Late Pleistocene–Holocene rise and collapse of Lake Suguta, northern Kenya Rift

Quaternary Science Reviews 28 (2009) 911–925

Contents lists available at ScienceDirect

Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev

Late Pleistocene–Holocene rise and collapse of Lake Suguta, northern Kenya Rift Yannick Garcin a, *, Annett Junginger a, Daniel Melnick a, Daniel O. Olago b, Manfred R. Strecker a, Martin H. Trauth a a b

¨ t Potsdam, 14476 Potsdam, Germany ¨ r Geowissenschaften and DFG Leibniz Centre for Surface Process and Climate Studies, Universita Institut fu Department of Geology, University of Nairobi, Nairobi, Kenya

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 June 2008 Received in revised form 21 November 2008 Accepted 5 December 2008

The Late Pleistocene to Middle Holocene African Humid Period (AHP) was characterized by dramatic hydrologic fluctuations in the tropics. A better knowledge of the timing, spatial extent, and magnitude of these hydrological fluctuations is essential to decipher the climate-forcing mechanisms that controlled them. The Suguta Valley (2 N, northern Kenya Rift) has recorded extreme environmental changes during the AHP. Extensive outcrops of lacustrine sediments, ubiquitous wave-cut notches, shorelines, and broad terrace treads along the valley margins are the vestiges of Lake Suguta, which once filled an 80 km long and 20 km wide volcano–tectonic depression. Lake Suguta was deep between 16.5 and 8.5 cal ka BP. During its maximum highstand, it attained a water depth of ca 300 m, a surface area of ca 2150 km2, and a volume of ca 390 km3. The spatial distribution of lake sediments, the elevation of palaeo-shorelines, and other geomorphic evidences suggest that palaeo-Lake Suguta had an overflow towards the Turkana basin to the north. After 8.5 cal ka BP, Lake Suguta abruptly disappeared. A comparison of the Lake Suguta water-level curve with other reconstructed water levels from the northern part of the East African Rift System shows that local insolation, which is dominated by precessional cycles, may have controlled the timing of lake highstands in this region. Our data show that changes of lake levels close to the Equator seem to be driven by fluctuations of spring insolation, while fluctuations north of the Equator are apparently related to variations in summer insolation. However, since these inferred timings of lake-level changes are mostly based on the radiocarbon dating of carbonate shells, which may have been affected by a local age reservoir, alternative dating methods are needed to support this regional synthesis. Between 12.7 and 11.8 cal ka BP, approximately during the Northern Hemisphere high-latitude Younger Dryas, the water level of Lake Suguta fell by ca 50 m, suggesting that remote influences also affected local hydrology. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction One of the major problems in understanding the long-term climate history in tectonically active areas in the tropics involves the correct identification, assessment, and timing of environmental changes. Such changes may have a very different expression in the sedimentary basins that also record local tectonic subsidence and regional to local-scale catchment-evolution histories. Local perturbations in the depositional systems due to changes in tectonically controlled topography may modulate the water balance of the basins and thus the effects of regional/global climatic changes. In addition to unambiguously documenting the influence of climate change on environmental conditions, the lack of well * Corresponding author. Tel.: þ49 331 977 5837; fax: þ49 331 977 5700. E-mail address: [email protected] (Y. Garcin). 0277-3791/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2008.12.006

preserved, datable, and reliable palaeo-environmental records often make such assessments a daunting task. At the end of the last glacial period, the generally dry climate of tropical Africa was superseded by a wetter climate. This wet period, also known as the African Humid Period (AHP), is thought to have prevailed between 14.8 and 5.5 cal ka BP (Ritchie et al., 1985; Street-Perrott and Perrott, 1990; deMenocal et al., 2000; Gasse, 2000; Le´zine and Cazet, 2005). The rejuvenation of the AfricanIndian monsoonal circulation at this time is generally attributed to the increase in Northern Hemisphere summer insolation (Kutzbach, 1981). Climate modelling indicates that increased summer insolation yielded an increase in monsoonal rainfall by 35–45% in northern tropical Africa (Prell and Kutzbach, 1987). However, apart from insolation, other mechanisms seem to have influenced the tropical African climate. Indeed, climate-proxy data from tropical Africa do not closely follow the gradual increase in summer

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

2. General characteristics of the Suguta Valley The Suguta Valley is an integral part of the Kenya Rift (or Gregory Rift), one of the most spectacular topographic expressions of active continental extension, with a length of ca 500 km, a width of 60–80 km, and an average cross-sectional relief of more than 1000 m. The Suguta Valley is approximately at 300 m elevation; it consists of a NNE-trending trough, where Quaternary volcanism, faulting, and coeval sedimentation processes are ubiquitous (Figs. 1 and 2). The width of the trough varies between 21 km in the south and 17 km in the north (Dunkley et al., 1993). The Suguta Valley is an asymmetric graben delimited in the west by a major border fault system, and in the east by a series of smaller fault-bounded escarpments, which are part of a westward dipping monocline (Baker, 1963; Dodson, 1963; Makinouchi et al., 1984; Dunkley et al., 1993). The western margin of the Suguta Valley is topographically lower than the eastern margin by ca 500 m. Although a depression

LAKE TURKANA

2.5°N

The Barrier LAKE LOGIPI

Valley

Loriu

Plate

au

Kalolenyang Kakorinya

Namarunu

2.0°N

Sugu

ta

Fig. 2a

Emuruangogolak 1.5°N

Rive

Laikip

0.5°N

Topography (a.s.l.) 2600 m

teau

LAKE BARINGO

Paka Korosi

ia Pla

1.0°N

r

Silali

uta

insolation during the Late Pleistocene–Early Holocene and its subsequent gradual decrease during the Mid–Late Holocene (Kutzbach and Street-Perrott, 1985). Rather, they indicate abrupt climatic fluctuations such as the abrupt onset and termination of the AHP, which have been generally attributed to strong biogeophysical feedbacks (Claussen et al., 1999; deMenocal et al., 2000; Renssen et al., 2006). In order to test hypotheses of the factors determining the timing and expression of the AHP, new detailed studies of palaeo-records from tropical Africa are needed. The lacustrine rift basins of East Africa are potentially excellent recorders of past climate changes in the tropical-equatorial realm, but often these records can only be accessed either by coring or by technically challenging drilling (e.g., Scholz et al., 2007). However, in some areas of the rift system lakes have vanished, resulting in exposure of long-term lacustrine records, which are often accessible in relatively well-preserved sedimentary sections (e.g., Trauth et al., 2005; Deino et al., 2006). The Suguta Valley (ca 2 000 N, 36 300 E) of the northern Kenya Rift hosts a rich record of palaeoenvironmental information exposed in well-preserved strata. Here, evidence of pronounced repeated hydrological changes during the Late Pleistocene and Holocene is recorded by fluvio-lacustrine sediments and multiple palaeo-lake shorelines (e.g., Baker and Lovenbury, 1971; Truckle, 1976). To better determine the magnitude of lake-level fluctuations in the Suguta Valley, we mapped shoreline elevations and extents using a high-precision differential global positioning system (DGPS). The timing of hydrological changes was constrained by 17 accelerator mass spectrometry (AMS) 14C ages from fossil shells collected on shoreline, littoral, nearshore, and offshore deposits. In this study, we present an original palaeo-climatic record providing new clues about the size of Lake Suguta and its hydrologic connectivity with adjacent lake-basins in the past, building on an earlier study by Truckle (1976). Furthermore, we compare the Lake Suguta record with other palaeo-records from the northern part of the East African Rift System (EARS) in order to differentiate between the effects of local and regional climate change and local tectonic forcing on sedimentary processes during the Late Pleistocene and Holocene. We show that the Suguta Valley basin is very sensitive to climate change. Reconstructed lake-level fluctuations suggest that local insolation has mainly controlled past hydrological changes in this region. These results help better understand high- versus low-latitude influences on tropical climate. In addition, ongoing tectonic extension of the EARS has deformed the palaeo-Lake Suguta shorelines, including the region of a palaeooverflow sill that once connected this basin with the Turkana drainage to the north, thus emphasizing the need to consider basinscale processes in palaeo-climatic reconstructions.

Sug

912

270 m Volcano Fault Monocline

LAKE BOGORIA

0

10

50 km

0.0° 36.0°E

36.5°E

37.0°E

Fig. 1. Digital elevation model derived from the SRTM data of the northern Kenya Rift including the Suguta Valley and adjacent basins. Major faults, monoclines and volcanic eruptive centres are also displayed. Inset shows a simplified structural map of the EARS.

existed in the northern sector of the Kenya Rift during the Miocene and Pliocene (Matsuda et al., 1984; Williamson and Savage, 1986; Saneyoshi et al., 2006), the main phase of relief formation resulting in development of the Suguta trough began later, between 3 and 1.8 Ma (Truckle, 1976; Dunkley et al., 1993). Antithetic faulting, basinward tilting, coeval landslipping at the rift margins, as well as an active volcano-tectonic axis characterize this region (Dunkley et al., 1993). To the south, the large Emuruangogolak trachyte shield volcano (Figs. 1 and 2) started activity between 1.3 and 0.5 Ma (Weaver, 1973; Dunkley et al., 1993). In the centre of the Suguta main depression, trachytic and rhyolitic activity of the Pliocene Namarunu volcano was renewed at 0.8 Ma (Dunkley et al., 1993). To the north, the construction of the Barrier volcanoes (Champion, 1935) began at 1.3 Ma (Dunkley et al., 1993), thus initiating the hydrologic isolation of the Suguta Valley from the Turkana basin. The Suguta Valley was definitively hydrologically isolated from the Turkana basin, when the Kakorinya volcano was constructed on the Barrier, between 0.2 and 0.1 Ma (Dunkley et al., 1993). During the tectonic and topographic development of the northern Kenya Rift, several episodes of fluvio-lacustrine sedimentation partly filled the inner trough, recording past lake-level

