T101 African Environments across Space and through Time: Integrating Modern and Ancient Climate Data for Insights into Terrestrial Ecosystem Dynamics

Monday, 2 November 2015
8:00 AM - 12:00 PM
Room 331/332

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

Paper number: T101-6
Program: Technical Sessions
Time: 9:25 AM – 9:40 AM

Authors

Abstract

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.