates of temperature change and previous rates of lake level fall, Lake Victoria could have no outlet to the White Nile within 10 years, and Kenya could lose access to the lake in <400 years, which would significantly affect the economic resources supplied by Lake Victoria to the East African Community.

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... Importantly, fossils of R. atopocranion co-occur with hominin fossils attributed to H. sapiens (Grine, 2016;Pearson et al., 2020) and Middle Stone Age (MSA) artifacts (Tryon et al., 2010(Tryon et al., , 2012Faith et al., 2015;Blegen et al., 2017;Jenkins et al., 2017), the latter providing the archaeological context of early modern humans in eastern Africa (Tryon and Faith, 2013;Tryon, 2019). Past work in the Lake Victoria Basin has documented the expansion of Serengeti-like grasslands across the region in the late Pleistocene (e.g., Tryon et al., 2010Tryon et al., , 2012Tryon et al., , 2016Faith et al., 2015;Garrett et al., 2015), likely in response to increased aridity and desiccation of the lake (e.g., Beverly et al., 2015aBeverly et al., , 2017Beverly et al., , 2020. This interpretation has been heavily influenced by the fossil faunas, including inferences based on the dominance of R. atopocranion, which was assumed to have had an affinity for open grassland habitats similar to extant alcelaphins (e.g., Faith et al., 2011;Faith, 2014). ...
... Analyses of ancient soils, associated fossil taxa, and bathymetric reconstructions suggest that open and grassy habitats were widespread throughout much of the late Pleistocene in the Lake Victoria Basin. The region was considerably drier than modern times from ∼100-36 ka, which probably resulted in the complete desiccation of Lake Victoria and an expansion of a Serengeti-like ecosystem across the basin Beverly et al., 2015aBeverly et al., , b, 2017Beverly et al., , 2020Faith et al., 2015). ...
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Article
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Article
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Article
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The cichlid fishes of the East African Great Lakes are the largest extant vertebrate radiation identified to date. These lakes and their surrounding waters support over 2,000 species of cichlid fish, many of which are descended from a single common ancestor within the past 10 Ma. The extraordinary East African cichlid diversity is intricately linked to the highly variable geologic and paleoclimatic history of this region. Greater than 10 Ma, the western arm of the East African rift system began to separate, thereby creating a series of rift basins that would come to contain several water bodies, including the extremely deep Lakes Tanganyika and Malawi. Uplifting associated with this rifting backponded many rivers and created the extremely large, but shallow Lake Victoria. Since their creation, the size, shape, and existence of these lakes have changed dramatically which has, in turn, significantly influenced the evolutionary history of the lakes' cichlids. This paper reviews the geologic history and paleoclimate of the East African Great Lakes and the impact of these forces on the region's endemic cichlid flocks.
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The degree of chemical weathering in soils increases with mean annual precipitation (P; mm) and mean annual temperature (T; C). We have quantified these relationships using a database of major-element chemical analyses of 126 North American soils. The most robust relationship found was between P and the chemical index of alteration without potash (CIA-K): with . Another strong relationship was found between P and 0.0197(CIA-K) 2 P p 221.12e Rp 0.72 the molecular ratio of bases/alumina (B): with . A Mollisol-specific relationship 2 P p 259.34 ln (B) 759.05 R p 0.66 was found relating P to the molar ratio of calcium to aluminum (C) as follows: with P p 130.93 ln (C) 467.4 . Relationships between weathering ratios and T are less robust, but a potentially useful one was found 2 R p 0.59 between T and the molecular ratio of potash and soda to alumina (S) where with 2 T p 18.516(S) 17.298 R p . Our data also showed that most Alfisols can be distinguished from Ultisols by a molecular weathering ratio of 0.37 bases/alumina of !0.5 or by a chemical index of alteration without potassium !80. Application of these data to a sequence of Eocene and Oligocene paleosols from central Oregon yielded refined paleoprecipitation and paleotem-perature estimates consistent with those from other pedogenic and paleobotanical transfer functions for paleoclimate.