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

20 km

late au

m 2.3°N

SANC NAMC 275 m LAKE LOGIPI

1000 m

ERN MAR GIN

1000

KALO

Lori uP

Valley

2.2°N

LORI

1500 m

EAST

300 m

Namarunu

ta

WES TERN

2°N

Kakorinya

Kalolenyang

LAKE LOGIPI

MAR GIN

Minor fault Major fault Monocline Caldera

b

The Barrier

Sugu

10

Recent alluvium Volcanic centers Precambrian basement

Loriu Plate au

0

2.4°N

LAKE TURKANA

N

500 m

a

913

2.1°N

Emuruangogolak GPS base station (2008)

2.0°N

36.5°E

10

00

m

NAMS

late

NARU

au

1.9°N

Tirr Tirr P

HCRA

Ba ra

Kerio River

Namarunu NAKI

i go

er Riv

SILL 1.8°N

Riv er

STT2 GPS base station (2007)

Sugu ta

KAMU

1.7°N

ge River

10

00

m

Ka mu

STTR

500 m

BARA

NAMR

LOS1

1.6°N

LOSC+LOSE

5

0

10 km

Volcanic eruptive centre

EMU1

Surveyed site

00

36.1°E

36.2°E

36.3°E

Maximum highstand shoreline (MHS ca 567 m)

Emuruangogolak

10

m

1.5°N

Contours at 100 m intervals

36.4°E

36.5°E

36.6°E

36.7°E

36.8°E

Fig. 2. (a) Structural map of the Suguta Valley adapted from Dunkley et al. (1993). (b) Topographic map of the study area based on SRTM data (Farr et al., 2007). Also shown are locations of surveyed sites and a contour line at 567 m indicating the maximum extent of the palaeo-Lake Suguta (MHS, blue line).

914

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

fluctuations in the Suguta Valley. The oldest lacustrine sediments range between 0.8 and 0.5 Ma (Dunkley et al., 1993), while the youngest sediments are of Early Holocene age (Bishop, 1975; Truckle, 1976; Williams and Johnson, 1976; Casanova et al., 1988). The Suguta Valley constitutes the most arid environment in Kenya (Ojany and Ogenoo, 1973) and is probably the most arid locality on Earth in the immediate vicinity of the Equator. Exact data on climate conditions are not available due to the remoteness of the region. However, limited available data document a rainfall amount of <300 mm yr1, which is distributed following a unimodal cycle, with a peak between March and May, but both the periodictity and the quantity of rainfall are highly variable (Morgan, 1971; East African Meteorological Department, 1975). The mean annual temperature ranges between 28 and 31 C, and mean annual evaporation ranges between 3000 and 4000 mm yr1 (Morgan, 1971; Dunkley et al., 1993). A sparse and xerophytic vegetative cover, as well as barchan dunes in the centre of the basin, furthermore underscore the arid character of the Suguta Valley. The wide flood plain on the valley floor supports a sparse grass cover (Gramineae, Cyperaceae) and palm trees (Hyphaene). The rift flanks are characterized by a more developed cover of deciduous trees dominated by Acacia, Commiphora, Hyphaene palms, Salvadora and small euphorbias (Hemming, 1972; Vincens, 1982). The Suguta River, draining a catchment area of ca 13,000 km2, is the only perennial stream in this environment. It is 175 km long and originates on Paka volcano to the south; it terminates in the swamps south of the Barrier volcanoes to the north (Figs. 1 and 2). This sector of the rift is 80 km long and 20 km wide, and consists of a wide flood plain and swamp area rising at elevations between 340 m in the south and 275 m in the north. The northernmost part of this sediment-filled depression is occupied by the temporary shallow Lake Logipi. It is an alkaline, 0.5–5 m deep lake primarily fed by the Suguta River, ephemeral streams from the flanks during the rainy season, and possible seepage from Lake Turkana, as well as hot springs along the volcano-tectonic axis (Casanova et al., 1988; Castanier et al., 1993). Due to climatic variability on interannual time scales, Lake Logipi may grow considerably in extent, such as during the wet years of 1975 and 2007, while it may virtually disappear during protracted drought, such as during the extremely dry year of 1987 (Castanier et al., 1993). 3. Materials and methods 3.1. Kinematic DGPS survey of palaeo-shorelines In June 2007 and June 2008 we carried out high-precision topographic surveys of the prominent palaeo-shorelines using a DGPS (Leica GPS 1200, single-frequency receiver GX1210, L1 ¼ 1575.46 MHz) aided by a helicopter to reconstruct past lake levels. The palaeo-shorelines of the former Lake Suguta are ubiquitous, wellexpressed geomorphic features exposed along most of the rift flanks as well as on cinder cones along the centre of the rift (Fig. 3). The shorelines on the rift flanks consist of terrace surfaces with treads up to 40 m wide, which dip 1–5 towards the centre of the rift, and rise up to an elevation of 577 m. At some localities, flights of up to 40 terraces are well expressed, forming a staircase morphology. The shorelines on the cinder cones mainly constitute prominent wavecut notches (Fig. 3a–e), locally offset by normal faults. The use of a differential positioning system of at least two GPS receivers working simultaneously (with one as a static base station) keeps measurement uncertainties due to clock errors, number and geometry of satellites, and atmospheric effects, low. The base station was firmly fixed above a bedrock platform, and we ensured that the base-rover distance never exceeded 90 km. Kinematic GPS data were continuously collected at 1 Hz observation rate. We measured

a total of 13 km of shorelines on cinder cones and basaltic flows at 18 sites, up to 90 km apart, both inside and on the rift flanks. At each site, we surveyed the elevations of the most prominent shorelines by moving parallel to them. We also measured elevation profiles along the slope axis, i.e., perpendicularly to the shorelines. We estimated the mean elevation of each shoreline by inspecting the entire set of elevations and by averaging the elevations of points measured on geomorphic features, which were only related to the palaeo-lakelevel position, i.e., wave-cut notches and shoreline angles (indicated by blue crosses on the maps from the Supplementary material). The survey data were post-processed using the Leica GeoOffice (v6.0) software, with a 15 satellite elevation cut-off and broadcasted ephemeris with automatic selection of solution and frequency. The ionospheric activity used in the analysis was the Hopfield tropospheric model (Hopfield, 1969) with an automatic setting using a stochastic approach. The type of solution obtained using the processing software was all in code, indicating that only the coded information transmitted by the satellites was used during the processing and not the phase of the signals. The averaged height quality (standard deviation of the height element) of our 35,000 post-processed field-measurements is 26.5 cm. This value likely represents the average analytical error of our survey setting, which is very similar to the theoretical value of 25 cm given by Leica (Leica Geosystems, 2007). Note that the analytical errors of this measurement method are smaller than the natural variability in the elevations of inherited shoreline features (ca 0.5–1.5 m) as also suggested by similar surveys of lacustrine geomorphic markers of Late Pleistocene–Holocene age (e.g., Adams, 1999). These uncertainties associated with the variability of shoreline elevations also include the fact that some of the shoreline features have been locally altered by erosional processes (e.g., rill incision or deposition of colluvium). We systematically tested the reliability of our elevation data from site to site by comparing the vertical offset of several fault scarps (of ca 1– 10 m height) measured with both the DGPS and a tape. In addition, we surveyed several identical sites during both years 2007 and 2008. For these two periods of measurement, the base-rover distance as well as both satellite and atmospheric conditions were different; however, we obtained similar elevations within the analytical error range. In order to validate our height information, and due to the virtual absence of geodetic benchmarks in this area, we compared the elevation of relatively flat areas (e.g., fluvial terraces and floodplains) obtained by our averaged DGPS data with the corresponding elevation of the Shuttle Radar Topography Mission 3 arc second data (SRTM of ca 90 m horizontal spacing). We added the computed height separations (EGM96 geoid–WGS84 ellipsoid) to our DGPS elevations in order to match the SRTM reference frame (Farr et al., 2007). In the Suguta Valley, these elevation differences (height separations) between the geoid and ellipsoid range from 15.9 to 16.5 m. The elevation differences between our DGPS data and the SRTM data never exceed 5 m (Fig. 4), which is slightly above the SRTM instrumental error for Africa of 1.54 m (Becek, 2008). These differences are probably related to the terrain roughness and to the lack of surveyed areas that are truly horizontal. However, this comparison suggests that our DGPS elevations are robust and may be compared both between each other and with the global geodetic reference frame. 3.2. Age control Seventeen AMS 14C dates were obtained from freshwater snails (Melanoides tuberculata), oysters (Etheria elliptica), and ostracods collected on lacustrine deposits (e.g., nearshore gravels and sands, and former deeper-water clays and diatomites), locally associated with palaeo-shorelines. The radiocarbon dates were processed at the Leibniz Laboratory for Radiometric Dating and Isotope

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

915

Fig. 3. Photographs of prominent wave-cut notches and wide terrace treads corresponding to the maximum highstand shoreline (MHS, shown with white arrowheads). (a) Site KALO (aerial view looking northeast). (b) Site NAMC (ground view looking north); the Barrier volcanoes are visible in the background. (c) Site NAMS (aerial view looking north). (d) Sites LOSC and LOSE (aerial view looking south); the normal fault (F) cutting through this volcano accounts for ca 14 m offset of the MHS. (e) Site LOS1 (aerial view looking south); Emuruangogolak volcano is visible in the background. (f) Site NAMR (ground view looking southeast). Whitish deposits in fore- and background correspond to lacustrine claystones and diatomites that prograde basinward (indicated by a black arrow). These flat-toped deposits have a mean elevation of 564 m and extend for ca 5 km2.

