PLAYA LAKE AND MARGINAL WETLAND SEDIMENTS RECORD OVER 1000 YEARS OF CLIMATE SHIFTS AT LAKE SOLAI, EAST AFRICAN RIFT, KENYA

Playa lakes associated with perennial wetlands are unusual but important archives of paleoclimatic and environmental changes in semi-arid environments. We present sediment and organic geochemical data from a lake core and a delta-wetland core collected from Lake Solai, a small (<9 km²) shallow (<1 m) playa lake located near the equator in the East African Rift (EAR), Kenya. The lake is situated in a small graben; its watershed (175 km²) is isolated from other EAR lakes. Lake Solai is fed by two perennial spring-fed rivers sourced in the highlands (~2100 m) to the east and south, and by groundwater emerging as springs and seeps from the flanks of the adjacent escarpment. P in the watershed (~700-900 mm/yr) is less than ET on the valley floor (~1800 mm/yr). Irrigation pressure limits surface and groundwater input into the lake and it has dried completely several times in the last 40 years. Recently, water levels have risen (2011-2014) displacing local residents. These lake-level fluctuations are the result of intra-annual-to-decadal climate variations (e.g., ENSO).

The wetland core is divided based on texture, organic matter (OM) content, and biological proxies. Zone 1 (187-67 cm) consists of OM-poor (<0.5 wt% C) crumbly-textured silty clays, scattered carbonate nodules with δ¹³C and δ¹⁸O consistent with pedogenesis, and interbedded laminated clays, suggesting periodic exposure and flooding of a playa lake. An abrupt change to OM-rich black clays and peaty clays occurs at the start of Zone 2 (67-40 cm) and marks the inception of the wetland at ~ AD 1155. Zone 1 (40-0 cm) is more OM-rich (2-7 wt% C) indicating better wetland development and higher lake levels beginning at ~ AD 1490. The lake core (86 cm) consists of crumbly silts interbedded with laminated silty clays consistent with periodic exposure and flooding events. At 65 cm, an abrupt increase in OM content (~2 wt% C) indicates possible correlation with the onset of wetland formation. Greater proportions of detrital kaolinite and illite clay-minerals (highland source) relative to smectite (rift-valley source) occurs in the wetland and upper lake sediments consistent with greater runoff and erosion of the highlands as a result of wetter conditions. Both the wetland and lake records in this small basin indicate inception of wetter conditions at the start of the Little Ice Age.