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Lingering debate among evolutionary biologists over whether or not Lake Victoria dried out during the late Pleistocene focuses on perceived conflicts between biological and geological evidence for the age of its endemic species. This article reviews and updates the geophysical and paleoecological evidence for lake-wide desiccation and describes the environmental conditions that aquatic species likely experienced during the low stand. Lake Victoria was at its lowest between 18,000 and 14,000 calendar years ago, and it dried out at least once during that time. There is no evidence of remnant ponds or marshes persisting within the desiccated basin. If such features existed, then they would have been small, shallow, turbid, and/or saline, and therefore markedly different from the lake to which today’s species are adapted. The existence of Lake Victoria’s diverse endemic biota must be reconciled with the incontrovertible geophysical and paleoecological evidence of a ca. 15,000year age for the lake, and not vice versa.
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Palaeoclimatic estimates of mean annual rainfall in the equatorial highlands of Central East Africa have been established for the last 40 kyr. The values are inferred from nine fossil pollen sequences, collected from six peat bogs located between 2° and 4°S latitude, in the forest belt, from 1800 to 2240 m a.s.l. The transformation of pollen data into climatic parameters is achieved by the best analogues statistical method, using a modern pollen data set of East and Central Africa and calibration by meteorological data. The climatic reconstructions are first performed for each individual sequences. They are transformed into time-series using 88 of the 125 available radiocarbon dates (including 41 AMS dates). The synthesis of the results is presented as a single curve, illustrating the precipitation values obtained for 682 dated stratigraphic layers plotted on a radiocarbon timescale. The precipitation changes are presented at about a century resolution during the Holocene, and about a millennium for the glacial period.
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Climate variability during the present interglacial, the Holocene, has been rather smooth in comparison with the last glacial. Nevertheless, there were some rather abrupt climate changes. One of these changes, the desertification of the Saharan and Arabian region some 4 - 6 thousand years ago, was presumably quite important for human society. It could have been the stimulus leading to the foundation of civilizations along the Nile, Euphrat and Tigris rivers. Here we argue that Saharan and Arabian desertification was triggered by subtle variations in the Earth's orbit which were strongly amplified by atmosphere- vegetation feedbacks in the subtropics. The timing of this transition, however, was mainly governed by a global interplay between atmosphere, ocean, sea ice, and vegetation.
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We present here a new solution for the astronomical computation of the insolation quantities on Earth spanning from $-250$ Myr to 250 Myr. This solution has been improved with respect to La93 (Laskar et al. [CITE]) by using a direct integration of the gravitational equations for the orbital motion, and by improving the dissipative contributions, in particular in the evolution of the Earth–Moon System. The orbital solution has been used for the calibration of the Neogene period (Lourens et al.  [CITE]), and is expected to be used for age calibrations of paleoclimatic data over 40 to 50 Myr, eventually over the full Palaeogene period (65 Myr) with caution. Beyond this time span, the chaotic evolution of the orbits prevents a precise determination of the Earth's motion. However, the most regular components of the orbital solution could still be used over a much longer time span, which is why we provide here the solution over 250 Myr. Over this time interval, the most striking feature of the obliquity solution, apart from a secular global increase due to tidal dissipation, is a strong decrease of about $0.38$ degree in the next few millions of years, due to the crossing of the $s_6+g_5-g_6$ resonance (Laskar et al. [CITE]). For the calibration of the Mesozoic time scale (about 65 to 250 Myr), we propose to use the term of largest amplitude in the eccentricity, related to $g_2-g_5$, with a fixed frequency of $3.200''$/yr, corresponding to a period of 405 000 yr. The uncertainty of this time scale over 100 Myr should be about $0.1\%$, and $0.2\%$ over the full Mesozoic era.