Research, Kiel, Germany (Table 1). Exclusively samples showing no sign of dissolution or local abrasion were selected for dating. The samples were first cleaned with 30% H2O2 in an ultrasonic bath, rinsed with distilled water and dried to remove adhering dust and detrital carbonate as well as organic surface coating. This was followed by a second cleaning step with 15% H2O2 in an ultrasonic bath. Dates were calibrated using the Calib 5.0.1 program (Stuiver and Reimer, 1993) with the IntCal04 curve (Reimer et al., 2004). All calibrated radiocarbon ages are reported in cal yr BP and in cal ka BP (calendar year and kilo year before Year 1950, respectively).

As the shells of many freshwater organisms are composed of aragonite, a thermodynamically unstable mineral, the shells of snails and oysters were tested for calcite and aragonite content by XRD. The shells of snails and oysters used in our analysis are composed of ca 100% aragonite. Furthermore, the drainage basin of the Suguta Valley mainly consists of volcanic rocks (basalts, trachytes) as well as gneisses, granites and metasediments. No units containing carbonate rocks crop out in the catchment area, although, carbonate travertines are locally associated with some of the hydrothermal springs surveyed by Dunkley et al. (1993). We

916

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

feature, which is virtually continuous along the rift flanks, as the maximum highstand shoreline (MHS). Because of its ubiquity, correlation of this geomorphic marker among sites of the entire Suguta basin is straightforward. We generally found that the MHS in the inner rift sector is carved into cinder cones of palagonitized basalt, related to phreatomagmatic eruptions (e.g., Heiken, 1971). This observation, in addition to the occurrence of pillow lavas throughout the Suguta Valley (Dunkley et al., 1993), suggests interaction between the palaeo-Lake Suguta and Late Quaternary volcanic processes along the axis of the rift. The local DGPS elevation of the MHS for each surveyed site is recorded in Table 2. Our data indicate differential elevations of the MHS in the Suguta rift basin, with a range between 552 and 577 m (blue lines in Fig. 6a). When viewed along an across-rift profile, this shoreline is clearly tilted eastward by ca 0.03 (Fig. 6c). Because of its very low amplitude/wavelength ratio, this deformation pattern is not readily visible in the field, but was revealed by our highresolution DGPS survey. Site KALO (Fig. 3a) is a 200-m-high cinder cone of phreatomagmatic origin, located on the southern flank of Kalolenyang volcano (western sector of the Barrier). It measures 800 m in diameter and has a prominent wave-cut notch at 570 m, corresponding to the MHS. Here, numerous chert and obsidian artefacts were found adjacent to the MHS, which presently can only be accessed by helicopter due to the precipitous cliffs delimiting the cone. The presence of artefacts suggests that humans must have accessed and occupied this shoreline when this cone was an island of Lake Suguta. Site SANC is a cinder cone of phreatomagmatic origin situated 3 km south of Andrews Cone (Champion, 1935). It measures 1 km in diameter and rises ca 100 m above the surrounding flanks of the Barrier. Several shorelines occur on its inner and outer flanks between 492 and 560 m. The highest shoreline is a prominent wave-cut notch corresponding to the MHS. Fossil snails (Melanoides tuberculata) collected at elevations of 481, 528 and 533 m yielded three radiocarbon ages of 11,850, 11,240 and 10,575 cal yr BP, respectively. Site NAMC (Fig. 3b) is a steep-sided cinder cone (known as Namurinyang) with an elliptical, breached central crater that is located above a N-striking fault zone. It measures 190 m in height and 1.4 km in diameter. A prominent wave-cut notch at 570 m, which corresponds to the MHS, carves it. Another, less

5

DGPS - SRTM elevation (m)

4 3 2 1 0 -1 -2 -3 -4 -5 300

350

400

450

500

550

600

DGPS elevation (m a.s.l.) Fig. 4. Comparison between DGPS elevations and DGPS minus SRTM elevations for selected sub-horizontal surveyed areas.

thus assume that the radiocarbon age reservoir, and hence the uptake of old carbon by biogenic carbonate is likely negligible in this environment. However, we cannot rule out the influence of a radiocarbon age reservoir on the dated shells. 4. Results 4.1. Description of the surveyed sites In this section we present the 18 surveyed sites. In the interest of readability, a detailed description of all sites is provided in the Supplementary material (Appendix A). A location map of the surveyed palaeo-shoreline sites is provided in Fig. 2, whereas Figs. 3 and 5 contain field views of selected sites, and Fig. 6 summarizes the entire dataset of surveyed shorelines. Our DGPS survey reveals the existence of a nearly continuous sequence of shorelines, which starts at the shore of Lake Logipi at 275 m and reaches 577 m (Fig. 6a). The most prominent shoreline of the palaeo-Lake Suguta systematically reaches the highest elevations in the entire valley. This shoreline also has the widest terrace tread of up to 40 m, typically very well expressed on the western flanks of the basin. In the following sections we refer to this

Table 1 14 C AMS ages from the Suguta Valley. Kiel ID

Elevation (m a.s.l.)

Site

Lat. (N)

Lon. (E)

Material

14

C age [yr BP]

d13C (&)

Calibrated age [cal yr BP]

Calibrated age 2s error bounds

KIA KIA KIA KIA KIA KIA KIA KIA KIA KIA KIA KIA KIA KIA KIA KIA KIA

528 m** 481 m** 533 m** 530 m* 532 m** 519 m** 532 m* 529 m** 544 m** 556 m** 507 m** 564 m** 564 m** 312 m** 557 m** 559 m** 565 m**

SANC SANC SANC NAMR LOSC LOS1 NAMR NAMC NAMC NAMC NAKI NAMS NAMS HCRA BARA STT2 EMU1

2 160 20.600 2 160 11.600 2 160 21.100 1320 14.200 1340 41.700 1340 31.800 1320 14.300 2 140 45.800 2 140 52.700 2 140 52.400 1560 59.500 1570 19.700 1570 19.700 1540 46.000 1370 08.300 1470 23.100 1330 39.100

36 340 41.100 36 340 27.400 36 340 41.600 36 130 32.300 36 230 59.500 36 220 22.000 36 130 32.400 36 360 51.700 36 360 47.300 36 360 49.400 36 220 49.900 36 240 56.100 36 240 56.100 36 270 54.500 36 270 39.200 36 300 55.300 36 200 03.200

M M M E M M M M M M M M M O M M M

9840  45 10,205  45 9365  50 13,900  60 10,000  45 9775  45 13,725  60 10,795  50 10,930  50 10,760  50 9970  45 10,025  45 10,750  50 6345  40 8510  60 9980  55 7850  50

5.15  0.42 1.69  0.73 0.47  1.3 8.43  0.42 4.86  0.21 5.38  0.42 1.3  1.02 8.54  0.81 5.22  0.55 8.96  0.35 0.88  0.97 11.11  1.51 0.39  0.41 2.32  0.3 7.93  0.44 5.82  0.29 3.37  1.3

11,240 11,850 10,575 16,560 11,460 11,205 16,320 12,830 12,880 12,815 11,370 11,540 12,810 7270 9525 11,380 8620

11,187/11,328 11,750/12,083 10,484/10,718 16,181/16,943 11,270/11,646 11,133/11,254 15,991/16,751 12,758/12,884 12,831/12,949 12,712/12,862 11,250/11,618 11,318/11,752 12,704/12,857 7170/7332 9414/9559 11,250/11,645 8538/8789

33901 33902 33903 33907 33908 33909 33910 33911 33912 33913 33916 33917 33918 35809 36866 36872 36873

(*) Measured with a Garmin GPS, (**) measured with a Leica 1200 DGPS. M is Melanoides tuberculata, E is Etheria elliptica, and O is ostracods. Radiocarbon ages given before Year 1950, 14C calibration method: Program CALIB 5.0.1 (Stuiver and Reimer, 1993), IntCal04 curve (Reimer et al., 2004).

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

917

a

N

Suguta Valley

Sill TowardsTurkana

basin

Toward

s Sugu

ta basin

N

0

250

500 m

c

DGPS data Streams Drainage divide Dark grey silty clays 176000 m

b’

a’

200000 m

201000 m

201500m 201500m

202000 m

Overflow sill

a’

b’

580.5 580.0 579.5

Kamuge headwaters Kerio headwaters

581.5 581.0

Elevation (m a.s.l.)

basin

Suguta ba

Turkana

b

sin

b

579.0 0

100

200

300 400 Distance (m)

500

600

700

Fig. 5. Detail of the palaeo-overflow sill of Lake Suguta (site SILL). (a) Oblique aerial photograph looking east. (b) Geomorphic interpretation based on field observations, aerial photographs and ASTER satellite imagery. (c) Plot of DGPS measurements along a north-south profile (profile a0 –b0 on panel b) indicating an elevation of ca 581 m for the overflow sill. Error bars correspond to the height quality (standard deviation of the height element).

