RECORD OF CLIMATE CHANGE IN A TROPICAL GROUNDWATER-FED WETLAND, KISIMA NGEDA, EYASI BASIN, TANZANIA

The lakes of arid regions often dry up causing gaps in sediment records. Previous researchers identified groundwater discharge (GWD) sites, which unlike lakes are unaffected by precipitation levels as groundwater can be sourced from great distances. GWD occurs at the surface as springs and seeps, supporting wetlands and continuously recording climate despite low rainfall or droughts. This study used remote sensing to identify GWD sites, which were visited and cored to determine longevity and climate record potential of GWD sites.

The study site is on the NE margin of Lake Eyasi in the Eastern Gregory Rift of Northern Tanzania. Landsat and GoogleEarth images were analyzed to locate GWD sites. In July 2011 logistically feasible sites were visited and 7 cores collected. All cores underwent initial analyses (magnetic susceptibility, density, and physical description), and Core 1A selected for radiocarbon dating and sedimentological, geochemical, organic biomarker (n-alkanes), and microfaunal (diatoms and testate amoebae) analysis.

All cores recovered sediment with 2 layers distinguishable by color and content. The lower layer, light (Munsell: 5Y 4/1) organic-rich clay, further characterized in 1A by depleted microfauna, 5% OM, low C (</= 1%) and N (<0.1%), and avg. C/N of ~11.4. The upper layer, darker (Munsell: 10YR 2/1) and very organic-rich, characterized in 1A by increased microfauna, 10-35% OM increasing up core, C 3-22% and N 0.2-1.2% both increasing up core, and avg. C/N of ~14.5. The upper layer base (28-29 cm depth) of 1A was dated to 270 +/- 30 yrs BP.

Lower layer is interpreted as lake margin sediment, while upper is freshwater wetland deposit. Contact between layers represents GWD onset. The upper layer contains 3 zones representing stages of GWD development and wetland growth. The age date coincides with the end of the Maunder Minimum, shown to be a period of increased rainfall in this region of East Africa. As rainfall decreased GWD began, fewer lake transgressions occurred allowing plants to grow and a wetland to form.

This study shows GWD sites contain usable climate records. Additionally, remote sensing offers a low-cost way to identify and monitor GWD sites, key in arid regions where GWD can provide continuous potable water for people, livestock, and crops.