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External climate forcings-such as long-term changes in solar insolation-generate different climate responses in tropical and high latitude regions. Documenting the spatial and temporal variability of past climates is therefore critical for understanding how such forcings are translated into regional climate variability. In contrast to the data-rich middle and high latitudes, high-quality climate-proxy records from equatorial regions are relatively few, especially from regions experiencing the bimodal seasonal rainfall distribution associated with twice-annual passage of the Intertropical Convergence Zone. Here we present a continuous and well-resolved climate-proxy record of hydrological variability during the past 25,000 years from equatorial East Africa. Our results, based on complementary evidence from seismic-reflection stratigraphy and organic biomarker molecules in the sediment record of Lake Challa near Mount Kilimanjaro, reveal that monsoon rainfall in this region varied at half-precessional ( approximately 11,500-year) intervals in phase with orbitally controlled insolation forcing. The southeasterly and northeasterly monsoons that advect moisture from the western Indian Ocean were strengthened in alternation when the inter-hemispheric insolation gradient was at a maximum; dry conditions prevailed when neither monsoon was intensified and modest local March or September insolation weakened the rain season that followed. On sub-millennial timescales, the temporal pattern of hydrological change on the East African Equator bears clear high-northern-latitude signatures, but on the orbital timescale it mainly responded to low-latitude insolation forcing. Predominance of low-latitude climate processes in this monsoon region can be attributed to the low-latitude position of its continental regions of surface air flow convergence, and its relative isolation from the Atlantic Ocean, where prominent meridional overturning circulation more tightly couples low-latitude climate regimes to high-latitude boundary conditions.
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The Earth's climate system is highly nonlinear: inputs and outputs are not proportional, change is often episodic and abrupt, rather than slow and gradual, and multiple equilibria are the norm. While this is widely accepted, there is a relatively poor understanding of the different types of nonlinearities, how they manifest under various conditions, and whether they reflect a climate system driven by astronomical forcings, by internal feedbacks, or by a combination of both. In this paper, after a brief tutorial on the basics of climate nonlinearity, we provide a number of illustrative examples and highlight key mechanisms that give rise to nonlinear behavior, address scale and methodological issues, suggest a robust alternative to prediction that is based on using integrated assessments within the framework of vulnerability studies and, lastly, recommend a number of research priorities and the establishment of education programs in Earth Systems Science. It is imperative that the Earth's climate system research community embraces this nonlinear paradigm if we are to move forward in the assessment of the human influence on climate.
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The processes that control climate in the tropics are poorly understood. We applied compound-specific hydrogen isotopes (δD) and the TEX86 (tetraether index of 86 carbon atoms) temperature proxy to sediment cores from Lake Tanganyika to independently reconstruct precipitation and temperature variations during the past 60,000 years. Tanganyika temperatures follow Northern Hemisphere insolation and indicate that warming in tropical southeast Africa during the last glacial termination began to increase ∼3000 years before atmospheric carbon dioxide concentrations. δD data show that this region experienced abrupt changes in hydrology coeval with orbital and millennial-scale events recorded in Northern Hemisphere monsoonal climate records. This implies that precipitation in tropical southeast Africa is more strongly controlled by changes in Indian Ocean sea surface temperatures and the winter Indian monsoon than by migration of the Intertropical Convergence Zone.
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Lake Victoria is the largest lake in Africa and harbors more than 300 endemic species of haplochromine cichlid fish. Seismic reflection profiles and piston cores show that the lake not only was at a low stand but dried up completely during the Late Pleistocene, before 12,400 carbon-14 years before the present. These results imply that the rate of speciation of cichlid fish in this tropical lake has been extremely rapid.