pronounced shoreline occurs at 552 m. Lacustrine deposits, including carbonate nodules, reworked fragments of stromatolites, and freshwater snails, are found between 528 m and the MHS. Fossil snails yielded three radiocarbon ages of 12,830, 12,880 and 12,815 cal yr BP, at elevations of 529, 544 and 566 m, respectively. Site LORI corresponds to the northernmost part of the western escarpment of the Suguta Valley. Along the eastern Loriu escarpment, a continuous succession of terraces forming a staircase morphology is exposed in the sector between the valley floor at 275 m and the MHS at 575 m. The latter surface has the widest tread of ca 20 m, whereas treads of lower-elevation terraces only attain a width of a few meters. Site NAMS (Fig. 3c) is located on a steep-sided basaltic cinder cone, south of the Namarunu main volcanic centre. Two marked shorelines occur on the outer slopes of the cone. The MHS is expressed as a prominent wave-cut notch at 577 m. Several less pronounced shorelines occur at the lowest part of the cone. Fossil snails collected at 564 m yielded two radiocarbon ages of 12,810 and 11,540 cal yr BP. Site NAKI is part of the western escarpment of the Suguta Valley along the main border fault zone. A succession of well-developed

terraces occurs on the eastern flanks of Nakitoekirion between 490 m and the MHS, which reaches 570 m. One sample collected from lacustrine deposits located at 507 m, provided an age of 11,370 cal yr BP. Site NARU is a gently inclined basaltic lava cone located south of Namarunu. Multiple shorelines occur between 400 and 446 m. On the eastern flank of the cone, two wide terrace treads, delineated by continuous beach berms, are located at 431 and 442 m. Site HCRA is a crescent-shaped, asymmetric cinder ring located southeast of Namarunu, on the edge of the Suguta flood plain. A succession of prominent terraces occurs along its eastern flank between 319 and 348 m. A 5-cm-thick ostracod-rich layer associated with sandy lenses covered by gravely fluvial deposits occurs at 312 m. The ostracods, which were probably deposited when Lake Suguta was very shallow, yielded a radiocarbon age of 7270 cal yr BP. Site SILL (Fig. 5) is located in the area between the headwaters of the Kamuge and Kerio rivers that drain towards the Suguta and Turkana basins, respectively. The elevation of the overflow sill connecting these two basins is estimated approximately at 581 m (Fig. 5c). This major drainage divide is situated inside an elongated flat-floored valley. At least 2-m-thick dark grey silty clays, which are light brown coloured at surface due to

918

S Fig. 3d

580

EMU1

Fig. 3f 8620

NAMR

Fig. 3e

BARA

LOSE

KAMU

LORI

Fig. 5

Fig. 3c

SILL

NAMS

Fig. 3b

NAMC

LOS1 11,380

KALO

SANC

11,540 12,810

9525

LOSC

N Fig. 3a

12,815

16,560 16,320

12,880

STT2

Fault

540

10,575

11,460

12,830

STTR

11,240

11,205

200 m 11,370

a

500

11,850

Elevation (m a.s.l.)

11,380

Calibrated radiocarbon age (cal yr BP)

NAKI

Local elevation of the maximum highstand shoreline (MHS) Prominent shoreline 460

Lacustrine sediment NARU

b 0

c 10

1600

20 km

a’ 420 mean MHS ca 567 m

595

a’

380 Overflow sill

b’

Elevation profile (m a.s.l.)

590 1200

585 580

1000

575 800

570 565

600

560

HCRA

555

400

340

MHS elevation (m a.s.l.)

2°N

b’

1400

550 200 0

1.5°N

36.5°E

20

40

Distance (km)

60

7270

300 Fig. 6. Summary plot of the DGPS survey. (a) Synthetic diagram showing the vertical and horizontal distribution of measured shorelines for each site, displayed along elevation profiles. Also shown are the MHS and calibrated radiocarbon ages. (b) Map showing locations of the MHS survey sites. (c) Plot of the MHS elevations. Grey area corresponds to ESE–WNW elevation profile across the Suguta Valley (profile a0 –b0 on panel b, vertical exaggeration: ca 55, axis to the left). Blue dots denote the mean elevation of the MHS given for each surveyed site (axis to the right), and projected onto a0 –b0 profile. Error bars correspond to sum of uncertainties related to natural variability of the shoreline features and to DGPS measurement errors (MHS SD and Hq SD, respectively, see Table 2). The blue area shows the apparent eastward tilting of the MHS sites including the overflow sill (orange circle).

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

Scale for elevation profiles, vertical exaggeration is ×6

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

919

Table 2 DGPS results for the maximum highstand shoreline (MHS). Site

Lat. (N)

Lon. (E)

D (km)

N

G

n

Hq (m)

Hq SD (m)

MHS (m)

MHS SD (m)

KALO SANC NAMC LORI NAMS NAKI SILL STT2 STTR KAMU BARA LOSC LOSE LOS1 EMU1 NAMR

2 160 58.400 2 160 16.600 2 140 52.000 2 090 50.800 1570 18.500 1560 45.900 1480 55.900 1470 29.900 1450 35.800 1430 24.700 1370 03.600 1340 42.700 1340 44.100 1340 21.100 1330 39.500 1320 58.200

36 300 51.200 36 340 36.800 36 360 51.600 36 230 50.000 36 240 51.500 36 220 38.500 36 05017.100 36 300 55.300 36 300 36.600 36 100 36.100 36 270 37.100 36 240 02.400 36 240 09.600 36 220 22.400 36 200 03.100 36 130 14.600

60.04 57.01 54 53.77 36.27 39.21 85.7 72.17 19.43 53.8 61.7 37.7 37.6 40.62 76.34 56.67

9 7–9 7–9 8 6 6 10 8 8 5–7 8 6 7 6 10 7–9

2.5 3 2.8 2.3 3.3 2.7 2.3 2.2 2.3 3.7 2.3 3.5 2.5 3 3 2.4–3

528 1327 1461 56 52 172 74 362 85 291 140 222 187 75 46 752

0.23 0.26 0.27 0.26 0.24 0.5 0.28 0.22 0.25 0.17 0.33 0.14 0.34 0.34 0.4 0.23

0.1 0.09 0.15 0.07 0.2 0.17 0.08 0.05 0.07 0.1 0.03 0.05 0.1 0.15 0.04 0.08

569.73 560.03 569.8 574.52 577.38 570.65 580.73 566.44 567.7 574.36 571.51 552.64 566.42 561.2 566 563.96

0.26 1.3 0.96 0.3 0.7 0.41 0.06 0.42 0.25 1.08 0.56 0.21 0.37 1.54 0.07 0.3

D is the distance between the rover and the base station. N is the number of GPS satellites at elevations >15 above the horizon that were recorded during each survey. G is the mean geometric dilution of precision (GDOP), used to describe the geometric strength of satellite configuration on GPS accuracy. n is the number of kinematic measurements (1 Hz rate) made on the MHS. Hq and Hq SD are respectively the mean and standard deviation of the height quality element. MHS and MHS SD are respectively the mean and standard deviation of the MHS elevation.

alteration, are present in the vicinity of the sill and only occur on the Suguta basin headwaters (Fig. 5a and b). These organic and fine-grained sediments of inferred lacustrine origin may have been deposited when Lake Suguta was overflowing towards the Turkana basin. Site STT2 is located south of the Tirr Tirr Plateau, on the eastern escarpment of the Suguta Valley. The MHS is here expressed as a prominent terrace, which reaches 566 m. Fossil snails collected at an elevation of 559 m yielded one radiocarbon age of 11,380 cal yr BP. Site STTR is situated 3.5 km south of site STT2. Several lacustrine terrace surfaces occur between 540 m and the MHS, which is located at 568 m. The MHS terrace is continuous over 300 m and has 20-m-wide treads. Site KAMU is an isolated volcanic hill about 110 m above the Kamuge River plain, southwest of the Suguta Valley. At mid-slope of the steep (40 ) and regular flanks of this hill, a 20-m-wide terrace surface breaks the slope, highlighted in the landscape by a continuous grass cover. This prominent terrace corresponds to the MHS reaching an elevation of 574 m. Site BARA is located on the mouth of the Baragoi River. It comprises a 25-m-wide terrace rising at 572 m, corresponding to the MHS. Fossil snails collected at an elevation of 557 m yielded a radiocarbon age of 9525 cal yr BP. Sites LOSC and LOSE are located on the western and eastern side of the Losotem basaltic pyroclastic cone, respectively (Fig. 3d). The upper part of this cone has been abraded by wave action, resulting in an 40-m-wide terrace tread corresponding to the MHS. This indicates a protracted high-elevation lake-level of the former Lake Suguta. Subsequently, the MHS was faulted and displaced as much as 14 m along a NNE-striking normal fault. On site LOSC, the elevation of the MHS is 553 m, whereas it rises at 566 m on site LOSE. Numerous chert and obsidian artefacts were found scattered over the shoreline surface, indicating human occupation. Other less pronounced shorelines are found in the lowest part of the cone. Radiocarbon dating of fossil snails located at 532 m on site LOSC yielded an age of 11,460 cal yr BP. Site LOS1 is situated on a cinder cone of phreatomagmatic origin named Losetum (Fig. 3e). A prominent, nearly vertical wave-cut notch, with a 5- to 10-m-high cliff, occurs at an elevation of 561 m and corresponds to the MHS. Fossil snails sampled at 519 m provided an age of 11,205 cal yr BP. Site EMU1 is located on the northern flank of the Emuruangogolak volcanic centre. It consists of a lava flow, which has

been truncated by wave action, indicating the location of the MHS at 566 m. Fossil snails collected along this shoreline yielded an age of 8620 cal yr BP. Site NAMR is located in the southwestern part of the Suguta Valley, at Namruy (Fig. 3f). It consists of light coloured lacustrine claystones and laminated diatomites, indicating nearshore to offshore environments. These units prograde towards the deepest part of the basin east of Namruy and correspond to a former delta. The upper units constantly crop out at an elevation of 564 m and cover a surface area in excess of 5 km2. This flat accumulation surface developed probably at an elevation equal to the position of the MHS. The lowermost units occur at 500 m elevation. Fossil snails and oysters at an elevation of ca 531 m from the lowermost units yielded two similar ages of 16,320 and 16,560 cal yr BP, respectively. 4.2. Lake Suguta fluctuations since the last 17 ka Stratigraphic and geomorphic data collected in the Suguta Valley were interpreted in terms of palaeo-environments and lake-water depth. Wave-cut notches and terraces associated with coarse carbonate nodules indicate high-energy environments, while fine-grained claystones and laminated diatomites indicate low-energy nearshore and offshore environments. Radiocarbon dating of shells collected on former lake deposits helps reconstruct spatiotemporal patterns of past fluctuations of Lake Suguta (Fig. 7). Thick claystones and laminated diatomites from site NAMR indicate that the level of Lake Suguta was already high from 16.5 to 15 cal ka BP, reaching a minimum elevation of 535 m. Between 15 and 12.8 cal ka BP, the absence of dated sediments and other lakelevel indicators prevents us from providing an unambiguous assessment of the lake-level curve at that time. At ca 12.8 cal ka BP, the lake was at its maximum highstand, dated on sites NAMC and NAMS. This maximum highstand shoreline (MHS, blue lines on Fig. 6a) is a prominent geomorphic feature of the Suguta Valley, highlighted by wide terrace treads on the valley margins as well as by pronounced wave-cut notches on cinder cones. Tectonic movements have deformed the MHS, which has resulted in differential elevations for this originally horizontal geomorphic feature (see further explanations for these tectonic movements in Section 5.1). Here, we assume that the 567-m-average elevation of our MHS surveyed sites closely approximates the pre-deformation elevation of the MHS. Based on SRTM data, we calculated the size of this lake

920

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

600 580

NAMS

STT2 NAMS

Overflow to Lake Turkana basin

560 EMU1 540

NAMC BARA

NAMC

LOSC

520

SANC

SANC

NAMR

NAMC

LOS1

500

Elevation (m a.s.l.)

