Characterization of groundwater discharge sites using remote sensing and wetland cores, Lake Eyasi Basin, Tanzania.

摘要

Lakes in arid regions commonly dry up creating gaps in the sediment records. Groundwater discharge (GWD) sites are sourced from aquifers that may be recharged, at great distances and unaffected by local precipitation levels. GWD occurs as surface springs and seeps supporting continuously aggrading wetlands with the potential to record environmental and climatic change.;The study site is the NE margin of Lake Eyasi in the Eastern Gregory Rift of Northern Tanzania. Landsat and GoogleEarth images were analyzed to locate GWD. In July 2011 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, and biological analysis. All cores contained 2 intervals distinguishable by color and content. The base of the upper interval (28-29 cm depth) of Core 1A was AMS dated to 270 +/- 30 yrs BP (conventional radiocarbon age) using seeds.;The lower interval, light (Munsell: 5Y 4/1) organic-poor clay, impoverished microfauna, 5% organic matter, low total organic carbon (/= 1%) and total nitrogen (0.1%), and average TOC/TN of ~11.4. The upper interval, darker (Munsell: 10YR 2/1) organic-rich, microfauna consisting of diatoms and testate amoebae, 10-35% organic matter, total organic carbon 3-22% and total nitrogen 0.2-1.2%, and average TOC/TN of ∼14.5.;The lower interval records pre-wetland, lake margin sedimentation, while the upper interval is a freshwater wetland deposit. Contact between intervals represents onset of groundwater flow to the site. When calibrated to calendar years the age date indicates GWD reached the site between 200-500 years before today (2012). When the groundwater travel time, estimated to be 400 years, is factored in the date of the rainfall responsible for recharging groundwater coincides with the start of the Little Ice Age, a period of increased rainfall in East Africa.;This study shows GWD sites contain usable climate records. Additionally, remote sensing offers a low-cost way to identify and monitor GWD, critical in arid regions where GWD means potable water. Lastly, radiocarbon dating shows these sites are not ephemeral features and have the capacity to persist for hundreds of years.