EVIDENCE FOR CENTENNIAL-SCALE CLIMATE VARIATION RECORDED BY ORGANIC MATTER IN A FRESHWATER WETLAND, LAKE SOLAI, KENYA

Lake Solai is a small (9 km²) shallow (< 1.5 m) playa lake located near the equator in the East African Rift Valley, Kenya. The lake is situated on the eastern side of a half-graben bounded by the Solai escarpment (>2000 m). The lake is fed by groundwater sourced in the highlands, which emerges as springs and seeps on the flanks of the escarpment, and by two perennial rivers, which drain the highlands to the east and south. A small freshwater wetland (~0.3 km²) dominated by Typha sp. developed on the delta formed by the Maji Tamu river at the southern end of the Lake during the late Holocene. The climate is semi-arid. P is ~650 mm/yr on valley floor and ~1000 mm/yr in the river catchments; ET ~1800 mm/yr. The area is heavily populated; irrigation pressure limits freshwater input into the lake and it has dried out several times in the last 40 years.

A push core (~2 m) collected in the wetland contains organic rich clays and peat in the upper 68 cm. Wetland formation occurred at ~880±40 ¹⁴C yr BP and likely is related to a shift to wetter climate recorded by similar dates in other wetlands within the East African Rift. The high bulk organic matter (BOM) content of the wetland (1.1-7.5 wt% C) separates it from the underlying organic poor (< 0.5 wt% C) laminated lacustrine and lake margin silty clays. C/N ratios are consistent with an increase in the contribution of vascular relative to non-vascular (algae, plankton) plants to BOM as the wetland developed. δ¹³C ratios of BOM in the wetland (-16.8 to -21.0 ‰) are depleted relative to the underlying sediment (δ¹³C=-13.5 to -19 ‰) and indicate an increase in the proportion of C₄ relative to C₃ plants. δ¹⁵N ratios of BOM in the wetland (5.0 to 7.1 ‰) are enriched relative to the underlying sediment (δ¹⁵N=2.6 to 5.4‰) and indicate a change from dominantly terrestrial to dominantly aquatic nitrogen sources. Within the wetland interval itself (~-20 to -30 cm), a period of drying has been identified based on depleted δ¹⁵N ratios (~5.4 ‰). A slight reddening (oxidation?) of the sediment and an increase in the abundance of seeds in this interval are consistent. Other climatic proxies such as diatoms or ostracods were not identified. This study documents periods of wetter, then dryer, then wetter climate conditions occurred over centennial time scales in the Lake Solai wetland and are preserved by bulk organic matter.