NAMR

?

NAKI

480

SANC

460 440 420 Lake-level reconstruction

400

14

380

C ages from shoreline and littoral deposits

YD

14

C ages from nearshore and offshore deposits

360 340 320

HCRA

300

6

7

8

9

10

11

12

13

14

15

16

17

18

Age (cal ka BP) Fig. 7. Reconstruction of Lake Suguta levels for the period 18–6 cal ka BP. Radiocarbon ages (Table 1) are shown with probability curves and 2s error bounds. Ages marked in red are from shoreline and littoral deposits. Ages marked in green are from nearshore and offshore deposits. Vertical yellow band highlights a lower lake-level during the Younger Dryas (YD) event.

with a maximum depth of ca 300 m, an area of ca 2150 km2, and a volume of ca 390 km3. During this highstand, Lake Suguta was an open basin overflowing into the Lake Turkana basin. The overflow sill, which is located southwest of the Suguta Valley, rises ca 14 m above the MHS in the inner part of the valley. This elevation difference is likely a result of subsequent deformation by tectonic movements (see Section 5.1). Soon after the construction of the MHS, dated shorelines from sites NAMC, SANC and NAKI indicate that the lake level dropped by ca 50 m during an episode that lasted ca 1 ka between 12.7 and 11.8 cal ka BP. During this time the basin was isolated. Interestingly, the short-lived lake-level drop seems synchronous with the Northern Hemisphere high-latitude Younger Dryas (YD) event (e.g., Hughen et al., 2000; Rasmussen et al., 2006). Subsequently to this drop, the water level rose again to reach the former maximum highstand. Dated littoral and nearshore deposits at elevations between 557 and 565 m from the sites NAMS, BARA, STT2 and EMU1 indicate that Lake Suguta was sustained as a deep lake, with an open basin intermittently overflowing into the Lake Turkana basin during the Early Holocene (11.8–8.5 cal ka BP). Lake Suguta started to disappear, however, after ca 8.5 cal ka BP, following a ca 240 m drop that occurred over a time span of 1 ka. The lowstand is supported by ostracod-bearing sandstones from site HCRA, dated at 7.3 cal ka BP and located at an elevation of 312 m. From 7 cal ka BP until today, the lake-level curve is poorly constrained, because of the absence of unambiguous lake-level indicators. In addition, the water level must have remained very low since that time: subsequent lake transgressions in the Suguta Valley would have reworked and overprinted the prominent former beach ridges, broad terraces, and wave-cut notches. However, further field data would be needed to support our hypothesis of a protracted lake lowstand since the Middle Holocene.

5. Discussion 5.1. Tectonic deformation of the palaeo-shorelines Based on our topographic survey, we have shown that the palaeo-shorelines of the Suguta Valley have been deformed. The deformation of the MHS in the Suguta basin suggests that tectonic movements in that area are ongoing as previously noted by Casanova (1986) and Casanova et al. (1988). We interpret the eastward tilting of the MHS as a result of the active tectonism in the EARS (Fig. 6c). The Suguta Valley is asymmetric, both in terms of topography and geological structure (Fig. 2a). Extension in this sector of the EARS is controlled by a major east-dipping fault associated with a rollover monocline. This structural scenario results in continued rollover folding and subsequent segmentation of the hanging wall by antithetic normal faulting, as well as back tilting and flexural uplift of the footwall (Buck, 1988; Weissel and Karner, 1989). Therefore, extension leads to subsidence along the inner rift and eastern escarpment, and simultaneous flexural uplift of the western flank. This structural model successfully explains the across-rift tilting pattern of the MHS inside and along the flanks of the Suguta Valley by as much as ca 25 m. It also provides a plausible explanation for the present-day 14–m–difference in the elevation of the overflow sill and the MHS around the lake. This interpretation reconciles inconsistencies in the position of the overflow sill with respect of the MHS in the Suguta Valley. 5.2. Fluvial connectivity in the northern Kenya Rift during the Late Pleistocene–Early Holocene Our DGPS survey combined with SRTM data allowed us to assess the possible past hydrological connections between palaeo-Lake

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

921

0.9°N

To Suguta basin

m 994

0.8°N

900

m

0m

95

Lobat Gap 0.7°N

994

1150 m

0.6°N

LAKE BARINGO (971 m)

m

Kokwob Formation

Contours at 50 m intervals 0 0.5°N 35.9°E

36.0°E

10 km 36.1°E

36.2°E

Fig. 9. Hydrological connection between the highest lake stand of the Lake Baringo and the Suguta drainage basin via the Lobat Gap (located today at ca 994 m, blue contour line, SRTM data). Highest stand in Baringo basin inferred from outcrops of lacustrine Kokwob Formation (see text), reaches 993 m, according to SRTM data. This supports the hypothesis of a former overflow of Lake Baringo into the Lake Suguta basin.

Fig. 8. Location map of the Suguta Valley and adjacent basins. Rivers, catchment boundaries, and overflow sills are also shown. Thick blue line indicates the probable path of the palaeo-river connecting lakes Baringo, Suguta and Turkana during overflow stage. Light blue surfaces denote extent of present-day lakes, dark blue surfaces that of Late Pleistocene–Early Holocene lakes.

Suguta and neighbouring rift-lake basins in the Turkana and the Baringo-Bogoria regions to the north and south, respectively (Figs. 8–10). The Lake Suguta overflow hypothesis was first suggested by Bishop (1975) and Truckle (1976) but it had never been firmly validated. Truckle (1976) indicated an altitude of 600 m for the highest shoreline during the Early Holocene, suggesting overflow of Lake Suguta into Lake Turkana basin. In contrast, Casanova (1986) and Casanova et al. (1988) estimated the elevation of the maximum highstand shoreline at 484 m, i.e., more than 100 m below the potential overflow sill, which would have made an overflow impossible. From our DGPS survey, however, we clearly demonstrate that Lake Suguta was overflowing into the Lake Turkana basin at least from 13 to 8.5 cal ka BP, probably interrupted during transient lowstand periods. Details of this connection are shown in Fig. 5. Lake Suguta may have also received periodic overflow from the Baringo-Bogoria basin during the Late Pleistocene–Early Holocene, as previously proposed by Renaut and Owen (1980) and Renaut

(1982). The Lobat Gap at 994 m (SRTM data, Fig. 9) is the Lake Baringo overflow sill. Previous mapping surveys of the Lobat Gap topography resulted in contradictory elevations (Gregory, 1921; Nilsson, 1931). The highest shoreline in the Lake Baringo basin is dated at ca 16.3–13.8 cal ka BP (Williams and Johnson, 1976) manifested in the deposits of the Kokwob Formation. Reports on elevations of this shoreline diverge (Bishop, 1971; Williams and Johnson, 1976; Young and Renaut, 1979; Renaut and Owen, 1980; Tiercelin and Vincens, 1987), but generally suggest a close association between it and the overflow level. SRTM data indicate a mean elevation of 993 m for the uppermost sediments of the lacustrine Kokwob Formation, which outcrop 1 m below the Lobat Gap, thus supporting the notion of an overflow during former lake highstands. Analogous to our observations made in the Suguta Valley, recent tectonic movements in the Baringo basin may have subsequently deformed the shoreline and sill elevations (Nilsson, 1931; Tiercelin, 1981; Tiercelin and Vincens, 1987). Given the topographic characteristics of the Baringo basin, it is thus conceivable that Lake Baringo and the adjacent Lake Bogoria were connected and overflowed into the Suguta Valley during past high lake-levels that attained comparable heights to the Late Pleistocene–Early Holocene highstand (Young and Renaut, 1979; Renaut and Owen, 1980; Tiercelin and Vincens, 1987). Our new results thus support the hypothesis of a transient fluvial connectivity between the northern Kenya Rift lake-basins, at least during the Late Pleistocene–Early Holocene (Fig. 11). Since it has been suggested that Lake Turkana overflowed into the Nile River hydrologic system at the same time (e.g., Butzer et al., 1972; Harvey and Grove, 1982; Nyamweru, 1989; Owen et al., 1982; Owen and Renaut, 1986), the entire northern Kenya

922

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

Wh it e N

ile

Blue Nile

So

ba

t Lake ZiwayShalla

ETHIOPIA SUDAN o Om Lake Abaya Lake Chamo

Lake Chew Bahir

Lake Turkana

UGANDA km

Fig. 8

KENYA

Rivers 0

50 100

Modern lakes Late PleistoceneEarly Holocene lakes 0°

Lake Suguta Lake Baringo

Late PleistoceneEarly Holocene overflows

Fig. 10. Present-day and Late Pleistocene–Early Holocene physiographic settings of the northern part of the EARS, adapted from Owen and Renaut (1986).

