Early Holocene Water Budget of the Nakuru-Elmenteita Basin, Central Kenya Rift
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by RERO DOC Digital Library J Paleolimnol (2006) 36:281–294 DOI 10.1007/s10933-006-9003-z ORIGINAL ARTICLE Early Holocene water budget of the Nakuru-Elmenteita basin, Central Kenya Rift Miriam Du¨ hnforth Æ Andreas G. N. Bergner Æ Martin H. Trauth Received: 11 July 2005 / Accepted: 5 March 2006 / Published online: 25 August 2006 Ó Springer Science+Business Media B.V. 2006 Abstract The Nakuru-Elmenteita basin in the negative hydrological budget of this basin. Since Central Kenya Rift, contains two shallow, alka- we did not find any field evidence for a surface line lakes, Lake Nakuru (1770 m above sea level) connection, as often proposed during the last and Lake Elmenteita (1786 m). Ancient shore- 70 years, the hydrological deficit of the Nakuru- lines and lake sediments at ~1940 m suggest that Elmenteita basin could have also been compen- these two lakes formed a single large and deep sated by a subsurface water exchange. lake as a result of a wetter climate during the early Holocene. Here, we used a hydrological Keywords East Africa Æ Hydrological model to compare the precipitation–evaporation modeling Æ Paleoclimate Æ Nakuru-Elmenteita balance during the early Holocene to today. Assuming that the Nakuru-Elmenteita basin was hydrologically closed, as it is today, the most Introduction likely climate scenario includes a 45% increase in mean-annual precipitation, a 0.5°C decrease in air In East Africa, most climate records document temperature, and an increase of 9% in cloud dramatic hydrological changes since the Last coverage from the modern values. Compared to Glacial Maximum (LGM) (e.g. Street-Perrott and the modeling results from other East African lake Perrott 1993; Gasse 2000; Barker et al. 2003; basins, this dramatic increase in precipitation Trauth et al. 2003). Orbitally-induced changes in seems to be unrealistic. Therefore, we propose a the strength of the monsoon system and the significant flow of water from the early Holocene intertropical convergence account for most of Lake Naivasha in the south towards the Nakuru- these long-term changes in tropical Africa. Dur- Elmenteita basin to compensate the extremely ing the LGM (23–18 kyr BP), most records indi- cate a relatively dry climate, followed by a series of arid-humid transitions at 17–16, 15–14.5 and 12–11 cal. kyr BP (Barker et al. 2003). In contrast M. Du¨ hnforth (&) Institute of Geology, ETH Zu¨ rich, 8092 Zu¨ rich, to these fluctuating conditions reflecting complex Switzerland linkages between orbital forcing and ocean- e-mail: [email protected] atmospheric interactions, the early Holocene cli- mate was generally very wet in East Africa and A. G. N. Bergner Æ M. H. Trauth Institut fu¨ r Geowissenschaften, Universita¨t Potsdam, surrounding areas (Gasse and van Campo 1994; Potsdam, Germany Gasse 2000; Thompson et al. 2002). The wetter 123 282 J Paleolimnol (2006) 36:281–294 climate at 10–6 kyr BP is generally attributed to a high-resolution modeling for the Naivasha basin lower-latitude maximum in solar radiation at support these results (16–32% more rain during around 10.4 kyr BP, an increase of the land-ocean the early Holocene, Bergner et al. 2003). pressure gradient, stronger lateral moisture Employing the same high-resolution model used transport and higher precipitation (Kutzbach and by Bergner et al. (2003), we have now estimated Street-Perrott 1985). the hydrological conditions in the Nakuru-Elmenteita Paleoshorelines and associated sediments also basin and compared these with the hydrology of document high lake levels in the Central Kenya the Naivasha basin. The comparison of the two Rift during the early Holocene (Washbourn hydrological budgets allows us to assess a poten- 1967a; Washbourn-Kamau 1970; 1971; Richard- tial water exchange between the two high lakes son and Richardson 1972; Richardson and and will provide a better estimate for the pre- Dussinger 1986). Sediment characteristics and cipitation–evaporation balance during the early diatom assemblages contained in sediment cores Holocene. provide further evidence for deep freshwater lakes at that time (Richardson and Dussinger 1986). Whereas there is no doubt about large Regional setting lakes in the Nakuru-Elmenteita and Naivasha basins, the existence of an even larger Lake The Nakuru-Elmenteita basin is located in the Nakuru-Elmenteita-Naivasha between 15 and Central Kenya Rift between 0°S36¢Eand1°S 5 kyr BP has been intensely discussed for more 36.5¢E (Figs. 1 and 2). The deepest part of the than 70 years (e.g. Nilsson 1931; Butzer et al. basins contains two lakes, Lake Nakuru (1770 m 1972). Nilsson (1931) suggested that both lake above sea level) and Lake Elmenteita (1786 m). basins were connected and contained one large The catchment area of the Nakuru-Elmenteita Gamblian Lake (named after the Gamble’s Cave basin is about 2390 