Lake Naivasha (central Rift Valley, Kenya) is regionally unique in having accumulated a continuous sediment record of climate-driven palaeohydrological change over the past 1650 years, attesting to dramatic shifts between freshwater high-stands and either fresh or saline low-stands. This study employs fossil ostracod assemblages and stable-isotope (δ18O and δ13C) signatures to disclose the lake's ecohydrological history and explain the evolving non-linearity between lake level and salinity. The original age model and lithology-based lake-level reconstruction are revised, and the diatom-inferred salinity reconstruction is expanded to the lower (pre-770 CE) part of the record. The stable-isotope time series showed limited coherence with the other proxies, probably due to complexities related to hydrological connections between different surface- and groundwater components shifting through time. Ostracod palaeoecological data indicate that between ca 350 and 1200 CE the system often veered towards hydrological closure, with frequent and high-amplitude shifts in lake level and salinity preventing long-term establishment of wetland vegetation. Low lake-level and saline conditions dominated between ca 900 and 1200 CE. After that time and particularly since the onset of more permanent Malewa River inflow ca 1430 CE, hydrologically open conditions prevailed. Salinity shifts exhibited lower amplitude and frequency, promoting the expansion of wetland vegetation and associated biota including benthivorous fish. With lake water balance strongly influenced by variation in river inflow, high-amplitude lake-level fluctuations continued throughout the freshwater phase of the past 600 years, notably creating a pronounced high-stand dated to ca 1600–1780 CE and the well-documented extreme low-stand of the early 1800s.
Bibliographical noteFunding Information:
Fieldwork and original analyses on the recovered sediment cores, including the extended diatom-inferred salinity reconstruction and the generation of ostracod data presented here, were supported by NSF-DEB 93-20324 , NSF-ATM 95–31222 , NSF-RTG 90–14277 and the Quaternary Paleoecology program at the University of Minnesota. Additional analyses of those data were supported by the Federal Science Policy of Belgium (BelSPO) through Brain-be project BR/121/A2/PAMEXEA ‘Patterns and mechanisms of climate extremes in East Africa’. We thank Karin Holmgren for laboratory assistance, Koen Martens for advice on ostracod taxonomy and Bob Rumes for background information on ostracod surveys in Kenya and Uganda. We further thank two anonymous reviewers for detailed and constructive comments which improved the manuscript.
- East Africa
- Lake Naivasha
- Lake level
- Salinity fluctuations