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Book
This book presents an account of the hydrology of the whole Nile basin, dealing with each tributary in turn but drawing attention to links between reaches. The Nile is shown to be a set of very different tributaries which came together by geological accident. Nevertheless, evidence from one part of the basin often throws light on a different area. Recent changes are discussed, in particular the dramatic change of regime of Lake Victoria and other lakes that occurred post-1961. The relationship between hydrology and vegetation affects the important wetlands of the White Nile Basin and discussion of this relationship includes the effect of increased lake flows. The authors draw on records collected throughout the basin to paint a detailed hydrological picture of the Nile. The book is illustrated with over 100 diagrams and photographs.
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Late Pleistocene sedimentary, biogeochemical, and fossil data from the Lake Victoria basin (the largest lake in Africa) suggest that its reduction or desiccation during periods of increased aridity repeatedly facilitated the dispersal of C4 grassland ecosystems across the basin. Archaeological evidence from Middle Stone Age and Later Stone Age sites suggest that human groups diffused into the basin during intervals of declining lake levels, likely tracking the movement of the dense and predictable resources of shoreline environments, as well as the dense but less predictable C4 grass grazing herbivores. Repeated cycles of lake expansion and contraction provide a push–pull mechanism for the isolation and combination of populations in Equatorial Africa that may contribute to the Late Pleistocene human biological variability suggested by the fossil and genetic records. Latitudinal differences in the timing of environmental change between the Lake Victoria basin and surrounding regions may have promoted movements across, within, and possibly out of Africa.
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The "long rains" season of East Africa has recently experienced a series of devastating droughts, whereas the majority of climate models predict increasing rainfall for the coming decades. This has been termed the East African climate paradox and has implications for developing viable adaptation policies. A logical framework is adopted that leads to six key hypotheses that could explain this paradox. The first hypothesis that the recent observed trend is due to poor quality data is promptly rejected. An initial judgment on the second hypothesis that the projected trend is founded on poor modeling is beyond the scope of a single study. Analysis of a natural variability hypothesis suggests this is unlikely to have been the dominant driver of recent droughts, although it may have contributed. The next two hypotheses explore whether the balance between competing forcings could be changing. Regarding the possibility that the past trend could be due to changing anthropogenic aerosol emissions, the results of sensitivity experiments are highly model dependent, but some show a significant impact on the patterns of tropical SST trends, aspects of which likely caused the recent long rains droughts. Further experiments suggest land-use changes are unlikely to have caused the recent droughts. The last hypothesis that the response to CO2 emissions is nonlinear explains no more than 10% of the contrast between recent and projected trends. In conclusion, it is recommended that research priorities now focus on providing a process-based expert judgment of the reliability of East Africa projections, improving the modeling of aerosol impacts on rainfall, and better understanding the relevant natural variability.
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Construction of the Jonglei Canal began in 1977 to reduce absorption and evaporation from the White Nile, and to provide water for irrigation and hydroelectric power in the wetland area of southern Sudan. Based on scientific studies carried out in the early 1950s and in 1978 and 1983, the book aims to present a multi-disciplinary survey of the complex interrelated hydrological, ecological, biological and human problems involved with the scheme. Construction was halted in 1983 by civil war, and the book is intended to provide a foundation upon which further research can proceed with minimum delay should political circumstances allow work on the canal and development of the area to begin again. -after Publisher
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This paper presents new calculations of Lake Victoria's water balance. Evaporation is estimated using both the Penman formula and the energy balance approach, and sensitivity studies are performed to determine the influence of input data on the estimates. Rainfall over the lake is estimated from catchment rainfall using a relationship between the two that was derived using satellite data. The results, using the reference period 1956-1978, indicate that mean annual rainfall over the lake is 1791 mm, compared to mean annual evaporation of 1551 mm. When compared with lake level changes, tributary inflow, and discharge during this period, there is a resultant imbalance of 19 mm. Adding this amount to the calculated evaporation, we are able to reproduce with great accuracy the lake level changes during the period 1956-1978 utilizing precipitation estimates of this study plus measured inflow and discharge. Sensitivity studies show that the discrepancy in the balance of 19 mm is considerably smaller than the error in evaporation calculations that can be introduced by uncertainties in the input data. Of particular concern is cloudiness. The diurnal cycle of cloudiness is quite different over the lake than at shoreline stations and the total cloud cover over the lake is probably lower than at these stations. A change from 50% cloudiness to 30% can increase evaporation by about 30%. Thus, this study underscores the need for adequate cloud data, sufficient to resolve the diurnal cycle, as well as direct estimates of lake rainfall in assessing the lake's water balance.
