Fossil diatoms and the mid to late holocene paleolimnology of Lake Turkana, Kenya: a reconnaissance study

Abstract

A 12 m sediment core recovered from the south basin of Lake Turkana, northwestern Kenya, reveals four major diatom assemblages that span approximately 5450 to 1070 years BP based on AMS radiocarbon analyses. The oldest assemblage, Zone D (5450 to 4850 yr BP), is dominated by Melosira nyassensis and Stephanodiscus spp. and is interpreted to reflect higher lake levels, fresher water and more variable seasonal mixing of the water column than the modern lake. Melosira dominates the assemblage in Zone C (4850 to 3900 yr BP) with some Surirella engleri and Stephanodiscus. This assemblage indicates a continuation of relatively high lake levels and seasonal mixing of a stratified lake. The brief peak of Surirella, interpreted as benthic, suggests an episode of slightly lower lake level. Thalassiosira rudolfi and Surirella predominate since the beginning of Zone B (3900 to 1900 yr BP), reflecting a decrease in lake level and increase in water column salinity. Increasing dominance of Surirella in Zone A (1900 to 1070 yr BP) may suggest that the lake continued to decrease in depth. Salinity probably rose to levels comparable with the modern lake. These results are consistent with paleoclimatic interpretations based on carbonate abundance, lamination thickness, oxygen isotope and bulk geochemistry profiles from this core and cores recovered from the north basin. It extends the known paleolimnology beyond 4000 yr BP of the earlier research to 5450 yr BP and into the early to mid Holocene pluvial phase in northern intertropical east Africa.

Lakeside View: Sociocultural Responses to Changing Water Levels of Lake Turkana, Kenya

“…There are few data to constrain lake level between 4,000 and 200 years ago besides a sediment core extracted from the southeastern portion of the lake and seismic data (Halfman et al 1992;Halfman and Johnson 1988;Morrissey and Scholz 2014). Seismic unconformities dated to approximately 2,300 years BP indicate that the lake may have been 40 m below its current level (Morrissey and Scholz 2014).…”

Environmental assessment of the East African Rift Valley lakes

“…Lake Turkana subsequently rose rapidly and achieved open lake status at approximately 9,500 yr BP, standing at about 70 m above the present day lake-level (Butzer, 1980). Regressions in Lake Turkana occurred at 7,500 (a 60 m lowering of lake level - Owen et al, 1982), 4,500 yr BP (Halfman et al, 1992) and 4,000 yr BP (a 30 m lowering of lake level - Owen et al, 1982), respectively. From about 400,000 years BP, the Lake Victoria basin seems to have experienced several major climatic events which included the lake drying up and most recently has seen the present Lake Victoria arising from a dry landscape about 14,600 years ago (Johnson et al, 2000).…”

Collapse, environment, and society

“…Prominent and dated beach ridges indicate a sporadic, early to mid-Holocene overflow of this nonoutlet lake, through a series of swamps to the Sobat and White Nile rivers (20). Together with diatom assemblages, alkalinity levels, and Omo detrital silicates, the lake levels offer a proxy record for Blue Nile behavior, but the chronology is only approximate (21). A decrease in Omo material approximately 2800 (calibrated years) BCE was followed by an abrupt change in water chemistry, to a closed, alkaline-saline lake, approximately 2400 BCE.…”