Rift may have been an integral part of the headwaters of the White Nile as previously hypothesized (Champion, 1935; Butzer et al., 1972; Truckle, 1976; Adamson et al., 1980; Harvey and Grove, 1982; Owen et al., 1982; Owen and Renaut, 1986; Vacelet et al., 1991). 5.3. Comparison of the Suguta Valley lacustrine optimum with existing regional/global palaeo-data Our shoreline data and associated lacustrine sediments provide an unprecedented record of hydrological fluctuations in the northern Kenya Rift, when the effect of active tectonic deformation is taken into account. In order to examine the temporal variability of the AHP in East Africa, we will compare the Suguta record with other lake-level fluctuations in the region. Most of these lake-level reconstructions relied on the radiocarbon dating of carbonate shells. Due to the fact that the catchment areas of the compared lake-basins only comprise siliceous basement rocks and volcanics, we hypothesize that the radiocarbon age reservoir for all of them was minor and probably within the uncertainties of the dating method. However, we cannot discard that a radiocarbon age reservoir might have existed for each of the lake-basins compared below. In this context, the chronology of these lake-level reconstructions should be tested with alternative dating methods. In the northern part of the EARS, lakes have undergone highamplitude fluctuations during the last 20 ka (Figs. 10 and 11), thus recording important regional climatic and hydrologic changes (e.g., Butzer et al., 1972; Adamson et al., 1980; Gasse, 2000). Interestingly, no lacustrine deposits corresponding with the Last Glacial Maximum climatic period (LGM: 23–19 cal ka BP) have been recovered from the Suguta Valley. The apparent absence of lakelevel indicators from the LGM in the Suguta Valley is probably due to a very low or vanished lake at that time, similar to many other

lakes in tropical Africa (e.g., Street and Grove, 1976; Farrera et al., 1999). Indeed, the climate during the LGM in tropical Africa was generally arid, at least in the Northern Hemisphere and at the Equator (e.g., Gasse, 2000). Subsequent lacustrine optima occurred approximately between 16.3 and 12 cal ka BP at Lake Baringo (Williams and Johnson, 1976; Renaut and Owen, 1980; Tiercelin and Vincens, 1987) and between ca 16.5 and 8.5 cal ka BP at Lake Suguta. In contrast, to the north, Lake Turkana records a highstand between 13 and 3 cal ka BP (Butzer and Thurber, 1969; Harvey and Grove, 1982; Owen et al., 1982), and Lake Ziway-Shalla in Ethiopia was high between 13 and 5 cal ka BP (Gillespie et al., 1983). Stratigraphic evidence suggests that both Lake Turkana and Lake Ziway-Shalla were probably low or had dried out before 15 cal ka BP (Owen et al., 1982; Gillespie et al., 1983; Johnson, 1987). Our comparison of lake-level curves thus indicate that the Late Pleistocene–Holocene lacustrine optimum occurred earlier at Lake Baringo (0.5 N) and at Lake Suguta (2 N), which are sub-equatorial lakes, while it occurred later at Lake Turkana (2.5–4.5 N) and at Lake Ziway-Shalla (10 N), farther north. We thus interpret these regional differences in the timing of lake highstands in terms of spatial and temporal variations in the climate system of East Africa. A comparison between reconstructed lake-level curves and local insolation curves further helps isolate potential forcing mechanisms driving the observed hydrological changes in East Africa. Late Pleistocene–Early Holocene lake highstands in the Suguta and Baringo basins are approximately in phase with the maximum of spring (March) insolation for this period (Fig. 11). This suggests that a strongly positive water balance at this time was probably associated with intensified spring rains due to increased March insolation at the Equator. As presently observed in the Suguta Valley, the period of equatorial long rains starts during spring. In much of equatorial East Africa the annual cycle of rainfall is strongly bimodal (Nicholson, 1996), with a main rainy season between March and May (‘‘long rains’’) and a short rainy season between October and December (‘‘short rains’’). In turn, towards the northern part of the EARS, Late Pleistocene to Mid-Late Holocene lake highstands in the Turkana and ZiwayShalla basins are roughly in phase with the maximum of summer (June) insolation at 10 N. Today, more than 80% of Lake Turkana water originates from the Omo River, much of which is supplied during the July flood season (i.e., during the boreal summer insolation peak), when rainfall occurs in the Ethiopian Highlands at a latitude of ca 10 N (Owen and Renaut, 1986). This suggests that large changes in water levels in the Turkana basin are mostly controlled by climatic changes, which occur at the same latitudes of the Ziway-Shalla basin. In this region, rains are mainly unimodal and occur between July and September, although significant rains can occur locally during March to May (Griffiths, 1972). In summary, our compilation of regional lake records shows that fluctuations of lakes close to the Equator appear to be driven by cycles of spring insolation, while fluctuations of lakes located away from the Equator are apparently influenced by cycles of summer insolation. This may have strong implications for the evaluation of the moisture history of East Africa in general and may be important for the assessment of different mechanisms driving climate change in the tropics. Our identification of local insolation as the principal forcing mechanism for the past hydrological changes in East Africa is in agreement with the present-day insolation-forcing mechanism that controls rainfall behaviour in this region. Orbital change in local insolation is probably one of the main drivers of hydrological changes in East Africa, which happened after the last glacial period (e.g., Trauth et al., 2003). The timing for the AHP (14.8–5.5 cal ka BP) has been widely used to decipher the regional/global climatic drivers responsible for environmental changes in tropical Africa as a whole during the Late

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

460

Summer insolation (10°N)

June

450 440

(W/m2)

a

923

Elevation (m a.s.l.)

430

b

Lake Ziway-Shalla (7.5-8.5°N)

c

Lake Turkana (2.5-4.5°N)

Overflow to Awash River

1670 1630 1590 1550

460 420 380

Elevation (m a.s.l.)

Transient fluvial connectivity

d

600

340 300

Elevation (m a.s.l.)

Overflow to Nile River

Overflow to Lake Turkana basin

Lake Suguta (2°N)

500

?

400 ?

300

1000

Lake Baringo (0.5°N)

990 980 970

?

f

Elevation (m a.s.l.)

e

Overflow to Suguta Valley

960

Spring and fall insolation (Equator)

(W/m2)

450 440 430 March

420 0

2

4

6

September

8

10

12

14

16

18

20

Age (cal ka BP) Fig. 11. Comparison of Lake Suguta record with other lake-level records from the northern part of the EARS and with low-latitude insolation curves for the period 20–0 cal ka BP. Insolation data (a, f) from Berger (1978). Lake-level curves are dashed where fluctuations are uncertain. (b) Lake Ziway-Shalla curve (Gillespie et al., 1983). (c) Lake Turkana curve (Butzer and Thurber, 1969; Harvey and Grove, 1982; Owen et al., 1982; Johnson, 1987). (d) Lake Suguta curve (this study). (e) Lake Baringo curve (Williams and Johnson, 1976; Renaut and Owen, 1980; Tiercelin and Vincens, 1987; Renaut et al., 2000). All radiocarbon ages were calibrated using Calib 5.0.1 program (Stuiver and Reimer, 1993) with the IntCal04 curve (Reimer et al., 2004) to ensure optimal condition for comparison. Calibrated ages are shown with 2s error bounds. The grey-shaded sectors highlight the periods of probable fluvial connectivity between lakes Baringo, Suguta and Turkana.

Pleistocene–Holocene (e.g., deMenocal et al., 2000; Le´zine and Cazet, 2005). Initially, the AHP timing was defined by compiling palaeo-data mainly from West Africa and almost exclusively from the Northern Hemisphere, explaining the AHP with the 15–20 N summer insolation curve (deMenocal et al., 2000). However, in East Africa the timing of lake highstands during the AHP was apparently diachronous (Fig. 11), suggesting that local (e.g., equatorial) insolation may have influenced the timing and expression of available moisture and resulting lake highstands.

In addition to these long-term humidity changes during the Late Pleistocene–Holocene in tropical Africa, lake highstands were also interrupted by short-term, high-amplitude water-level fluctuations. For example, a sharp 50 m lake-level drop is recorded at Lake Suguta during the YD. The YD consisted of a brief return into full-glacial conditions in the Northern Atlantic region (Broecker, 2003). Several African lakes have also recorded such a pronounced lowstand or even dried out during this time (Gillespie et al., 1983; Street-Perrott and Perrott, 1990; Talbot and Johannessen, 1992; Roberts et al.,