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The African Humid Period of the early to mid-Holocene (12,000–5000 years ago) had dramatic ecological and societal consequences, including the expansion of vegetation and civilization into the “green Sahara.” While the humid period itself is well documented throughout northern and equatorial Africa, mechanisms behind observed regional variability in the timing and magnitude of the humid period remain disputed. This paper presents a new hydrogen isotope record from leaf waxes (δDwax) in a 15,000-year sediment core from Lake Tana, Ethiopia (12°N, 37°E) to provide insight into the timing, duration, and intensity of the African Humid Period over northeastern Africa. δDwax at Lake Tana ranges between −80‰ and −170‰, with an abrupt transition from D-enriched to D-depleted waxes between 13,000–11,500 years before present (13–11.5 ka). A similarly abrupt transition from D-depleted to D-enriched waxes occurs ca 8.5–8 ka and is followed by generally D-enriched waxes throughout the late Holocene. Trends in δDwax covary with changes in Northern Hemisphere summer insolation and reflect increased precipitation at Lake Tana during the AHP; however, the transition from D-depleted to D-enriched waxes occurs earlier at Lake Tana (ca 8 ka, vs 5 ka) than in many other regional records, and the amplitude of D-depletion during the AHP is larger at Lake Tana as well. We attribute this early enrichment to a reduction of moisture derived from westerly sources (the Congo Basin and Atlantic Ocean) which we suggest are D-depleted relative to moisture sourced from the east (Indian Ocean) and the north (Red Sea and Mediterranean Sea). Our new record highlights the importance of both the northward migration of the tropical rain belt as well as east-west migration of the Congo Air Boundary to precipitation source and amount during the African Humid Period.
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Improved tables for the albedo of water as a function of latitude for latitudes from 0 to 90 deg and for 10 and 30 deg latitude belts are presented. Water albedos averaged over all radiation received at all solar elevation angles, as a function of month, and yearly averages are calculated from the Fresnel equation and from the data set of Grishchenko (1959) in which diffuse radiation is taken into account. The albedos calculated on the basis of Grishchenko's data are found to be 2 to 4% higher at low latitudes and up to 20% lower near the pole than albedos calculated by Sivkov (1952) for midday at the middle of each month and presently in general use.
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Vertisols are clayey, shrink-swell soils that are widely recognized in the rock record, thus generating the need to better understand the dynamics of elemental concentrations on the development of weathering indexes for climate interpretations. We assessed the weathering performance of the four major base-forming oxides (CaO, MgO, Na2O, K2O) along a modern Vertisol climosequence spanning a strong precipitation gradient, and discovered that the concentration of bulk soil CaO and MgO yields the strongest correlation to mean annual precipitation (MAP). Based on this finding, we introduce the CALMAG weathering index, defined as Al2O3 / (Al2O3 + CaO + MgO) × 100, which improves rainfall estimates for Vertisols relative to the well-established CIA-K (chemical index of alteration minus potassium) weathering index. Rather than documenting the hydrolysis of weatherable minerals common in many other soil orders, in Vertisols CALMAG principally tracks the fl ux of calcium and magnesium sourced from calcium carbonate, detrital clay, and exchangeable Ca2+ and Mg2+. Application to two Mesozoic paleosols reveals that in drier climates CIA-K yields higher MAP estimates than CALMAG, but that the reverse is true in wetter climates. This work improves paleorainfall estimates from Vertisols and suggests that a family of weathering indexes is needed for different paleosol types.