km2 and the elevation ranges south of modern Lake Nakuru), but paleoshore- from 1760 to 3080 m. On the western side, the lines at the same elevation as in the Naivasha basin is bounded by east-dipping normal faults of basin (2000 m above sea level, compared to the the Mau Escarpment (3080 m). To the east, the modern lake level at 1890 m) have never been basin is bordered by west-dipping faults that mapped in the Nakuru-Elmenteita basin (highest separate the intra-rift Bahati-Kinangop Plateau shorelines at ~1940 m). Hence, Butzer et al. (2740 m) from the eastern rift shoulder and the (1972) published a map showing the outline of Aberdare Ranges (3999 m). The northern two separate early Holocene lakes with ~60 m boundary comprises the Menengai Caldera different lake levels in the Central Kenya Rift. (2278 m). To the south, the Mt. Eburru volcano The hydrological connection of the two basins (2820 m) separates the Nakuru-Elmenteita basin has important consequences for lake-balance from the Naivasha Basin, which contains the modeling. Lake-balance models have become a modern Lake Naivasha (1890 m). widely accepted tool to link lake-level Regional climate is strongly influenced by the fluctuations with changes in the precipitation– seasonal migration of the Intertropical Conver- evaporation ratio in the catchment (Kutzbach gence Zone (ITCZ) and the coinciding precipi- 1980; Hastenrath and Kutzbach 1983; Bookhagen tation pattern (Nicholson 1996; 2000). Rainfall et al. 2001; Bergner et al. 2003). Ancient shore- associated with the transition of the ITCZ follows lines, beach gravels and the spatial distribution of the highest sun in March and September with a lake sediments help to reconstruct the dimensions lag of three to four weeks (Nicholson 2000). of paleolakes, which are then simulated using a Therefore, the basin receives most of its precipi- hydrological model. Early low-resolution hydro- tation during the ‘‘long rains’’ in April to May and logical models applied to the lakes in the Central the ‘‘short rains’’ in November. A third subordi- Kenya Rift during the early Holocene suggest an nate precipitation maximum occurs in September, increase of 15–35% in precipitation as compared when the Congo air masses transport moisture to today (Hastenrath and Kutzbach 1983). Recent from the Congo basin to East Africa (Vincent 123 J Paleolimnol (2006) 36:281–294 283 Fig. 1 Topographic map of the East African Riftsystem and the location of the Central Kenya Rift (modified from Strecker 1991) et al. 1979). Mean annual rainfall is 920 mm/y, the precipitation at high altitudes of the Aberdare whereas evaporation is about 1736 mm/y (Kenya Ranges (Richardson and Richardson 1972). In Meteorological Department 2000). The amount contrast, the lakes Nakuru and Elmenteita are of precipitation is strongly linked to topography. highly alkaline with values between 120 and Thus, the highest rates are obtained in the high- 1440 meq/l for Lake Nakuru (Beadle 1932; elevation parts of the basin in the west and east, Talling and Talling 1965) and 290 meq/l for Lake whereas the lower areas are relatively dry. Elmenteita (Talling and Talling 1965). The mod- The long term hydrologic budget of the basin is ern lakes Nakuru and Elmenteita cover an area of primarily controlled by the spatial and temporal 26 and 40 km2, respectively. Both are shallow distribution of rainfall, superimposed on tectoni- with an average water depth of less than 3 m, cally-driven influences on the drainage network fluctuating between 3 and 5 m since water levels and basin geometry (Strecker et al. 1990). Gen- have been recorded (Talling and Talling 1965). erally, north–south trending fault blocks deflect most of the rivers draining the Aberdare Ranges and Kinangop Plateau towards the south into the Methods Naivasha basin (Fig. 2). Only minor rivers peri- odically drain into the Nakuru-Elmenteita basin. The hydrologic modeling involved the (1) recon- The freshwater character of modern Lake struction of the 3D basin geometry and (2) the Naivasha is primarily attributed to this excep- modeling of the modern and early Holocene tional drainage pattern and strongly depends on hydrological budgets. To reconstruct the early 123 284 J Paleolimnol (2006) 36:281–294 Fig. 2 Topographic map of the Nakuru-Elmenteita and the adjacent Naivasha basin showing Lakes Nakuru, Elmenteita and Naivasha and their catchments Holocene lake-level highstand, we mapped lake measurements were corrected for daily sediments and paleoshorelines in the Nakuru-Elmenteita barometric trends. We compared our results with basin. The ages of the lake deposits were deter- measurements by Washbourn (1967a) who used a mined by 14C dating of snail shells sampled from Zeiss self-aligning level, the Survey of Kenya diatomaceous silts at Lemulug crater, Karterit trigonometrical stations and benchmarks. Chan- volcano and in a section east of Lake Elmenteita ges in the basin geometry in the course of (Figs.