924

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925

1993; Williamson et al., 1993; Gasse, 2000; Shanahan et al., 2006). The synchronicity between pronounced palaeo-environmental changes in tropical Africa and the YD event indicates that extratropical forcing mechanisms may have imprinted the regional hydrological cycle (e.g., Garcin et al., 2006; Gasse et al., 2008). 6. Conclusions Relict shorelines along the Suguta Valley margins in the northern Kenya Rift document that this presently extreme arid environment once accommodated a large lake. A high-precision DGPS survey associated with age determination of lacustrine deposits enabled us to reconstruct the fluctuations of Lake Suguta during the Late Pleistocene–Holocene. Lake Suguta started to rise at least from ca 16.5 cal ka BP, possibly after a pronounced period of aridity, which generally characterized tropical Africa during the LGM (e.g., Gasse, 2000). Lake Suguta was a deep and large lake between 16.5 and 8.5 cal ka BP, intermittently overflowing into the Lake Turkana basin. During its maximum highstand it had a water depth of ca 300 m, a surface area of ca 2150 km2, and a volume of ca 390 km3. The comparison of water levels from Lake Suguta and other lakes of the northern EARS suggests that the timing of the Late Pleistocene–Holocene lake highstands was diachronous in this region. The Suguta lake episode was interrupted by a short-lived regression of ca 50 m between 12.7 and 11.8 cal ka BP, synchronous with the YD event of the Northern Hemisphere. Several other lakes from tropical Africa were low or dry during this period as well, suggesting that remote influences rooted in Northern Hemisphere high-latitude processes may have also influenced climatic changes in this region. Between 8.5 and 7.3 cal ka BP, the water level of Lake Suguta fell by ca 240 m. Since 7 cal ka BP, it remained low or dry, only forming the transient shallow Lake Logipi in the north during episodes of increased rainfall in the catchment areas. Results of our study support the hypothesis that the timing of hydrological changes in East Africa since the Late Pleistocene was mainly controlled by local insolation changes associated with other global climatic forcing mechanisms. Finally, since most of the lake-level reconstructions in the northern EARS are based on radiocarbon dating of carbonate shells, which may be subjected to a reservoir effect, alternative dating methods are needed to develop an unambiguous chronology of environmental changes in this region. Acknowledgements Funding was provided by the German Research Foundation (Deutsche Forschungsgemeinschaft) to projects GRK 1364, TR419/ 6-1 (M.R.S. and M.H.T.) and STR373/16-1 (M.R.S.). Y.G. is supported by an Alexander von Humboldt research fellowship and the DFG Leibniz Center for Surface Process and Climate Studies. We thank the Government of Kenya (Research Permit MOST 13/001/30C 59/ 22), the University of Nairobi and the National Museums of Kenya for research permits and support. We thank C. Gu¨nter for laboratory assistance with the XRD analyses; A. Vincens and M. Taieb for sharing their knowledge about the Suguta Valley; A. Deino, P. Omenda, and M. Maslin who participated to the fieldwork and for inspiring discussions. We are grateful to H. Douglas-Dufresne, P. Ilsley, B. Simpson, M. Magonga and M. Watson for logistical help during fieldwork. The assistance of the Samburu and Turkana people is also greatly appreciated. Two reviewers are thanked for their constructive comments. Appendix A. Supplementary material Supplementary information for this manuscript can be downloaded at doi:10.1016/j.quascirev.2008.12.006.

References Adams, K.D., 1999. Isostatic rebound, active faulting, and potential geomorphic effects in the Lake Lahontan basin, Nevada and California. Geol. Soc. Am. Bull. 111, 1739–1756. Adamson, D.A., Gasse, F., Street, F.A., Williams, M.A.J., 1980. Late Quaternary history of the Nile. Nature 288, 50–55. Baker, B.H., 1963. Geology of the Baragoi area. Geological Survey of Kenya, Report 53, Nairobi, 74 pp. Baker, M.J., Lovenbury, H.T., 1971. The South Turkana Expedition: Scientific Papers VII: The 1969 Season Survey. Geogr. J. 137, 349–360. Berger, A., 1978. Long-term variations of caloric insolation resulting from the earth’s orbital elements. Quat. Res. 9, 139–167. Bishop, W.W., 1971. Succession of Cainozoic vertebrate assemblages from northern Kenya Rift Valley. Nature 233, 389–394. Bishop, W.W., 1975. Geological reconnaissance of the lower Suguta Valley and its eastern flank, north Kenya rift Valley. In: Grove, A.T. (Ed.), Cambridge Meeting on Desertification, pp. 62–63. Broecker, W.S., 2003. Does the trigger for abrupt climate change reside in the ocean or in the atmosphere? Science 300, 1519–1522. Becek, K., 2008. Investigating error structure of shuttle radar topography mission elevation data product. Geophys. Res. Lett. 35. doi:10.1029/2008GL034592 L15403. Buck, W.R., 1988. Flexural rotation of normal faults. Tectonics 7, 959–973. Butzer, K.W., Thurber, D.L., 1969. Some Late Cenozoic sedimentary formations of the lower Omo Basin. Nature 222, 1138–1143. Butzer, K.W., Isaac, G.L., Richardson, J.L., Washbourn-Kamau, C., 1972. Radiocarbon dating of East African lake levels. Science 175, 1069–1076. Casanova, J., 1986. Les stromatolites continentaux: pale´oe´cologie, pale´ohydrologie, pale´oclimotologie. Application au Rift Gregory. Ph.D. thesis, Univ. Aix-Marseille II, Marseille, France, 256 pp. Casanova, J., Hillaire-Marcel, C., Page, N., Taieb, M., Vincens, A., 1988. Palaeohydrology and Late Quaternary stratigraphy of lacustrine deposits in the Suguta Rift (Kenya). C. R. Acad. Sci. Paris Se´r. II 307, 1251–1258. Castanier, S., Bernet-Rollande, M.-C., Maurin, A., Perthuisot, J.-P., 1993. Effects of microbial activity on the hydrochemistry and sedimentology of Lake Logipi, Kenya. Hydrobiologia 267, 99–112. Champion, A.M., 1935. Teleki’s volcano and the lava fields at the southern end of Lake Rudolf. Geogr. J. 85, 323–336. Claussen, M., Kubatzki, C., Brovkin, V., Ganopolski, A., Hoelzmann, P., Pachur, H.J., 1999. Simulation of an abrupt change in Saharan vegetation in the mid-Holocene. Geophys. Res. Lett. 26, 2037–2040. Deino, A.L., Kingston, J.D., Glen, J.M., Edgar, R.K., Hill, A., 2006. Precessional forcing of lacustrine sedimentation in the late Cenozoic Chemeron Basin, Central Kenya Rift, and calibration of the Gauss/Matuyama boundary. Earth Planet. Sci. Lett. 247, 41–60. deMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M., Baker, L., Yarusinsky, M., 2000. Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing. Quat. Sci. Rev. 19, 347–361. Dodson, R.G., 1963. Geology of the South Horr area. Geological Survey of Kenya, Report 60, Nairobi, 53 pp. Dunkley, P.N., Smith, M., Allen, D.J., Darling, W.G., 1993. The Geothermal Activity and Geology of the Northern Sector of the Kenya Rift Valley. British Geological Survey Research Report SC/93/1, NERC, Nottingham, 185 pp. East African Meteorological Department, 1975. Climatological statistics for East Africa, Part 1: Kenya. East African Community/East African Meteorological Department, Nairobi, 92 pp. Farr, T.G., Rosen, P.A., Caro, E., Crippen, R., Duren, R., Hensley, S., Kobrick, M., Paller, M., Rodriguez, E., Roth, L., Seal, D., Shaffer, S., Shimada, J., Umland, J., Werner, M., Oskin, M., Burbank, D., Alsdorf, D., 2007. The shuttle radar topography mission. Rev. Geophys. 45. doi:10.1029/2005RG000183 RG2004. Farrera, I., Harrison, S.P., Prentice, I.C., Ramstein, G., Guiot, J., Bartlein, P.J., Bonnefille, R., Bush, M., Cramer, W., von Grafenstein, U., Holmgren, K., Hooghiemstra, H., Hope, G., Jolly, D., Lauritzen, S.-E., Ono, Y., Pinot, S., Stute, M., Yu, G., 1999. Tropical climates at the Last Glacial Maximum: a new synthesis of terrestrial palaeoclimate data. I. Vegetation, lake-levels and geochemistry. Clim. Dyn. 15, 823–856. Garcin, Y., Vincens, A., Williamson, D., Guiot, J., Buchet, G., 2006. Wet phases in tropical Southern Africa during the last glacial period. Geophys. Res. Lett. 33. doi:10.1029/2005GL025531 L07703. Gasse, F., 2000. Hydrological changes in the African tropics since the Last Glacial Maximum. Quat. Sci. Rev. 19, 189–211. Gasse, F., Chalie´, F., Vincens, A., Williams, M.A.J., Williamson, D., 2008. Climatic patterns in equatorial and southern Africa from 30,000 to 10,000 years ago reconstructed from terrestrial and near-shore proxy data. Quat. Sci. Rev. 27, 2316–2340. Gillespie, R., Street-Perrott, F.A., Switsur, R., 1983. Post-glacial arid episodes in Ethiopia have implications for climate prediction. Nature 306, 680–683. Gregory, J.W., 1921. The Rift Valleys and Geology of East Africa. London, 479 pp. Griffiths, J.F., 1972. Climates of Africa. Elsevier, Amsterdam, 604 pp. Harvey, C.P.D., Grove, A.T., 1982. A prehistoric source of the Nile. Geogr. J. 148, 327–336. Heiken, G.H., 1971. Cinder rings: examples from Fort Rock-Christmas Lake Valley Basin, south central Oregon. J. Geophys. Res. 76, 5615–5626.