Article
We propose a generally applicable formula for estimating evaporation rate from open water bodies which utilizes readily available land-based meteorological data. We follow the well-known aerodynamic approach in which evaporation rate is modelled as the product of a vapour pressure deficit between the water surface and upwind air and a wind function which depends on wind speed. Importantly, such a technique should account for the size of a given water body as evaporation rate is known to decrease with distance downwind from the shore due to the increasing entrainment of water vapour. Our area-dependent relationship is based on a compilation of site-specific wind functions reported for individual water bodies covering a large range of sizes from 0.07 m2 to 33.5 km2. The proposed wind function (mm d−1 kPa−1) is f(U2) = (2.36 + 1.67U2)A−0.05 where U2 is wind speed (m s−1) measured over land at a height of 2 m and A is the surface area of the water body (m2). The areal dependence of our relationship was found to be similar to that proposed in other experimental and theoretical studies. Our proposed area-dependent wind function might be expected to be a best estimate in the absence of more specific site related information. From the scatter of reported site-specific wind functions around our proposed relationship, we develop an estimate of its uncertainty. We also explore the propagation of this wind function uncertainty through aerodynamic and combination type evaporation estimation techniques. We use Rushy Billabong, a 5 ha pond in Australia, as a case study and show how uncertainty can be reduced by more than 60% by using a combination equation approach.
Article
The relative timings of the last deglacial warming in the Southern and Northern hemispheres are not well constrained, but are a crucial component in understanding the mechanisms of deglaciation1. A clearer picture of the degree of interhemispheric synchrony has been obscured by a dearth of high-resolution temperature records that can be tied to the absolute calendar timescale. Moreover, the quantification of tropical temperatures during the last glacial cycle is controversial2-8. Here we apply the alkenone method of sea surface temperature reconstruction9,10 to several high-resolution sediment cores recovered from the tropical Indian Ocean between 20° N and 20° S. The inferred initial sea surface temperature warming ~15,000 calendar years ago at 20° S is in phase with Northern Hemisphere sea (this study) and air11 temperature changes, but lags Antarctic warming12-14 by several millennia. This finding, along with the results of recent modelling studies15,16, provides strong support for the idea that changes in the ocean's global thermohaline circulation were not the only cause of interhemispheric climate teleconnection during the last deglaciation.
Article
The Toro-Ankole volcanics in the northern sector of the Western Branch of the East African Rift are generally assumed to be of Quaternary age. New field observations and comparative KAr and ArAr analyses on whole rock samples and phenocrysts of perpotassic lavas reveal the presence of excess Ar and led to the conclusion that apparently no volcanics have been erupted prior to 50 ka.The present geochronological data, compared with previous ones for the Virunga and South Kivu volcanic fields, suggest that a Late Quaternary pulse of volcanic activity occured in the three areas, with the emission of perpotassic, mildly potassic or sodic alkaline magmas. The genetic implications of such a synchronous activity with wide compositional variation are briefly discussed with respect to basement age and assumed lithosphere characteristics.
Article
Lake Victoria water levels provide an important indicator of climate; since 1954 these levels have been maintained natural by agreement, but after recent increased releases require adjustment to retain a naturalised series. It is possible to compute the impact of recent over-abstraction on declining water levels. This impact has been significant: of the order of 0.6 m.RésuméLes niveaux d'eau du Lac Victoria constituent un indicateur important du climat; depuis 1954, ces niveaux sont maintenus naturels par convention, mais nécessitent d'être corrigés en une série naturalisée suite à une récente augmentation des prélèvements. Il est possible de calculer l'impact des sur-prélèvements récents sur les niveaux d'eau. L'impact est significatif: de l'ordre de 0.6 m.
Article
Past research on the climate response to orbital forcing has emphasized the glacial-interglacial variations in global ice volume, global-mean temperature, and the global hydrologic cycle. This emphasis may be inappropriate in the tropics, where the response to precessional forcing is likely to be somewhat independent of the glacial-interglacial variations, particularly in variables relating to the hydrologic cycle. To illustrate this point, we use an atmospheric general circulation model coupled to a slab ocean model, performing experiments that quantify the tropical climates response to (1) opposite phases of precessional forcing, and (2) Last Glacial Maximum boundary conditions. While the glacially-forced tropical temperature changes are typically more than an order of magnitude larger than those arising from precessional forcing, the hydrologic signals stemming from the two forcings are comparable in magnitude. The mechanisms behind these signals are investigated and shown to be quite distinct for the precessional and glacial forcing. Because of strong dynamical linkages in the tropics, the model results illustrate the impossibility of predicting the local hydrologic response to external forcing without understanding the response at much larger spatial scales. Examples from the paleoclimate record are presented as additional evidence for the importance of precessional signals in past variations of the tropical climate.
Chapter
The Cenozoic evolution of the Nile basin reflects a complex interaction between tectonic, volcanic and climatic events. The Ethiopian and Ugandan headwaters of the Nile attain elevations in excess of 2 km, while the watersheds rise to over 5 km. The Ethiopian tributaries of the Nile (notably the Blue Nile/Abbai and the Atbara/Tekezze) provide the bulk of the flood discharge and sediment load, but the Ugandan tributaries, via the White Nile, provide the reliable dry season discharge responsible for maintaining perennial flow in the main Nile. The hydrologi-cal differences between the Blue and White Nile rivers reflect their very different geomorphic histories, reflecting the Cenozoic tectonic and volcanic evolution of their respective headwaters.
Article
The beginning of the Bølling-Allerød warm period was marked in Greenland ice by an abrupt rise in δ18O, an abrupt drop in dust rain, and an abrupt increase in atmospheric methane content. The surface waters in the Norwegian Sea underwent a simultaneous abrupt warming. At about this time, a major change in the pattern of global rainfall occurred. Lake Victoria (latitude 0°), which prior to this time was dry, was rejuvenated. The Red Sea, which prior to this time was hypersaline, freshened. Lake Lahontan, which prior to this time had achieved its largest size, desiccated. Whereas the chronologic support for the abruptness of the hydrologic changes is firm only for the Red Sea, in keeping with evidence obtained well away from the northern Atlantic in the Santa Barbara Basin and the Cariaco Trench, the onset and end of the millennial-duration climate events were globally abrupt. If so, the proposed linkage between the size of African closed basin lakes and insolation cycles must be reexamined.
Article
An energy-balanced hydrologic model is used to quantitatively assess atmosphere–water budget relationships across the Lake Malawi catchment, a hydrologically-open lake within the East African Rift System. The model first simulates the historical lake-level record over the last 100 years using climate station and vegetation data as inputs. Atmospheric conditions required to sustain equilibrium water balance are then estimated at known critical lake-levels: modern (700 m maximum water depth), basin closure (696 m maximum water depth), 500 m, 350 m, 200 m, and 150 m maximum water depth. The critical low lake stages were determined from analysis of seismic-reflection and deep lake drill-core data. The model predicts modern precipitation rate to be 955 mm/yr, which is consistent with observed climate station precipitation records. The minimum lowstand observed in geophysical records is 150 m water depth (550 m below present lake-level), and occurred about 95,000 years before present. The precipitation rate required to sustain equilibrium conditions at this low lake stage is 557 mm/yr, assuming modern Lake Malawi temperature and vegetation, and 374 mm/yr using modern temperature and vegetation data from the Little Karoo Basin, an analogue for the Malawi paleo-environment during severe arid intervals that resulted in major lake lowstands. The latter result is consistent with the range of precipitation measured from the Little Karoo Basin (100 to 500 mm/yr), and from interpretations of drill-core data sets (Cohen et al., 2007). The time required to drop lake-level from its modern maximum to the most severe low lake stage determined from paleoclimate data sets (from 700 m to 150 m maximum water depth) is less than 2500 years, even when accounting for additional water volume loss stored as groundwater. A lake-level fall of this magnitude reduces the lake surface area by 94% and reduces the total lake volume by 99%.