Y. Garcin et al. / Quaternary Science Reviews 28 (2009) 911–925 Hemming, C.F., 1972. The South Turkana Expedition: Scientific Papers VIII: The ecology of South Turkana: a reconnaissance classification. Geogr. J. 138, 15–40. Hopfield, H.S., 1969. Two-Quartic tropospheric refractivity profile for correcting satellite data. J. Geophys. Res. 74, 4487–4499. Hughen, K.A., Southon, J.R., Lehman, S.J., Overpeck, J.T., 2000. Synchronous radiocarbon and climate shifts during the last deglaciation. Science 290, 1951–1954. Johnson, T.C., Halfman, J.D., Rosendahl, B.R., Lister, G.S., 1987. Climatic and tectonic effects on sedimentation in a rift-valley lake: Evidence from high-resolution seismic profiles, Lake Turkana, Kenya. Geol. Soc. Am. Bull. 98, 439–447. Kutzbach, J.E., 1981. Monsoon climate of the Early Holocene: Climate experiment with the Earth’s orbital parameters for 9000 years ago. Science 214, 59–61. Kutzbach, J.E., Street-Perrott, F.A., 1985. Milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr BP. Nature 317, 130–134. Leica Geosystems, 2007. Leica GPS1200 Series Technical Data. Technical report, Heerbrugg, Switzerland, 15 pp. Le´zine, A.-M., Cazet, J.-P., 2005. High-resolution pollen record from core KW31, Gulf of Guinea, documents the history of the lowland forests of West Equatorial Africa since 40,000 yr ago. Quat. Res. 64, 432–443. Makinouchi, T., Koyaguchi, T., Matsuda, T., Mitsushio, H., Ishida, S., 1984. Geology of the Nachola area and the Samburu Hills, west of Baragoi, Northern Kenya. Afr. Stud. Monogr. 2 (Suppl. Issue), 15–44. Matsuda, T., Torii, M., Koyaguchi, T., Makinouchi, T., Mitsushio, H., Ishida, S., 1984. Fission-track, K-Ar age determinations and palaeomagnetic measurements of Miocene volcanic rocks in the western area of Baragoi, Northern Kenya: ages of hominoids. Afr. Stud. Monogr. 2 (Suppl. Issue), 57–56. Morgan, W.T.W., 1971. The South Turkana Expedition: Scientific Papers IV: Land Units of the Lokori Area. Geogr. J. 137, 14–28. Nicholson, S.E., 1996. A review of climate dynamics and climate variability in eastern Africa. In: Johnson, T.C., Odada, E.O. (Eds.), The Limnology, Climatology and Paleoclimatology of the East African Lakes. Gordon and Breach Publishers, Amsterdam, pp. 25–56. Nilsson, E., 1931. Quaternary glaciations and pluvial lakes in British East Africa. Geografiska Annaler 13, 249–349. Nyamweru, C., 1989. New evidence for the former extent of the Nile drainage system. Geogr. J. 155, 179–188. Ojany, F.F., Ogenoo, R.B., 1973. Kenya: A Study in Physical and Human Geography. Longman, Nairobi, 228 pp. Owen, R.B., Renaut, R.W., 1986. Sedimentology, stratigraphy and palaeoenvironments of the Holocene Galana Boi Formation, NE Lake Turkana, Kenya. In: Frostick, L.E. (Ed.), Sedimentation in the African Rifts. Geol. Soc. London, Spec. Publ. 25, pp. 311–322. Owen, R.B., Barthelme, J.W., Renaut, R.W., Vincens, A., 1982. Paleolimnology and archaeology of Holocene deposits north-east of Lake Turkana, Kenya. Nature 298, 523–529. Prell, W.L., Kutzbach, J.E., 1987. Monsoon variability over the past 150,000 Years. J. Geophys. Res. 92, 8411–8425. Rasmussen, S.O., Andersen, K.K., Svensson, A.M., Steffensen, J.P., Vinther, B.M., Clausen, H.B., Siggaard-Andersen, M.L., Johnsen, S.J., Larsen, L.B., DahlJensen, D., Bigler, M., Rothlisberger, R., Fischer, H., Goto-Azuma, K., Hansson, M.E., Ruth, U., 2006. A new Greenland ice core chronology for the last glacial termination. J. Geophys. Res. 111. doi:10.1029/2005JD006079 D06102. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Bertrand, C.J.H., Blackwell, P.G., Buck, C.E., Burr, G.S., Cutler, K.B., Damon, P.E., Edwards, R.L., Fairbanks, R.G., Friedrich, M., Guilderson, T.P., Hogg, A.G., Hughen, K.A., Kromer, B., McCormac, G., Manning, S., Ramsey, C.B., Reimer, R.W., Remmele, S., Southon, J.R., Stuiver, M., Talamo, S., Taylor, F.W., van der Plicht, J., Weyhenmeyer, C.E., 2004. IntCal04 terrestrial radiocarbon age calibration, 0-26 cal kyr BP. Radiocarbon 46, 1029–1058. Renaut, R.W., 1982. Late Quaternary Geology of the Lake Bogoria Fault-Trough, Kenya Rift Valley. Ph.D. thesis, Univ. of London, 498 pp. Renaut, R.W., Owen, R.B., 1980. Late Quaternary fluvio-lacustrine sedimentation and lake levels in the Baringo basin, northern Kenya, Rift Valley. Recherches Ge´ologiques en Afrique 5, 130–133. Renaut, R.W., Tiercelin, J.-J., Owen, R.B., 2000. Lake Baringo, Kenya Rift Valley, and its Pleistocene precursors. In: Gierlowski-Kordesch, E., Kelts, K. (Eds.), Lake Basins Through Space and Time. Am. Assoc. Petrol. Geol. Stud. Geol. 46 Tulsa, pp. 561–568.

925

Renssen, H., Brovkin, V., Fichefet, T., Goosse, H., 2006. Simulation of the Holocene climate evolution in Northern Africa: The termination of the African Humid Period. Quat. Int. 150, 95–102. Ritchie, J.C., Eyles, C.H., Haynes, C.V., 1985. Sediment and pollen evidence for an Early to Mid-Holocene humid period in the eastern Sahara. Nature 314, 352–355. Roberts, N., Taieb, M., Barker, P., Damnati, B., Icole, M., Williamson, D., 1993. Timing of the Younger Dryas Event in East Africa from lake-level changes. Nature 366, 146–148. Saneyoshi, M., Nakayama, K., Sakai, T., Sawada, Y., Ishida, H., 2006. Half graben filling processes in the early phase of continental rifting: The Miocene Namurungule Formation of the Kenya Rift. Sediment. Geo. 186, 111–131. Scholz, C.A., Johnson, T.C., Cohen, A.S., King, J.W., Peck, J.A., Overpeck, J.T., Talbot, M.R., Brown, E.T., Kalindekafe, L., Amoako, P.Y.O., Lyons, R.P., ˜ eda, I.S., Heil, C.W., Forman, S.L., McHargue, L.R., Shanahan, T.M., Castan Beuning, K.R., Gomez, J., Pierson, J., 2007. East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins. Proc. Natl. Acad. Sci. U.S.A. 104, 16416–16421. Shanahan, T.M., Overpeck, J.T., Wheeler, C.W., Beek, J.W., Pigati, J.S., Talbot, M.R., Scholz, C.A., Peck, J., King, J.W., 2006. Paleoclimatic variations in West Africa from a record of Late Pleistocene and Holocene lake level stands of Lake Bosumtwi, Ghana. Palaeogeogr. Palaeoclimatol. Palaeoecol. 242, 287–302. Street, F.A., Grove, A.T., 1976. Environmental and climatic implications of Late Quaternary lake-level fluctuations in Africa. Nature 261, 385–390. Street-Perrott, F.A., Perrott, R.A., 1990. Abrupt climate fluctuations in the tropics: the influence of Atlantic Ocean circulation. Nature 343, 607–612. Stuiver, M., Reimer, P.J., 1993. Extended 14C data base and revised CALIB 3.0 14C age calibration program. Radiocarbon 35, 215–230. Talbot, M.R., Johannessen, T., 1992. A high-resolution paleoclimatic record for the last 27,500 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustrine organic matter. Earth Planet. Sci. Lett. 110, 23–37. Tiercelin, J.-J., 1981. Rifts continentaux, tectonique, climats, se´diments. Exemples: la se´dimentation dans le Nord du Rift Gregory (Kenya) et dans le Rift de l’Afar (Ethiopie) depuis le Mioce`ne. Ph.D. thesis, Univ. Aix-Marseille II, Marseille, France, 260 pp. Tiercelin, J.-J., Vincens, A., 1987. Le Demi-Graben de Baringo-Bogoria, Rift Gregory, Kenya, 30,000 Ans d’Histoire Hydrologique et Se´dimentaire, Bull. Centres de Rech. Expl. Prod. Elf-Aquitaine, Pau, France 11, pp. 249–540. Trauth, M.H., Deino, A.L., Bergner, A.G.N., Strecker, M.R., 2003. East African climate change and orbital forcing during the last 175 kyr BP. Earth Planet. Sci. Lett. 206, 297–313. Trauth, M.H., Maslin, M.A., Deino, A.L., Strecker, M.R., 2005. Late Cenozoic moisture history of East Africa. Science 309, 2051–2053. Truckle, P.H., 1976. Geology and Late Cainozoic lake-sediments of the Suguta Trough, Kenya. Nature 263, 380–383. Vacelet, J., Tiercelin, J.-J., Gasse, F., 1991. The sponge Dosilia brouni (Spongillidae) in Lake Baringo, Gregory Rift, Kenya. Hydrobiologia 211, 11–18. Vincens, A., 1982. Palynologie, environnements actuels et plio-ple´istoce`nes a` l’Est du lac Turkana (Kenya). Ph.D. thesis, Univ. Aix-Marseille II, Marseille, France, 244 pp. Weaver, S.D., 1973. The Quaternary caldera volcano Emuruangogolak, Kenya Rift, and the petrology of a bimodal ferrobasalt-pantelleritic trachyte association. Bull. Volcanol. 40, 209–230. Weissel, J.K., Karner, G.D., 1989. Flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension. J. Geophys. Res. 94, 13919–13950. Williams, R.E.G., Johnson, A.S., 1976. Birmingham University radiocarbon dates X. Radiocarbon 18, 249–267. Williamson, P.G., Savage, R.J.G., 1986. Early rift sedimentation in the Turkana basin, northern Kenya. In: Frostick, L.E. (Ed.), Sedimentation in the African Rifts. Geol. Soc. London, Spec. Publ. 25, pp. 267–283. Williamson, D., Taieb, M., Damnati, B., Icole, M., Thouveny, N., 1993. Equatorial extension of the younger Dryas event: rock magnetic evidence from Lake Magadi (Kenya). Global Planet. Change 7, 235–242. Young, J.A.T., Renaut, R.W., 1979. Radiocarbon date from Lake Bogoria, Kenya Rift Valley. Nature 278, 243–244.