Article
Seismic-reflection data from crater lake Challa (Mt. Kilimanjaro, equatorial East Africa) reveal a ∼ 210-m thick sedimentary infill containing distinct seismic-stratigraphic signatures of late-Quaternary lake-level fluctuations. Extrapolation of a well-constrained age model on the cored upper part of the sequence suggests that these lake-level fluctuations represent a detailed and continuous record of moisture-balance variation in equatorial East Africa over the last 140 kyr. This record indicates that the most severe aridity occurred during peak Penultimate glaciation immediately before ∼ 128 kyr BP (coeval with Heinrich event 11) and during a Last Interglacial ‘megadrought’ period between ∼ 114 and ∼ 97 kyr BP; in comparison, Last Glacial Maximum (LGM) aridity was modest. It was preceded by ∼ 75 000 years of relatively stable and moist climate conditions interrupted by eleven short-lived dry spells, five of which match the timing of Heinrich events 2 to 6. Climate history near the East African equator reflects variation in the precessional forcing of monsoon rainfall modulated by orbital eccentricity, but precession-driven moisture fluctuations were less extreme than those observed in northern and southern tropical Africa. The near-continuous moist climate from ∼ 97 to 20.5 kyr BP recorded in the Lake Challa record contrasts with the trend towards greater aridity after ∼ 70 kyr BP documented in equatorial West Africa. This long period of moist glacial climate and a short, relatively modest LGM drought can be attributed to greater independence of western Indian Ocean monsoon dynamics from northern high-latitude glaciation than those in the tropical Atlantic Ocean. This rather persistent moist glacial climate regime may have helped maintain high biodiversity in the tropical forest ecosystems of the Eastern Arc mountains in Tanzania.
Article
Variations in the temporal and spatial distribution of solar radiation caused by orbital changes provide a partial explanation for the observed long-term fluctuations in African lake levels. The understanding of such relationships is essential for designing climate-prediction models for the tropics. Our assessment of the nature and timing of East African climate change is based on lake-level fluctuations of Lake Naivasha in the Central Kenya Rift (0°55′S 36°20′E), inferred from sediment characteristics, diatoms, authigenic mineral assemblages and 17 single-crystal 40Ar/39Ar age determinations. Assuming that these fluctuations reflect climate changes, the Lake Naivasha record demonstrates that periods of increased humidity in East Africa mainly followed maximum equatorial solar radiation in March or September. Interestingly, the most dramatic change in the Naivasha Basin occurred as early as 146 kyr BP and the highest lake level was recorded at about 139–133 kyr BP. This is consistent with other well-dated low-latitude climate records, but does not correspond to peaks in Northern Hemisphere summer insolation as the trigger for the ice-age cycles. The Naivasha record therefore provides evidence for low-latitude forcing of the ice-age climate cycles.
Article
Between 15,000 and 18,000 years ago, large amounts of ice and meltwater entered the North Atlantic during Heinrich stadial 1. This caused substantial regional cooling, but major climatic impacts also occurred in the tropics. Here, we demonstrate that the height of this stadial, about 16,000 to 17,000 years ago (Heinrich event 1), coincided with one of the most extreme and widespread megadroughts of the past 50,000 years or more in the Afro-Asian monsoon region, with potentially serious consequences for Paleolithic cultures. Late Quaternary tropical drying commonly is attributed to southward drift of the intertropical convergence zone, but the broad geographic range of the Heinrich event 1 megadrought suggests that severe, systemic weakening of Afro-Asian rainfall systems also occurred, probably in response to sea surface cooling.